RFC 8130 PROPOSED STANDARD AUDIO · RTP · CODECS
MELPe RTP Payload: RTP Payload Format for the Mixed Excitation Linear Prediction Enhanced (MELPe) Codec
How to carry the NATO-standard MELPe low-bitrate speech codec inside RTP packets, at 2400, 1200 or 600 bps.
RFC EDITOR
Demjanenko, V. and D. Satterlee, "RTP Payload Format for the Mixed Excitation Linear Prediction Enhanced (MELPe) Codec", RFC 8130, DOI 10.17487/RFC8130, March 2017, <https://www.rfc-editor.org/info/rfc8130>.
BIBTEX
@misc{rfc8130,
series = {Request for Comments},
number = 8130,
howpublished = {RFC 8130},
publisher = {RFC Editor},
doi = {10.17487/RFC8130},
url = {https://www.rfc-editor.org/info/rfc8130},
author = {V. Demjanenko and D. Satterlee},
title = {{RTP Payload Format for the Mixed Excitation Linear Prediction Enhanced (MELPe) Codec}},
pagetotal = 30,
year = 2017,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) V. Demjanenko
Request for Comments: 8130 D. Satterlee
Category: Standards Track VOCAL Technologies, Ltd.
ISSN: 2070-1721 March 2017
RTP Payload Format
for the Mixed Excitation Linear Prediction Enhanced (MELPe) Codec
Abstract
This document describes the RTP payload format for the Mixed
Excitation Linear Prediction Enhanced (MELPe) speech coder. MELPe's
three different speech encoding rates and sample frame sizes are
supported. Comfort noise procedures and packet loss concealment are
described in detail.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8130.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions ................................................2
2. Background ......................................................3
3. Payload Format ..................................................4
3.1. MELPe Bitstream Definitions ................................5
3.1.1. 2400 bps Bitstream Structure ........................6
3.1.2. 1200 bps Bitstream Structure ........................9
3.1.3. 600 bps Bitstream Structure ........................13
3.2. MELPe Comfort Noise Bitstream Definition ..................18
3.3. Multiple MELPe Frames in an RTP Packet ....................20
3.4. Congestion Control Considerations .........................21
4. Payload Format Parameters ......................................22
4.1. Media Type Definitions ....................................22
4.2. Mapping to SDP ............................................23
4.3. Declarative SDP Considerations ............................25
4.4. Offer/Answer SDP Considerations ...........................25
5. Discontinuous Transmissions ....................................26
6. Packet Loss Concealment ........................................26
7. IANA Considerations ............................................26
8. Security Considerations ........................................27
9. References .....................................................27
9.1. Normative References ......................................27
9.2. Informative References ....................................29
Authors' Addresses ................................................30
1. Introduction
This document describes how compressed Mixed Excitation Linear
Prediction Enhanced (MELPe) speech as produced by the MELPe codec
may be formatted for use as an RTP payload. Details are provided to
packetize the three different codec bitrate data frames (2400, 1200,
and 600) into RTP packets. The sender may send one or more codec
data frames per packet, depending on the application scenario or
based on transport network conditions, bandwidth restrictions, delay
requirements, and packet loss tolerance.
1.1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Best current practices for writing an RTP payload format
specification were followed [RFC2736].
2. Background
The MELP speech coder was developed by the US military as an upgrade
from the LPC-based CELP standard vocoder for low-bitrate
communications [MELP]. ("LPC" stands for "Linear-Predictive Coding",
and "CELP" stands for "Code-Excited Linear Prediction".) MELP was
further enhanced and subsequently adopted by NATO as MELPe for use by
its members and Partnership for Peace countries for military and
other governmental communications [MELPE]. The MELP speech coder
algorithm was developed by Atlanta Signal Processing (ASPI), Texas
Instruments (TI), SignalCom (now Microsoft), and Thales
Communications, with noise preprocessor contributions from AT&T,
under contract with NSA/DOD as international NATO Standard
STANAG 4591 [MELPE].
Commercial/civilian applications have arisen because of the
low-bitrate property of MELPe with its (relatively) high
intelligibility. As such, MELPe is being used in a variety of wired
and radio communications systems. Voice over IP (VoIP) / SIP systems
need to transport MELPe without decoding and re-encoding in order to
preserve its intelligibility. Hence, it is desirable and necessary
to define the proper payload formatting and use conventions of MELPe
in RTP payloads.
The MELPe codec [MELPE] supports three different vocoder bitrates:
2400, 1200, and 600 bps. The basic 2400 bps bitrate vocoder uses a
22.5 ms frame of speech consisting of 180 8000-Hz, 16-bit speech
samples. The 1200 and 600 bps bitrate vocoders each use three and
four 22.5 ms frames of speech, respectively. These reduced-bitrate
vocoders internally use multiple 2400 bps parameter sets with further
processing to strategically remove redundancy. The payload sizes for
each of the bitrates are 54, 81, and 54 bits for the 2400, 1200, and
600 bps frames, respectively. Dynamic bitrate switching is permitted
but only if supported by both endpoints.
The MELPe algorithm distinguishes between voiced and unvoiced speech
and encodes each differently. Unvoiced speech can be coded with
fewer information bits for the same quality. Forward error
correction (FEC) is applied to the 2400 bps codec unvoiced speech for
better protection of the subtle differences in signal reconstruction.
The lower-bitrate coders do not allocate any bits for FEC and rely on
strong error protection and correction in the communications channel.
Comfort noise handling for MELPe follows the procedures in Appendix B
of SCIP-210 [SCIP210]. After Voice Activity Detection (VAD)
no longer indicates the presence of speech/voice, a minimum of two
comfort noise vocoder frames (serving as a grace period) are to be
transmitted. The contents of the comfort noise frames are described
in the next section.
Packet loss concealment (PLC) exploits the FEC (and, more precisely,
any combination of two set bits in the pitch/voicing parameter) of
the 2400 bps speech coder. The pitch/voicing parameter has a sparse
set of permitted values. A value of zero indicates a non-voiced
frame. At least three bits are set for all valid pitch parameters.
The PLC erasure indication utilizes any errored/erasure encodings of
the pitch/voicing parameter with exactly two set bits, as described
below.
3. Payload Format
The MELPe codec uses 22.5, 67.5, or 90 ms frames with a sampling rate
clock of 8 kHz, so the RTP timestamp MUST be in units of 1/8000 of a
second.
The RTP payload for MELPe has the format shown in Figure 1. No
additional header specific to this payload format is needed. This
format is intended for situations where the sender and the receiver
send one or more codec data frames per packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ one or more frames of MELPe |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Packet Format Diagram
The RTP header of the packetized encoded MELPe speech has the
expected values as described in [RFC3550]. The usage of the M bit
SHOULD be as specified in the applicable RTP profile -- for example,
[RFC3551], where [RFC3551] specifies that if the sender does not
suppress silence (i.e., sends a frame on every frame interval), the
M bit will always be zero. When more than one codec data frame is
present in a single RTP packet, the timestamp is, as always, that of
the oldest data frame represented in the RTP packet.
The assignment of an RTP payload type for this new packet format is
outside the scope of this document and will not be specified here.
It is expected that the RTP profile for a particular class of
applications will assign a payload type for this encoding, or if that
is not done, then a payload type in the dynamic range shall be chosen
by the sender.
3.1. MELPe Bitstream Definitions
The total number of bits used to describe one frame of 2400 bps
speech is 54, which fits in 7 octets (with two unused bits). For
1200 bps speech, the total number of bits used is 81, which fits in
11 octets (with seven unused bits). For 600 bps speech, the total
number of bits used is 54, which fits in 7 octets (with two unused
bits). Unused bits, shown below as RSVA, RSVB, etc., are coded as
described in Section 3.3 in support of dynamic bitrate switching.
In the MELPe bitstream definitions, the most significant bits are
considered priority bits. The intention was that these bits receive
greater protection in the underlying communications channel. For IP
networks, such additional protection is irrelevant. However, for the
convenience of interoperable gateway devices, the bitstreams will be
presented identically in IP networks.
3.1.1. 2400 bps Bitstream Structure
According to Table 3 of [MELPE], the 2400 bps MELPe bit transmission
order (for clarity, the bit priority is not shown) is as follows:
+--------+-------------+-------------+
| Bit | Voiced | Unvoiced |
+--------+-------------+-------------+
| B_01 | g20 | g20 |
| B_02 | BP0 | FEC10 |
| B_03 | P0 | P0 |
| B_04 | LSF20 | LSF20 |
| B_05 | LSF30 | LSF30 |
| B_06 | g23 | g23 |
| B_07 | g24 | g24 |
| B_08 | LSF35 | LSF35 |
+--------+-------------+-------------+
| B_09 | g21 | g21 |
| B_10 | g22 | g22 |
| B_11 | P4 | P4 |
| B_12 | LSF34 | LSF34 |
| B_13 | P5 | P5 |
| B_14 | P1 | P1 |
| B_15 | P2 | P2 |
| B_16 | LSF40 | LSF40 |
+--------+-------------+-------------+
| B_17 | P6 | P6 |
| B_18 | LSF10 | LSF10 |
| B_19 | LSF16 | LSF16 |
| B_20 | LSF45 | LSF45 |
| B_21 | P3 | P3 |
| B_22 | LSF15 | LSF15 |
| B_23 | LSF14 | LSF14 |
| B_24 | LSF25 | LSF25 |
+--------+-------------+-------------+
| B_25 | BP3 | FEC13 |
| B_26 | LSF13 | LSF13 |
| B_27 | LSF12 | LSF12 |
| B_28 | LSF24 | LSF24 |
| B_29 | LSF44 | LSF44 |
| B_30 | FM0 | FEC40 |
| B_31 | LSF11 | LSF11 |
| B_32 | LSF23 | LSF23 |
+--------+-------------+-------------+
| B_33 | FM7 | FEC22 |
| B_34 | FM6 | FEC21 |
| B_35 | FM5 | FEC20 |
| B_36 | g11 | g11 |
| B_37 | g10 | g10 |
| B_38 | BP2 | FEC12 |
| B_39 | BP1 | FEC11 |
| B_40 | LSF21 | LSF21 |
+--------+-------------+-------------+
| B_41 | LSF33 | LSF33 |
| B_42 | LSF22 | LSF22 |
| B_43 | LSF32 | LSF32 |
| B_44 | LSF31 | LSF31 |
| B_45 | LSF43 | LSF43 |
| B_46 | LSF42 | LSF42 |
| B_47 | AF | FEC42 |
| B_48 | LSF41 | LSF41 |
+--------+-------------+-------------+
| B_49 | FM4 | FEC32 |
| B_50 | FM3 | FEC31 |
| B_51 | FM2 | FEC30 |
| B_52 | FM1 | FEC41 |
| B_53 | g12 | g12 |
| B_54 | SYNC | SYNC |
+--------+-------------+-------------+
Notes:
g = Gain
BP = Bandpass Voicing
P = Pitch/Voicing
LSF = Line Spectral Frequencies
FEC = Forward Error Correction Parity Bits
FM = Fourier Magnitudes
AF = Aperiodic Flag
B_01 = least significant bit of data set
Table 1: Bitstream Definition for MELPe 2400 bps
The 2400 bps MELPe RTP payload is constructed as per Figure 2. Note
that bit B_01 is placed in the least significant bit (LSB) of the
first byte with all other bits in sequence. When filling octets, the
least significant bits of the seventh octet are filled with bits B_49
to B_54, respectively.
MSB LSB
0 1 2 3 4 5 6 7
+------+------+------+------+------+------+------+------+
| B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
+------+------+------+------+------+------+------+------+
| B_16 | B_15 | B_14 | B_13 | B_12 | B_11 | B_10 | B_09 |
+------+------+------+------+------+------+------+------+
| B_24 | B_23 | B_22 | B_21 | B_20 | B_19 | B_18 | B_17 |
+------+------+------+------+------+------+------+------+
| B_32 | B_31 | B_30 | B_29 | B_28 | B_27 | B_26 | B_25 |
+------+------+------+------+------+------+------+------+
| B_40 | B_39 | B_38 | B_37 | B_36 | B_35 | B_34 | B_33 |
+------+------+------+------+------+------+------+------+
| B_48 | B_47 | B_46 | B_45 | B_44 | B_43 | B_42 | B_41 |
+------+------+------+------+------+------+------+------+
| RSVA | RSVB | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
+------+------+------+------+------+------+------+------+
Figure 2: Packed MELPe 2400 bps Payload Octets
3.1.2. 1200 bps Bitstream Structure
According to Tables D-9a and D-9b of [MELPE], the 1200 bps MELPe bit
transmission order is as follows:
+--------+-------------+-------------+
| Bit | Modes 1-4 | Mode 5 |
| | (Voiced) | (Unvoiced) |
+--------+-------------+-------------+
| B_01 | Syn | Syn |
| B_02 | Pitch&UV0 | Pitch&UV0 |
| B_03 | Pitch&UV1 | Pitch&UV1 |
| B_04 | Pitch&UV2 | Pitch&UV2 |
| B_05 | Pitch&UV3 | Pitch&UV3 |
| B_06 | Pitch&UV4 | Pitch&UV4 |
| B_07 | Pitch&UV5 | Pitch&UV5 |
| B_08 | Pitch&UV6 | Pitch&UV6 |
+--------+-------------+-------------+
| B_09 | Pitch&UV7 | Pitch&UV7 |
| B_10 | Pitch&UV8 | Pitch&UV8 |
| B_11 | Pitch&UV9 | Pitch&UV9 |
| B_12 | Pitch&UV10 | Pitch&UV10 |
| B_13 | Pitch&UV11 | Pitch&UV11 |
| B_14 | LSP0 | LSP0 |
| B_15 | LSP1 | LSP1 |
| B_16 | LSP2 | LSP2 |
+--------+-------------+-------------+
| B_17 | LSP3 | LSP3 |
| B_18 | LSP4 | LSP4 |
| B_19 | LSP5 | LSP5 |
| B_20 | LSP6 | LSP6 |
| B_21 | LSP7 | LSP7 |
| B_22 | LSP8 | LSP8 |
| B_23 | LSP9 | LSP9 |
| B_24 | LSP10 | LSP10 |
+--------+-------------+-------------+
| B_25 | LSP11 | LSP11 |
| B_26 | LSP12 | LSP12 |
| B_27 | LSP13 | LSP13 |
| B_28 | LSP14 | LSP14 |
| B_29 | LSP15 | LSP15 |
| B_30 | LSP16 | LSP16 |
| B_31 | LSP17 | LSP17 |
| B_32 | LSP18 | LSP18 |
+--------+-------------+-------------+
| B_33 | LSP19 | LSP19 |
| B_34 | LSP20 | LSP20 |
| B_35 | LSP21 | LSP21 |
| B_36 | LSP22 | LSP22 |
| B_37 | LSP23 | LSP23 |
| B_38 | LSP24 | LSP24 |
| B_39 | LSP25 | LSP25 |
| B_40 | LSP26 | LSP26 |
+--------+-------------+-------------+
| B_41 | LSP27 | GAIN0 |
| B_42 | LSP28 | GAIN1 |
| B_43 | LSP29 | GAIN2 |
| B_44 | LSP30 | GAIN3 |
| B_45 | LSP31 | GAIN4 |
| B_46 | LSP32 | GAIN5 |
| B_47 | LSP33 | GAIN6 |
| B_48 | LSP34 | GAIN7 |
+--------+-------------+-------------+
| B_49 | LSP35 | GAIN8 |
| B_50 | LSP36 | GAIN9 |
| B_51 | LSP37 | |
| B_52 | LSP38 | |
| B_53 | LSP39 | |
| B_54 | LSP40 | |
| B_55 | LSP41 | |
| B_56 | LSP42 | |
+--------+-------------+-------------+
| B_57 | GAIN0 | |
| B_58 | GAIN1 | |
| B_59 | GAIN2 | |
| B_60 | GAIN3 | |
| B_61 | GAIN4 | |
| B_62 | GAIN5 | |
| B_63 | GAIN6 | |
| B_64 | GAIN7 | |
+--------+-------------+-------------+
| B_65 | GAIN8 | |
| B_66 | GAIN9 | |
| B_67 | BP0 | |
| B_68 | BP1 | |
| B_69 | BP2 | |
| B_70 | BP3 | |
| B_71 | BP4 | |
| B_72 | BP5 | |
+--------+-------------+-------------+
| B_73 | JITTER | |
| B_74 | FS0 | |
| B_75 | FS1 | |
| B_76 | FS2 | |
| B_77 | FS3 | |
| B_78 | FS4 | |
| B_79 | FS5 | |
| B_80 | FS6 | |
+--------+-------------+-------------+
| B_81 | FS7 | |
+--------+-------------+-------------+
Notes:
BP = Bandpass voicing
FS = Fourier magnitudes
LSP = Line Spectral Pair
Pitch&UV = Pitch/voicing
GAIN = Gain
JITTER = Jitter
Table 2: Bitstream Definition for MELPe 1200 bps
The 1200 bps MELPe RTP payload is constructed as per Figure 3. Note
that bit B_01 is placed in the LSB of the first byte with all other
bits in sequence. When filling octets, the least significant bit of
the eleventh octet is filled with bit B_81.
MSB LSB
0 1 2 3 4 5 6 7
+------+------+------+------+------+------+------+------+
| B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
+------+------+------+------+------+------+------+------+
| B_16 | B_15 | B_14 | B_13 | B_12 | B_11 | B_10 | B_09 |
+------+------+------+------+------+------+------+------+
| B_24 | B_23 | B_22 | B_21 | B_20 | B_19 | B_18 | B_17 |
+------+------+------+------+------+------+------+------+
| B_32 | B_31 | B_30 | B_29 | B_28 | B_27 | B_26 | B_25 |
+------+------+------+------+------+------+------+------+
| B_40 | B_39 | B_38 | B_37 | B_36 | B_35 | B_34 | B_33 |
+------+------+------+------+------+------+------+------+
| B_48 | B_47 | B_46 | B_45 | B_44 | B_43 | B_42 | B_41 |
+------+------+------+------+------+------+------+------+
| B_56 | B_55 | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
+------+------+------+------+------+------+------+------+
| B_64 | B_63 | B_62 | B_61 | B_60 | B_59 | B_58 | B_57 |
+------+------+------+------+------+------+------+------+
| B_72 | B_71 | B_70 | B_69 | B_68 | B_67 | B_66 | B_65 |
+------+------+------+------+------+------+------+------+
| B_80 | B_79 | B_78 | B_77 | B_76 | B_75 | B_74 | B_73 |
+------+------+------+------+------+------+------+------+
| RSVA | RSVB | RSVC | RSV0 | RSV0 | RSV0 | RSV0 | B_81 |
+------+------+------+------+------+------+------+------+
Figure 3: Packed MELPe 1200 bps Payload Octets
3.1.3. 600 bps Bitstream Structure
According to Tables M-11 to M-16 of [MELPE], the 600 bps MELPe bit
transmission order (for clarity, the bit priority is not shown) is as
follows:
+--------+-------------+-------------+-------------+
| Bit | Mode 1 | Mode 2 | Mode 3 |
| | (Voiced) | (voiced) | (voiced) |
+--------+-------------+-------------+-------------+
| B_01 | Voicing (4) | Voicing (4) | Voicing (4) |
| B_02 | Voicing (3) | Voicing (3) | Voicing (3) |
| B_03 | Voicing (2) | Voicing (2) | Voicing (2) |
| B_04 | Voicing (1) | Voicing (1) | Voicing (1) |
| B_05 | Voicing (0) | Voicing (0) | Voicing (0) |
| B_06 | LSF1,4 (3) | Pitch (5) | Pitch (7) |
| B_07 | LSF1,4 (2) | Pitch (4) | Pitch (6) |
| B_08 | LSF1,4 (1) | Pitch (3) | Pitch (5) |
+--------+-------------+-------------+-------------+
| B_09 | LSF1,4 (0) | Pitch (2) | Pitch (4) |
| B_10 | LSF1,3 (3) | Pitch (1) | Pitch (3) |
| B_11 | LSF1,3 (2) | Pitch (0) | Pitch (2) |
| B_12 | LSF1,3 (1) | LSF1,3 (3) | Pitch (1) |
| B_13 | LSF1,3 (0) | LSF1,3 (2) | Pitch (0) |
| B_14 | LSF1,2 (3) | LSF1,3 (1) | LSF1,3 (3) |
| B_15 | LSF1,2 (2) | LSF1,3 (0) | LSF1,3 (2) |
| B_16 | LSF1,2 (1) | LSF1,2 (3) | LSF1,3 (1) |
+--------+-------------+-------------+-------------+
| B_17 | LSF1,2 (0) | LSF1,2 (2) | LSF1,3 (0) |
| B_18 | LSF1,1 (5) | LSF1,2 (1) | LSF1,2 (4) |
| B_19 | LSF1,1 (4) | LSF1,2 (0) | LSF1,2 (3) |
| B_20 | LSF1,1 (3) | LSF1,1 (5) | LSF1,2 (2) |
| B_21 | LSF1,1 (2) | LSF1,1 (4) | LSF1,2 (1) |
| B_22 | LSF1,1 (1) | LSF1,1 (3) | LSF1,2 (0) |
| B_23 | LSF1,1 (0) | LSF1,1 (2) | LSF1,1 (5) |
| B_24 | LSF2,4 (3) | LSF1,1 (1) | LSF1,1 (4) |
+--------+-------------+-------------+-------------+
| B_25 | LSF2,4 (2) | LSF1,1 (0) | LSF1,1 (3) |
| B_26 | LSF2,4 (1) | LSF2,3 (3) | LSF1,1 (2) |
| B_27 | LSF2,4 (0) | LSF2,3 (2) | LSF1,1 (1) |
| B_28 | LSF2,3 (3) | LSF2,3 (1) | LSF1,1 (0) |
| B_29 | LSF2,3 (2) | LSF2,3 (0) | LSF2,3 (3) |
| B_30 | LSF2,3 (1) | LSF2,2 (4) | LSF2,3 (2) |
| B_31 | LSF2,3 (0) | LSF2,2 (3) | LSF2,3 (1) |
| B_32 | LSF2,2 (3) | LSF2,2 (2) | LSF2,3 (0) |
+--------+-------------+-------------+-------------+
| B_33 | LSF2,2 (2) | LSF2,2 (1) | LSF2,2 (4) |
| B_34 | LSF2,2 (1) | LSF2,2 (0) | LSF2,2 (3) |
| B_35 | LSF2,2 (0) | LSF2,1 (6) | LSF2,2 (2) |
| B_36 | LSF2,1 (5) | LSF2,1 (5) | LSF2,2 (1) |
| B_37 | LSF2,1 (4) | LSF2,1 (4) | LSF2,2 (0) |
| B_38 | LSF2,1 (3) | LSF2,1 (3) | LSF2,1 (5) |
| B_39 | LSF2,1 (2) | LSF2,1 (2) | LSF2,1 (4) |
| B_40 | LSF2,1 (1) | LSF2,1 (1) | LSF2,1 (3) |
+--------+-------------+-------------+-------------+
| B_41 | LSF2,1 (0) | LSF2,1 (0) | LSF2,1 (2) |
| B_42 | GAIN2 (5) | GAIN2 (5) | LSF2,1 (1) |
| B_43 | GAIN2 (4) | GAIN2 (4) | LSF2,1 (0) |
| B_44 | GAIN2 (3) | GAIN2 (3) | GAIN2 (4) |
| B_45 | GAIN2 (2) | GAIN2 (2) | GAIN2 (3) |
| B_46 | GAIN2 (1) | GAIN2 (1) | GAIN2 (2) |
| B_47 | GAIN2 (0) | GAIN2 (0) | GAIN2 (1) |
| B_48 | GAIN1 (6) | GAIN1 (6) | GAIN2 (0) |
+--------+-------------+-------------+-------------+
| B_49 | GAIN1 (5) | GAIN1 (5) | GAIN1 (5) |
| B_50 | GAIN1 (4) | GAIN1 (4) | GAIN1 (4) |
| B_51 | GAIN1 (3) | GAIN1 (3) | GAIN1 (3) |
| B_52 | GAIN1 (2) | GAIN1 (2) | GAIN1 (2) |
| B_53 | GAIN1 (1) | GAIN1 (1) | GAIN1 (1) |
| B_54 | GAIN1 (0) | GAIN1 (0) | GAIN1 (0) |
+--------+-------------+-------------+-------------+
Table 3: Bitstream Definition for MELPe 600 bps (Part 1 of 2)
+--------+-------------+-------------+-------------+
| Bit | Mode 4 | Mode 5 | Mode 6 |
| | (voiced) | (voiced) | (voiced) |
+--------+-------------+-------------+-------------+
| B_01 | Voicing (4) | Voicing (4) | Voicing (4) |
| B_02 | Voicing (3) | Voicing (3) | Voicing (3) |
| B_03 | Voicing (2) | Voicing (2) | Voicing (2) |
| B_04 | Voicing (1) | Voicing (1) | Voicing (1) |
| B_05 | Voicing (0) | Voicing (0) | Voicing (0) |
| B_06 | Pitch (7) | Pitch (7) | Pitch (7) |
| B_07 | Pitch (6) | Pitch (6) | Pitch (6) |
| B_08 | Pitch (5) | Pitch (5) | Pitch (5) |
+--------+-------------+-------------+-------------+
| B_09 | Pitch (4) | Pitch (4) | Pitch (4) |
| B_10 | Pitch (3) | Pitch (3) | Pitch (3) |
| B_11 | Pitch (2) | Pitch (2) | Pitch (2) |
| B_12 | Pitch (1) | Pitch (1) | Pitch (1) |
| B_13 | Pitch (0) | Pitch (0) | Pitch (0) |
| B_14 | LSF1,3 (3) | LSF1,3 (3) | LSF1,3 (3) |
| B_15 | LSF1,3 (2) | LSF1,3 (2) | LSF1,3 (2) |
| B_16 | LSF1,3 (1) | LSF1,3 (1) | LSF1,3 (1) |
+--------+-------------+-------------+-------------+
| B_17 | LSF1,3 (0) | LSF1,3 (0) | LSF1,3 (0) |
| B_18 | LSF1,2 (3) | LSF1,2 (4) | LSF1,2 (4) |
| B_19 | LSF1,2 (2) | LSF1,2 (3) | LSF1,2 (3) |
| B_20 | LSF1,2 (1) | LSF1,2 (2) | LSF1,2 (2) |
| B_21 | LSF1,2 (0) | LSF1,2 (1) | LSF1,2 (1) |
| B_22 | LSF1,1 (5) | LSF1,2 (0) | LSF1,2 (0) |
| B_23 | LSF1,1 (4) | LSF1,1 (5) | LSF1,1 (6) |
| B_24 | LSF1,1 (3) | LSF1,1 (4) | LSF1,1 (5) |
+--------+-------------+-------------+-------------+
| B_25 | LSF1,1 (2) | LSF1,1 (3) | LSF1,1 (4) |
| B_26 | LSF1,1 (1) | LSF1,1 (2) | LSF1,1 (3) |
| B_27 | LSF1,1 (0) | LSF1,1 (1) | LSF1,1 (2) |
| B_28 | LSF2,3 (3) | LSF1,1 (0) | LSF1,1 (1) |
| B_29 | LSF2,3 (2) | LSF2,3 (3) | LSF1,1 (0) |
| B_30 | LSF2,3 (1) | LSF2,3 (2) | LSF2,3 (3) |
| B_31 | LSF2,3 (0) | LSF2,3 (1) | LSF2,3 (2) |
| B_32 | LSF2,2 (4) | LSF2,3 (0) | LSF2,3 (1) |
+--------+-------------+-------------+-------------+
| B_33 | LSF2,2 (3) | LSF2,2 (4) | LSF2,3 (0) |
| B_34 | LSF2,2 (2) | LSF2,2 (3) | LSF2,2 (4) |
| B_35 | LSF2,2 (1) | LSF2,2 (2) | LSF2,2 (3) |
| B_36 | LSF2,2 (0) | LSF2,2 (1) | LSF2,2 (2) |
| B_37 | LSF2,1 (6) | LSF2,2 (0) | LSF2,2 (1) |
| B_38 | LSF2,1 (5) | LSF2,1 (5) | LSF2,2 (0) |
| B_39 | LSF2,1 (4) | LSF2,1 (4) | LSF2,1 (6) |
| B_40 | LSF2,1 (3) | LSF2,1 (3) | LSF2,1 (5) |
+--------+-------------+-------------+-------------+
| B_41 | LSF2,1 (2) | LSF2,1 (2) | LSF2,1 (4) |
| B_42 | LSF2,1 (1) | LSF2,1 (1) | LSF2,1 (3) |
| B_43 | LSF2,1 (0) | LSF2,1 (0) | LSF2,1 (2) |
| B_44 | GAIN2 (4) | GAIN2 (4) | LSF2,1 (1) |
| B_45 | GAIN2 (3) | GAIN2 (3) | LSF2,1 (0) |
| B_46 | GAIN2 (2) | GAIN2 (2) | GAIN1 (8) |
| B_47 | GAIN2 (1) | GAIN2 (1) | GAIN1 (7) |
| B_48 | GAIN2 (0) | GAIN2 (0) | GAIN1 (6) |
+--------+-------------+-------------+-------------+
| B_49 | GAIN1 (5) | GAIN1 (5) | GAIN1 (5) |
| B_50 | GAIN1 (4) | GAIN1 (4) | GAIN1 (4) |
| B_51 | GAIN1 (3) | GAIN1 (3) | GAIN1 (3) |
| B_52 | GAIN1 (2) | GAIN1 (2) | GAIN1 (2) |
| B_53 | GAIN1 (1) | GAIN1 (1) | GAIN1 (1) |
| B_54 | GAIN1 (0) | GAIN1 (0) | GAIN1 (0) |
+--------+-------------+-------------+-------------+
Notes:
xxxx (0) = LSB
xxxx (nbits-1) = MSB
LSF1,p = MSVQ* index of the pth stage of the two first frames
LSF2,p = MSVQ index of the pth stage of the two last frames
GAIN1 = VQ/MSVQ index of the 1st stage
GAIN2 = MSVQ index of the 2nd stage
* MSVQ: Multi-Stage Vector Quantizer
Table 4: Bitstream Definition for MELPe 600 bps (Part 2 of 2)
The 600 bps MELPe RTP payload is constructed as per Figure 4. Note
that bit B_01 is placed in the LSB of the first byte with all other
bits in sequence. When filling octets, the least significant bits of
the seventh octet are filled with bits B_49 to B_54, respectively.
MSB LSB
0 1 2 3 4 5 6 7
+------+------+------+------+------+------+------+------+
| B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
+------+------+------+------+------+------+------+------+
| B_16 | B_15 | B_14 | B_13 | B_12 | B_11 | B_10 | B_09 |
+------+------+------+------+------+------+------+------+
| B_24 | B_23 | B_22 | B_21 | B_20 | B_19 | B_18 | B_17 |
+------+------+------+------+------+------+------+------+
| B_32 | B_31 | B_30 | B_29 | B_28 | B_27 | B_26 | B_25 |
+------+------+------+------+------+------+------+------+
| B_40 | B_39 | B_38 | B_37 | B_36 | B_35 | B_34 | B_33 |
+------+------+------+------+------+------+------+------+
| B_48 | B_47 | B_46 | B_45 | B_44 | B_43 | B_42 | B_41 |
+------+------+------+------+------+------+------+------+
| RSVA | RSVB | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
+------+------+------+------+------+------+------+------+
Figure 4: Packed MELPe 600 bps Payload Octets
3.2. MELPe Comfort Noise Bitstream Definition
Table B.3-1 of [SCIP210] identifies the usage of MELPe 2400 bps
parameters for conveying comfort noise.
+-------------------------------------+----------------+
| MELPe Parameter | Value |
+-------------------------------------+----------------+
| msvq[0] (line spectral frequencies) | * See Note |
+-------------------------------------+----------------+
| msvq[1] (line spectral frequencies) | Set to 0 |
+-------------------------------------+----------------+
| msvq[2] (line spectral frequencies) | Set to 0 |
+-------------------------------------+----------------+
| msvq[3] (line spectral frequencies) | Set to 0 |
+-------------------------------------+----------------+
| fsvq (Fourier magnitudes) | Set to 0 |
+-------------------------------------+----------------+
| gain[0] (gain) | Set to 0 |
+-------------------------------------+----------------+
| gain[1] (gain) | * See Note |
+-------------------------------------+----------------+
| pitch (pitch - overall voicing) | Set to 0 |
+-------------------------------------+----------------+
| bp (bandpass voicing) | Set to 0 |
+-------------------------------------+----------------+
| af (aperiodic flag/jitter index) | Set to 0 |
+-------------------------------------+----------------+
| sync (sync bit) | Alternations |
+-------------------------------------+----------------+
Note:
The default values are the respective parameters from the
vocoder frame. It is preferred that msvq[0] and gain[1]
values be derived by averaging the respective parameter from
some number of previous vocoder frames.
Table 5: MELPe Comfort Noise Parameters
Since only msvq[0] (also known as LSF1x or the first LSP) and gain[1]
(also known as g2x or the second gain) are needed, the following bit
order is used for comfort noise frames:
+--------+-------------+
| Bit | Comfort |
| | Noise |
+--------+-------------+
| B_01 | LSF10 |
| B_02 | LSF11 |
| B_03 | LSF12 |
| B_04 | LSF13 |
| B_05 | LSF14 |
| B_06 | LSF15 |
| B_07 | LSF16 |
| B_08 | g20 |
+--------+-------------+
| B_09 | g21 |
| B_10 | g22 |
| B_11 | g23 |
| B_12 | g24 |
| B_13 | SYNC |
+--------+-------------+
Notes:
g = Gain
LSF = Line Spectral Frequencies
Table 6: Bitstream Definition for MELPe Comfort Noise
The comfort noise MELPe RTP payload is constructed as per Figure 5.
Note that bit B_01 is placed in the LSB of the first byte with all
other bits in sequence. When filling octets, the least significant
bits of the second octet are filled with bits B_09 to B_13,
respectively.
MSB LSB
0 1 2 3 4 5 6 7
+------+------+------+------+------+------+------+------+
| B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
+------+------+------+------+------+------+------+------+
| RSVA | RSVB | RSVC | B_13 | B_12 | B_11 | B_10 | B_09 |
+------+------+------+------+------+------+------+------+
Figure 5: Packed MELPe Comfort Noise Payload Octets
3.3. Multiple MELPe Frames in an RTP Packet
A MELPe RTP packet MAY consist of zero or more MELPe coder frames
followed by zero or one MELPe comfort noise frame. The presence of a
comfort noise frame can be deduced from the length of the RTP
payload. The default packetization interval is one coder frame
(22.5, 67.5, or 90 ms) according to the coder bitrate (2400, 1200, or
600 bps). For some applications, a longer packetization interval is
used to reduce the packet rate.
A MELPe RTP packet comprised of no coder frame and no comfort noise
frame MAY be used periodically by an endpoint to indicate
connectivity by an otherwise idle receiver.
All MELPe frames in a single RTP packet MUST be of the same coder
bitrate. Dynamic switching between frame rates within an RTP stream
may be permitted (if supported by both ends) provided that reserved
bits RSVA, RSVB, and RSVC are filled in as per Table 7. If bitrate
switching is not used, all reserved bits are encoded as 0 by the
sender and ignored by the receiver. (RSV0 is always coded as 0.)
+-------------------+------+------+------+
| Coder Bitrate | RSVA | RSVB | RSVC |
+-------------------+------+------+------+
| 2400 bps | 0 | 0 | N/A |
+-------------------+------+------+------+
| 1200 bps | 1 | 0 | 0 |
+-------------------+------+------+------+
| 600 bps | 0 | 1 | N/A |
+-------------------+------+------+------+
| Comfort Noise | 1 | 0 | 1 |
+-------------------+------+------+------+
| (reserved) | 1 | 1 | N/A |
+-------------------+------+------+------+
Table 7: MELPe Frame Bitrate Indicators
It is important to observe that senders have the following additional
restrictions:
Senders SHOULD NOT include more MELPe frames in a single RTP packet
than will fit in the MTU of the RTP transport protocol.
Frames MUST NOT be split between RTP packets.
It is RECOMMENDED that the number of frames contained within an RTP
packet be consistent with the application. For example, in telephony
and other real-time applications where delay is important, then the
fewer frames per packet the lower the delay, whereas for bandwidth-
constrained links or delay-insensitive streaming messaging
applications, more than one frame per packet or many frames per
packet would be acceptable.
Information describing the number of frames contained in an RTP
packet is not transmitted as part of the RTP payload. The way to
determine the number of MELPe frames is to count the total number of
octets within the RTP packet and divide the octet count by the number
of expected octets per frame (7/11/7 per frame). Keep in mind that
the last frame can be a 2-octet comfort noise frame.
When dynamic bitrate switching is used and more than one frame is
contained in an RTP packet, it is RECOMMENDED that the coder rate
bits contained in the last octet be inspected. If the coder bitrate
indicates a comfort noise frame, then inspect the third last octet
for the coder bitrate. All MELPe speech frames in the RTP packet
will be of this same coder bitrate.
3.4. Congestion Control Considerations
The target bitrate of MELPe can be adjusted at any point in time,
thus allowing congestion management. Furthermore, the amount of
encoded speech or audio data encoded in a single packet can be used
for congestion control, since the packet rate is inversely
proportional to the packet duration. A lower packet transmission
rate reduces the amount of header overhead but at the same time
increases latency and loss sensitivity, so it ought to be used
with care.
Since UDP does not provide congestion control, applications that use
RTP over UDP SHOULD implement their own congestion control above the
UDP layer [RFC8085] and MAY also implement a transport circuit
breaker [RFC8083]. Work in the RMCAT working group [RMCAT] describes
the interactions and conceptual interfaces necessary between the
application components that relate to congestion control, including
the RTP layer, the higher-level media codec control layer, and the
lower-level transport interface, as well as components dedicated to
congestion control functions.
4. Payload Format Parameters
This RTP payload format is identified using the MELP, MELP2400,
MELP1200, and MELP600 media subtypes, which are registered in
accordance with RFC 4855 [RFC4855] and per the media type
registration template from RFC 6838 [RFC6838].
4.1. Media Type Definitions
Type name: audio
Subtype names: MELP, MELP2400, MELP1200, and MELP600
Required parameters: N/A
Optional parameters:
ptime: the recommended length of time (in milliseconds)
represented by the media in a packet. It SHALL use the nearest
rounded-up ms integer packet duration. For MELPe, this
corresponds to the following values: 23, 45, 68, 90, 112, 135,
156, and 180. Larger values can be used as long as they are
properly rounded. See Section 6 of RFC 4566 [RFC4566].
maxptime: the maximum length of time (in milliseconds) that can be
encapsulated in a packet. It SHALL use the nearest rounded-up
ms integer packet duration. For MELPe, this corresponds to the
following values: 23, 45, 68, 90, 112, 135, 156, and 180.
Larger values can be used as long as they are properly rounded.
See Section 6 of RFC 4566 [RFC4566].
bitrate: specifies the MELPe coder bitrates supported. Possible
values are a comma-separated list of rates from the following
set: 2400, 1200, 600. The modes are listed in order of
preference; first is preferred. If "bitrate" is not present,
the fixed coder bitrate of 2400 MUST be used. The alternate
encoding names "MELP2400", "MELP1200", and "MELP600" directly
specify the MELPe coder bitrates of 2400, 1200, and 600,
respectively, and MUST NOT specify a "bitrate" parameter.
Encoding considerations: These media subtypes are framed and binary;
see Section 4.8 of RFC 6838 [RFC6838].
Security considerations: Please see Section 8 of RFC 8130.
Interoperability considerations: Early implementations used MELP2400,
MELP1200, and MELP600 to indicate both coder type and bitrate.
These media type names should be preserved with this registration.
Published specification: N/A
Applications that use this media type: N/A
Additional information: N/A
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information:
Victor Demjanenko, Ph.D.
VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Buffalo, NY 14228
United States of America
Phone: +1 716 688 4675
Email: victor.demjanenko@vocal.com
Intended usage: COMMON
Restrictions on usage: These media subtypes depend on RTP framing and
hence are only defined for transfer via RTP [RFC3550]. Transport
within other framing protocols is not defined at this time.
Author: Victor Demjanenko
Change controller: IETF Payload working group delegated from the
IESG.
Provisional registration? (standards tree only): No
4.2. Mapping to SDP
The mapping of the above-defined payload format media subtypes and
their parameters SHALL be done according to Section 3 of RFC 4855
[RFC4855].
The information carried in the media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[RFC4566], which is commonly used to describe RTP sessions. When SDP
is used to specify sessions employing the MELPe codec, the mapping is
as follows:
o The media type ("audio") goes in SDP "m=" as the media name.
o The media subtype (payload format name) goes in SDP "a=rtpmap" as
the encoding name.
o The parameter "bitrate" goes in the SDP "a=fmtp" attribute by
copying it as a "bitrate=<value>" string.
o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and
"a=maxptime" attributes, respectively.
When conveying information via SDP, the encoding name SHALL be "MELP"
(the same as the media subtype). Alternate encoding name subtypes
"MELP2400", "MELP1200", and "MELP600" MAY be used in SDP to convey
fixed-bitrate configurations. These names have been observed in
systems that do not support dynamic frame-rate switching as specified
by the parameter "bitrate".
An example of the media representation in SDP for describing MELPe
might be:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 MELP/8000
An alternative example of SDP for fixed-bitrate configurations
might be:
m=audio 49120 RTP/AVP 97 100 101 102
a=rtpmap:97 MELP/8000
a=rtpmap:100 MELP2400/8000
a=rtpmap:101 MELP1200/8000
a=rtpmap:102 MELP600/8000
If the encoding name "MELP" is received without a "bitrate"
parameter, the fixed coder bitrate of 2400 MUST be used. The
alternate encoding names "MELP2400", "MELP1200", and "MELP600"
directly specify the MELPe coder bitrates of 2400, 1200, and 600,
respectively, and MUST NOT specify a "bitrate" parameter.
The optional media type parameter "bitrate", when present, MUST be
included in the "a=fmtp" attribute in the SDP, expressed as a media
type string in the form of a semicolon-separated list of
parameter=value pairs. The string "value" can be one or more of
2400, 1200, and 600, separated by commas (where each bitrate value
indicates the corresponding MELPe coder). An example of the media
representation in SDP for describing MELPe when all three coder
bitrates are supported might be:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 MELP/8000
a=fmtp:97 bitrate=2400,600,1200
Parameter "ptime" cannot be used for the purpose of specifying the
MELPe operating mode, due to the fact that for certain values it will
be impossible to distinguish which mode is about to be used (e.g.,
when ptime=68, it would be impossible to distinguish if the packet is
carrying one frame of 67.5 ms or three frames of 22.5 ms).
Note that the payload format (encoding) names are commonly shown in
upper case. Media subtypes are commonly shown in lower case. These
names are case insensitive in both places. Similarly, parameter
names are case insensitive in both the media subtype name and the
default mapping to the SDP a=fmtp attribute.
4.3. Declarative SDP Considerations
For declarative media, the "bitrate" parameter specifies the possible
bitrates used by the sender. Multiple MELPe rtpmap values (such as
97, 98, and 99, as used below) MAY be used to convey MELPe-coded
voice at different bitrates. The receiver can then select an
appropriate MELPe codec by using 97, 98, or 99.
m=audio 49120 RTP/AVP 97 98 99
a=rtpmap:97 MELP/8000
a=fmtp:97 bitrate=2400
a=rtpmap:98 MELP/8000
a=fmtp:98 bitrate=1200
a=rtpmap:99 MELP/8000
a=fmtp:99 bitrate=600
4.4. Offer/Answer SDP Considerations
In the Offer/Answer model [RFC3264], "bitrate" is a bidirectional
parameter. Both sides MUST use a common "bitrate" value or values.
The offer contains the bitrates supported by the offerer, listed in
its preferred order. The answerer MAY agree to any bitrate by
listing the bitrate first in the answerer response. Additionally,
the answerer MAY indicate any secondary bitrate or bitrates that it
supports. The initial bitrate used by both parties SHALL be the
first bitrate specified in the answerer response.
For example, if offerer bitrates are "2400,600" and answer bitrates
are "600,2400", the initial bitrate is 600. If other bitrates are
provided by the answerer, any common bitrate between the offer and
answer MAY be used at any time in the future. Activation of these
other common bitrates is beyond the scope of this document.
The use of a lower bitrate is often important for a case such as when
one endpoint utilizes a bandwidth-constrained link (e.g., 1200 bps
radio link or slower), where only the lower coder bitrate will work.
5. Discontinuous Transmissions
A primary application of MELPe is for radio communications of voice
conversations, and discontinuous transmissions are normal. When
MELPe is used in an IP network, MELPe RTP packet transmissions may
cease and resume frequently. RTP synchronization source (SSRC)
sequence number gaps indicate lost packets to be filled by PLC, while
abrupt loss of RTP packets indicates intended discontinuous
transmissions.
If a MELPe coder so desires, it may send a comfort noise frame as per
Appendix B of [SCIP210] prior to ceasing transmission. A receiver
may optionally use comfort noise during its silence periods. No SDP
negotiations are required.
6. Packet Loss Concealment
MELPe packet loss concealment (PLC) uses the special properties and
coding for the pitch/voicing parameter of the MELPe 2400 bps coder.
The PLC erasure indication utilizes any of the errored encodings of a
non-voiced frame as identified in Table 1 of [MELPE]. For the sake
of simplicity, it is preferred that a code value of 3 for the
pitch/voicing parameter (represented by the bits P6 to P0 in Table 1
of this document) be used. Hence, set bits P0 and P1 to one and bits
P2, P3, P4, P5, and P6 to zero.
When using PLC in 1200 bps or 600 bps mode, the MELPe 2400 bps
decoder is called three or four times, respectively, to cover the
loss of a MELPe frame.
7. IANA Considerations
IANA has registered MELP, MELP2400, MELP1200, and MELP600 as
specified in Section 4.1. IANA has also added these media subtypes
to the "RTP Payload Format media types" registry
(http://www.iana.org/assignments/rtp-parameters).
8. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550] and in any applicable RTP profile such as
RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or
RTP/SAVPF [RFC5124]. However, as discussed in [RFC7202], it is not
an RTP payload format's responsibility to discuss or mandate what
solutions are used to meet such basic security goals as
confidentiality, integrity, and source authenticity for RTP in
general. This responsibility lies with anyone using RTP in an
application. They can find guidance on available security mechanisms
and important considerations in [RFC7201]. Applications SHOULD use
one or more appropriate strong security mechanisms. The rest of this
section discusses the security-impacting properties of the payload
format itself.
This RTP payload format and the MELPe decoder do not exhibit any
significant non-uniformity in the receiver-side computational
complexity for packet processing and thus are unlikely to pose a
denial-of-service threat due to the receipt of pathological data.
Additionally, the RTP payload format does not contain any active
content.
Please see the security considerations discussed in [RFC6562]
regarding VAD and its effect on bitrates.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2736] Handley, M. and C. Perkins, "Guidelines for Writers of RTP
Payload Format Specifications", BCP 36, RFC 2736,
DOI 10.17487/RFC2736, December 1999,
<http://www.rfc-editor.org/info/rfc2736>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<http://www.rfc-editor.org/info/rfc3551>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<http://www.rfc-editor.org/info/rfc4855>.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124,
February 2008, <http://www.rfc-editor.org/info/rfc5124>.
[RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of
Variable Bit Rate Audio with Secure RTP", RFC 6562,
DOI 10.17487/RFC6562, March 2012,
<http://www.rfc-editor.org/info/rfc6562>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017,
<http://www.rfc-editor.org/info/rfc8083>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", RFC 8085, DOI 10.17487/RFC8085, March 2017,
<http://www.rfc-editor.org/info/rfc8085>.
[MELP] Department of Defense Telecommunications Standard,
"Analog-to-Digital Conversion of Voice by 2,400 Bit/Second
Mixed Excitation Linear Prediction (MELP)", MIL-STD-3005,
December 1999.
[MELPE] North Atlantic Treaty Organization (NATO), "The 600 Bit/S,
1200 Bit/S and 2400 Bit/S NATO Interoperable Narrow Band
Voice Coder", STANAG No. 4591, January 2006.
[SCIP210] National Security Agency, "SCIP Signaling Plan", SCIP-210,
December 2007.
9.2. Informative References
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>.
[RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC7202,
April 2014, <http://www.rfc-editor.org/info/rfc7202>.
[RMCAT] IETF, RTP Media Congestion Avoidance Techniques (rmcat)
Working Group,
<https://datatracker.ietf.org/wg/rmcat/about/>.
Authors' Addresses
Victor Demjanenko, Ph.D.
VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Buffalo, NY 14228
United States of America
Phone: +1 716 688 4675
Email: victor.demjanenko@vocal.com
David Satterlee
VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Buffalo, NY 14228
United States of America
Phone: +1 716 688 4675
Email: david.satterlee@vocal.com
AI ELI5
On this page
PROBLEMMELPe is a very low-bitrate speech codec used in military and government communications, and VoIP systems need to carry it without decoding and re-encoding it, which would hurt intelligibility.
MECHANISMEach MELPe frame is packed into whole octets (7, 11 or 7 octets for the 2400, 1200 and 600 bps rates) and sent as the RTP payload, with no extra payload header. Reserved bits in each frame identify the bitrate, and special rules cover comfort noise and packet loss concealment.
WHY IT MATTERSEndpoints can negotiate the bitrate in SDP, send one or more frames per packet, and switch rates dynamically when both sides support it.
§1Introduction
RFC 8130 describes how speech compressed by the Mixed Excitation Linear Prediction Enhanced (MELPe) codec is formatted as an RTP payload. It covers the three codec bitrates (2400, 1200 and 600 bps) and how to put their data frames into RTP packets.
A sender may put one or more codec frames in each packet, depending on the application, network conditions, bandwidth limits, delay requirements and tolerance for packet loss.
MELPe is a way of shrinking a voice into very few bits. This document is the packing list for putting those bits into internet packets.
§2The MELPe Codec
MELP was developed by the US military as an upgrade from the LPC-based CELP standard vocoder for low-bitrate communications. It was further enhanced and adopted by NATO as MELPe for military and other governmental communications, under the name STANAG 4591. Commercial and civilian uses have since arisen because of its low bitrate and relatively high intelligibility.
VoIP and SIP systems need to transport MELPe without decoding and re-encoding it, to preserve intelligibility. That is why a payload format is needed.
The basic 2400 bps coder uses a 22.5 ms frame of 180 samples (8000 Hz, 16 bits each). The 1200 and 600 bps coders use three and four 22.5 ms frames respectively. Payload sizes are 54, 81 and 54 bits for the 2400, 1200 and 600 bps frames. Dynamic bitrate switching is permitted, but only if both endpoints support it.
The coder treats voiced and unvoiced speech differently. Forward error correction (FEC) is applied to unvoiced speech at 2400 bps only; the lower-rate coders rely on error protection in the channel.
The codec chops speech into tiny slices and describes each slice with very few numbers. A slow setting uses fewer numbers per second but sounds less clear.
| Bitrate | Frame length | Bits per frame | Octets per frame |
|---|---|---|---|
| 2400 bps | 22.5 ms | 54 | 7 |
| 1200 bps | 67.5 ms | 81 | 11 |
| 600 bps | 90 ms | 54 | 7 |
§3The Payload and Bitstream Layout
The RTP payload is simply one or more MELPe frames. No payload-specific header is needed. Because the codec uses 22.5, 67.5 or 90 ms frames at an 8 kHz clock, the RTP timestamp MUST be in units of 1/8000 of a second. When a packet holds several frames, the timestamp is that of the oldest frame. The RTP payload type is not assigned here; a profile may assign one, or a dynamic one is chosen by the sender.
A 2400 bps frame uses 54 bits, which fit in 7 octets with two unused bits. A 1200 bps frame uses 81 bits in 11 octets (seven unused bits). A 600 bps frame uses 54 bits in 7 octets. Bit B_01 goes in the least significant bit of the first octet and the rest follow in order. The unused bits, named RSVA, RSVB and RSVC, carry the bitrate indicator described below.
The RFC gives the bit-by-bit meaning of each bit position for all three rates (gain, pitch and voicing, line spectral frequencies, Fourier magnitudes and so on), taken from the NATO specification. In the original bitstream the most significant bits were priority bits meant for stronger channel protection; that is irrelevant on IP, but the bitstreams are presented identically so gateways stay interoperable.
Each slice of speech becomes a tiny parcel of 54 or 81 bits, padded out to whole bytes. The parcels are always packed in the same order so everyone can unpack them.
§3.3Several Frames per Packet
A packet may hold zero or more coder frames followed by zero or one comfort noise frame. The default packetization interval is one coder frame (22.5, 67.5 or 90 ms depending on the rate); a longer interval reduces the packet rate. A packet with no frames at all MAY be sent periodically to show connectivity to an otherwise idle receiver.
All frames in one packet MUST be the same bitrate, and frames MUST NOT be split between packets. The number of frames is not sent: the receiver counts the payload octets and divides by the expected size per frame (7, 11 or 7 octets). The last frame can be a 2-octet comfort noise frame. Fewer frames per packet means lower delay; more frames suit bandwidth-limited or delay-insensitive uses.
When bitrate switching is used, the reserved bits tell the receiver which kind of frame it has (see the table). If switching is not used, reserved bits are sent as 0 and ignored. When a packet has several frames, it is RECOMMENDED to inspect the last octet, and if that indicates comfort noise, the third last octet.
On congestion, the target bitrate can be changed at any time, and the amount of speech per packet can be changed too. Applications running RTP over UDP SHOULD add their own congestion control and MAY use a transport circuit breaker.
You can put one parcel or several in an envelope, but never cut a parcel in half. Sending fewer parcels per envelope is faster but uses more envelopes.
- A=0, B=02400 bps — RSVC is not applicable.
- A=1, B=0, C=01200 bps — All three reserved bits are used.
- A=0, B=1600 bps — RSVC is not applicable.
- A=1, B=0, C=1Comfort noise — Marks a comfort noise frame.
- A=1, B=1Reserved — RSVC is not applicable.
§4Media Types and SDP
The format is registered as the audio subtypes MELP, MELP2400, MELP1200 and MELP600. The names with a rate in them were used by early implementations and are kept; they fix the rate and MUST NOT carry a bitrate parameter. Optional parameters are ptime, maxptime and bitrate, a comma-separated list chosen from 2400, 1200 and 600 in order of preference. If bitrate is absent, the fixed rate of 2400 MUST be used.
In SDP, the encoding name SHALL be MELP, and bitrate goes in the a=fmtp line. The ptime value cannot select the mode, because some values are ambiguous: ptime=68 could mean one 67.5 ms frame or three 22.5 ms frames.
In offer/answer, bitrate is bidirectional. Both sides MUST use a common value, the offer lists the offerer's rates in preference order, and the initial rate is the first one in the answer. With an offer of 2400,600 and an answer of 600,2400, the initial bitrate is 600. A lower rate matters when an endpoint is on a bandwidth-constrained link, such as a 1200 bps radio link or slower.
Before talking, the two sides swap a list of speeds they can handle and agree on one, like picking a walking speed you can both keep up.
SDP example
m=audio 49120 RTP/AVP 97
a=rtpmap:97 MELP/8000
a=fmtp:97 bitrate=2400,600,1200§5Comfort Noise, Silence and Packet Loss
MELPe is mainly used for radio voice, where transmissions normally stop and start. Gaps in RTP sequence numbers mean lost packets, which are filled by packet loss concealment (PLC), while an abrupt end of packets means an intended pause. A coder may send a comfort noise frame before it stops, and a receiver may play comfort noise during silence. No SDP negotiation is needed.
Comfort noise follows Appendix B of SCIP-210. Only two parameters are carried (the first line spectral frequency set and the second gain), in a 13-bit frame sent in 2 octets. After voice activity detection stops indicating speech, at least two comfort noise frames are transmitted as a grace period.
PLC uses the pitch/voicing parameter of the 2400 bps coder. A value of zero means non-voiced, and valid pitch values have at least three bits set, so a value with exactly two bits set can signal an erasure. The preferred erasure code is 3: bits P0 and P1 set to one and P2 to P6 set to zero. In 1200 and 600 bps modes, the 2400 bps decoder is called three or four times to cover one lost frame.
When nobody is talking, the codec can send a tiny note saying 'it is quiet here'. If a packet goes missing, a special marker tells the player to fill the hole instead of playing garbage.
§6Security Considerations
Packets using this format are subject to the security considerations of RTP and any applicable profile (RTP/AVP, RTP/AVPF, RTP/SAVP, RTP/SAVPF). The RFC notes that securing RTP is the responsibility of whoever uses RTP in an application, and that applications SHOULD use one or more appropriate strong security mechanisms.
Specific to the payload format: neither it nor the MELPe decoder shows significant non-uniformity in receiver-side computational complexity, so pathological data is unlikely to cause denial of service, and the format has no active content. The RFC also points to RFC 6562 on voice activity detection and its effect on bitrates.
The envelopes themselves are boring and safe to open. If you want them sealed and signed, use the usual RTP security tools.
§8References
[RFC3550]RFC 3550Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.July 2003[RFC3551]RFC 3551Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003.July 2003[RFC3264]RFC 3264Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.July 2002[RFC4566]RFC 4566Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.July 2006[RFC4855]RFC 4855Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, February 2007.February 2007[RFC6562]RFC 6562Perkins, C. and JM. Valin, "Guidelines for the Use of Variable Bit Rate Audio with Secure RTP", RFC 6562, March 2012.March 2012[RFC8085]RFC 8085Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", RFC 8085, March 2017.March 2017
[MELPE]North Atlantic Treaty Organization (NATO), "The 600 Bit/S, 1200 Bit/S and 2400 Bit/S NATO Interoperable Narrow Band Voice Coder", STANAG No. 4591, January 2006.