Internet DRAFT - draft-demjanenko-payload-melp
draft-demjanenko-payload-melp
INTERNET-DRAFT Victor Demjanenko
David Satterlee
Intended Status: Experimental VOCAL Technologies, Ltd.
Expires: September 24, 2015 March 23, 2015
RTP Payload Format for MELPe Codec
draft-demjanenko-payload-melp-03
Abstract
This document describes the RTP payload format for the Mixed
Excitation Linear Prediction Enhanced (MELPe) speech coder algorithm
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. All three different speech
encoding rates and sample frames sizes are included. Comfort noise
procedures and packet loss concealment are detailed. Also, within
the document there are included necessary details for the use of MELP
with SDP.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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Copyright and License Notice
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Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 RTP Payload Format . . . . . . . . . . . . . . . . . . . . . . 4
3.1 MELPe Bitstream Definition . . . . . . . . . . . . . . . . 5
3.1.1 2400 bps Bitstream Structure . . . . . . . . . . . . . . 5
3.1.2 1200 bps Bitstream Structure . . . . . . . . . . . . . . 8
3.1.3 600 bps Bitstream Structure . . . . . . . . . . . . . . 11
3.2 MELPe Comfort Noise Bitstream Definition . . . . . . . . . 14
3.3 Multiple MELPe frames in a RTP packet . . . . . . . . . . . 16
4 Mapping to SDP Parameters . . . . . . . . . . . . . . . . . . . 18
5 Discontinious Transmission . . . . . . . . . . . . . . . . . . 20
6 Packet Loss Concealment . . . . . . . . . . . . . . . . . . . . 20
7 Security Considerations . . . . . . . . . . . . . . . . . . . . 21
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 21
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1 Normative References . . . . . . . . . . . . . . . . . . . 21
9.2 Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1 Introduction
This document describes how compressed 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 rate 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 the transport network condition, bandwidth
restriction, delay requirements and packet-loss tolerance.
1.1 Terminology
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].
2 Background
The MELP speech coder was developed by the US military as an upgrade
from LPC-based CELP standard vocoder for low bit-rate communications
[MELP]. 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].
Commercial/civilian applications have arisen because of the low bit-
rate property of MELPe with its (relatively) high intelligibility.
As such MELPe is being used in a variety of wired and radio
communications systems. 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 rates; 2400,
1200, and 600 bps. The basic 2400 bps rate vocoder uses a 22.5 ms
frame of speech consisting of 180 8000 Hz, 16-bit speech samples.
The 1200 and 600 bps rate vocoders uses respectively three and four
22.5 ms frames of speech each. These reduced rate vocoders
internally use multiple 2400 bps parameter sets with further
processing to strategically remove redundancy. The payload sizes for
each of the rates are 54, 81, and 54 bits respectively for the 2400,
1200, and 600 bps frames. Dynamic rate switching is permitted but
only if supported by both endpoints.
The MELPe algorithm distinguishes between voiced and un-voiced speech
and encodes each differently. Unvoiced speech can be coded with
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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 bit rate 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 is recommended to follow SCIP-210
Appendix B [SCIP210]. After VAD no longer indicates the presence of
speech/voice, a grace period of a minimum of two comfort noise
vocoder fames are to be transmitted. The contents of the comfort
noise frames is described in the next section.
Packet loss concealment (PLC) exploits the FEC (and more precisely,
double bits errors of 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 of the errored encodings of a non-
voiced frame as will be described infra.
3 RTP 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 the figure below.
No additional header specific to this payload format is required.
This format is intended for the situations where the sender and the
receiver send one or more codec data frames per packet. The RTP
packet looks as follows:
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
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The RTP header of the packetized encoded MELPe speech has the
expected values as described in [RFC3550]. The usage of M bit SHOULD
be as specified in the applicable RTP profile, for example, RFC 3551
[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 then 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 Definition
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 the
1200 bps speech the total number of bits used is 81, which fits in 11
octets (with seven unused bits). For the 600 bps speech the total
number of bits used is 54, which fits in 7 octets (with two unused
bits). Unused bits are coded as described in 3.3 in support of
dynamic rate switching.
In the MELPe bitstream definition, 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
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 bit/s MELPe bit
transmission order (bit priority is not shown for clarity) is the
following:
+--------+-------------+-------------+
| Bit | Voiced | Unvoiced |
+--------+-------------+-------------+
| B_01 | g20 | g20 |
| B_02 | BP0 | FEC10 |
| B_03 | P0 | P0 |
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| 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 |
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| 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 3.1 - The 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 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 |
+------+------+------+------+------+------+------+------+
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Figure 2 - Packed MELPe 2400 bps payload octets.
3.1.2 1200 bps Bitstream Structure
According to Tables D9a and D9b of [MELPE], the 1200 bit/s MELPe bit
transmission order is the following:
+--------+-------------+-------------+
| 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 |
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+--------+-------------+-------------+
| 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 | |
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| B_75 | FS1 | |
| B_76 | FS2 | |
| B_77 | FS3 | |
| B_78 | FS4 | |
| B_79 | FS5 | |
| B_80 | FS6 | |
+--------+-------------+-------------+
| B_81 | FS7 | |
+--------+-------------+-------------+
NOTES:
BP = Band pass voicing
FS = Fourier magnitudes
Table 3.2 - The 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 |
+------+------+------+------+------+------+------+------+
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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 bit/s MELPe bit
transmission order (bit priority is not shown for clarity) is the
following:
+--------+-------------+-------------+-------------+
| 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) |
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| 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 3.3a - The 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) |
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| 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) |
+--------+-------------+-------------+-------------+
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Notes:
xxxx (0) = LSB
xxxx (nbits-1) = MSB
LSF1,p = MSVQ indice of the pth stage of the two first frames
LSF2,p = MSVQ indice of the pth stage of the two last frames
GAIN1 = VQ/MSVQ indice of the 1st stage
GAIN2 = MSVQ indice of the 2nd stage
Table 3.3b - The 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 |
+-------------------------------------+----------------+
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| 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 value shall be the respective parameters
from the vocoder frame. It is recommended that msvq[0] and
gain[1] values be derived by averaging the respective
parameter from some number of previous vocoder frames.
Table 3.4 - 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 required, 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 |
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+--------+-------------+
| B_09 | g21 |
| B_10 | g22 |
| B_11 | g23 |
| B_12 | g24 |
| B_13 | SYNC |
+--------+-------------+
NOTES:
g = Gain
LSF = Line Spectral Frequencies
Table 3.5 - The 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 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 a RTP packet
A MELPe RTP packet may consist of zero or more MELPe coder frames,
followed by zero or one MELPe Comfort Noise frames. 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 rate (2400, 1200 or 600
bps). For some applications, a longer packetization interval may be
required to reduce the packet rate.
All MELPe frames in a single RTP packet MUST be of the same coder
rate. 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 3.6. If rate
switching is not used, all reserved bits are encoded as 0 by the
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sender and ignored by the receiver. (RSV0 is always coded as 0).
+-------------------+------+------+------+
| Coder Rate | 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 3.6 - MELPe Frame Rate Indicators.
It is important to observe that senders have the following additional
restrictions:
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 is consistent with the application. For example, in a
telephony and other real time applications where delay is important,
then the fewer frames per packet the lower the delay, whereas for a
bandwidth constrained links or delay insensitive streaming messaging
application, more than one 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
the last frame may be a 2 octet comfort noise frame.
When dynamic rate switching is used and more than one frame is
contained in a RTP packet, it is recommended to inspect the coder
rate bits contained in the last octet. If the coder rate indicates a
Comfort Noise frame, then inspect the third last octet for the coder
rate. All MELPe speech frames in the RTP packet will be of this same
coder rate.
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4 Mapping to SDP Parameters
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[RFC2327], 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 MIME type ("audio") goes in SDP "m=" as the media name.
o The MIME subtype (payload format name) goes in SDP "a=rtpmap"
as the encoding name.
o The parameter "rate" goes in the SDP "a=fmtp" attribute by
copying it directly from the MIME media type string as "rate=value"
or "rate=value1,value2" or "rate=value1,value2,value3".
When conveying information by SDP, the encoding name SHALL be "MELP"
(the same as the MIME subtype). Alternative encoding name types,
"MELP2400", "MELP1200", and "MELP600", may be used in SDP to convey
fixed rate configurations. These names have been observed in systems
that do not support dynamic frame rate switching as specified by the
parameter, "rate".
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 rate 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 "rate" parameter,
the fixed coder rate of 2400 MUST be used. The alternate encoding
names, "MELP2400", "MELP1200", and "MELP600" directly specify the
MELPe coder rate of 2400, 1200, and 600 respectively and MUST NOT
specify a "rate" parameter.
A remote MELPe encoder SHALL receive "rate" parameter in the SDP
"a=fmtp" attribute by copying them directly from the MIME media type
string as a semicolon separated with parameter=value, where parameter
is "rate", and value can be one or more of 2400, 1200, and 600
separated by commas (where each rate value indicates the
corresponding MELPe coder). An example of the media representation in
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SDP for describing MELPe when all three coder rates are supported
might be:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 MELP/8000
a=fmtp:97 rate=2400,600,1200
For streaming media, the "rate" parameter specifes the possible rates
used by the sender. In an Offer/Answer mode [RFC3264], "rate" is a
bi-directional parameter. Both sides MUST use a common "rate" value
or values.
The offer contains the rates supported by the offerer listed in its
preferred order. The answerer may agree to any rate by listing the
rate first in the answerer response. Additionally the answerer may
indicate any secondary rate or rates that it supports. The initial
rate used by both parties SHALL be the first bandwidth rate specified
in the answerer response.
For example if offerer rates are "2400,600", and answer rates are
"600,2400", the initial rate is 600. If other rates are provided by
the answerer, any common rate between offer and answer may be used at
any time in the future. Activation of these other common rates is
beyond the scope of this document.
The use of a lower rate is often important for a case such as when
one end point utilizes a bandwidth constrained link (e.g. 1200 bps
radio link or slower), where only the lower coder rate will work.
Parameter ptime can not be used for the purpose of specifying MELPe
operating mode, due to fact that for the 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 packet is carrying
1 frames of 67.5 ms or 3 frames of 22.5 ms etc.).
When SDP is used for broadcast MELPe audio, multiple MELPe rtpmap
values (such as 97, 98, and 99 as used below) may be used to convey
MELPe coded voice at different rates. 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 rate=2400
a=rtpmap:98 MELP/8000
a=fmtp:98 rate=1200
a=rtpmap:99 MELP/8000
a=fmtp:99 rate=600
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Note that the payload format (encoding) names are commonly shown in
upper case. MIME subtypes are commonly shown in lower case. These
names are case-insensitive in both places. Similarly, parameter
names are case-insensitive both in MIME types and in the default
mapping to the SDP a=fmtp attribute
The value for "packet time" and "maximum packet time" parameters of
the "ptime" and "maxptime" SDP attributes respectively, SHALL use the
nearest rounded-up ms integer packet duration. For MELPe, this
corresponds to the values: 23, 45, 68, 90, 112, 135, 156, and 180.
Larger values may be used as long as they are properly rounded.
5 Discontinious Transmission
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 SSRC sequence number gaps indicate lost
packets to be filled by PLC while abrupt loss of RTP packets indicate
intended discontinuous transmission.
If a MELPe coder so desires, it may send a comfort noise frame as per
SCIP-210 Appendix B [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 may utilize any of the errored encodings
of a non-voiced frame as identified in Table 1 of [MELPE]. For the
sake of simplicity it is recommended to use a code value of 3 for the
pitch/voicing parameter (represented by the bits P6 to P0 in Table
3.1). Hence, set bits P0 and P1 to one and bits P2, P3, P4, P5, and
P6 to zero.
When using PLC in a 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.
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7 Security Considerations
RTP packets using the payload format defined in this specification
are subject to the general security considerations discussed in
[RFC3550] and any appropriate profile (e.g. [RFC2736]).
As this format transports encoded speech, the main security issues
include confidentiality and authentication of the speech itself. The
payload format itself does not have any built-in security mechanisms.
Confidentiality of the media streams is achieved by encryption,
therefore external mechanisms, such as SRTP [RFC3711], MAY be used
for that purpose. The data compression used with this payload format
is applied end-to-end; hence encryption may be performed after
compression with no conflict between the two operations.
A potential denial-of-service threat exists for data encoding using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the stream which are complex to decode and cause the receiver to
become overloaded. However, the encodings covered in this document do
not exhibit any significant non-uniformity.
<Security considerations text>
8 IANA Considerations
<IANA considerations text>
9 References
9.1 Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", IETF RFC
3550, July 2003.
[RFC3551] H. Schulzrinne, S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control" IETF RFC 3551, July 2003.
[RFC2327] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", IETF RFC 2327, April 1998
[RFC3264] J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with
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the Session Description Protocol (SDP)" IETF RFC 3264, June 2002.
[RFC2736] M. Handley and C. Perkins, "Guidelines for Writers of RTP
Payload Format Specifications", BCP 36, RFC 2736, December 1999.
[RFC3711] Baugher, et al., "The Secure Real Time Transport Protocol",
IETF RFC 3711, March 2004.
9.2 Informative References
[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.
Authors' Addresses
Victor Demjanenko, Ph.D.
VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Buffalo, NY 14228
USA
Phone: +1 716 688 4675
Email: victor.demjanenko@vocal.com
David Satterlee
VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Buffalo, NY 14228
USA
Phone: +1 716 688 4675
Email: david.satterlee@vocal.com
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