ETSI TS 146 020 V17.0.0 (2022-05)

Digital cellular telecommunications system (Phase 2+) (GSM); Half rate speech; Half rate speech transcoding (3GPP TS 46.020 version 17.0.0 Release 17)

RTS/TSGS-0446020vh00

General Information

• Status: Not Published
• Technical Committee: 3GPP SA 4 - Codec
• Current Stage: 12 - Completion
• Completion Date: 03-May-2022

Standards Content (Sample)

ETSI TS 146 020 V17.0.0 (2022-05)






TECHNICAL SPECIFICATION
Digital cellular telecommunications system (Phase 2+) (GSM);
Half rate speech;
Half rate speech transcoding
(3GPP TS 46.020 version 17.0.0 Release 17)

R
GLOBAL SYSTEM FOR
MOBILE COMMUNICATIONS

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3GPP TS 46.020 version 17.0.0 Release 17 1 ETSI TS 146 020 V17.0.0 (2022-05)

Reference
RTS/TSGS-0446020vh00
Keywords
GSM
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ETSI

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3GPP TS 46.020 version 17.0.0 Release 17 2 ETSI TS 146 020 V17.0.0 (2022-05)
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ETSI

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3GPP TS 46.020 version 17.0.0 Release 17 3 ETSI TS 146 020 V17.0.0 (2022-05)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 5
1 Scope . 6
2 References . 6
3 Definitions, symbols and abbreviations . 6
3.1 Definitions . 6
3.2 Symbols . 8
3.3 Abbreviations . 9
4 Functional description of the GSM half rate speech codec . 10
4.1 GSM half rate speech encoder . 10
4.1.1 High-pass filter . 12
4.1.2 Segmentation . 12
4.1.3 Fixed Point Lattice Technique (FLAT) . 13
4.1.4 Spectral quantization. 14
4.1.4.1 Autocorrelation Fixed Point Lattice Technique (AFLAT) . 14
4.1.5 Frame energy calculation and quantization . 16
4.1.6 Soft interpolation of the spectral parameters . 16
4.1.7 Spectral noise weighting filter coefficients . 17
4.1.8 Long Term Predictor lag determination . 18
4.1.8.1 Open loop long term search initialization . 19
4.1.8.2 Open loop lag search . 20
4.1.8.3 Frame lag trajectory search (Mode ≠ 0) . 25
4.1.8.4 Voicing mode selection . 27
4.1.8.5 Closed loop lag search . 27
4.1.9 Harmonic noise weighting . 28
4.1.10 Code search algorithm . 30
4.1.10.1 Decorrelation of filtered basis vectors. 31
4.1.10.2 Fast search technique . 32
4.1.11 Multimode gain vector quantization . 33
4.1.11.1 Coding GS and P0 . 33
4.2 GSM half rate speech decoder . 36
4.2.1 Excitation generation . 37
4.2.2 Adaptive pitch prefilter . 37
4.2.3 Synthesis Filter . 37
4.2.4 Adaptive spectral postfilter . 37
4.2.5 Updating decoder states . 39
5 Homing sequences . 39
5.1 Functional description . 39
5.2 Definitions . 39
5.3 Encoder homing . 40
5.4 Decoder homing . 40
5.5 Encoder home state . 40
5.6 Decoder home state . 40
Annex A (normative): Codec parameter description . 41
A.1 Codec parameter description . 41
A.1.1 MODE . 41
A.1.2 R0 . 41
A.1.3 LPC1 - LPC3 . 42
A.1.4 LAG_1 - LAG_4 . 42
ETSI

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3GPP TS 46.020 version 17.0.0 Release 17 4 ETSI TS 146 020 V17.0.0 (2022-05)
A.1.5 CODEx_1 - CODEx_4 . 42
A.1.6 GSP0_1 - GSP0_4 . 42
A.2 Basic coder parameters . 42
Annex B (normative): Order of occurrence of the codec parameters over Abis . 43
Annex C (informative): Bibliography . 44
Annex D (informative): Change history . 45
History . 46

ETSI

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3GPP TS 46.020 version 17.0.0 Release 17 5 ETSI TS 146 020 V17.0.0 (2022-05)
Foreword
rd
This Technical Specification has been produced by the 3 Generation Partnership Project (3GPP).
The present document specifies the speech codec to be used for the GSM half rate channel for the digital cellular
telecommunications system. The present document is part of a series covering the half rate speech traffic channels as
described below:
GSM 06.02 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate speech
processing functions".
GSM 06.06 "Digital cellular telecommunications system (Phase 2+); Half rate speech; ANSI-C code for the
GSM half rate speech codec".
GSM 06.07 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Test sequences for the
GSM half rate speech codec".
GSM 06.20 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate speech
transcoding".
GSM 06.21 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Substitution and muting
of lost frames for half rate speech traffic channels".
GSM 06.22 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Comfort noise aspects
for half rate speech traffic channels".
GSM 06.41 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Discontinuous
Transmission (DTX) for half rate speech traffic channels".
GSM 06.42 "Digital cellular telecommunications system (Phase 2+); Half rate speech; Voice Activity Detector
(VAD) for half rate speech traffic channels".
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
ETSI

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3GPP TS 46.020 version 17.0.0 Release 17 6 ETSI TS 146 020 V17.0.0 (2022-05)
1 Scope
The present document specifies the speech codec to be used for the GSM half rate channel. It also specifies the test
methods to be used to verify that the codec implementation complies with the present document.
The requirements are mandatory for the codec to be used either in GSM Mobile Stations (MS)s or Base Station Systems
(BSS)s that utilize the half rate GSM speech traffic channel.
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
• References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a
GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] GSM 06.02: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate
speech processing functions".
[2] GSM 06.06: "Digital cellular telecommunications system (Phase 2+); Half rate speech; ANSI-C
code for the GSM half rate speech codec".
[3] GSM 06.07: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Test
sequences for the GSM half rate speech codec".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following definitions apply:
adaptive codebook: adaptive codebook is derived from the long term filter state. The lag value can be viewed as an
index into the adaptive codebook.
adaptive pitch prefilter: in the GSM half rate speech decoder, this filter is applied to the excitation signal to enhance
the periodicity of the reconstructed speech. Note that this is done prior to the application of the short term filter.
adaptive spectral postfilter: in the GSM half rate speech decoder, this filter is applied to the output of the short term
filter to enhance the perceptual quality of the reconstructed speech.
allowable lags: set of lag values which may be coded by the GSM half rate speech encoder and transmitted to the GSM
half rate speech decoder. This set contains both integer and fractional values (see table 3).
analysis window: for each frame, the short term filter coefficients are computed using the high pass filtered speech
samples within the analysis window. The analysis window is 170 samples in length, and is centered about the last 100
samples in the frame.
basis vectors: set of M, M1, or M2 vectors of length Ns used to generate the VSELP codebook vectors. These vectors
are not necessarily orthogonal.
closed loop lag search: process of determining the near optimal lag value from the weighted input speech and the long
term filter state.
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3GPP TS 46.020 version 17.0.0 Release 17 7 ETSI TS 146 020 V17.0.0 (2022-05)
closed loop lag trajectory: for a given frame, the sequence of near optimal lag values whose elements correspond to
each of the four subframes as determined by the closed loop lag search.
codebook: set of vectors used in a vector quantizer.
Codeword (OR Code): M, M1, or M2 bit symbol indicating the vector to be selected from a VSELP codebook.
Delta (LAG) code: four bit code indicating the change in lag value for a subframe relative to the previous subframe's
coded lag. For frames in which the long term predictor is enabled (MODE 1, 2, or 3), the lag for subframe 1 is
independently coded using eight bits, and delta codes are used for subframes 2, 3, and 4.
direct form coefficients: one of the formats for storing the short term filter parameters. All filters which are used to
modify speech samples use direct form coefficients.
fractional lags: set of lag values having sub-sample resolution. Note that not every fractional lag value considered in
the GSM half rate speech encoder is an allowable lag value.
frame: time interval equal to 20 ms, or 160 samples at an 8 kHz sampling rate.
harmonic noise weighting filter: this filter exploits the noise masking properties of the spectral peaks which occur at
harmonics of the pitch frequency by weighting the residual error less in regions near the pitch harmonics and more in
regions away from them. Note that this filter is only used when the long term filter is enabled (MODE = 1, 2 or 3).
high pass filter: this filter is used to de-emphasize the low frequency components of the input speech signal.
integer lags: set of lag values having whole sample resolution.
interpolating filter: FIR filter used to estimate sub-sample resolution samples, given an input sampled with integer
sample resolution.
lag: long term filter delay. This is typically the pitch period, or a multiple or sub-multiple of it.
long term filter: this filter is used to generate the periodic component in the excitation for the current subframe. This
filter is only enabled for MODE = 1, 2 or 3.
LPC coefficients: Linear Predictive Coding (LPC) coefficients is a generic descriptive term for describing the short
term filter coefficients.
open loop lag search: process of estimating the near optimal lag directly from the weighted speech input. This is done
to narrow the range of lag values over which the closed loop lag search shall be performed.
open loop lag trajectory: for a given frame, the sequence of near optimal lag values whose elements correspond to the
four subframes as determined by the open loop lag search.
reflection coefficients: alternative representation of the information contained in the short term filter parameters.
residual: output signal resulting from an inverse filtering operation.
short term filter: this filter introduces, into the excitation signal, short term correlation which models the impulse
response of the vocal tract.
soft interpolation: process wherein a decision is made for each frame to use either interpolated or uninterpolated short
term filter parameters for the four subframes in that frame.
soft interpolation bit: one bit code indicating whether or not interpolation of the short term parameters is to be used in
the current frame.
spectral noise weighting filter: this filter exploits the noise masking properties of the formants (vocal tract resonances)
by weighting the residual error less in regions near the formant frequencies and more in regions away from them.
subframe: time interval equal to 5 ms, or 40 samples at an 8 kHz sampling rate.
vector quantization: method of grouping several parameters into a vector and quantizing them simultaneously.
GSP0 vector quantizer: process of vector quantization, its intermediate parameters (GS and P0) for the coding of the
excitation gains β and γ.
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3GPP TS 46.020 version 17.0.0 Release 17 8 ETSI TS 146 020 V17.0.0 (2022-05)
VSELP codebook: Vector-Sum Excited Linear Predictive (VSELP) codebook, used in the GSM half rate speech coder,
wherein each codebook vector is constructed as a linear combination of the fixed basis vectors.
zero input response: output of a filter due to all past inputs, i.e. due to the present state of the filter, given that an input
of zeros is applied.
zero state response: output of a filter due to the present input, given that no past inputs have been applied, i.e. given
the state information in the filter is all zeroes.
3.2 Symbols
For the purposes of the present document, the following symbols apply:
A(z) Short term spectral filter.
α The LPC coefficients.
i
b (n) The output of the long term filter state (adaptive codebook) for lag L.
L
β The long term filter coefficient.
C(z) Second weighting filter.
e(n) Weighted error signal
th
f (i) The coefficients of the j phase of the 10th order interpolating filter used to evaluate candidate
j
fractional lag values; i ranges from 0 to P -1.
f
th
g (i) The coefficients of the j phase of the 6th order interpolating filter used to interpolate C's and G's
j
as well as fractional lags in the harmonic noise weighting; i ranges from 0 to Pg-1.
γ The gain applied to the vector(s) selected from the VSELP codebook(s).
H A M2 bit code indicating the vector to be selected from the second VSELP codebook (when
operating in mode 0).
I A M or M1 bit code indicating the vector to be selected from one of the two first VSELP
codebooks.
L The long term filter lag value.
L 142 (samples), the maximum possible value for the long term filter lag.
max
L 21 (samples), the minimum possible value for the long term filter lag.
min
M 9, the number of basis vectors, and the number of bits in a codeword, for the VSELP codebook
used in modes 1, 2, and 3.
M1 7, the number of basis vectors, and the number of bits in a codeword, for the first VSELP
codebook used in mode 0.
M2 7, the number of basis vectors, and the number of bits in a codeword, for the second VSELP
codebook used in mode 0.
MODE A two bit code indicating the mode for the current frame (see annex A).
N 170, the length of the analysis window. This is the number of high pass filtered speech samples
A
used to compute the short term filter parameters for each frame.
N 160, the number of samples per frame (at a sampling rate of 8 kHz).
F
N 10, the short term filter order.
p
N 40, the number of samples per subframe (at a sampling rate of 8 kHz).
s
P1 6, the number of bits in the prequantizer for the r1 - r3 vector quantizer.
P2 5, the number of bits in the prequantizer for the r4 - r6 vector quantizer.
P3 4, the number of bits in the prequantizer for the r7 - r10 vector quantizer.
P The order of one phase of an interpolating filter used to evaluate candidate fractional lag values. P
f f
equals 10 for j ≠ 0 and equal to 1 for j = 0.
P The order of one phase of an interpolating filter, f (n), used to interpolate C's and G's as well as
g j
fractional lags in the harmonic noise weighting, P equals 6.
g
pitch The time duration between the glottal pulses which result when the vocal chords vibrate during
speech production.
Q1 11, the number of bits in the r1 - r3 reflection coefficient vector quantizer.
Q2 9, the number of bits in the r4 - r6 reflection coefficient vector quantizer.
Q3 8, the number of bits in the r7 - r10 reflection coefficient vector quantizer.
R0 A five bit code used to indicate the energy level in the current frame.
r(n) The long term filter state (the history of the excitation signal); n < 0
r (n) The long term filter state with the adaptive codebook output for lag L appended.
L
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3GPP TS 46.020 version 17.0.0 Release 17 9 ETSI TS 146 020 V17.0.0 (2022-05)
s'(n) Synthesized speech.
W(z) Spectral weighting filter.
λ The harmonic noise weighting filter coefficient.
hnw
ξ The adaptive pitch prefilter coefficient.
x Ceiling function: the largest integer y where y < x + 1,0.
x Floor function: the largest integer y where y ≤ x.
K
xi Summation: x(j)+x(j+1)+.+x(K).
 ()
ij=
K
xi Product: x(j)(x(j+1)).(x(K))
()
∏
ij=
max(x,y) Find the larger of two numbers x and y.
min(x,y) Find the smaller of two numbers x and y.
round(x) Round the non-integer x to the closest integer yy:,=+x 05 y: y=x+0,5.
3.3 Abbreviations
For the purposes of the present document, the following abbreviati
...