SIST EN 300 973 V6.0.1:2003

• Abstract:

  Upgrade from Phase 2+ to Release 1997

• Status: Published
• Publication Date: 30-Nov-2003
• ICS: 33.070.50 - Global System for Mobile Communication (GSM)
• Technical Committee: SS EIT - Technical Board for the electrotechnical, information technology and telecommunications fields
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Dec-2003
• Due Date: 01-Dec-2003
• Completion Date: 01-Dec-2003
• Mandate: T-172
• Ref Project: ETSI EN 300 973 V6.0.1 (1999-06) - Digital cellular telecommunications system (Phase 2+) (GSM); Half rate speech; Voice Activity Detector (VAD) for half rate speech traffic channels (GSM 06.42 version 6.0.1 Release 1997)

SIST EN 300 973 V6.0.1:2003 is the Slovenian edition of ETSI EN 300 973 V6.0.1, a European Telecommunications standard for the GSM half-rate speech Voice Activity Detector (VAD). It specifies how the VAD decides whether each 20 ms speech frame contains speech, how the decision is adapted for Discontinuous Transmission (DTX), and how implementations are tested in GSM Mobile Stations (MS) and Base Station Systems (BSS). The document is written for people implementing, integrating, or verifying half-rate GSM speech traffic channels.

What does SIST EN 300 973 V6.0.1:2003 specify?

SIST EN 300 973 V6.0.1:2003 specifies the Voice Activity Detector (VAD) used in Discontinuous Transmission (DTX) for GSM half-rate speech traffic channels. It also specifies the test methods used to check whether a VAD implementation complies with the document.

The scope makes the requirements mandatory for any VAD used in GSM Mobile Stations (MS) or Base Station Systems (BSS) that use the half-rate speech traffic channel. The document is organized into a short technical structure: scope, references, definitions and abbreviations, general principles, functional description, computational description overview, and four informative annexes.

Annex	What it covers
Annex A	VAD performance
Annex B	Simplified block filtering operation
Annex C	Pole frequency calculation
Annex D	Change Request History

What are the key requirements of SIST EN 300 973 V6.0.1:2003?

SIST EN 300 973 V6.0.1:2003 defines a frame-by-frame decision process for speech activity in GSM half-rate channels. The important point in practice is that implementers do not just detect energy - they combine filtering, spectral checks, tone checks, threshold adaptation, and hangover logic before producing the final VAD output.

• Clause 4 states that the VAD indicates whether each 20 ms frame produced by the speech encoder contains speech or not. The output is the Boolean flag vadflag, which is then used by the TX DTX handler defined in GSM 06.41. In practice, this means the VAD must interface cleanly with the transmit-side DTX logic.

• Clause 5.1 describes the core principle: the VAD compares the energy of a filtered version of the input signal with a threshold. The input is adaptively filtered so that noise is reduced before the decision is made. This matters because the VAD is meant to work in mobile environments where speech and noise can be close in level.

• Clause 5.2.1 defines the filtered-frame energy pvad and links it to an 8th order block filtering operation, with the derivation explained in Annex B. For implementers, this means the VAD is not using a simple raw energy check - it uses filtered signal energy based on speech encoder data.

• Clause 5.2.2 averages ACF (Autocorrelation Function) vectors over several frames to capture spectral properties beyond a single 20 ms frame. This is important because speech and noise characteristics often need a short time window to be distinguished reliably.

• Clause 5.2.4 compares the spectrum derived from predictor values with the averaged ACF data and produces the Boolean flag stat. A stationary spectrum is one of the conditions used later for adapting the threshold. In practice, this helps the VAD avoid changing behavior too quickly when the signal is stable.

• Clause 5.2.5 defines information tone detection using prediction error and a pole-frequency check, with the pole-frequency derivation given in Annex C. The document treats tones and certain resonant noise patterns separately from ordinary speech, which matters in telephony because tones can otherwise look speech-like to a simple detector.

• Clause 5.2.6, 5.2.7, 5.2.8, and 5.2.9 define the threshold adaptation, the pre-hangover decision vvad, the final decision vadflag, and periodicity detection through LTP lags. In practice, the VAD only adapts its threshold when conditions suggest speech and tones are unlikely, and it adds hangover to avoid chopping short speech bursts.

What terms does SIST EN 300 973 V6.0.1:2003 define?

SIST EN 300 973 V6.0.1:2003 defines the key terms and abbreviations needed to read the algorithm and its equations.

• Voice Activity Detector (VAD) - The function that decides whether a 20 ms frame contains speech.
• Discontinuous Transmission (DTX) - The transmit mode that uses the VAD decision to reduce transmission during non-speech periods.
• Autocorrelation Function (ACF) - The correlation data from the speech encoder that the VAD uses for averaging and spectral comparison.
• Long Term Predictor (LTP) - The speech-encoder predictor whose lag values are used for periodicity detection.
• Transmit (TX) - The transmit side that uses vadflag for DTX handling.
• AFLAT - The Autocorrelation Fixed point LAttice Technique used in the speech encoder before the VAD algorithm can run.
• MS - Mobile Stations, one of the mandatory implementation targets.
• BSS - Base Station Systems, the other mandatory implementation target.

Who uses SIST EN 300 973 V6.0.1:2003?

SIST EN 300 973 V6.0.1:2003 is used by GSM equipment manufacturers, codec developers, and test laboratories working on half-rate speech traffic channels. It is also relevant to quality managers and buyers who need to check whether a product supports the GSM half-rate VAD defined for DTX operation.

In practice, the document is used for:

• implementing the VAD software or fixed-point algorithm,
• integrating VAD output with GSM 06.41 transmit-side DTX handling,
• verifying implementations with the digital test sequences from GSM 06.07,
• checking conformance in Mobile Stations and Base Station Systems,
• reviewing whether a half-rate speech product follows the expected GSM speech-processing chain.

What changed in SIST EN 300 973 V6.0.1:2003 from the previous edition?

SIST EN 300 973 V6.0.1:2003 is identified as an upgrade from Phase 2+ to Release 1997. The document is the GSM 06.42 version 6.0.1 Release 1997 text.

Which standards are used with SIST EN 300 973 V6.0.1:2003?

SIST EN 300 973 V6.0.1:2003 refers to the following standards and companion parts:

Standard	What it contributes
GSM 01.04	Abbreviations and acronyms used in the GSM series
GSM 06.20	Half rate speech transcoding, including encoder data such as ACF, rc, and lags
GSM 06.22	Comfort noise aspects for half rate speech traffic channels
GSM 06.41	Discontinuous Transmission (DTX) for half rate speech traffic channels, including the TX DTX handler
GSM 06.06	ANSI C code for the GSM half rate speech codec, used for the fixed-point computational description
GSM 06.07	Test sequences for the GSM half rate speech codec, used for verification

What does the SIST EN 300 973 V6.0.1:2003 document contain?

SIST EN 300 973 V6.0.1:2003 contains a functional description of the VAD, a computational overview, and informative annexes that support implementation and understanding. The main body includes the block diagram of the algorithm, tables of variables and constants, and the step-by-step logic for adaptive filtering, ACF averaging, predictor computation, spectral comparison, tone detection, threshold adaptation, hangover addition, and periodicity detection.

The document also explains the fixed-point implementation structure through three ANSI C modules: vad_reset, vad_algorithm, and periodicity_update. Annex B gives the derivation behind the block-filtering equation, Annex C gives the pole-frequency test used in tone detection, and Annex A summarizes observed VAD performance and the speech clipping versus activity trade-off.

SLOVENSKI STANDARD
01-december-2003
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'HWHNWRUJRYRUQLKGHMDYQRVWL9$'SULSURPHWQLKNDQDOLKVSRORYLþQRKLWURVWMR
JRYRUD*60UD]OLþLFDL]GDMD
Digital cellular telecommunications system (Phase 2+) (GSM); Half rate speech; Voice
Activity Detector (VAD) for half rate speech traffic channels (GSM 06.42 version 6.0.1
Release 1997)
Ta slovenski standard je istoveten z: EN 300 973 Version 6.0.1
ICS:
33.070.50 Globalni sistem za mobilno Global System for Mobile
telekomunikacijo (GSM) Communication (GSM)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 300 973
V6.0.1 (1999-06)
European Standard (Telecommunications series)
Digital cellular telecommunications system (Phase 2+);
Half rate speech;
Voice Activity Detector (VAD)
for half rate speech traffic channels
(GSM 06.42 version 6.0.1 Release 1997)
R
GLOBAL SYSTEM FOR
MOBILE COMMUNICATIONS
GSM 06.42 version 6.0.1 Release 1997 2 EN 300 973 V6.0.1 (1999-06)
Reference
DEN/SMG-110642Q6 (90c0300o.PDF)
Keywords
Digital cellular telecommunications system,
Global System for Mobile communications (GSM)
ETSI
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Copyright Notification
No part may be reproduced except as authorized by written permission.
The copyright and the foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1999.
All rights reserved.
ETSI
GSM 06.42 version 6.0.1 Release 1997 3 EN 300 973 V6.0.1 (1999-06)
Contents
Intellectual Property Rights.4
Foreword .4
1 Scope.6
2 References.6
3 Definitions, symbols and abbreviations.6
3.1 Definitions . 6
3.2 Symbols . 6
3.2.1 Variables . 7
3.2.2 Constants. 7
3.2.3 Functions. 7
3.3 Abbreviations. 8
4 General.8
5 Functional description.8
5.1 Overview and principles of operation. 8
5.2 Algorithm description . 9
5.2.1 Adaptive filtering and energy computation . 10
5.2.2 ACF averaging . 10
5.2.3 Predictor values computation . 10
5.2.4 Spectral comparison . 11
5.2.5 Information tone detection . 11
5.2.6 Threshold adaptation. 12
5.2.7 VAD decision. 14
5.2.8 VAD hangover addition . 14
5.2.9 Periodicity detection. 14
6 Computational description overview .15
6.1 VAD modules . 15
6.2 Pseudo-floating point arithmetic. 15
Annex A (informative): VAD performance .17
Annex B (informative): Simplified block filtering operation.18
Annex C (informative): Pole frequency calculation.19
Annex D (informative): Change Request History .20
History.21
ETSI
GSM 06.42 version 6.0.1 Release 1997 4 EN 300 973 V6.0.1 (1999-06)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect
of ETSI standards", which is available free of charge from the ETSI Secretariat. Latest updates are available on the
ETSI Web server (http://www.etsi.org/ipr).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in SR 000 314 (or the updates on the ETSI Web server)
which are, or may be, or may become, essential to the present document.
Foreword
This European Standard (Telecommunications series) has been produced by ETSI Technical Committee Special Mobile
Group (SMG).
The present document specifies the Voice Activity Detector (VAD) to be used in the Discontinuous Transmission
(DTX) within 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 is subject to continuing work within SMG and may change following formal SMG
approval. Should SMG modify the contents of the present document it will be re-released with an identifying change of
release date and an increase in version number as follows:
Version 6.x.y
where:
6 indicates Release 1997 of GSM Phase 2+
x the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections, updates,
etc.
y the third digit is incremented when editorial only changes have been incorporated in the specification.
ETSI
GSM 06.42 version 6.0.1 Release 1997 5 EN 300 973 V6.0.1 (1999-06)
Proposed national transposition dates
Date of adoption of this EN: 05 June 1999
Date of latest announcement of this EN (doa): 30 September 1999
Date of latest publication of new National Standard
or endorsement of this EN (dop/e): 31 March 2000
Date of withdrawal of any conflicting National Standard (dow): 31 March 2000
ETSI
GSM 06.42 version 6.0.1 Release 1997 6 EN 300 973 V6.0.1 (1999-06)
1 Scope
This European Standard (EN) specifies the Voice Activity Detector (VAD) to be used in the Discontinuous
Transmission (DTX) as described in GSM 06.41 [4]. It also specifies the test methods to be used to verify that a VAD
implementation complies with the present document.
The requirements are mandatory on any VAD 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.
• A non-specific reference to an ETS shall also be taken to refer to later versions published as an EN with the same
number.
[1] GSM 01.04: "Digital cellular telecommunications system (Phase 2+); Abbreviations and
acronyms".
[2] GSM 06.20: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Half rate
speech transcoding".
[3] GSM 06.22: "Digital cellular telecommunications system (Phase 2+); Half rate speech; Comfort
noise aspects for half rate speech traffic channels".
[4] GSM 06.41: "Digital cellular telecommunications system (Phase 2+); Half rate speech;
Discontinuous Transmission (DTX) for half rate speech traffic channels".
[5] GSM 06.06: "Digital cellular telecommunications system (Phase 2+); Half rate speech; ANSI C
code for the GSM half rate speech codec".
[6] 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 purpose of the present document, the following definitions apply:
mobile environment: Any environment in which MSs may be used.
noise: The signal component resulting from acoustic environmental noise.
3.2 Symbols
For the purpose of the present document, the following symbols apply:
ETSI
GSM 06.42 version 6.0.1 Release 1997 7 EN 300 973 V6.0.1 (1999-06)
3.2.1 Variables
aav1 filter predictor values, see subclause 5.2.3
acf the ACF vector which is calculated in the speech encoder (GSM 06.20 [2])
adaptcount secondary hangover counter, see subclause 5.2.6
av0 averaged ACF vector, see subclause 5.2.2
av1 a previous value of av0, see subclause 5.2.2
burstcount speech burst length counter, see subclause 5.2.7
den denominator of left hand side of equation 8 in annex C, see subclause 5.2.5
difference difference between consecutive values of dm, see subclause 5.2.4
dm spectral distortion measure, see subclause 5.2.4
hangcount primary hangover counter, see subclause 5.2.7
lagcount number of subframes in current frame meeting periodicity criterion, see subclause 5.2.9
lastdm previous value of dm, see subclause 5.2.4
lags the open loop long term predictor lags for the four speech encoder subframes (GSM 06.20 [2].)
num numerator of left hand side of equation 8 in annex C, see subclause 5.2.5
oldlagcount previous value of lagcount, see subclause 5.2.9
prederr fourth order short term prediction error, see subclause 5.2.5
ptch Boolean flag indicating the presence of a periodic signal component, see subclause 5.2.9
pvad energy in the current filtered signal frame, see subclause 5.2.1
rav1 autocorrelation vector obtained from av1, see subclause 5.2.3
rc the first four unquantized reflection coefficients calculated in the speech encoder (GSM 06.20 [2])
rvad autocorrelation vector of the adaptive filter predictor values, see subclause 5.2.6
smallag difference between consecutive lag values, see subclause 5.2.9
stat Boolean flag indicating that the frequency spectrum of the input signal is stationary, see subclause
5.2.4
thvad adaptive primary VAD threshold, see subclause 5.2.6
tone Boolean flag indicating the presence of an information tone, see subclause 5.2.5
vadflag Boolean VAD decision with hangover included, see subclause 5.2.8
veryoldlagcount previous value of oldlagcount, see subclause 5.2.9
vvad Boolean VAD decision before hangover, see subclause 5.2.7
3.2.2 Constants
adp number of frames of hangover for secondary VAD, see subclause 5.2.6
burstconst minimum length of speech burst to which hangover is added, see subclause 5.2.8
dec determines rate of decrease in adaptive threshold, see subclause 5.2.6
fac determines steady state adaptive threshold, see subclause 5.2.6
frames number of frames over which av0 and av1 are calculated, see subclause 5.2.2
freqth threshold for pole frequency decision, see subclause 5.2.5
hangconst number of frames of hangover for primary VAD, see subclause 5.2.8
inc determines rate of increase in adaptive threshold, see subclause 5.2.6
lthresh lag difference threshold for periodicity decision, see subclause 5.2.9
margin determines upper limit for adaptive threshold, see subclause 5.2.6
nthresh frame count threshold for periodicity decision, see subclause 5.2.9
plev lower limit for adaptive threshold, see subclause 5.2.6
predth threshold for short term prediction error, see subclause 5.2.5
pth energy threshold, see subclause 5.2.6
thresh decision threshold for evaluation of stat flag, subclause 5.2.4
3.2.3 Functions
+ addition
- subtraction
* multiplication
/ division
ETSI
GSM 06.42 version 6.0.1 Release 1997 8 EN 300 973 V6.0.1 (1999-06)
| x | absolute value of x
AND Boolean AND
OR Boolean OR
b
MULT(x(i)) the product of the series x(i) for i=a to b
i=a
b
SUM(x(i)) the sum of the series x(i) for i=a to b
i=a
3.3 Abbreviations
For the purposes of the present document the following abbreviations apply:
ACF Autocorrelation Function
AFLAT Autocorrelation Fixed point LAttice Technique
ANSI American National Standards Institute
DTX Discontinuous Transmission
LTP Long Term Predictor
TX Transmission
VAD Voice Activity Detector
For abbreviations not given in this subclause see GSM 01.04 [1].
4 General
The function of the VAD is to indicate whether each 20 ms frame produced by the speech encoder contains speech or
not. The output is a Boolean flag (vadflag) which is used by the Transmit (TX) DTX handler defined in GSM 06.41 [4].
The present document is organized as follows:
Clause 5 describes the principles of operation of the VAD. Clause 6 provides an overview of the computational
description of the VAD. The computational details necessary for the fixed point implementation of the VAD algorithm
are given in the form of an American National Standards Institute (ANSI) C program contained in GSM 06.06 [5].
The verification of the VAD is based on the use of digital test sequences which are described in GSM 06.07 [6].
The performance of the VAD algorithm is characterized by the amount of audible speech clipping it introduces and the
percentage activity it indicates. The characteristics for the VAD defined in the present document have been established
by extensive testing under a wide range of operating conditions. The results are summarized in annex A.
5 Functional description
The purpose of this clause is to give the reader an understanding of the principles of operation of the VAD, whereas
GSM 06.06 [5] contains the fixed point computational description of the VAD. In the case of discrepancy between the
two descriptions, the description in GSM 06.06 [5] will prevail.
5.1 Overview and principles of operation
The function of the VAD is to distinguish between noise with speech present and noise without speech present. This is
achieved by comparing the energy of a filtered version of the input signal with a threshold. The presence of speech is
indicated whenever the threshold is exceeded.
ETSI
GSM 06.42 version 6.0.1 Release 1997 9 EN 300 973 V6.0.1 (1999-06)
The detection of speech in mobile environments is difficult due to the low speech/noise ratios which are encountered,
particularly in moving vehicles. To increase the probability of detecting speech, the input signal is adaptively filtered
(see subclause 5.2.1) to reduce its noise content before the voice activity decision is made (see subclause 5.2.7).
The frequency spectrum and level of the noise may vary within a given environment as well as between different
environments. It is therefore necessary to adapt the input filter coefficients and energy threshold at regular intervals as
described in subclause 5.2.6.
5.2 Algorithm description
The block diagram of the VAD algorithm is shown in figure 1. The individual blocks are described in the following
subclauses. The global variables shown in the block diagram are described in table 1.
Table 1: Description of variables in figure 1
Var Description
acf The ACF vector which is calculated in the speech encoder
(GSM 06.20 [2]).
av0 Averaged ACF vector.
av1 A previous value of av0.
lags The open loop long term predictor lags for the four speech encoder subframes
(GSM 06.20 [2])
...