Content for TS 46.042 Word version: 18.0.0

3.1 Definitions 3.2 Symbols 3.3 Abbreviations
...

1 Scope p. 5

The present document 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 p. 5

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 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 p. 5

3.1 Definitions p. 5

For the purposes of the present document, the following terms and definitions apply:

mobile environment:

any environment in which MSs may be used.

noise:

signal component resulting from acoustic environmental noise.

3.2 Symbols p. 6

For the purposes of the present document, the following symbols apply:

3.2.1 Variables p. 6

aav1

filter predictor values, see clause 5.2.3

acf

the ACF vector which is calculated in the speech encoder (GSM 06.20 [2])

adaptcount

secondary hangover counter, see clause 5.2.6

av0

averaged ACF vector, see clause 5.2.2

av1

a previous value of av0, see clause 5.2.2

burstcount

speech burst length counter, see clause 5.2.7

den

denominator of left hand side of equation 8 in annex C, see clause 5.2.5

difference

difference between consecutive values of dm, see clause 5.2.4

spectral distortion measure, see clause 5.2.4

hangcount

primary hangover counter, see clause 5.2.7

lagcount

number of subframes in current frame meeting periodicity criterion, see clause 5.2.9

lastdm

previous value of dm, see clause 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 clause 5.2.5

oldlagcount

previous value of lagcount, see clause 5.2.9

prederr

fourth order short term prediction error, see clause 5.2.5

ptch

Boolean flag indicating the presence of a periodic signal component, see clause 5.2.9

pvad

energy in the current filtered signal frame, see clause 5.2.1

rav1

autocorrelation vector obtained from av1, see clause 5.2.3

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 clause 5.2.6

smallag

difference between consecutive lag values, see clause 5.2.9

stat

Boolean flag indicating that the frequency spectrum of the input signal is stationary, see clause 5.2.4

thvad

adaptive primary VAD threshold, see clause 5.2.6

tone

Boolean flag indicating the presence of an information tone, see clause 5.2.5

vadflag

Boolean VAD decision with hangover included, see clause 5.2.8

veryoldlagcount

previous value of oldlagcount, see clause 5.2.9

vvad

Boolean VAD decision before hangover, see clause 5.2.7

3.2.2 Constants p. 6

adp

number of frames of hangover for secondary VAD, see clause 5.2.6

burstconst

minimum length of speech burst to which hangover is added, see clause 5.2.8

dec

determines rate of decrease in adaptive threshold, see clause 5.2.6

fac

determines steady state adaptive threshold, see clause 5.2.6

frames

number of frames over which av0 and av1 are calculated, see clause 5.2.2

freqth

threshold for pole frequency decision, see clause 5.2.5

hangconst

number of frames of hangover for primary VAD, see clause 5.2.8

inc

determines rate of increase in adaptive threshold, see clause 5.2.6

lthresh

lag difference threshold for periodicity decision, see clause 5.2.9

margin

determines upper limit for adaptive threshold, see clause 5.2.6

nthresh

frame count threshold for periodicity decision, see clause 5.2.9

plev

lower limit for adaptive threshold, see clause 5.2.6

predth

threshold for short term prediction error, see clause 5.2.5

pth

energy threshold, see clause 5.2.6

thresh

decision threshold for evaluation of stat flag, clause 5.2.4

3.2.3 Functions p. 7

addition

subtraction

multiplication

division

| x |

absolute value of x

AND

Boolean AND

Boolean OR

MULT(x(i))

the product of the series x(i) for i=a to b

SUM(x(i))

the sum of the series x(i) for i=a to b

3.3 Abbreviations p. 7

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

Transmission

VAD

Voice Activity Detector

For abbreviations not given in this clause see GSM 01.04 [1].

4 General p. 7

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.