Figure 6.20-3 Vad Data Diagram - Nuvoton ISD94124BYI Technical Reference Manual

Isd arm cortex-m4f soc

Hide thumbs

page of 928

/ 928
Contents
Table of Contents
Bookmarks

Table of Contents

6.20.5.3 SINC Filter

For the SINC filter, it has three over sampling rate (OSR) configuration that controlled by register

SINCOSR (VAD_SINC_CTL[11:8]): OSR48, OSR64, OSR96. For the three OSR options, the DMIC

bus clock (DMIC_CLK) will be (Fs x 48) kHz, (Fs x 64) kHz or (Fs x 96) kHz, where Fs is sample

rate. The DMIC_CLK will be controlled by VAD module when VADEN (VAD_SINCCTL[31]) is

enabled. The frequency of DMIC_CLK is obtained according to the following equation.

where F_DMIC_CLK is the frequency of DMIC_CLK and F_DMIC_MCLK is the frequency of

The frequency of DMIC working main clock (DMIC_MCLK) should be F_DMIC_CLK x 4.

6.20.5.4 Biquad Filter

The biquad filter is a second-order recursive linear filter with two poles and two zeros. Its transfer

function is the Z-domain consists of two quadratic functions:

Each Biquad Coefficient (a1, a2, b0, b1 and b2) has 16 bits in Sxx.13 format where

1. S is the sign bit (1 bit)

2. xx are integers (2bits)

3. 13 fractional bits (13 bits)

6.20.5.5 VAD Configuration

VAD analyses the PCM data from DMIC channel 0. In order to use the VAD function, the parameters

need to be set correctly. First it's to set the attack time. For the attack time setting, the bigger we

set, the faster the energy we calculated. So the default value of the LTAT (VAD_CTL0[19:16]) (Long

term attack time) is 0x7, and the STAT (VAD_CLT0 [7:0]) (short term attack time) is 0xCC. If you

want to calculate the energy faster, you can set it bigger, and then the waveform of the energy will

be more similar with the input. But the STAT (VAD_CLT0[7:0]) should be always bigger than the

LTAT (VAD_CTL0[19:16]).

Then we need to set the threshold of the energy, including the short term energy threshold, long

term energy threshold and the deviation energy threshold. The threshold setting is based on the

input level. The bigger the input level, the larger the threshold setting. If the threshold is set too

high, then the VAD cannot detect the voice, and if the threshold is set too low, then it may have

some wrong detection.

In order to use the VAD detection, user can have some example such as some actual voice file to

tune the parameters.

6.20.5.6 VAD Decision Tree

Figure 6.20-4 illustrates the operation flow of VAD.

ISD94100 Series Technical Reference Manual