TrueSpeech® Co-Processor
PRELIMINARY/CONFIDENTIAL
Version: 1.18
Table 9-1: Time Constant Table
Time Constant Parameter
attack/decay fraction per 125
microseconds = 1/2^(n+1)
Time Constant inms
(assuming 8 KHz sample rate)
U(n+1) = U(n) - U(n) * fraction
0
1
2
0.5
0.25
0.125
250 us
512 us
1ms
3
4
0.0625
0.03125
2ms
4ms
5
0.015625
8ms
6
0.0078125
16ms
7
0.00390625
32ms
8
0.001953125
64ms
9
0.0009765625
0.00048828125
0.000244140625
0.0001220703125
0.00006103515625
0.00003051757813
0.00001525878906
128ms
256ms
512ms
1024ms
2048ms
4096ms
8192ms
10
11
12
13
14
15
In the Attack phase (gain decreases) , the AGC gain is recalculated every CODEC sample (125 us) such that:
AGC gain = AGC gain - ( ( AGC gain - min AGC gain) * 1/2^(n+1) )
This behaves as an approximately exponential decay from the current instantaneous gain value towards the min AGC
value.
In the Decay phase (gain increases), the AGC gain is recalculated every CODEC sample (125 us) such that:
AGC gain = AGC gain + ( (max AGC gain - AGC gain) * 1/2^(n+1) )
This behaves as an approximately exponential decay from the current instantaneous gain value towards the max
AGC value.
Note that the time constant, is the time that it takes for an initial value to decay to 1/e of its initial value where:
e = 2.71828.
92
DSP GROUP, INC., 3120 SCOTT BOULEVARD CT8022A11AQC FW Revision 0118
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
All specifications are subject to change without prior notice.
ETC [ ETC ]
