Version: 1.18
PRELIMINARY/CONFIDENTIAL
TrueSpeech® Co-Processor
The following table shows the parameter order and sample values for Fixed Filter Set 4 (300-640 Hz):
Word
Sample Value
28967 A1 (Q14) = 1.768028
Description
0
1
2
3
4
-28798 A2 (Q15) = -0.878854
-1988 B2 (Q15) = -0.060682
0 B1 (Q14) = 0
1988 B0 (Q15) = 0.060682
5
6
7
8
9
28017 A1 (Q14) = 1.710051
-30963 A2 (Q15) = -0.944905
12605 B2 (Q15) = 0.384666
0 B1 (Q14) = -0.620271
12607 B0 (Q15) = 0.384720
10
11
12
13
14
31398 A1 (Q14) = 1.916387
-31859 A2 (Q15) = -0.972276
30541 B2 (Q15) = 0.932033
-30020 B1 (Q14) = -1.832294
30542 B0 (Q15) = 0.932074
15
16
17
18
7 Internal Filter numeric scaling
159 Minimum in-band energy threshold
21 21/32 in-band to broad-band ratio parameter
0FF5H shift-mask = 0FFH, bit count=5 for History test
Q15 format uses 32768 to represent 1.000
Q14 format uses 16384 to represent 1.000
The internal filter numeric scaling is set to compensate for the internal gain within the filter to prevent numeric
overflow. The higher the internal gain (Q factor), the lower the numeric scaling value required. For a narrow
bandpass filter (high Q) this should be set to around 5; for a wide bandpass filter this should be set to around 7. The
value used is best determined by experimentation, by looking for non-linearity in the filter output value as a function
of input signal frequency and level.
The minimum in-band energy is squared by the CT8022, and used for comparison with the in-band filter energy.
The in-band to broadband ratio parameter is used to create a scaling ratio by dividing by 32 and using the resulting
fractional multiplier to scale the broadband energy for comparison with the in-band energy.
The final parameter is split into two parts. These 12-bits create an AND mask to control how many History Register
bits are counted. The least significant 4 bits provide the bit count threshold 0-15 for counting the “1” bits in the
History Register.
The filter coefficients themselves have to be created using special Digital Filter Design Software. This software is
available from a number of sources specializing in DSP design software. Filter design packages vary in the
nomenclature used to label the A and B parameters as well the sign value +/- of the constants. The best approach to
take when designing a new filter is to first create a design for the 300-640Hz filter defined above. The filter
coefficients generated can usually be approximately compared against the sample given above to determine if any of
the parameters need to be re-ordered or have their sign +/- inverted.
CT8022A11AQC FW Revision 0118 DSP GROUP, INC., 3120 SCOTT BOULEVARD
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
139
All specifications are subject to change without prior notice.
ETC [ ETC ]
