TWL1110 Datasheet, PDF (25/37 Pages) Texas Instruments – VOICE-BAND AUDIO PROCESSOR VBAPE

TWL1110
VOICE-BAND AUDIO PROCESSOR (VBAP™)
PRINCIPLES OF OPERATION
SLWS103 – NOVEMBER 2000
DTMF generator operation and interface (continued)
Table 7. Typical DTMF and Single Tone Control
DT7 DT6 DT5 DT4 DT3 DT2 DT1 DT0
0
1
0
1
1
0
0
1
0
1
1
0
0
0
1
1
0
1
1
0
1
1
0
1
0
1
1
1
1
0
0
0
1
0
0
1
1
0
1
1
1
0
1
0
1
0
1
1
1
0
1
1
1
1
0
1
1
1
0
1
0
0
0
1
INTEGER
VALUE
89
99
109
120
155
171
189
209
TONE
FUNCTION
DTMF Low
DTMF Low
DTMF Low
DTMF Low
DTMF HIgh
DTMF HIgh
DTMF HIgh
DTMF HIgh
TONE/HZ
697
770
852
941
1209
1336
1477
1633
Tones from the DTMF generator block are present at all outputs and are controlled by enabling or disabling the
individual output ports. The values that determine the tone frequency are loaded into the tone registers (high
and low) as two separate values.
The values loaded into the tone registers initiate an iterative table look-up function, placing a 6-bit or 7-bit in
twos-complement value into the the tone registers. There is a 2-dB difference in the resulting output of the two
registers, the high-tone register having the greater result.
In the case of low-tone signal, the tone generator outputs a 6-bit integer with a maximum code of 31 (011111).
However, the DTMF output is an 8-bit integer. Therefore, two zeros are padded to the MSB position which results
in 31 (00011111). On the other hand, the receive channel requires a 15-bit integer, the input 3968
(000111110000000). Since the maximum digital value of receive channel is 16383 (011111111111111), the
maximum low-tone signal is designed to be – 12.32 dB below the full digital scale.
ǒ Ǔ²
20 log
3968
16383
+ –12.32 dB
(1)
In the case of high-tone signal, the tone generator outputs a 7-bit integer with a maximum code of 39 (0100111).
The DTMF, therefore, pads a zero to the MSB and generates an 8-bit integer (00100111). In order to send the
digital code to receive channel, it is converted to a 15-bit integer with seven more zeros padded to LSB position
and biased as 4992 (001001110000000). Therefore, the maximum high-tone signal is designed to be – 10.32 dB
below the full digital scale.
ǒ Ǔ _{20 log}
4992
16383
2
+ –10.32 dB
(2)
In the case of DTMF output, the tone generator outputs an 8-bit integer with the maximum code level of 70
(01000110). This output is converted to a 15-bit code with the value of 8960 (010001100000000). Therefore,
the maximum output of DTMF is designed to be – 5.24 dB below the full digital scale.
ǒ Ǔ²
20 log
8960
16383
+ –5.24 dB
(3)
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