TDA7402 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'TDA7402' PDF : 69 Pages View PDF
TDA7402
Audioprocessor part
3.11 Compander
Signal compression
A fully integrated signal compressor with programmable attack and decay times is present in
the TDA7402 (see Figure 17).
The compander consists of a signal level detection, an A/D Converter plus adder and the
normal Soft Step volume stage. First of all the left and the right InGain-signal is rectified,
respectively, and the logarithm is build from the summed signal. The following low-pass
smooth the output-signal of the logarithm amplifier and improves the low frequency
suppression. The low pass output-voltage then is A/D converted an added to the current
volume-word defined by the I2C bus. Assuming reference level or higher at the compander
input, the output of the ADC is 0. At lower levels the voltage is increasing with 1Bit/dB. It is
obvious that with this configuration and a 0.5dB-step volume stage the compression rate is
) fixed to 2:1 (1dB less at the input leads to 0.5dB less at the output).
ct(s The internal reference level of the compander is programmable in three steps from 0.5VRMS
u to 2VRMS. For a proper behavior of the compression circuit it is mandatory to have at a 0dB
d input signal exactly the programmed reference level after the InGain-stage. E.g. at a
ro configured reference-level of 0.5VRMS the output of the InGain stage has to have also
P 0.5VRMS at 0dB source-signal (Usually the 0dB for CD is defined as the maximum possible
signal-level). To adapt the external level to the internal reference level the programmable
te attenuation in the differential stages and the InGain can be used.
ole Figure 17. Compander block diagram
bs SOFT STEP VOLUME
- O INL
INL
OUTL
OUTL
) INR
INR
OUTR
OUTR
ct(s 8Bit
IIC-BUS
u VOLUME
rod CONTROL
8 Bit
6 Bit
ADDER
CLKATT
CLKREL
D
A
LIN
STEREO FULL
P WAVE RECTIFIER
50Hz LOW-PASS
lete ANTI-CLIP
RIN
o ENABLE
+
LOG
AMPL
-
s+
b AM
O MPIN
CLPIN
INPUT
SELECT
D00AU1147
3.11.1
Anti-clipping
In a second application the compander-circuit can be used for a anti-clipping or limiting
function. In this case one of the dedicated inputs (AM or MPin) is connected directly to the
clip-detector of the power-amplifier. if no clipping is detected, the open-collector output of
the power-amplifier is highohmic and the input-voltage of the rectifier is VREF. The level
detector interprets this as a very small signal and reacts with the maximum programmed
compander gain. In the application this gain has to be compensated by decreasing the
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