TDA7402 Datasheet, PDF (20/59 Pages) STMicroelectronics – CARRADIO SIGNAL PROCESSOR

TDA7402
1.11 Compander
Signal-Compression
A fully integrated signal-compressor with programmable Attack- and Decay-times is present in the A619 (see
Figure 15).
The compander consists of a signal-level detection, an A/D-Converter plus adder and the normal SoftStep-Vol-
ume-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 vol-
ume-word defined by the IIC-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).
The internal reference-level of the compander is programmable in three steps from 0.5V_RMS to 2V_RMS. For a
proper behavior of the compression-circuit it is mandatory to have at a 0dB input-signal exactly the programmed
reference-level after the InGain-stage. E.g. at a configured reference-level of 0.5V_RMS the output of the InGain-
stage has to have also 0.5V_RMS at 0dB source-signal (Usually the 0dB for CD is defined as the maximum pos-
sible signal-level). To adapt the external level to the internal reference-level the programmable attenuation in
the differential-stages and the InGain can be used.
Figure 15. Compander Block Diagram
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 V_REF. The level-detector interprets this as a very small signal and reacts with the maximum pro-
grammed compander-gain. In the application this gain has to be compensated by decreasing the volume with
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