TDA7405_04 Datasheet, PDF (20/57 Pages) STMicroelectronics – EQUALIZER CARRADIO SIGNAL PROCESSOR

TDA7405
5.10 Compander
5.10.1 Signal-Compression
A fully integrated signal-compressor with programmable Attack- and Decay-times is present in the A631 (see
Figure 20). The compander consists of a signal-level detection, an A/D-Converter plus adder and the normal
SoftStep-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 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 1 Bit/dB. It is obvious that with this config-
uration 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 out-
put 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 possible 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 21. Compander Block Diagram
5.10.2 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 programmed compander-gain.
In the application this gain has to be compensated by decreasing the volume with the same value in order to get the
desired output-level. In clipping situation the open-collector-current generates a voltage-drop at the rectifier-input,
which forces the compander to decrease the gain until the clipping disappears.
It is even possible to run the compression-mode and the Anti-Clipping mode in parallel. In this case the maxi-
mum compander-Gain should be set to 29 dB.
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