TDA7402 datasheet(20/59 Pages) STMICROELECTRONICS

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TDA7402

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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 0.5VRMS to 2VRMS. 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 0.5VRMS the output of the InGain-

stage has to have also 0.5VRMS 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 VREF. 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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