TDA7402 datasheet(26/71 Pages) STMICROELECTRONICS

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Audioprocessor part

TDA7402

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4.10

Compander

Signal compression

A fully integrated signal compressor with programmable attack and decay times is present in

the A619 (see

Figure 17).

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 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).

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 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 17.

Compander block diagram

4.10.1

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

INL

INR

INL

INR

CLKATT

CLKREL

OUTL

OUTR

OUTL

8 Bit

SOFT STEP VOLUME

ADDER

IIC-BUS

VOLUME

CONTROL

OUTR

8 Bit

6 Bit

LOG

AMPL

INPUT

SELECT

ANTI-CLIP

ENABLE

CLPIN

RIN

LIN

MPIN

50Hz LOW-PASS

STEREO FULL

WAVE RECTIFIER

D00AU1147

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