TDA7405 datasheet(20/56 Pages) STMICROELECTRONICS

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TDA7405

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2.10 Compander

2.10.1Signal-Compression

A fully integrated signal-compressor with programmable Attack- and Decay-times is present in the A631 (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 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 0.5VRMS

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 out-

put 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

2.10.2Anti-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 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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