Hi all,

I am curious of the operation of the two main subsystems of the NE570/SA571 compandor IC (averaging rectifier and variable gain cell).

Figure 9 shows a "simplified" schematic of the rectifier subsystem but I have no idea what half of those transistors are... i.e. Q3/Q7 and Q9...

Here is my take at what is going on, say for compression:

- the input signal is tied to both the rectifier input and the variable gain cell.
- the rectifier creates a unidirectional signal using a diode from the input and a smoothing capacitor Cr flattens out the ripple.
- the output of the rectifier is a current Ig proportional to that flattened input signal divided by the resistor R5
- the current Ig created by the rectifier then moves into the variable gain cell as a current source while the same segment of input signal is flowing through
- the current Ig then determines the transconductance of one of the transistors being used in gain cell's inverting amplifier configuration.
- the gain is then the inverse of the transconductance divided by R2, which will vary based on the input signal's level.

Could this same gain cell effect also be achieved with an LED (lit with current from rectifier) and an LDR in an inverting amplifier configuration?

Also, don't know how they arrive at the gain equation as being a square root...

Cheers!

 

The rectifier determines the amplitude of the signal

The LED circuit will not goose up the weak signals and attenuate the strong to compress the dynamic range.

 

The rectifier determines the amplitude of the signal

The LED circuit will not goose up the weak signals and attenuate the strong to compress the dynamic range.

I was thinking more along the lines of an isocoupler where the LDR is controlled by the LED's brightness which is proportional to the signal's amplitude.