question about - PT2399 Digital Delay

Thread Starter


Joined Mar 29, 2018
Ill be more specific:
1. mod + demod : cant understand whats the purpose of this part.
2. there's an op amp that dont act like low pass (the one that goes into Din)- cant understand his purpose to.
3. I don't understand how these are low pass filters - cant analyse this circuit.


Joined May 4, 2013
The IC converts the audio to a digital bit stream. This bit stream is delayed by a shift register and then converted back to audio.

The analog to digital converter appears to be some kind of delta modulator. A delta modulator has a single bit output that says whether the input is increasing or decreasing in voltage. The demodulator is simply a low pas filter.

The length of time for the delay is set by the number of bits in the shift register and the clock rate going into the shift register. The shift register is 44 K bits long so the delay would be 1 second for a 44 KHz clock.

Thread Starter


Joined Mar 29, 2018
First of all thanks ! that answer really helped ! :)
but I still have some questions :
I attached here schematics of echo circuit using this chip ( all the op amps and the delay unit)
1. I understand that op-1 (left) is a low pass filter, but what is the purpose of the one next to him ?
2. I think I just dont understand how the circuit function, I understand all what you said but all the function of the op amps isnt clear to me. if you can explain it to me it would be great !

Thanks !! :)