Hi Andy,

Ten years ago I build a (Philips 87c552 based) full duplex voice scrambler machine.
The processing was in the time domain (packet scrambler) and was done entirely with the 87c552. I don’t exactly remember but I think I forced a little the clock (about 16MHz).
The 87c552 has a multiplexed 50uS 10-bits ADC and two 8-bits PWM based DAC.
I used 2 of the ADC’s channels, both the PWM DACs and an external 8KB RAM memory for 0.5-second voice acquisition in both directions. The program was in the internal 8K eprom.
Also I used 4 identical 2-poles Chebyshev LPF in a Sallen-Key configuration.
The voice band was limited 3.4kHz and the sampling frequency was 8 ksps.
The PWM frequency was ~24KHz (at the upper limit of the uP).
The scrambling process was over 0.5 s in 16ms packets in full duplex mode.
The full processing chain was as follows:

(Microphone) ->(LPF)->(8bit ADC) ->(uP scrambler)->(8bit PWM)->(LPF)-> (phone-line)-> (LPF)->(8bit ADC) ->(uP descrambler)->(8bit PWM)->(LPF)->(speaker).

Conclusions:
1. The S/N ratio for 8- bits PWM DAC was identical as for an 8-bits parallel DAC.
2. The quality of the voice was near to an 8-bit PCM system (without scrambling)
3. The quality is more dependent from the input antialiasing LPF then from the output LPF witch may be an also a 1-pole RC LPF.
4. 8-bits 8Ksps PWM is good enough for a telephone voice player.

Regards,
Cristian Ceausescu