All.

Somewhat long description, but I thought background info may help.

I am using a PIC16F88, 25LC1025 EEPROM, and MCP4822 DAC to record and play back spoken audio. Communication between the PIC, EEPROM, and DAC is SPI. Input audio is from a microphone and amp and is approximately 3-4Vpp, depending on how loud one talks. Input and output filters are 4kHz passive.This circuit uses an internal 4MHz oscillator and the ADC clock is fosc/8, or 500kHz (2us). 

Only the upper 8 bits of the 10-bit conversion result are saved to the EEPROM, and there is a delay of 200us between conversions. The reason for the delay is because the code for retrieving the data from the EEPROM and running it through the DAC is much slower than the on the ADC side. Originally, the ADC was filling the EEPROM in about 16 or 17 seconds, while the DAC side took about 45-48 seconds. This resulted in the audio output sounding like an old tape player running very slowly. The ADC delay resulted in a more realistic speed.

The overall quality of the audio is poor. There is a lot of noise; dropouts, hiss, and some higher frequency (digital?) background. I tried changing the input filter to an active filter but, if anything, the sound quality was worse. Dropouts increased and the sound seemed to shift (phasing problem?) more frequently.

I can connect headphones to the output of the output filter (the DAC output is sufficiently robust for that) to hear the audio, but I sent it to an amplifier (simple op-amp) so I have more volume. If I listen during record, I have a low-frequency 'motorboat' sound at about 100Hz. Experimentation has shown that if I disconnect power to the EEPROM, the sould clears up. Of course, no data is being recorded. I have been unable to determine precisely what is causing that noise.

Here is where I think I am:
1. SPI comms - ok
2. Writing to/reading from EEPROM - ok
3. ADC - working, but may be timeing problem
4. DAC - working, but may be timing problem

I think a large part of the problem is that the ADC and DAC are not running at exactly the same speed/phase. However, I have been unable to figure out how to fix this without resorting to changing delays over and over until I get it right. I know there has to be an easier way to figure this out. Eventually, I would like to use an SD card to increase the storage time but, first, I have to get this working.

I have found a lot of information on the internet about ADC in general, but very little related to converting, storing, and retrieving basic audio. With all the info out there, I have likely overlooked that key piece.

Any suggestions and links will be appreciated.

Thanks for taking the time to wade through this post.

Richard Seriani

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