At 12:16 PM 4/26/2005, DCFluX wrote:
>Progress Report on this dead thread:
>
>I got some free time to experiment. I was able to make a attempt
>using a ATtiny26. I used the atmel's internal ADC biased to 2.5V with
>100K resistors to +5 and GND and audio coupled in with a 0.22uF
>capacitor. For the DAC I used a resistor ladder built with 20K and
>10K resistors and audio coupled out with a 0.22uF. And I am using a
>256Kbit 25MHz bus speed serial nvram for the storage.
>
>It works, But...
>
>Because of the succesive aproximation nature of the atmel's ADC there
>is not enough speed to make the project usable. Each conversion takes
>between 16uS and 256uS. and a different variable means a different
>sampling time. I was able to sustain a sampling rate of about 2.8kHz,
> which if you know anything about digital audio means that the best
>frequency that can be passed is 1.4kHz. I am doing some math to
>convert the 10bit ADC to 8bit for the DAC and to save space in the
>RAM, but it is not what is slowing it down as it is a simple Shift
>statement.
Use a timer to space out the ADC samples.
The timer runs as long as the longest ADC conversion (BTW you can
violate the aDC clock, you just get less resolution)
You don't start a new sample till the timer runs out.
>In the future I will use a dual op-amp for the audio in and out as it
>will do a better job of biasing to 2.5V than the 2 resistors.
Stay away from the LM324, nasty crossover distortion.
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