rolf wrote:
> Well, I took the sage advice from the list and implemented
> the Roman Black algorithm on my microcontroller.
>
> The results were mixed.
>   

When you say that it seems like the noise in your implementation is due 
to the sampling frequency, do you mean that the sampling frequency was 
too low or that your sampling frequency was not stable?

For this approach to work, it's absolutely necessary that the output 
samples are generated at a rate that exactly matches the pseudo analog 
node. The easy way to achieve this is by running the sampler at a 
constant rate and predicting how the output voltage will change given 
the current filtered output voltage, the next pulse, the duration of the 
pulse, and the filter's frequency response.

BTW, I have not built this type of D/A before - so my insight lacks any 
practical backing. But here are a couple of suggestions that might help.

First, the RC summing junction is the one that the algorithm attempts to 
model. As you note, to drive any kind of load an amplifier like the 
LM386 is needed. If you make the LM386 a low pass filter whose cutoff is 
above the RC but below the output sample rate, and at the same time 
allow it to have gain, then you might improve the sound quality. 

Second, do you have access to an audio-bandwidth spectrum analyzer? If 
so you may try synthesizing a sine wave (e.g. 500Hz) and measure its 
spectral purity. It would be interesting to see how narrow the 500Hz 
spike is, how low the noise floor it, what the harmonics look like, etc. 
Also, it'd be interesting to see how strong the output sampling 
frequency is in the spectrum.

Third, I'm not sure how you structured your code, but if you want to get 
some tips on obscure single-cycle resolution isochronous timing 
generation, then check out:
http://www.dattalo.com/technical/software/pic/pwm256.txt

Fourth, not a suggestion, but a question; did you experiment with 
different RC time constants?

Scott
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