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I have an c5502 texas low cost DSP. I can capture sound trough its P2 input and process every separated sample after sending it to the P2 output. Im trying to implement some kind of effect in the sound, but this DSP has very low memory so i cant storage the signal to process it as a whole after.

There is any way of implementing effects like overdrive or even low pass filters in real time? I have tried somethings and it seems they arent enough to change the sound by modifying only sample by sample (using past results and samples)...

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I'm sure you can implement a simple fuzz or overdrive effect on that board. Apply asymmetrical clipping to you your data, and filter the result with a biquad IIR filter. You'll need a lowpass filter to smooth out some of the nasty high frequencies after clipping (try a cut-off frequency $f_c\in [4..7]$kHz), and you might want a parametric EQ to boost some mid frequencies. You can compute the filter coefficients using the Audio EQ Cookbook. You can probably do much more than that but this should get you started.

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It would actually be a big problem if you implemented a guitar effect by recording the signal in a buffer, processing the entire block, and playing it back - there would be a long latency between the moment the note is played and the effect is heard!

Actually, you can do a lot with little memory.

  • You don't need memory at all for very rough fuzz / distortion effects. In the same category are octaving effects based on rectification and modulation.

  • With just a few words of memory to store the filter state you can implement IIR filters - and thus all kinds of EQ / tone-controller. A few more words to store a phase accumulator and you have a low-frequency oscillator for auto-wah or other modulated effects.

  • Another class of effects that only require a handful of words of memory are dynamic processing effects - compression, limiting...

  • With a few kilowords of memory, you can get into (short) delay-based effects: flanger, chorus, phaser, and time-domain pitch-shifting (and by combining a few instances of those: harmonization).

  • 10-20 kilowords is long enough for actual echo effects.

  • You can get a decent reverb in 30-40 kilowords.

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