[sdiy] Raspberry Pi 2 Synthesizer Project

[Richie Burnett]

> I've been able to get a period size of 8 frames and I'm using a polling 
> function instead of a callback. Either way, the period size is much larger 
> than 2 frames.

Okay.  By "frames" do you mean audio samples?  It looks like I'll have to 
read up on ALSA if I'm going to make my R-Pi do anything impressive audio 
related.  I've had it playing and recording WAV files via an external Edirol 
USB audio interface (the on-board Audio output is awful quality,) but not 
actually processing or synthesising anything in real-time.

> I prefer interrupt per DAC frame, but this worked surprisingly well so I'm 
> encouraged to continue.

The block processing scheme is what I am used to myself.  One interrupt 
generated every 48 samples, or 64 samples or whatever, then you synthesise 
another block of 64 samples to re-fill the buffer before they're played out. 
Usually in two separate halves:  The buffer write pointer and the playback 
pointer ping-pong between the halves, so that you are always writing to the 
other buffer to the one that the DAC is playing back the audio from.  It 
increases latency over the one interrupt per sample scheme for sure, but you 
generate a lot less interrupts and it's a much more efficient way of working 
once you get used to it.

-Richie,


-----Original Message----- 
From: Scott Gravenhorst
Sent: Sunday, February 07, 2016 6:36 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Raspberry Pi 2 Synthesizer Project

Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>Great stuff.
>
>Was it hard to get the ping-pong audio buffer stuff working
>properly with the OS and audio driver? I've never programed
>real-time audio on such a high-level platform before. I've always
>had to set up the buffers and DMA controller myself to make this
>sort of thing happen, on dspic33f etc.
>
>Look forward to seeing how you take the project forward.
>
>-Richie,

With the Linux ALSA lib, at least for this system, I've not been able to
get an interrupt or flag per sample time like dsPIC33F easily does.
That's what I'm accustomed to developing with, so ALSA is new/different
for me.  I've been able to get a period size of 8 frames and I'm using a
polling function instead of a callback.  Either way, the period size is
much larger than 2 frames.  Polling is just an infinite loop that calls
a function to try to write already generated data.  The function can
return a value that indicates try-again-later.  So you loop on
generate-write.  I prefer interrupt per DAC frame, but this worked
surprisingly well so I'm encouraged to continue.

-- ScottG
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