[sdiy] Raspberry Pi 2 B Synthesizer Project Progress

Richie Burnett rburnett at richieburnett.co.uk
Tue Mar 15 20:37:58 CET 2016


This is very interesting Scott, thanks for sharing your results. 

-Richie, 

Sent from my Xperia SP on O2

---- Scott Gravenhorst wrote ----

>More progress - the current version works with latency (MIDI note-on 
>receipt to start of sound) of 2 milliseconds.
>
>Now that I finally understand how the buffer usage algorithm works for 
>ALSA, I started reducing the buffer size until it stopped working.  4 
>periods caused error messages from ALSA where 6 periods ran smoothly.
>
>6 periods or 48 frames at 44.1 ksps is about 1.09 milliseconds.
>
>It turned out that for the example program I started with (from the ALSA 
>project) the buffer had originally been set up to provide flawless audio 
>in a system with a lot of user stuff going on.  It's not a problem in 
>that environment for someone's MP3 to start playing some few 
>milliseconds after clicking the play button, so the buffer being rather 
>large allows for fits and starts of other CPU activity.  For the Rpi2, 
>I've been able to set audio to have a period size of 8 frames and a 
>buffer size of 6 periods.  This provides about 1 millisecond of latency 
>from ALSA and the rest comes from various sources including the MIDI 
>controller.  With these new settings I tested the synth for about 3 
>hours (pretty much continuously) and found no problems, no audio 
>glitches and no error indications from the synth or from ALSA.
>
>At 2 milliseconds, it's my opinion that the Raspberry Pi 2 B is a worthy 
>platform for synth development in C.
>
>Note that this is with Xorg running, so there's also the possibility of 
>a GUI based front panel with patch controls.
>
>-- Scott
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