Pi at the absolute limit!

Published on Oct 9, 2014 Pi Synth

"Brought a desktop environment back onto my Pi for this test, which saw it right on the edge of collapse. Apologies for the carnage on my desk by the way, I have a cable hell thing going on ...

But here is a Pi clocked at 'High' / 950MHz, *JUST* getting through the Slade test - in fact you can hear me laugh at one point when the handclap comes in and the 3D 'hello_triangle' test visibly stutters horribly! But you can't really hear any glitches, even though some events definitely got backed up as synthesis took too many cycles.

But a fine achievement for the little Pi - desktop environment, plus top -H, plus a 720p full-screen 3D demo, PLUS PIANA playing 7 notes of polyphony from 7 independently-configured synths.

A bit more detail for the obsessives -

The 'tune' was sequenced in Logic. Logic is playing it out of my Mac as a MIDI stream, which is routed for a variety of odd reasons through an M-Axiom Axiom 49 (USB input, out through Physical MIDI 5-pin DIN) into an inexpensive USB to MIDI adapter so that it can then be seen by the Pi as USB MIDI. The Pi synthesizes the sound and emits it through a USB audio adapter into my lovely little Minirig. The fact that a CME Pro, the USB adapter, a Bluetooth keyboard and USB audio are all hung off the Pi speaks volumes to how much they have improved USB performance in the last 2 years. The Pi is running whatever the stock desktop is that came with it, and to monitor CPU usage I have a terminal running top -H, and to really stress the cack out of it, particularly the memory interface, I have a full-screen OpenGL ES thing running.

And it works fine, but hell's bells it is redlining - the green CPU monitor bottom right shows nothign left, and that is reflected in both top -H and in my own monitoring within PIANA which tells me how close to 44100 the achieved sample render rate was, as a percentage. So the lower the better, and you can see numbers of 64-70% on screen as I lean in. 100% is bad, and it does exceed 100% at times, meaning some packets ate into the insurance policy, which is about 8 packets wide - so if 8 packets back to back average at 100% or greater, you will hear a glitch.

Impressive though, the Pi, despite being feeble and useless, is also pretty damn great. See how conflicted I am?!?!"