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Joe Armstrong - Erlang and other things

Controlling Sound with OSC Messages

In earlier articles I have talked about Controlling Live Music and A Badass Way to Connect Programs Together

In this article I'll show how I have interfaced Erlang to the Sonic Pi, the SuperCollider and Pure Data.

The code for described here is available at

The code I'm described is at “proof of concept” stage - you can download it and run it - but there are no build instructions, so familiarity with Erlang/OSC and the target systems is implied.

Act1 : Connecting Sonic PI to Erlang.

This all started when Sam Arron showed me how to do remotely control Sonic Pi.

“It's easy,” he said “Just open port 4557 and send it a run_code message.” Actually he didn't say *exactly* this but he might say this if I asked again.

So it's really really easy. Any of the programs you see in a sonic pi window can be turned into a fragment of Erlang code

Here's a very simple program in Sonic Pi:

To run this in Sonic Pi you just press the run button, and you'll hear a noise.

Here's the code to do the same thing in Erlang:

test1() -> run_code("use_synth :fm\nplay 50\n"). run_code(Prog) -> P1 = lists:flatten(Prog), M = ["/run-code" , "erl-id", P1], E = osc:encode(M), {ok, Socket} = gen_udp:open(0,[binary]), ok = gen_udp:send(Socket, "localhost", 4557, E), gen_udp:close(Socket).

The entire program is in sonic.erl the OSC encoder is in osc.erl.

Fantastic I say.

Note: I can collaborate with the Sonic PI without messing in the Sonic Pi source code tree - all I do is send it messages that it understands.

Act2 : Connecting The SuperCollider to Erlang.

Having connected the Sonic Pi to Erlang I started idly wondering “How does the Sonic Pi actually make sounds? does it do it itself or does in use some other code to do this” - I downloaded the sources to find out - the documentation doesn't say.

Low and behold the Sonic Pi makes sounds by sending OSC-over-UDP messages to the SuperCollider. I say the SuperCollider here, since this is the name of the program you have to download to perform these experiments. But actually the SuperCollider itself is actually two programs, which communicate by (guess what) OSC-over-UDP.

A program called scsynth which is part of the SuperCollider program generates the actual sounds. “Why is it done this way?” you're probably asking. It's because the author of the SuperCollider wanted to separate the program into a real-time part (scsynth) and a control part (which has less stringent control problems) - the music should play (via scsynth) even if the controller is busy or crashed for a short time - a very nice design.

Sam kindly told me how to trace the OSC messages to the SuperCollider and I turned this into an Erlang program.

So to control the SuperCollider I open UDP port 4556 and send it a stream of OSC messages - ie I'm doing exactly what the Sonic Pi did, only I'm doing it from Erlang and without the user interface.

The code is in sc.erl All I've done is start a scsynth server listening to port 4556 (this is done in and send it a load of OSC commands.

Working in the SuperCollider was an eye opener. It a REPL *without* a terminal and is similar to the Plan9 ACME shell/editor.

I'd read about ACME but never experienced it. It's a Visceral experience. You never type code twice. If it's on the screen you click it, the answer appears “somewhere else”.

Interestingly Pure data makes exactly the same design decision.

Why why why? Because it's faster to interact with. No typing just clicking - this is important for live performance. Musicians instruments are not laptops with keyboards - they are things you can hit and stroke.

Act 3 - Pure Data

Finally Pure Data. PD programs have no textual representation, there is only a graphic interface to a data-flow language.

The program is in

The Erlang code pd.erl is extremely simple:

play(N) -> run_code(["/playNote", N]). run_code(M) -> E = osc:encode(M), {ok, Socket} = gen_udp:open(0,[binary]), ok = gen_udp:send(Socket, "localhost", 6677, E), gen_udp:close(Socket).

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