Sound on BEAM: Music in the Land of Distributed Lisp

( Part II )

Overview

• Introduction
• undertone and Its Use Cases
• Architecture
• Supervision Trees
• Clients and Servers
• External Processes, Ports, and exec
• Languages
• A Custom REPL
• Demo / Performance / Walkthrough
• What’s Next for undertone
• Q & A

Who am I?

• Prinicpal software engineer
• Life-long hacker (started at age 9 in ‘81; never stopped)
• Habbitual explorer
• Core contributor to Robert Virding’s Lisp Flavoured Erlang (LFE)

Who am I?

• 4 yo - Messing about with Piano
• 9 or 10 yo - Formal piano lessons, casual guitar
• 14 yo - First synthesizer (analog Korg PolySix)
• 16 yo - Performed in a chamber music group
• 25 yo - Stopped music; replaced with physics/maths … then career
• 42 yo - Began exploring generative music
• 48 yo - Started playing seriously again

Re-entering the Musical World

Re-entering the Musical World

Re-entering the Musical World

Wherefore undertone?

Wherefore undertone?

Wherefore undertone?

Wherefore undertone?

Wherefore undertone?

Wherefore undertone?

• Create music in my preferred language
• Monitoring and automatically restarting OS processes
• Speak to Open Sound Control servers (i.e., controlling faders on software consoles/mixers)
• Potentially host my own OSC servers
• Send TCP packages to the Extempore compiler service
• Automtically reconnect (with backoff support) to required services
• Be able to restart any of these components in the event of partial or complete system failure

Wherefore undertone?

• Create music in my preferred language
• Monitoring and automatically restarting OS processes
• Speak to Open Sound Control servers (i.e., controlling faders on software consoles/mixers)
• Potentially host my own OSC servers
• Send TCP packages to the Extempore compiler service
• Automtically reconnect (with backoff support) to required services
• Be able to restart any of these components in the event of partial or complete system failure

Wherefore undertone?

Also:

• Maintain multiple, separate state contexts
• Support a familiar workflow (a REPL!)
• Provide basic session management (ETS + a handful of functions)

What is undertone?

Architecture

Architecture

System context for Extempore:

• talks to the OS / routes MIDI
• signals routed to external devices
• also routed to MIDI in the DAW (e.g., software synthesizers)

Architecture

System context for undertone:

• uses Erlang (starts up supervision tree, clients, servers)
• talks to Extempore (bitstrings over TCP)
• controls OSC servers (e.g., DAWs)

Architecture

Architecture

Architecture

Architecture

Architecture

Architecture

Architecture

Architecture

The undertone “container”:

• OTP app with supervisor and state server
• State server for mananging session commands and system config
• OSC clients for any OSC-enabled software running a UDP server
• TCP client for long-running connections to Extempore
• LFE REPL
• Extempore REPL

Progress Check

• ✅ Introduction
• ✅ undertone and Its Use Cases
• ✅ Architecture
• Supervision Trees
• Clients and Servers
• External Processes, Ports, and exec
• A Custom REPL
• Demo / Performance / Walkthrough
• What’s Next for undertone
• Q & A

Supervision Trees

Supervision Trees

Supervision Trees

Supervision Trees

Clients and Servers

• Extempore client
  • TCP client for sending messages to the compiler server
  • Scheme syntax as bitstrings
  • Started by the release

Clients and Servers

• Open Sound Control clients
  • potentially many
  • connecting to both software and hardware
  • e.g., digital audio workstations (DAWs) to control console faders

Clients and Servers

• Open Sound Control servers
  • none right now
  • could create an OSC/MIDI bridge in LFE/Erlang
  • create a custom Raspberry Pi sound device and export OSC methods
• erlsci/osc is UDP only

Clients and Servers

• gen_servers for state management
• The Extempore REPL is a simple looping server
• The undertone backends each have their own gen_server that’s responsible for managing the backend

External Processes & Erlang Ports

• Previously used erlang:open_port (spawn_executable)
• Switched to exec library (which still uses Erlang ports)
• Extempore backend:
  • capturing output from Extempore
• “Bevin” backend:
  • sending MIDI (OS process)
  • receiving MIDI (separate OS process)
  • stdout is captured for both and logged / parsed

External Processes & Erlang Ports

• Previously used erlang:open_port (spawn_executable)
• Switched to exec library (which still uses Erlang ports)
• Extempore backend:
  • capturing output from Extempore
• “Bevin” backend:
  • sending MIDI (OS process)
  • receiving MIDI (separate OS process)
  • stdout is captured for both and logged / parsed

External Processes & Erlang Ports

• Previously used erlang:open_port (spawn_executable)
• Switched to exec library (which still uses Erlang ports)
• Extempore backend:
  • capturing output from Extempore
• “Bevin” backend:
  • sending MIDI (OS process)
  • receiving MIDI (separate OS process)
  • stdout is captured for both and logged / parsed

External Processes & Erlang Ports

• Previously used erlang:open_port (spawn_executable)
• Switched to exec library (which still uses Erlang ports)
• Extempore backend:
  • capturing output from Extempore
• “Bevin” backend:
  • sending MIDI (OS process)
  • receiving MIDI (separate OS process)
  • stdout is captured for both and logged / parsed

Dependencies

• https://github.com/saleyn/erlexec
  • Addresses issues with terminating OS processes
  • Keeps to the spirit of Erlang’s clean Port API
  • Used to manage 2 of 3 backends in undertone

Dependencies

• https://github.com/erlsci/osc
  • Forked from https://github.com/marianoguerra/erlang-osc
  • 5-10 year old code, updated per rebar3 project best practices

Dependencies

• https://github.com/lfex/tcp-client
  • Originally used https://github.com/cabol/tcp_client
  • Abandoned that effort in favour of an implementation based upon gen_statem using https://andrealeopardi.com/posts/connection-managers-with-gen_statem/
  • Written in LFE

Dependencies

• https://github.com/ferd/backoff
  • A readily available and easy to use exponential backoff library

Progress Check

• ✅ Introduction
• ✅ undertone and Its Use Cases
• ✅ Architecture
• ✅ Supervision Trees
• ✅ Clients and Servers
• ✅ External Processes, Ports, and exec
• Languages
• A Custom REPL
• Demo / Performance / Walkthrough
• What’s Next for undertone
• Q & A

Erlang & LFE

Basics: a recursive function using pattern-matching in the function heads.

Erlang

ackermann(0, N) ->
  N+1;
ackermann(M, 0) ->
  ackermann(M-1, 1);
ackermann(M, N) ->
  ackermann(M-1, ackermann(M, N-1)).

LFE

(defun ackermann
  ((0 n) (+ n 1))
  ((m 0) (ackermann (- m 1) 1))
  ((m n) (ackermann (- m 1) (ackermann m (- n 1)))))

Erlang & LFE

OTP: Erlang supervisor

-module('undertone.sup').
-behaviour(supervisor).
-export([start_link/0]).
-export([init/1]).

start_link() ->
    supervisor:start_link({local, ?MODULE}, ?MODULE, []).
    
init([]) ->
    {ok, {sup_flags(),
          [child('undertone.server', start_link, [])]}}.
    
sup_flags() ->
    #{strategy => one_for_one,
      intensity => 3,
      period => 60}.

Erlang & LFE

OTP: LFE supervisor

(defmodule undertone.sup
  (behaviour supervisor)
  (export
   (start_link 0)
   (init 1)))
   
(defun start_link ()
  (supervisor:start_link `#(local ,(MODULE)) (MODULE) '()))

(defun init (_args)
  `#(ok #(,(sup-flags)
          (,(child 'undertone.server 'start_link '())))))
  
(defun sup-flags ()
  `#m(strategy one_for_one
      intensity 3

Extempore

xtlang:

(bind-func AudioBuffer_data_b64
  (lambda (ab:AudioBuffer*)
    (let ((b64size:i64 0)
          (datsize:i64 (* (AudioBuffer_frames ab)
                          (AudioBuffer_channels ab) 4)))
      (String (base64_encode (cast (tref ab 4) i8*)
                             datsize
                             (ref b64size))))))

Extempore

Scheme:

(sys:load "libs/external/portmidi.xtm")
(pm_initialize)
(define *midi-out* (pm_create_output_stream 3))

(define midi-loop
  (lambda (beat dur)
    (mplay *midi-out*
           (random (list 36 43 48 51 60 60 60 67 70 74 75))
           (random 60 80)
           dur 0)
    (callback (*metro* (+ beat (* .5 dur)))
              'midi-loop
              (+ beat dur)
              dur)))

(midi-loop (*metro* 'get-beat 4) 1/4)

A Custom REPL

A Custom REPL

A Custom REPL

A Custom REPL

A Custom REPL

Demo!

Demo!

Demo

Hi-res video of demo available here:

https://www.youtube.com/watch?v=neSGoz2F1d8

What’s Next for undertone?

Progress Check

• ✅ Introduction
• ✅ undertone and Its Use Cases
• ✅ Architecture
• ✅ Supervision Trees
• ✅ Clients and Servers
• ✅ External Processes, Ports, and exec
• ✅ Languages
• ✅ A Custom REPL
• ✅ Demo / Performance / Walkthrough
• ✅ What’s Next for undertone
• Q & A

Credits

Ent drawing by Talbot Jenkins.

Beginnig of first piece in demo adapted for Piano, based upon “Journey of the Sorcerer” by Bernie Leadon of The Eagles.

Contact

• oubiwann@gmail.com
• @oubiwann
• https://soundcloud.com/oubiwann/tracks
• linkedin.com/in/oubiwann
• @forgottentones
• https://soundcloud.com/forgotten-tones/tracks

undertone Resources

• https://github.com/ut-proj/undertone
• https://undertone.lfe.io/presentations
• https://undertone.lfe.io/book
• lfe.slack.com #algo-sound
• http://groups.google.com/group/lfe-undertone
• @lfeundertone
• https://www.instagram.com/lfeundertone/

LFE Resources

• https://lfe.io/
• https://github.com/rvirding/lfe
• lfe.slack.com
• http://groups.google.com/group/lisp-flavoured-erlang
• @ErlangLisp

Q & A