Re: [linux-audio-dev] seeking minimal C++ library for ear-training

From: David Olofson <david@email-addr-hidden>
Date: Sun Jan 23 2005 - 17:12:49 EET

On Sunday 23 January 2005 06.22, Jeffrey Brown wrote:
[...]
> All I'll ever need is a library that will let me
> playback a sample at a certain set of frequencies --
> like 6 at a time (bass tone, high tone, and a
> four-note chord) -- until keyboard input signals it to
> stop. In fact, even that's more than I need -- if
> playing a sample is hard, I'd be perfectly happy
> listening to square waves.

Square waves are actually quite hard to do well. :-) Sines are much
easier, and if you don't need loads of voices, you can construct
other waveforms - nicely bandlimited and alias free - by just adding
the appropriate harmonics or other overtones, stopping at half the
sample rate (Nyqvist). (You may need to roll off the amplitude of the
highest few overtones, if the ringing is too audible.)

If you *really* want to keep things simple, and don't need sampled
sounds (which are far from trivial to play back at arbitrary rates
without distortion), I think you should go with some nice and simple
"raw audio API", such as PortAudio or perhaps SDL.

SDL doesn't support audio or MIDI input, but does support audio output
and is very portable. It also makes it very easy to implement
"interactive" mouse and keyboard input. (It's designed for games and
multimedia applications, so one would expect nothing less...)
Finally, SDL provides a simple portable threading API, and there's a
dead simple way of doing safe interaction with the audio callback:
SDL_LockAudio()/SDL_UnlockAudio(), which ensures that the audio
callback doesn't fire while you're messing with the shared data.

(Real time programmers like myself will of course tell you to *never*
lock the audio thread like that, since that degrades determinism to
the level of the locking thread - but for a non-critical application,
it sure beats random data corruption! :-)

[...]
> Is there a non-threatening, perhaps even
> easy-to-install, C++ library that will do what I have
> in mind?

A library is probably overkill for this job, unless you need high
quality sample playback. It's just too simple to deserve a library of
it's own, so any library will do a lot more than what you need, which
means added complexity. It's probably easier to learn how to generate
sine waves and send them to the sound card than to learn any higher
level audio library.

As to threading, any audio API or audio library that's seriously
usable for interactive applications will have to use threads or
similar one way or another. PortAudio, SDL and most serious audio
APIs use callbacks that run in the context of a "hidden" audio
thread, or (on some platforms) in interrupt context.

This is because you *must* maintain a steady stream of data to the
audio driver at all times, or you'll get horrible sounding breaking
up or looping. When you want low latency as well (you don't want to
wait several seconds to hear the result of tweaking the EQ in your
media player, do you?), this just doesn't mix with the way "normal"
application programming is done - so you want to keep the actual
audio processing in it's own thread.

I have some SDL programming examples on my site. Nothing that does
quite what you need (the PC speaker emulator would be the closest
match), but if I get bored tonight, I might subvert one of them into
a very basic additive toy synth. :-)

 http://olofson.net/mixed.html (speaker.tar.gz)
  "Simple, useless PC speaker emulator/driver
  for SDL. It supports the audio API (that part
  is a speaker emulator, which should be portable)
  as well as the X bell API (X only) and the Linux
  console. Includes sound FX engine + sound effects
  from the DOS version of Project Spitfire."

 http://olofson.net/examples.html (simplemixer-1.1.tar.gz)
  "A very simple sound mixing example, using only
  SDL without any add-on libraries. It plays a drum
  pattern using 4 sounds and 4 voices and not too
  tight timing. ;-)"

//David Olofson - Programmer, Composer, Open Source Advocate

.- Audiality -----------------------------------------------.
| Free/Open Source audio engine for games and multimedia. |
| MIDI, modular synthesis, real time effects, scripting,... |
`-----------------------------------> http://audiality.org -'
   --- http://olofson.net --- http://www.reologica.se ---
Received on Sun Jan 23 20:15:09 2005

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