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Here's a complete library/toolkit for FM (Frequency Modulation) synthesis:

link1 link2

If you have time to spare... creating simple sound synthesis from scratch is actually a fun endeavor. If you create a small buffer of 256 16 bit samples which represent either a sine. a sawtooth, block or pulse, you can copy these to a live audiobuffer (e.g. a small buffer (say 16kb)) which constantly loops. By staying ahead of the playposition, and constantly filling up the buffer with new values, you can create the soundoutput. You can use the small buffers to combine these in interesting ways (simplest is just to add them together (additive synthesis)).

The frequency of the tone can be manipulated by using a bigger or smaller sampling step through the small buffers. Amplitude can be manipulated by scaling the samples before putting them into the output buffer.

Great fun experimenting with this!

If you have this step nailed, you can add more sophisticated effects like filters (low pass, high pass, etc) and effects (reverbs, echoes, etc)

R

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Have you looked at the synthesis toolkit (STK)? It's in C++ (I don't think ObjC is the right language for audio synthesis, in fact audio units, Apple's own way of doing audio stuff, including generators/filters/effects... is in C++).

STK will run on Mac OS X, and iOS no problem (CoreAudio is supported), but will also run on Linux and Windows (Direct sound and ASIO), using RtAudio. It's really nicely done and lightweight, these guys have spent a lot of time thinking about it and it will definitely give you a big head start. It can handle loads of different audio file formats + midi (and hopefully OSC soon...).

There is also Create and CLAM which is huge, these include GUI components and many other things which you might or might not want. If you're only interested in doing sound synthesis I really recommend STK.

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PortAudio is also a great C API that we used last semester in an audio programming course. It provides an audio callback...what more could you need!?

I haven't tried incorporating it with anything in Objective-C yet, but will report back when I do.

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Writing audio synthesis algorithms in C/obj-C is quite difficult in my opinion. I would recommend writing your signal processing algorithms using PureData and then use ZenGarden or libpd to embed and interpret the pd patches in your app.

Edit in 2024: Look for C++ libraries as others suggest in their answers, predata code is difficult to maintain, difficult to integrate with version control, and difficult to debug or optimize once the graphical abstraction is not enough.

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Another C++ library is nsound:

http://nsound.sourceforge.net

One can generate any kind of modulated signal using the Generator class or using the provided Sine class. Each time-step can have it's own instantaneous frequency and phase offset.

You can also experiment with the Python module to prototype your algorithm quickly, then implement in C++. It can produce pretty matplotlib plots from Python and even from C++!

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