A Core Audio "Hello World"
A440 is a "Hello World" application for Core Audio on for all variants of Mac OS X: desktop, iPhone, and iPad. The simplest thing that could possibly be done in audio is to play a simple tone. Thus, A440 plays a 440 Hz tone, also known as A440, Concert A, or Middle A (A4), the A above Middle C (C4).
The tone is played using both Audio Queue Services (AudioQueueRef and friends) and Audio Unit Processing Graph Services (AUGraph and friends). In order to keep the code as simple as possible, it does not use any of the "Public Utility" C++ APIs.
Getting the Code
You can download zips and tarballs of the source from the downloads page, or clone the repository:
% hg clone http://bitbucket.org/ddribin/a440