PortAudio - an Open Source Cross Platform Audio API
Ross Bencina, Phil Burk
email:
[email protected],
[email protected]
website: http: //www.portaudio. com/
Abstract
This paper describes a new open-source cross-platform 'C' 'C' Application
library for audio input and output. It is designed to simplify
the porting of audio applications between various platforms,
and also to simplify the development of audio programs inPortAudio
general by hiding the complexities of device interfacing. The
API was worked out through community discussions on the
music-dsp mailing list. A number of people have contributed
to the development of the API and are listed on the web-site. WMME OSS
Implementations of PortAudio for Windows MME and oth
DirectSound, the Macintosh Sound Manager, and Unix OSS DX aud
API
have been developed and are freely available on the web.
Support for other platforms is being planned. The paper
describes the use of PortAudio and discusses the issues Audio Hardware
involved in its development including the design philosophy,
latency, callbacks versus blocking read/write calls, and effi- Figure 1, PortAudio generally uses
ciency. audio APIs to access the audio har
existing
dware.
1. Introduction
Suppose you want to write a real-time audio application
and you want it to run on more than just one kind of computer. That could mean that you have to learn how to use several different audio APIs like DirectSoundTM on Microsoft
WindowsTM, and the MacintoshTM Sound Manager, and OSS
on Linux. And then you would have to write interfaces
between your program and each of those host-specific APIs.
Wouldn't it be nice to just write one simple audio interface
for your program and to have it work on the majority of computers. PortAudio is designed to help you do just that.
PortAudio is also useful if you are only writing for one
platform because PortAudio is often much simpler to use
than the native audio APIs. Thus PortAudio could be useful
for pedagogical purposes by allowing beginning students to
quickly obtain real-time audio input and output.
PortAudio provides a platform neutral interface to realtime audio streaming services in the form of a 'C' language
API. PortAudio has been implemented for a number of platforms by wrapping native audio services - these implementations are publicly available under a BSD style Open Source
license. PortAudio was selected as the audio component of a
larger initiative called PortMusic that combines audio, MIDI,
and audio file I/O.
2. The PortAudio Architecture
The PortAudio architecture includes two main abstractions: Audio Devices and Audio Streams.
Audio devices represent audio input and/or output ports
on the host platform. The PortAudio API provides functions
for enumerating available devices and querying them for
properties such as available sampling rates, number of supported channels and supported sample formats.
Audio streams manage active audio input and output
through at most one input device and one output device -
streams may be full duplex or half duplex. A PortAudio
implementation manages buffers internally and requests
audio processing from the client application via a callback
that is associated with a stream when it is opened.
A variety of sample formats are supported by PortAudio
including 16 and 32 bit integer and 32 bit floating point.
Where necessary PortAudio manages conversion between
the requested buffer formats and those available natively. If
requested, PortAudio can clamp out of range samples and/or
dither samples to a native format.
3. API Overview
This section presents an informal overview of the C language functions and structures that make up the PortAudio
