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-<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
-<html>
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- <meta name="GENERATOR" content="Mozilla/4.79 [en] (Windows NT 5.0; U) [Netscape]">
- <meta name="Author" content="Phil Burk">
- <meta name="Description" content="Internal docs. How a stream is started or stopped.">
- <meta name="KeyWords" content="audio, tutorial, library, portable, open-source, DirectSound,sound, music, JSyn, synthesis,">
- <title>PortAudio Implementation - Start/Stop</title>
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-<body>
-&nbsp;
-<center><table COLS=1 WIDTH="100%" BGCOLOR="#FADA7A" >
-<tr>
-<td>
-<center>
-<h1>
-<a href="http://www.portaudio.com">PortAudio</a> Latency</h1></center>
-</td>
-</tr>
-</table></center>
-
-<p>This page discusses the issues of audio latency for <a href="http://www.portaudio.com">PortAudio</a>
-. It offers suggestions on how to lower latency to improve the responsiveness
-of applications.
-<blockquote><b><a href="#what">What is Latency?</a></b>
-<br><b><a href="#portaudio">PortAudio and Latency</a></b>
-<br><b><a href="#macintosh">Macintosh</a></b>
-<br><b><a href="#unix">Unix</a></b>
-<br><b><a href="#windows">WIndows</a></b></blockquote>
-By Phil Burk, Copyright 2002 Phil Burk and Ross Bencina
-<h2>
-<a NAME="what"></a>What is Latency?</h2>
-Latency is basically longest time that you have to wait before you obtain
-a desired result. For digital audio output it is the time between making
-a sound in software and finally hearing it.
-<p>Consider the example of pressing a key on the ASCII keyboard to play
-a note. There are several stages in this process which each contribute
-their own latency. First the operating system must respond to the keypress.
-Then the audio signal generated must work its way through the PortAudio
-buffers. Then it must work its way through the audio card hardware. Then
-it must go through the audio amplifier which is very quick and then travel
-through the air. Sound travels at abous one foot per millisecond through
-air so placing speakers across the room can add 5-20 msec of delay.
-<p>The reverse process occurs when recording or responding to audio input.
-If you are processing audio, for example if you implement a software guitar
-fuzz box, then you have both the audio input and audio output latencies
-added together.
-<p>The audio buffers are used to prevent glitches in the audio stream.
-The user software writes audio into the output buffers. That audio is read
-by the low level audio driver or by DMA and sent to the DAC. If the computer
-gets busy doing something like reading the disk or redrawing the screen,
-then it may not have time to fill the audio buffer. The audio hardware
-then runs out of audio data, which causes a glitch. By using a large enough
-buffer we can ensure that there is always enough audio data for the audio
-hardware to play. But if the buffer is too large then the latency is high
-and the system feels sluggish. If you play notes on the keyboard then the
-"instrument" will feel unresponsive. So you want the buffers to be as small
-as possible without glitching.
-<h2>
-<a NAME="portaudio"></a>PortAudio and Latency</h2>
-The only delay that PortAudio can control is the total length of its buffers.
-The Pa_OpenStream() call takes two parameters: numBuffers and framesPerBuffer.
-The latency is also affected by the sample rate which we will call framesPerSecond.
-A frame is a set of samples that occur simultaneously. For a stereo stream,
-a frame is two samples.
-<p>The latency in milliseconds due to this buffering&nbsp; is:
-<blockquote><tt>latency_msec = 1000 * numBuffers * framesPerBuffer / framesPerSecond</tt></blockquote>
-This is not the total latency, as we have seen, but it is the part we can
-control.
-<p>If you call Pa_OpenStream() with numBuffers equal to zero, then PortAudio
-will select a conservative number that will prevent audio glitches. If
-you still get glitches, then you can pass a larger value for numBuffers
-until the glitching stops. if you try to pass a numBuffers value that is
-too small, then PortAudio will use its own idea of the minimum value.
-<p>PortAudio decides on the minimum number of buffers in a conservative
-way based on the frameRate, operating system and other variables. You can
-query the value that PortAudio will use by calling:
-<blockquote><tt>int Pa_GetMinNumBuffers( int framesPerBuffer, double sampleRate
-);</tt></blockquote>
-On some systems you can override the PortAudio minimum if you know your
-system can handle a lower value. You do this by setting an environment
-variable called PA_MIN_LATENCY_MSEC which is read by PortAudio when it
-starts up. This is supported on the PortAudio implementations for Windows
-MME, Windows DirectSound, and Unix OSS.
-<h2>
-<a NAME="macintosh"></a>Macintosh</h2>
-The best thing you can do to improve latency on Mac OS 8 and 9 is to turn
-off Virtual Memory. PortAudio V18 will detect that Virtual Memory is turned
-off and use a very low latency.
-<p>For Mac OS X the latency is very low because Apple Core Audio is so
-well written. You can set the PA_MIN_LATENCY_MSEC variable using:
-<blockquote><tt>setenv PA_MIN_LATENCY_MSEC 4</tt></blockquote>
-
-<h2>
-<a NAME="unix"></a>Unix</h2>
-PortAudio under Unix currently uses a backgroud thread that reads and writes
-to OSS. This gives you decent but not great latency. But if you raise the
-priority of the background thread to a very priority then you can get under
-10 milliseconds latency. In order to raise your priority you must run the
-PortAudio program as root! You must also set PA_MIN_LATENCY_MSEC using
-the appropriate command for your shell.
-<h2>
-<a NAME="windows"></a>Windows</h2>
-Latency under Windows is a complex issue because of all the alternative
-operating system versions and device drivers. I have seen latency range
-from 8 milliseconds to 400 milliseconds. The worst case is when using Windows
-NT. Windows 98 is a little better, and Windows XP can be quite good if
-properly tuned.
-<p>The underlying audio API also makes a lot of difference. If the audio
-device has its own DirectSound driver then DirectSound can often provide
-better latency than WMME. But if a real DirectSound driver is not available
-for your device then it is emulated using WMME and the latency can be very
-high. That's where I saw the 400 millisecond latency. The ASIO implementation
-is generally very good and will give the lowest latency if available.
-<p>You can set the PA_MIN_LATENCY_MSEC variable to 50, for example, by
-entering in MS-DOS:
-<blockquote><tt>set PA_MIN_LATENCY_MSEC=50</tt></blockquote>
-If you enter this in a DOS window then you must run the PortAudio program
-from that same window for the variable to have an effect. You can add that
-line to your C:\AUTOEXEC.BAT file and reboot if you want it to affect any
-PortAudio based program.
-<p>For Windows XP, you can set environment variables as follows:
-<ol>
-<li>
-Select "Control Panel" from the "Start Menu".</li>
-
-<li>
-Launch the "System" Control Panel</li>
-
-<li>
-Click on the "Advanced" tab.</li>
-
-<li>
-Click on the "Environment Variables" button.</li>
-
-<li>
-Click "New" button under&nbsp; User Variables.</li>
-
-<li>
-Enter PA_MIN_LATENCY_MSEC for the name and some optimistic number for the
-value.</li>
-
-<li>
-Click OK, OK, OK.</li>
-</ol>
-
-<h3>
-Improving Latency on Windows</h3>
-There are several steps you can take to improve latency under windows.
-<ol>
-<li>
-Avoid reading or writng to disk when doing audio.</li>
-
-<li>
-Turn off all automated background tasks such as email clients, virus scanners,
-backup programs, FTP servers, web servers, etc. when doing audio.</li>
-
-<li>
-Disconnect from the network to prevent network traffic from interrupting
-your CPU.</li>
-</ol>
-<b>Important: </b>Windows XP users can also tune the OS to favor background
-tasks, such as audio, over foreground tasks, such as word processing. I
-lowered my latency from 40 to 10 milliseconds using this simple technique.
-<ol>
-<li>
-Select "Control Panel" from the "Start Menu".</li>
-
-<li>
-Launch the "System" Control Panel</li>
-
-<li>
-Click on the "Advanced" tab.</li>
-
-<li>
-Click on the "Settings" button in the Performance area.</li>
-
-<li>
-Click on the "Advanced" tab.</li>
-
-<li>
-Select "Background services" in the Processor Scheduling area.</li>
-
-<li>
-Click OK, OK.</li>
-</ol>
-Please let us know if you have others sugestions for lowering latency.
-<br>&nbsp;
-<br>&nbsp;
-</body>
-</html>