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193 lines
8.2 KiB
HTML
193 lines
8.2 KiB
HTML
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Mozilla/4.79 [en] (Windows NT 5.0; U) [Netscape]">
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<meta name="Author" content="Phil Burk">
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<meta name="Description" content="Internal docs. How a stream is started or stopped.">
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<meta name="KeyWords" content="audio, tutorial, library, portable, open-source, DirectSound,sound, music, JSyn, synthesis,">
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<title>PortAudio Implementation - Start/Stop</title>
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</head>
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<body>
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<center><table COLS=1 WIDTH="100%" BGCOLOR="#FADA7A" >
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<tr>
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<td>
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<center>
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<h1>
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<a href="http://www.portaudio.com">PortAudio</a> Latency</h1></center>
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</td>
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</tr>
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</table></center>
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<p>This page discusses the issues of audio latency for <a href="http://www.portaudio.com">PortAudio</a>
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. It offers suggestions on how to lower latency to improve the responsiveness
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of applications.
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<blockquote><b><a href="#what">What is Latency?</a></b>
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<br><b><a href="#portaudio">PortAudio and Latency</a></b>
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<br><b><a href="#macintosh">Macintosh</a></b>
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<br><b><a href="#unix">Unix</a></b>
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<br><b><a href="#windows">WIndows</a></b></blockquote>
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By Phil Burk, Copyright 2002 Phil Burk and Ross Bencina
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<h2>
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<a NAME="what"></a>What is Latency?</h2>
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Latency is basically longest time that you have to wait before you obtain
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a desired result. For digital audio output it is the time between making
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a sound in software and finally hearing it.
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<p>Consider the example of pressing a key on the ASCII keyboard to play
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a note. There are several stages in this process which each contribute
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their own latency. First the operating system must respond to the keypress.
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Then the audio signal generated must work its way through the PortAudio
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buffers. Then it must work its way through the audio card hardware. Then
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it must go through the audio amplifier which is very quick and then travel
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through the air. Sound travels at abous one foot per millisecond through
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air so placing speakers across the room can add 5-20 msec of delay.
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<p>The reverse process occurs when recording or responding to audio input.
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If you are processing audio, for example if you implement a software guitar
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fuzz box, then you have both the audio input and audio output latencies
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added together.
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<p>The audio buffers are used to prevent glitches in the audio stream.
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The user software writes audio into the output buffers. That audio is read
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by the low level audio driver or by DMA and sent to the DAC. If the computer
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gets busy doing something like reading the disk or redrawing the screen,
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then it may not have time to fill the audio buffer. The audio hardware
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then runs out of audio data, which causes a glitch. By using a large enough
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buffer we can ensure that there is always enough audio data for the audio
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hardware to play. But if the buffer is too large then the latency is high
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and the system feels sluggish. If you play notes on the keyboard then the
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"instrument" will feel unresponsive. So you want the buffers to be as small
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as possible without glitching.
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<h2>
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<a NAME="portaudio"></a>PortAudio and Latency</h2>
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The only delay that PortAudio can control is the total length of its buffers.
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The Pa_OpenStream() call takes two parameters: numBuffers and framesPerBuffer.
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The latency is also affected by the sample rate which we will call framesPerSecond.
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A frame is a set of samples that occur simultaneously. For a stereo stream,
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a frame is two samples.
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<p>The latency in milliseconds due to this buffering is:
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<blockquote><tt>latency_msec = 1000 * numBuffers * framesPerBuffer / framesPerSecond</tt></blockquote>
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This is not the total latency, as we have seen, but it is the part we can
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control.
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<p>If you call Pa_OpenStream() with numBuffers equal to zero, then PortAudio
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will select a conservative number that will prevent audio glitches. If
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you still get glitches, then you can pass a larger value for numBuffers
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until the glitching stops. if you try to pass a numBuffers value that is
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too small, then PortAudio will use its own idea of the minimum value.
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<p>PortAudio decides on the minimum number of buffers in a conservative
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way based on the frameRate, operating system and other variables. You can
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query the value that PortAudio will use by calling:
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<blockquote><tt>int Pa_GetMinNumBuffers( int framesPerBuffer, double sampleRate
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);</tt></blockquote>
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On some systems you can override the PortAudio minimum if you know your
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system can handle a lower value. You do this by setting an environment
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variable called PA_MIN_LATENCY_MSEC which is read by PortAudio when it
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starts up. This is supported on the PortAudio implementations for Windows
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MME, Windows DirectSound, and Unix OSS.
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<h2>
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<a NAME="macintosh"></a>Macintosh</h2>
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The best thing you can do to improve latency on Mac OS 8 and 9 is to turn
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off Virtual Memory. PortAudio V18 will detect that Virtual Memory is turned
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off and use a very low latency.
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<p>For Mac OS X the latency is very low because Apple Core Audio is so
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well written. You can set the PA_MIN_LATENCY_MSEC variable using:
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<blockquote><tt>setenv PA_MIN_LATENCY_MSEC 4</tt></blockquote>
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<h2>
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<a NAME="unix"></a>Unix</h2>
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PortAudio under Unix currently uses a backgroud thread that reads and writes
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to OSS. This gives you decent but not great latency. But if you raise the
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priority of the background thread to a very priority then you can get under
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10 milliseconds latency. In order to raise your priority you must run the
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PortAudio program as root! You must also set PA_MIN_LATENCY_MSEC using
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the appropriate command for your shell.
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<h2>
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<a NAME="windows"></a>Windows</h2>
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Latency under Windows is a complex issue because of all the alternative
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operating system versions and device drivers. I have seen latency range
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from 8 milliseconds to 400 milliseconds. The worst case is when using Windows
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NT. Windows 98 is a little better, and Windows XP can be quite good if
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properly tuned.
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<p>The underlying audio API also makes a lot of difference. If the audio
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device has its own DirectSound driver then DirectSound can often provide
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better latency than WMME. But if a real DirectSound driver is not available
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for your device then it is emulated using WMME and the latency can be very
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high. That's where I saw the 400 millisecond latency. The ASIO implementation
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is generally very good and will give the lowest latency if available.
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<p>You can set the PA_MIN_LATENCY_MSEC variable to 50, for example, by
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entering in MS-DOS:
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<blockquote><tt>set PA_MIN_LATENCY_MSEC=50</tt></blockquote>
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If you enter this in a DOS window then you must run the PortAudio program
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from that same window for the variable to have an effect. You can add that
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line to your C:\AUTOEXEC.BAT file and reboot if you want it to affect any
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PortAudio based program.
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<p>For Windows XP, you can set environment variables as follows:
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<ol>
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<li>
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Select "Control Panel" from the "Start Menu".</li>
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<li>
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Launch the "System" Control Panel</li>
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<li>
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Click on the "Advanced" tab.</li>
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<li>
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Click on the "Environment Variables" button.</li>
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<li>
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Click "New" button under User Variables.</li>
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<li>
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Enter PA_MIN_LATENCY_MSEC for the name and some optimistic number for the
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value.</li>
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<li>
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Click OK, OK, OK.</li>
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</ol>
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<h3>
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Improving Latency on Windows</h3>
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There are several steps you can take to improve latency under windows.
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<ol>
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<li>
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Avoid reading or writng to disk when doing audio.</li>
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<li>
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Turn off all automated background tasks such as email clients, virus scanners,
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backup programs, FTP servers, web servers, etc. when doing audio.</li>
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<li>
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Disconnect from the network to prevent network traffic from interrupting
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your CPU.</li>
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</ol>
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<b>Important: </b>Windows XP users can also tune the OS to favor background
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tasks, such as audio, over foreground tasks, such as word processing. I
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lowered my latency from 40 to 10 milliseconds using this simple technique.
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<ol>
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<li>
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Select "Control Panel" from the "Start Menu".</li>
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<li>
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Launch the "System" Control Panel</li>
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<li>
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Click on the "Advanced" tab.</li>
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<li>
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Click on the "Settings" button in the Performance area.</li>
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<li>
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Click on the "Advanced" tab.</li>
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<li>
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Select "Background services" in the Processor Scheduling area.</li>
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<li>
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Click OK, OK.</li>
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</ol>
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Please let us know if you have others sugestions for lowering latency.
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<br>
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<br>
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</body>
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</html>
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