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diff --git a/pd/portaudio/docs/latency.html b/pd/portaudio/docs/latency.html deleted file mode 100644 index 87f1d122..00000000 --- a/pd/portaudio/docs/latency.html +++ /dev/null @@ -1,192 +0,0 @@ -<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> -<html> -<head> - <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> - <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> -</head> -<body> - -<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 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 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> -<br> -</body> -</html> |