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| author | Guenter Geiger <ggeiger@users.sourceforge.net> | 2004-02-02 11:28:02 +0000 |
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| committer | Guenter Geiger <ggeiger@users.sourceforge.net> | 2004-02-02 11:28:02 +0000 |
| commit | ae6b5d89ea93b95c2990895077cf5e8f0bba9ad9 (patch) | |
| tree | 1e7570f11ac688e94342968e90301c4684e61193 /pd/portaudio_v18/docs/latency.html | |
| parent | f26399eba6ee6ce9eb7bae9a4b60a90dc2ebca94 (diff) | |
This commit was generated by cvs2svn to compensate for changes in r1301,
which included commits to RCS files with non-trunk default branches.
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diff --git a/pd/portaudio_v18/docs/latency.html b/pd/portaudio_v18/docs/latency.html new file mode 100644 index 00000000..87f1d122 --- /dev/null +++ b/pd/portaudio_v18/docs/latency.html @@ -0,0 +1,192 @@ +<!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> |