From ae6b5d89ea93b95c2990895077cf5e8f0bba9ad9 Mon Sep 17 00:00:00 2001 From: Guenter Geiger Date: Mon, 2 Feb 2004 11:28:02 +0000 Subject: This commit was generated by cvs2svn to compensate for changes in r1301, which included commits to RCS files with non-trunk default branches. svn path=/trunk/; revision=1302 --- pd/portaudio_v18/docs/latency.html | 192 +++++++++++++++++++++++++++++++++++++ 1 file changed, 192 insertions(+) create mode 100644 pd/portaudio_v18/docs/latency.html (limited to 'pd/portaudio_v18/docs/latency.html') 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 @@ + + + + + + + + + PortAudio Implementation - Start/Stop + + +  +
+ + + +
+
+

+PortAudio Latency

+
+ +

This page discusses the issues of audio latency for PortAudio +. It offers suggestions on how to lower latency to improve the responsiveness +of applications. +

What is Latency? +
PortAudio and Latency +
Macintosh +
Unix +
WIndows
+By Phil Burk, Copyright 2002 Phil Burk and Ross Bencina +

+What is Latency?

+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. +

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. +

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. +

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. +

+PortAudio and Latency

+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. +

The latency in milliseconds due to this buffering  is: +

latency_msec = 1000 * numBuffers * framesPerBuffer / framesPerSecond
+This is not the total latency, as we have seen, but it is the part we can +control. +

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. +

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: +

int Pa_GetMinNumBuffers( int framesPerBuffer, double sampleRate +);
+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. +

+Macintosh

+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. +

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: +

setenv PA_MIN_LATENCY_MSEC 4
+ +

+Unix

+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. +

+Windows

+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. +

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. +

You can set the PA_MIN_LATENCY_MSEC variable to 50, for example, by +entering in MS-DOS: +

set PA_MIN_LATENCY_MSEC=50
+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. +

For Windows XP, you can set environment variables as follows: +

    +
  1. +Select "Control Panel" from the "Start Menu".
    2. + +
  2. +Launch the "System" Control Panel
    3. + +
  3. +Click on the "Advanced" tab.
    4. + +
  4. +Click on the "Environment Variables" button.
    5. + +
  5. +Click "New" button under  User Variables.
    6. + +
  6. +Enter PA_MIN_LATENCY_MSEC for the name and some optimistic number for the +value.
    7. + +
  7. +Click OK, OK, OK.
    8. +
+ +

+Improving Latency on Windows

+There are several steps you can take to improve latency under windows. +
    +
  1. +Avoid reading or writng to disk when doing audio.
    2. + +
  2. +Turn off all automated background tasks such as email clients, virus scanners, +backup programs, FTP servers, web servers, etc. when doing audio.
    3. + +
  3. +Disconnect from the network to prevent network traffic from interrupting +your CPU.
    4. +
+Important: 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. +
    +
  1. +Select "Control Panel" from the "Start Menu".
    2. + +
  2. +Launch the "System" Control Panel
    3. + +
  3. +Click on the "Advanced" tab.
    4. + +
  4. +Click on the "Settings" button in the Performance area.
    5. + +
  5. +Click on the "Advanced" tab.
    6. + +
  6. +Select "Background services" in the Processor Scheduling area.
    7. + +
  7. +Click OK, OK.
    8. +
+Please let us know if you have others sugestions for lowering latency. +
  +
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