From fe8aa4ce5e8eebc1c6f762f4fc40328718a13e22 Mon Sep 17 00:00:00 2001 From: Miller Puckette Date: Sat, 31 Dec 2005 01:32:12 +0000 Subject: Deleted unused (?) files svn path=/trunk/; revision=4318 --- pd/portaudio_v18/docs/latency.html | 192 ------------------------------------- 1 file changed, 192 deletions(-) delete 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 deleted file mode 100644 index 87f1d122..00000000 --- a/pd/portaudio_v18/docs/latency.html +++ /dev/null @@ -1,192 +0,0 @@ - - - - - - - - - PortAudio Implementation - Start/Stop - - - - - - - -

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-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

-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: -

-Select "Control Panel" from the "Start Menu".
-Launch the "System" Control Panel
-Click on the "Advanced" tab.
-Click on the "Environment Variables" button.
-Click "New" button under User Variables.
-Enter PA_MIN_LATENCY_MSEC for the name and some optimistic number for the -value.
-Click OK, OK, OK.

- -

-Improving Latency on Windows

-There are several steps you can take to improve latency under windows. -

-Avoid reading or writng to disk when doing audio.
-Turn off all automated background tasks such as email clients, virus scanners, -backup programs, FTP servers, web servers, etc. when doing audio.
-Disconnect from the network to prevent network traffic from interrupting -your CPU.

-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. -

-Select "Control Panel" from the "Start Menu".
-Launch the "System" Control Panel
-Click on the "Advanced" tab.
-Click on the "Settings" button in the Performance area.
-Click on the "Advanced" tab.
-Select "Background services" in the Processor Scheduling area.
-Click OK, OK.

-Please let us know if you have others sugestions for lowering latency. -
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