mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-19 10:32:02 -05:00
8d353a5b3b
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/trunk@189 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
193 lines
8.2 KiB
HTML
193 lines
8.2 KiB
HTML
<!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>
|