4.2 PCM Objects

From 2876869020584497e61a340d4ca3c4c8a8cbde7f Mon Sep 17 00:00:00 2001 From: Walter Bender Date: Sat, 21 Feb 2009 12:43:42 +0000 Subject: undoing pyalsaaudio patch --- (limited to 'arch_src/pyalsaaudio-0.2/doc/.svn/text-base/pcm-objects.html.svn-base') diff --git a/arch_src/pyalsaaudio-0.2/doc/.svn/text-base/pcm-objects.html.svn-base b/arch_src/pyalsaaudio-0.2/doc/.svn/text-base/pcm-objects.html.svn-base deleted file mode 100644 index 3fc5d78..0000000 --- a/arch_src/pyalsaaudio-0.2/doc/.svn/text-base/pcm-objects.html.svn-base +++ /dev/null @@ -1,331 +0,0 @@ - - - - - - - - - - - - - -4.2 PCM Objects - - - - - -

- -
-4.2 PCM Objects -

- -

-The acronym PCM is short for Pulse Code Modulation and is the method used in ALSA -and many other places to handle playback and capture of sampled sound data. - -

-PCM objects in alsaaudio are used to do exactly that, either play sample based -sound or capture sound from some input source (perhaps a microphone). The PCM object -constructor takes the following arguments: - -

-

- -
class PCM( [type], [mode], [cardname])

- -

-type - can be either PCM_CAPTURE or PCM_PLAYBACK (default). - -

-mode - can be either PCM_NONBLOCK, PCM_ASYNC, or PCM_NORMAL (the default). -In PCM_NONBLOCK mode, calls to read will return immediately independent of wether -there is any actual data to read. Similarly, write calls will return immediately -without actually writing anything to the playout buffer if the buffer is full. - -

-In the current version of alsaaudio PCM_ASYNC is useless, since it relies -on a callback procedure, which can't be specified from Python. - -

-cardname - specifies which card should be used (this is only relevant -if you have more than one sound card). Omit to use the default sound card - -

-This will construct a PCM object with default settings: - -

-Sample format: PCM_FORMAT_S16_LE -
-Rate: 8000 Hz -
-Channels: 2 -
-Period size: 32 frames -

- -

-PCM objects have the following methods: - -

-

- -
pcmtype( ): -Returns the type of PCM object. Either PCM_CAPTURE or PCM_PLAYBACK. -

- -

-

- -
pcmmode( ): -Return the mode of the PCM object. One of PCM_NONBLOCK, PCM_ASYNC, or PCM_NORMAL -

- -

-

- -
cardname( ): -Return the name of the sound card used by this PCM object. -

- -

-

- -
setchannels( nchannels): -Used to set the number of capture or playback channels. Common values are: 1 = mono, 2 = stereo, -and 6 = full 6 channel audio. Few sound cards support more than 2 channels -

- -

-

- -
setrate( rate): -Set the sample rate in Hz for the device. Typical values are 8000 (poor sound), 16000, 44100 (cd quality), -and 96000 -

- -

-

- -
setformat( )

-The sound format of the device. Sound format controls how the PCM device interpret data for playback, -and how data is encoded in captures. - -

-The following formats are provided by ALSA: -

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Format	Description
PCM_FORMAT_S8	Signed 8 bit samples for each channel
PCM_FORMAT_U8	Signed 8 bit samples for each channel
PCM_FORMAT_S16_LE	Signed 16 bit samples for each channel (Little Endian byte order)
PCM_FORMAT_S16_BE	Signed 16 bit samples for each channel (Big Endian byte order)
PCM_FORMAT_U16_LE	Unsigned 16 bit samples for each channel (Little Endian byte order)
PCM_FORMAT_U16_BE	Unsigned 16 bit samples for each channel (Big Endian byte order)
PCM_FORMAT_S24_LE	Signed 24 bit samples for each channel (Little Endian byte order)
PCM_FORMAT_S24_BE	Signed 24 bit samples for each channel (Big Endian byte order)
PCM_FORMAT_U24_LE	Unsigned 24 bit samples for each channel (Little Endian byte order)
PCM_FORMAT_U24_BE	Unsigned 24 bit samples for each channel (Big Endian byte order)
PCM_FORMAT_S32_LE	Signed 32 bit samples for each channel (Little Endian byte order)
PCM_FORMAT_S32_BE	Signed 32 bit samples for each channel (Big Endian byte order)
PCM_FORMAT_U32_LE	Unsigned 32 bit samples for each channel (Little Endian byte order)
PCM_FORMAT_U32_BE	Unsigned 32 bit samples for each channel (Big Endian byte order)
PCM_FORMAT_FLOAT_LE	32 bit samples encoded as float. (Little Endian byte order)
PCM_FORMAT_FLOAT_BE	32 bit samples encoded as float (Big Endian byte order)
PCM_FORMAT_FLOAT64_LE	64 bit samples encoded as float. (Little Endian byte order)
PCM_FORMAT_FLOAT64_BE	64 bit samples encoded as float. (Big Endian byte order)
PCM_FORMAT_MU_LAW	A logarithmic encoding (used by Sun .au files)
PCM_FORMAT_A_LAW	Another logarithmic encoding
PCM_FORMAT_IMA_ADPCM	a 4:1 compressed format defined by the Interactive Multimedia Association
PCM_FORMAT_MPEG	MPEG encoded audio?
PCM_FORMAT_GSM	9600 constant rate encoding well suitet for speech

- -

-

- -

-

- -
setperiodsize( period): -Sets the actual period size in frames. Each write should consist of exactly this number of frames, and -each read will return this number of frames (unless the device is in PCM_NONBLOCK mode, in which case -it may return nothing at all) -

- -

-

- -
read( ): -In PCM_NORMAL mode, this function blocks until a full period is available, and then returns a -tuple (length,data) where length is the size in bytes of the captured data, and data -is the captured sound frames as a string. The length of the returned data will be periodsize*framesize -bytes. - -
-In PCM_NONBLOCK mode, the call will not block, but will return (0,'') if no new period -has become available since the last call to read. -

- -

-

- -
write( data)

-Writes (plays) the sound in data. The length of data must be a multiple of the frame size, and -should be exactly the size of a period. If less than 'period size' frames are provided, the actual -playout will not happen until more data is written. - -

-If the device is not in PCM_NONBLOCK mode, this call will block if the kernel buffer is full, and -until enough sound has been played to allow the sound data to be buffered. The call always returns -the size of the data provided - -

-In PCM_NONBLOCK mode, the call will return immediately, with a return value of zero, if the buffer is -full. In this case, the data should be written at a later time. - -

-

- -

-A few hints on using PCM devices for playback - -

-The most common reason for problems with playback of PCM audio, is that the people don't properly understand -that writes to PCM devices must match exactly the data rate of the device. - -

-If too little data is written to the device, it will underrun, and ugly clicking sounds will occur. Conversely, -of too much data is written to the device, the write function will either block (PCM_NORMAL mode) or return zero -(PCM_NONBLOCK mode). - -

-If your program does nothing, but play sound, the easiest way is to put the device in PCM_NORMAL mode, and just -write as much data to the device as possible. This strategy can also be achieved by using a separate thread -with the sole task of playing out sound. - -

-In GUI programs, however, it may be a better strategy to setup the device, preload the buffer with a few -periods by calling write a couple of times, and then use some timer method to write one period size of data to -the device every period. The purpose of the preloading is to avoid underrun clicks if the used timer -doesn't expire exactly on time. - -

-Also note, that most timer API's that you can find for Python will cummulate time delays: If you set the timer -to expire after 1/10'th of a second, the actual timeout will happen slightly later, which will accumulate to -quite a lot after a few seconds. Hint: use time.time() to check how much time has really passed, and add -extra writes as nessecary. - -

- -

- - - - -- cgit v0.9.1