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101 lines
3.6 KiB
101 lines
3.6 KiB
4 years ago
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Audio capture with transcode using ALSA
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======================================================================
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Francesco Romani <fromani at gmail dot com> 12 May 2007
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*********************************************************************
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*** WARNING: ***
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*** import_alsa module is still EXPERIMENTAL! ***
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*** It may DON'T work for you or it can deliver BROKEN samples! ***
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*** If you experiences strange (= not explained here) behaviour, ***
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*** PLEASE drop a note on transcode-devel@exit1.org explaining ***
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*** your issue. Thanks! ***
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*********************************************************************
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1. What is this?
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----------------
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This module allow the user to capture audio streams from an ALSA
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device, that is the new default sound architecture for Linux 2.6.x
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and beyond. Former OSS API is officially deprecated.
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While ALSA API/drivers generally offers some OSS API emulation,
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is generally preferable to stay on touch with state of the art and
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use the new API directly.
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Audio capture is useful for doing screencasts (see import_x11 or
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import_vnc modules), or to analog video capture (see import_v4l(2)),
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or to record separate audio tracks, or to do whatever you want.
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2. Basic usage (gotchas, quirks, limitations)
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---------------------------------------------
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From an user point of view, usage of import_alsa should be
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straightforwarded compared with import_oss. Just select the import
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module (-x SOMETHING,alsa) setup import audio stream parameters using
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-e and have fun.
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A tricky step can be selecting the appropriate audio device to capture.
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import_alsa by default reads audio from "default" ALSA card (see ALSA
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documentation and/or your local configuration).
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You can specify the card to use using the "device" option of import_alsa:
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$ transcode -x SOMETHING,alsa=device=DEVICE
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You can use the `arecord' program from alsa-utils to list the recognized
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capture devices:
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$ arecord -l
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ALSA API strongly encourages applications to access audio through alsalib,
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not accessing the device directly. Usage of device file is not really
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supported (AFAIK), so import_alsa DOES NOT honours -i nor -p transcode
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options. Those values are just ignored, so the only way to change
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capture card is to use `device' option as outlined above.
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3. Known issues
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---------------
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* first and foremost, PLEASE NOTE that some audio capture device DOES NOT
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properly report the supported sample rate (and maybe others audio parameters
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too). They simply accept everything but without be able to fullfill
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the effective requests. SAA7134 devices is a known example of this
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behaviour, not being able to deliver sampling rate > 32000Hz.
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This is not a limitation of transcode, nor an easy way exists to
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recognize those situations. Please pay attention to this and try
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some different audio stream settings.
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* ALSA transparent resampling is not yet supported.
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* probing of audio device is not yet supported
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(require transcode core changes).
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* subdevice specification is not yet supported due to a transcode core
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limitation.
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* audio resync is still primitive.
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* almost no optimizations (see module version number :) )
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4. Internals
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------------
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See technical documentation in docs/tech/html/* and/or the comments
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on source code for details.
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5. Future plans
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---------------
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import_oss will NOT be deprecated in any foreseeable future.
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import_alsa will be improved in further versions to address
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all known issues above issues.
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During next releases, import_alsa will be pushed as preferred
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audio capture module over import_oss on Linux platforms.
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