Re: [Alsa-user] plughw versus hw

[Jaroslav K. <pe...@pe...>]

Date 21.6.2011 13:55, Pierre Habraken wrote:
> On 06/20/2011 10:06 PM, als...@li... wrote:
>>
>> Date: Mon, 20 Jun 2011 22:34:46 +0400
>> From: Vladimir Mosgalin<mosgalin@VM10124.spb.edu>
>> Subject: Re: [Alsa-user] plughw versus hw
>> To: als...@li...
>> Message-ID:<20110620183446.GA14919@VM10124.spb.edu>
>> Content-Type: text/plain; charset=us-ascii
>>
>> Hi Pierre Habraken!
>>
>>   On 2011.06.20 at 19:32:28 +0200, Pierre Habraken wrote next:
>>
>>> I can imagine that this is a FAQ, but I could not find a clear answer :
>>> which precise difference(s) distinguish(es) plughw and hw from each other ?
>>> Does plughw apply sound processing that hw does not ?
>>
>> plughw *might* apply simple sound processing if needed, mostly channels
>> conversion and rate conversion if required. It doesn't have to apply
>> processing.
>> hw doesn't support such processing only works when operating strictly in
>> mode that audio card support.
>>
>> If you have device that supports only 2 channel, 16 bit 48000 mode then
>> "hw" device won't be able to playback 2/16/44100 stream, or mono stream
>> for example; you'll get an error when you try. But plughw will accept
>> such streams and do the conversion. However, if you use plughw and
>> output 2/16/48000 stream then no conversion is needed and most likely
>> plughw won't be doing any processing.
>>
>> Note that using both hw and plughw can lead to specific problems, so
>> it's best to use "default" device unless you have very specific
>> requirements.
> 
> Hello Vladimir,
> 
> Thank you for your reply.
> 
> I just bought an Asus Xonar DX sound card, for sending 24bits/96KHz 
> stereo flac files to an external DAC.
> I am using Alsa 1.0.21 on a PC running Ubuntu 10.04 with Linux kernel 
> 2.6.32-32.
> Running aplay, I can't use hw for reading 24/96 files:
> 
> $ aplay -D hw:0,1 Prelude.wav
> Playing WAVE 'Prelude.wav' : Signed 24 bit Little Endian in 3bytes, Rate 
> 96000 Hz, Stereo
> aplay: set_params:990: Sample format non available
> Available formats:
> - S16_LE
> - S32_LE
> $
> 
> Adding the switch -f S32_LE does not help:
> 
> $ aplay -D hw:0,1 -f S32_LE Prelude.wav
> Warning: format is changed to S24_3LE
> Playing WAVE 'Prelude.wav' : Signed 24 bit Little Endian in 3bytes, Rate 
> 96000 Hz, Stereo
> aplay: set_params:990: Sample format non available
> Available formats:
> - S16_LE
> - S32_LE
> $
> 
> If I use plughw instead of hw, it works fine:
> 
> $ aplay -D plughw:0,1 Prelude.wav
> Playing WAVE 'Prelude.wav' : Signed 24 bit Little Endian in 3bytes, Rate 
> 96000 Hz, Stereo
> ^CAborted by signal Interrupt...
> $
> 
> Does it mean that the 24bits stream has to be converted to 16bits before 
> being sent to the device and then to the DAC ?

No, use 'aplay -v' to see all plugins used by plughw. Your hw supports
24-bit samples encoded in the 32-bit words. So alsa-lib does 3-byte to
4-byte conversion of 24-bit samples - audio bits are not lost.

				Jaroslav

-- 
Jaroslav Kysela <pe...@pe...>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.