[alsa-devel] Question about Conversion to S24_LE from S24_3LE

[Takashi Iwai]

At Thu, 16 Aug 2012 09:37:18 +0900 (JST),
etsuko suzuki wrote:
> 
> Hi.
> 
> I am Etsuko.
> Kaz is my coworker.
> 
> This was my question, so I had to  post this mail myself.
> However my mail was not posted from my E-mail address.

Did you subscribe to the ML?  Otherwise a post would need a manual
approval and it misses sometimes timeframe that the ML server keeps
a pending post.

> Therefore he had posted this instead of me.
> 
> I am challenging once again using the different mail address. 
> 
> *************************************************************
> 
> >It depends on what format is used in the original PCM stream.
> 
> I recorded this file from 16bits wav file.

OK, it's clear now.

> >It'd make sense to have 00 in LSB if it was converted from 16 to 24bit.
> 
> Does this mean the following? 
> 
> ex.)
> 
> > 0000030 ffffc300 ffffc600 ffffca00 ffffd300
>           ++====**
> 
>           ++ : padding
>           == : PCM data
>           ** : This is added when it was converted from 16 to 24bit.
> 

Yes.  When the original source is 16bit, ALSA-lib format conversion
just shifts the bits.  It's suboptimal, but fast.  That's the reason
of the left 00.

> I'd like to ask one more. 
> I read the following, in order to know about wav file. 
> 
> http://msdn.microsoft.com/en-us/windows/hardware/gg463006.aspx
> 
> In this, I found the statement 'left-alinged'. 
> (In 'Details about wValidBitsPerSample')
> 
> I understood 'left-alinged' was 'LSB padding'.
> So I thought S24_LE was LSB padding.

No.  S24_LE has the MSB padding of 8 bits in 32 bit container.

> Does the format in the ALSA driver differ from the format in wav file? 

You need to use S32_LE instead.


HTH,

Takashi