Re: Half length output with libsox for stereo -> mono

[Stephen.Paterson <stephen.paterson@synety.com>]

Hi Ulrich,

Thanks for that. All working now.

Cheers

Stephen Paterson
Developer   |  SYNETY
www.synety.com

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-----Original Message-----
From: Ulrich Klauer [mailto:ulrich@chirlu.de] 
Sent: 10 April 2013 16:48
To: sox-devel@lists.sourceforge.net
Subject: Re: [SoX-devel] Half length output with libsox for stereo -> mono

Stephen Paterson wrote:

> I can't get it to work with libsox. I've made a tiny modification to 
> example3.c that should do the same but no matter what I try I always 
> end up with a file that is exactly half the length of the one I get 
> from sox (or some crazy audio).

example3 is buggy. It doesn't take into account that sox_add_effect() modifies the "in" signalinfo, which is why it isn't a good idea to pass it a signalinfo that is still in use elsewhere, or to call
sox_add_effect() with identical in and out signalinfos.

What is happening here is that in->signal.length, after the sox_open_read(), contains some value like 882000, for a 10-second stereo 44100 Hz input file. After adding "rate", the value will have changed to 160000, and after "channels", to 80000. Because it is the same memory location all the time, the input format handler (from
sox_open_read()) will see this value as well and think that its file is only 80000 samples long. It will therefore stop reading after 80000 samples (/2 channels, /44100 Hz = 0.907 seconds), and that is all that is processed by the effects chain.

See the attached diff for how to use a copy of the signalinfo instead.

Ulrich
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