[LAD] Half speed audio playback issue

victor Victor.Lazzarini at nuim.ie
Mon Oct 5 21:59:42 UTC 2009

Humm, than that seems to be the reverse of what we would expect,
that must be then probably because of the missing samples.

If samples were not missing (say
samples from the two channels got distributed between four
channels) then it should go up in pitch, because each channel
would run through the entire data twice as fast, similarly
to what would happen if a mono file was opened as stereo.

If on the other hand you were opening a stereo file as mono,
then the pitch would go down (with some distortion possibly).
Maybe that is what is happening, your stereo file became 4
identical mono tracks.


----- Original Message ----- 
From: "Raymond Martin" <laseray at gmail.com>
To: "victor" <Victor.Lazzarini at nuim.ie>
Sent: Monday, October 05, 2009 10:39 PM
Subject: Re: [LAD] Half speed audio playback issue

> Hi Victor,
>> I thought something like that was happening. But I'm curious: if a
>>  2-channel file is played over 4-channels it should go up an octave, 
>> should
>>  it not? You'd
>> have half as much data for 4 channels as you'd have for 2 channels and so
>>  it would seem like it's going faster? Or my reasoning is wrong somewhere
>>  (it could
>> well be, I often get things like that mixed up)?
> Since you have half as much data your processing (e.g., D to A conversion)
> will interpret this as being at half the rate. Perhaps like it seeing 
> every
> second sample, with a blank one in between them, that could lead to
> interpolation that results in a signal at half the frequency. Sort of like
> every second sample is missing (in my case, two of every four are going
> to unused channels). I'm not too fresh on the details presently, but it
> is something this is easy to find mentioned in a standard DSP book.
> Best regards,
> Raymond 

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