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From: kork on 4 Feb 2010 08:51
Hi folks,

I'm going to develop a quality control application that inspects recently
imported audio files for a number of checks. One of them is the detection
of counterphase fragments in the file.
With counterphase I mean a 180 degrees (or pi rad, if you prefer) phase
shift between the two audio channels in the (stereo) file. In a radio
broadcast of the file this is killing when it is listened through a
mono-receiver.

I was thinking of subtracting one channel from the other (or reverse a
channel and add it to the other). Then flagging the audio fragments as
counterphase when the resulting signal differs a lot from zero during a
certain amount of time.
But since it is likely that the 2 channels are anything but equal, I may
never get to see a flatlioe.

I thought maybe you DSP guys can give me some insights on this? Maybe
there's a test in the frequency domain I can think of?

I'd appreciate you responses.

Thanks in advance,

Rob Vermeulen




From: Vladimir Vassilevsky on 4 Feb 2010 10:24


kork wrote:
> Hi folks,
>
> I'm going to develop a quality control application that inspects recently
> imported audio files for a number of checks. One of them is the detection
> of counterphase fragments in the file.
> With counterphase I mean a 180 degrees (or pi rad, if you prefer) phase
> shift between the two audio channels in the (stereo) file. In a radio
> broadcast of the file this is killing when it is listened through a
> mono-receiver.
>
> I was thinking of subtracting one channel from the other (or reverse a
> channel and add it to the other). Then flagging the audio fragments as
> counterphase when the resulting signal differs a lot from zero during a
> certain amount of time.
> But since it is likely that the 2 channels are anything but equal, I may
> never get to see a flatlioe.
>
> I thought maybe you DSP guys can give me some insights on this? Maybe
> there's a test in the frequency domain I can think of?

Compute (L+R) and (L-R), rectify, accumulate, compare.
It is very obvious if the stereo channels are in phase or out of phase.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

From: kork on 4 Feb 2010 10:57
>
>
>kork wrote:
>> Hi folks,
>>
>> I'm going to develop a quality control application that inspects
recently
>> imported audio files for a number of checks. One of them is the
detection
>> of counterphase fragments in the file.
>> With counterphase I mean a 180 degrees (or pi rad, if you prefer)
phase
>> shift between the two audio channels in the (stereo) file. In a radio
>> broadcast of the file this is killing when it is listened through a
>> mono-receiver.
>>
>> I was thinking of subtracting one channel from the other (or reverse a
>> channel and add it to the other). Then flagging the audio fragments as
>> counterphase when the resulting signal differs a lot from zero during
a
>> certain amount of time.
>> But since it is likely that the 2 channels are anything but equal, I
may
>> never get to see a flatlioe.
>>
>> I thought maybe you DSP guys can give me some insights on this? Maybe
>> there's a test in the frequency domain I can think of?
>
>Compute (L+R) and (L-R), rectify, accumulate, compare.
>It is very obvious if the stereo channels are in phase or out of phase.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com

Hi Vladimir,

Thanks for your answer.
Would you mind elaborating a bit on the "rectify" and "accumulate"
suggestions? They're not so obvious terms for me in this domain.
Thanks again.

Regards,
~Rob
From: Vladimir Vassilevsky on 4 Feb 2010 11:23


>>Compute (L+R) and (L-R), rectify, accumulate, compare.
>>It is very obvious if the stereo channels are in phase or out of phase.
>>
>
> Hi Vladimir,
>
> Thanks for your answer.
> Would you mind elaborating a bit on the "rectify" and "accumulate"
> suggestions? They're not so obvious terms for me in this domain.
> Thanks again.

The cost of elaborated answer is $500.
My contact info is at the web site.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com


From: kork on 4 Feb 2010 11:36
>It is very obvious if the stereo channels are in phase or out of phase.

... (cut)...

>The cost of elaborated answer is $500.
>My contact info is at the web site.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com


$500 for a "very obvious" solution? I wish I had your job ;-)


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