counterphase detection in stereo audio

kork wrote:

>>On Thu, 04 Feb 2010 09:57:49 -0600, kork wrote:
>>
>>
>>
>>>>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.
>>
>>"Rectify": take the absolute value.
>>
>>"Accumulate": sum up a bunch of samples.
>>
>>Then compare the relative strengths of the L+R and L-R channels -- 
>>normally L-R should be significantly smaller than L+R.  In fact, this is 
>>why the 'wrong' way is a broadcast-killer -- the FM stereo broadcast 
>>protocol depends on this property, won't work without it, etc.
>>
>>I'll charge you money for answers, too, but only if the question takes 
>>more than a few lines to answer.
>>
>>-- 
>>www.wescottdesign.com
>>
> 
> 
> An implementation of this has been running successfully for the past 15
> months so thanks a lot for the help. 
> 
> One nice anecdote to mention is that when an early version of the
> application checked live recordings from churches or big halls (especially
> with organs, flutes or sopranos) it gave a lot of false hits. Apparently
> these types of recordings (using a couple of microphones hanging high on
> the ceiling) resulted in a recording with not much difference between the
> accumulations of L+R and L-R. I checked for [L-R] > [L+R] when I should
> have checked for [L-R] > [L+R]*n. Where 2 <= 'n' <= 10.

You should have L and R lowpassed and then done the check.

> Hope this helps others. 

2 + 2 = 4

> Thanks again.

"Thank you" means a lot to me.
Just $100 would be enough.


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