counterphase detection in stereo audio

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

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

>
>
>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

>>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

>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 ;-)

kork wrote:

>>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 ;-)

That's why you don't have my job. If I was you, I woudn't ask idiotic 
questions.

VLV

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

Tim Wescott 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.

The accumulation should be lossy; i.e., include a "forgetting factor". 
alternatively, you could dump the result after a suitable time and start 
over.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

>Tim Wescott 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.
>
>The accumulation should be lossy; i.e., include a "forgetting factor". 
>alternatively, you could dump the result after a suitable time and start

>over.
>
>Jerry
>-- 

Thanks Tim and Jerry,

I appreciate the jargon explanation.
This sounds pretty straight-forward to implement. I'll have a go at it.

Jerry, your "forgetting factor" sounds logical. I was thinking of just
testing separate successive chunks of samples, so I won't have any
"memory-effect".

Best regards,
~Rob

kork wrote:
>> Tim Wescott 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.
>> The accumulation should be lossy; i.e., include a "forgetting factor". 
>> alternatively, you could dump the result after a suitable time and start
> 
>> over.
>>
>> Jerry
>> -- 
> 
> Thanks Tim and Jerry,
> 
> I appreciate the jargon explanation.
> This sounds pretty straight-forward to implement. I'll have a go at it.
> 
> Jerry, your "forgetting factor" sounds logical. I was thinking of just
> testing separate successive chunks of samples, so I won't have any
> "memory-effect".

That will require counting and branching. Forgetting is actually 
simpler. The convention is that x[n] is the input and y[n] is the 
output. Set y[n+1] = (1-a)*y[n] + a*x[n+1]. For stability, 0 > a > 1. 
Larger values forget faster. This is called an exponential averager.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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