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From: Jerry Avins on 5 Feb 2010 15:47 Richard Owlett wrote: > *DT* defined - DIVERGING from 'topic' > > > Jerry Avins wrote: >> 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 > > Why is this called an "exponential averager"? With this averager. the past decays away exponentially. Seen as an IIR filter, it is a first-order low-pass a "time" constant easily related to a. > I have heard of "boxcar" and "running" averages. > What is/are the difference(s)? A boxcar averager reports the average of the last N samples: N is your choice. A running average is the average of all past samples. Not quite trivial to implement. > What other averageres exist? As many more as you might have reason to invent. > The OP apparently says he is looking at *NON*overlapping chunks of data. > Is there not there an *INTRINSIC* forgetting factor? Past blocks are forgotten, but I call that a process. The forgetting *factor* in the exponential averager is 1-a, applied to each new result. Jerry -- Engineering is the art of making what you want from things you can get. ����������������������������������������������������������������������� |