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Audibility of group delay at low frequencies

Hayabusa

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What do you mean? 100Hz cycle is 1/100 so 10ms, 200Hz is 5ms so group delay seems to be less than one cycle. Unless the plots are wrong or the processed audio files have more group delay than the plots indicate.
Sorry I should have been more clear, I was reacting to the original (long ago) post.. there was +/-25msec for freq. 50-60hz..
 

BenB

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Bumping this thread and using a recent file discussed on ASR in another matter.

I used the original file and used two 2nd order allpass filter in Audacity (allpass2 s 100 1) and (allpass2 s 200 0.7). Both were tested in ABX against the original with the following results (the 100 Hz Q=1 was easy, the 200 Hz Q=0.7 was harder).

Questions: Are the files correctly made with respect to filter and is the result expected?


I compared the frequency magnitude and phase spectra for the 100 Hz allpass. I applied a bit of tukey shading on the ends of the waveforms, and performed a single FFT zero padded to 2^20. Results are attached. I believe the magnitudes are well matched, with the differences appearing in the nulls, which are always higly variable and don't impact our perception at all. The power spectra has 2 plots, with the orig in blue and the allpass in red. You can just notice a bit of blue extending lower than the red nulls at 35 Hz and 165 Hz for example.

The phase plot shows 360 degrees of phase rotation, centered at 100 Hz.

I would say this is a valid test.

Regarding the result, I believe that's expected as well. It's been established that phase differences can be heard in this frequency band. Do the same thing at 3000 Hz instead of 100, and you'll likely fail. However, while differences are audible, I am not sure it's been established that there's a clear preference, or that listeners have a clear sense of one being "right" and the other "wrong" or "distorted". At least not at the scale of phase distortions caused by filters people typically employ.

My concern is that people point to results like this and exaggerate the impact of phase, and give undeserved credit to manufacturers who address inconsequential phase distortions in the kHz range because that's easy, while doing nothing for lower frequencies, because that's more difficult. There's plenty of marketing material claiming that linear phase speaker systems are a cut above the others, because that one attribute is the secret to audio bliss. Sometimes these are great systems that have the other more important aspects right, and the reason for their greatness is misattributed. Sometimes it's systems that have more fundamental flaws, and don't approach greatness.
 

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Thomas_A

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So, in essencence, if you really want phase-coherent crossover, it is useless for the usual mid to tweeter crossover. If anything, such filters should be better used for sub to mid crossovers. A thing for AVRs to implement.
 

dasdoing

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I am not sure it's been established that there's a clear preference

If I have to bet, I bet that the majority will prefer the delayed bass. we are used to it, like we are used to a bass boost in our room; which ended up being a preference with Harman standard
 

dasdoing

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also the engineers produce the song with the (a) delay. So if we have a linear phase system we are not hearing it like them. I have a linear phase system and some synthetical kicks do sound strange
 

BenB

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So, in essencence, if you really want phase-coherent crossover, it is useless for the usual mid to tweeter crossover. If anything, such filters should be better used for sub to mid crossovers. A thing for AVRs to implement.

That's my understanding, but I would certainly suggest doing your own investigation. It looks as though Audacity is doing what it's supposed to with the allpass filters. Perhaps you could increase the frequency more and more and see at what point your ability to identify the difference goes away. I'd be interested to know that result.
Also, you could consider involving a third party to rename or load the files, so that you don't know which file is the original, and which had the allpass. It might be good if they DO know which is which, so you could guess about that and see how often you were right. At any rate, it would be interesting to hear your subjective assessment of the character (and magnitude) of the difference. Did you tell us yet what equipment you're using (speakers, headphones, etc)?
 

Thomas_A

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That's my understanding, but I would certainly suggest doing your own investigation. It looks as though Audacity is doing what it's supposed to with the allpass filters. Perhaps you could increase the frequency more and more and see at what point your ability to identify the difference goes away. I'd be interested to know that result.
Also, you could consider involving a third party to rename or load the files, so that you don't know which file is the original, and which had the allpass. It might be good if they DO know which is which, so you could guess about that and see how often you were right. At any rate, it would be interesting to hear your subjective assessment of the character (and magnitude) of the difference. Did you tell us yet what equipment you're using (speakers, headphones, etc)?
I use my headphones for this. Equipment list is below in all my posts. With respect to doing this with other files, there are threads with similar audible impact after polarity switching. It requires asymmetric signals in the bass region, while such signals above 500–1000 Hz are inaudible when flipped.
 

bennybbbx

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here is another test that show how diffrent it sound when the 40 hz sine is delay 0 ms , 4.1 ms or 8.2 ms in compare to 80 hz .

 
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