kraster wrote on Mon, 27 June 2005 04:02 |
Hi Dan,
I read Richard Black's 1999 AES paper on the effects of this phenomenon (inspired, I believe by Mr. Bob Katz's listening experiments). He maintains if there is(was) insufficient attenuation in Fs/2 at 44.1khz and that frequencies immediately above Fs/2 would be insufficiently filtered and cause Alisaing Intermodulation Distortion in the audible range.
Richard Black's experiments concluded that even small amounts of spurious frequencies could cause Intermodulation distortion: "(The tweeter) was found to give audible intermodulation when fed with 9kHz (approx.) at -12dBW and 21kHz at -47dBW".
Does the filtering on current chip designs sufficiently attenuate frequencies in the stop band to minimise Alisaing Intermodulation Distortion?
Thanks,
Karl
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Karl,
You can go to my web site at
www.lavryengineering.com and click on support. Look for my article named: "Sampling, Oversampling, Imaging, Aliasing". It is a PDF file.
In the file you will see some plots showing what happens when you have no oversampling, a X2 oversampling, X4 oversampling...
Notice the following:
When one begins with audio data that is limited to say 22KHz (CD format), and oversamples by say X2 to 88.2KHz, the frequency range between 22KHz and about 66KHz (88.2KHz-22KHz) is free of activity.
Say you up sample by X4 from 44.1KHz to 176.4KHz, there is a "dead zone" between 22KHz and 176-22=154KH.
Of course if you up sample higher, the "dead zone" increases.
The analog filter foe a DA needs to remove the high frequency image energy. In the case of X1 oversampling, you need to pass 20KHz and block 22.1KHz and that is a tough job - a 2KHz transition band. But in the case of X4, you need to pass 20KHz and block above 154KHz, Now you can have the ability to pass audio all the way to say even 54KHz, and still have 100KHz filter transition band. Moving the pass band to 54KHz helps gets you out of a big mess - the phase problems when the filter is right at 20KHz. Also, obviously, 100KHz transition band is much simpler filter than a 2KHz transition band!
Most DA's today have oversampling so high that one can set the filter way high above the audio and still reject the image energy by 120dB or more. The up sampling by some factor X pushes the image energy to very high frequencies by digital computation that can be phase linear.
A similar story, ending with a digital computation (that can be linear phase) hold for the AD side.
That is why I am saying: The analog 20KHz filter problem with it's associated phase problems is history and should be put aside. Your AD and DA, and everyone elses (unless it is very old gear) has oversampling and upsampling in it, therefore the problem of 20KHz analog filter is long gone.
Improving gear is not about 20KHz analog filters. We are long passed this bottleneck. Of course there are still issues to deal with, but we are making some progress.
Regards
Dan Lavry
www.lavryengineering.com