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-   -   Is There a "Standard" for CD Antialias Filtering? (http://www.diyaudio.com/forums/digital-source/210061-there-standard-cd-antialias-filtering.html)

dchisholm 3rd April 2012 07:49 AM

Is There a "Standard" for CD Antialias Filtering?
 
In the recording chain leading up to any digital media there has to be an implicit or explicit antialiasing filter. (OK, I guess you COULD leave it out, but from at least an information theory standpoint you wouldn't want to. I'm not going to quibble over nuances like that.)

Is there a formal standard, or even a working consensus, for the effective characteristics of the antialiasing filter in CD-quality audio? Yeah, we know it wants to be "almost flat" to about 20 KHz, and "reject everything" above 22.05 KHz. You can figure that out by the second day of your "Introduction to Digital Signal Processing" class. And I know that in a modern recording chain the overall antialias function will probably be distributed among analog and digital sections so you can't point to a handful of components and call it "THE antialiasing filter". ("Oversampling" and "Decimation" don't show up until at least the 3rd week; maybe half way through the class.)

But in realistic terms does the performance of the CD's antialias filter mean -3 dB at 20 KHz, and -20 dB above 22.05 KHz; or -1 dB at 20 KHz with -40 dB or better above 22.05 KHz; or something else? Where can I find a reference?

I thought Philips/Sony had published guidelines for this, or even a formal spec for anybody who wanted to paste the "CD" logo on their product, back around 1980 but after running my search engine at full throttle all evening I can't find it.

Dale

SoNic_real_one 3rd April 2012 10:51 AM

The alias image is centered on fs/2. Therefore you would need to reject as much as possible of the image. For CD you have a "guard space" 44.1/2-20=2.05kHz below and above Fs/2. That means that the alising products are full starting with 20+2x2.05=22.1kHz.

The only way to make a real analog aliasing filter work is to raise the sampling frequency in order to separate the images. In this way you don't need the brickwall filter but a more do-able one - every DAC manufacturer has in their datasheet a recomended filter.
Even from the first DAC's made by Philips and Sony, OS was seen as necessary (4x as minimum). They didn't "publish guidelines", they made the first DAC's for CD format and they made them 4x.
And... digital filtering is not possible without oversampling.

oshifis 3rd April 2012 02:05 PM

Does it mean there is oversampling during the recording (with a relaxed antialiasing filter), and the resulting digital stream is downsampled to 44.1 kHz before DC mastering?

jan.didden 3rd April 2012 02:33 PM

There's a CD 'Red Book' that is supposed to talk about such specs, probably somewhere on the 'net.
Then again, nothing stops anyone to implement what they think is the 'best' filter.
Some here run their DACs without any antialliasing filter and accept all the hf noise above 22kHz and even insist it sounds better that way. YMMV.

jan didden

SoNic_real_one 3rd April 2012 02:41 PM

Quote:

Originally Posted by oshifis (Post 2971124)
Does it mean there is oversampling during the recording (with a relaxed antialiasing filter), and the resulting digital stream is downsampled to 44.1 kHz before DC mastering?

Yes, all the multibit ADC that I know of, use oversampling and anti-alias filter combo analog+digital.
For modern delta-sigma single, the OS is implied.
If aliasing error was only ultrasonic noise, I could see one not using a filter. But unfortunatelly there are alias products that are "folded" back in to audio band.

AndrewT 3rd April 2012 02:43 PM

Some CD players came with a plug in filter block.
The manufacturer supplied different filter characteristics to suit the listener.
But, that that is at the opposite end to the anti-aliasing filtering.

dchisholm 3rd April 2012 06:46 PM

Thank you, Jan.
Quote:

Originally Posted by janneman (Post 2971158)
There's a CD 'Red Book' that is supposed to talk about such specs, probably somewhere on the 'net.

I found many references to the "Red Book" but didn't find any actual quotes, or even a formal title for it. And I couldn't find the document itself.


Quote:

Then again, nothing stops anyone to implement what they think is the 'best' filter.
One nice thing about "standards" is that you can always create a new one. Or at least arrogantly claim that your opinion should be treated as a "standard".

Dale

oshifis 3rd April 2012 06:47 PM

What type of ADC (principle and chip type/discrete?) and how many times oversampling is used at commercial recording facilities? Is there any publicly available info on it?

SoNic_real_one 3rd April 2012 08:32 PM

Today? The top of the line use delta-sigma DXD/DSD at 352.8 kHz.

SY 3rd April 2012 08:42 PM

Quote:

Originally Posted by janneman (Post 2971158)
Some here run their DACs without any antialliasing filter and accept all the hf noise above 22kHz and even insist it sounds better that way. YMMV.

I believe you're thinking of the anti-imaging filter, not the anti-aliasing. Other end of the digestive tract. :D

If the anti-aliasing and anti-imaging filters had been standardized by Red Book (they weren't), it would have allowed phase-correct CDs to be made. The phase shifts probably aren't that audibly significant, but if they could have been avoided, then why not?


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