Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

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lies, damn lies, statistics, upsampling/oversampling

Posted by Thorsten on June 26, 2001 at 07:09:27

In Reply to: 24/192 DAC posted by JO Jop on June 25, 2001 at 08:49:13:

Hi there,

I'll try to keep that brief and in laymans terms.

The information contained on the CD is in a format that has 16 Bit's with linear weightig and a sample rate of 44.1kHz. This is all there is. That is the grand total of information present on the CD.

As a result the CD Digital system is incapable of reliably describing the difference between a sinewave and a triangle wave above 5kHz (and trust me triangle and sinewaves at 5kHz sound very different and the difference between a squarewave and Triangele/sinewave above 10kHz.

In fact, 20kHz cannot be reliably encoded insofar that if the reproduction takes place without a very steep filter with extreme ringing at HF a 20kHz sinewave tone shows severe intermodulation effects.

Very steep LC analogue filters are hard to produce reliably and in an attempt to improve both the measured performance (who does not remember the "CD = 300% Distortion Headline in the British Mag "The Flat Response") and the consistency of HF response overampling was introduced. With the exception of a small number of comanies (Denon, Wadia, Pioneer) all oversampling designs used a specific Filter system, that interpolates (in effect "makes up") samples inbetween the original ones.

In fact, all digtal filters known to me actually DISCARD even the original samples and "rebuild" the signal based upon an algorythm that assumes steady state sinewaves. The result of this is that now the digital filter introduces severe levels of transient distortion (as much 10 - 20% are easily measurable) and that in effect the original signal has been corrupted.

The so called "Upsampling" works technically by oversampling to a very high oversampling rate and is ten downsampled again using a suitable (non integer) ratio. Again a classic (Brickwall) digital filter is present.

All in all, based upon repeat listening against anlogue sources I can only conclude that conventional Over/Upsampling methodes applied to CD Standard Digital Signals actually introduce audible distortions and are ideally avoided. Yes, Oversampling/Upsampling makes for nice looking measurements and if applied using a suitable (non brickwall, transient correct) algorythm can even provide a genuine improvement, I feel that operting without oversampling/upsampling in the digital domain applied DIRECTLY to the CD Datastream sounds better, more precisely, it sounds more natural.

I'm aware that I'm at odds with many and that a strong lobby in High End wishes to push these technologies. So what?

Please let's have either REAL High Resolution systems (SACD, DVD-A) without sound crippeling watermarking or let's really look at resolving some of CD's inadequacies.

However playing the number game is just a repetition of the late 80's/early 90's game of "bigger number = better" when usually the reverse is true.

Later T

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