Quick Question: Usb DAC or optical DAC - Page 2

Pit jitter on the disc does not make it to your digital output unless you have a really cheap or outdated CD player.

Even ignoring the fact that all data from a CD is buffered de-interleaved, decoded, error corrected in two stages and buffered again the output clock is coming from a stable crystal oscillator and not from the pits.

CD players in the late 1970s where deriving their clock from the incoming signal rate, so any jitter could enter into the output but this is no longer true. Just think about the CD player in your car. The drive mechanism is controlled completely asynchronously from the output. If you drive over a bump you re-read a sector and can't hear a skip. If you drive around corners you don't hear the music get faster or slower based on your corner speed.

This has been discussed endless times on this forum and I thought we had closed this a long time ago.

Cheers

Thomas

What about using USB soundcards for providing the optical out? Would it be possible the get the best of both worlds with a setup like that? (I'm talking about the ESI U24 waveterminal, actually.)

I disagree.
The reason that pit jiiter matters is related to the laser servo. I read that Laser servo is the a big source of jitter in the CD player. When the laser tries to re-read a sector, it negatively affects the whole clock circuit.
Land and pit jitter is a widespread phenomena.

Good for you, it does not make it right though :-)

Yes, the laser servo might be a great source of jitter which is exactly the reason you would never use it for clocking purposes.

You might want to read up on how the latest Philips chipsets that are inside most CD players actually work. A key idea is that the chip takes a master clock input which dictates at which rate it outputs the samples. The laser servo is driven asynchronously by a microcontroller on the chip.

Better players have completely independent power supplies and ground planes for the clock and conversion circuitry.

Sorry, but this is an urban myth that is being kept alive by high end drive manufacturers trying to charge prices similar to vinyl players. There is no such thing as pit jitter on a contemporary CD player and definitely not on a PC!

I remember some very strange Stereophile article on this which caused great humor in the community.

Cheers

Thomas

First, sorry for the wrong link, the correct link is www.digido.com

Second, while my technical knowledge is certainly not up to yours, thomaspf, I know what I'm hearing. And it's not only me, but also quite a few other people I know. So I've searched for a possible explanation, and the jitter seems the only one. Maybe I got it wrong, but this seems to be a confirmation: http://www.digido.com/portal/pmodule...er_page_id=28/

This is Bob Katz, I guy too well known to be that wrong, I think.

Regardless of the theoretical explanation, my conclusion is the same: different CD / CDR recordings of the same "bits" do sound different, at least through the $300-500 players I've tried and including the $300 player as a transport + Apogee Mini-Dac setup.

I think I already agreed that there is lots of jitter in the process of reading a CD drive with pit jitter and all that.

The question I was raising is how much of that jitter impacts the jitter you experience at the DACs inside the player and at the digital output. Let me add the link a Philips chip set. Variants of that are used in many players.

http://www.semiconductors.philips.co.../SAA7806_1.pdf

I maintain that most players work in I2S slave mode at this point which means they take their input from a fixed crystal oscillator whose contributions to jitter will not change between CDs or CDRs.

A completely orthogonal question to this is whether CDs or different CDRs sound different. There are many reasons why this could be the case with read problems being the most likely one.

And yes any circuit is dependent on clean power. I think I mentioned that many players do have a separate power supply and ground plane for the processor controlling the drive mechanism and laser and the parts of the circuit that have to do with the oscillator and the conversion.

In fact this mechanism is one of the reason it is hard to beat the sound of a decent CD player with an external DAC. Unless you have some kind of feedback mechanism to regulate the source or use an asynchronous resampler, a DAC can not be run from a fixed crystal oscillator.

Hope this clarifies. What you need to get right in a CD player is not reducing pit jitter but building a stable crystal oscillator and output stage.

Cheers

Thomas