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CD Transports: Data Drops Etc...

Please forgive the wordiness in advance. Having searched back and found a great series of posts on the technical aspects/sources of jitter (in a thread about differences in digital cable dating from last December), I find myself confronted with the following questions:

1) Is "jitter" purely a question of clock mismatch between the transmission of digital signal from the pickup and its reception by the DAC (whether separate or in-box)?

2) What is the source of so-called "data drops" (those data "errors" other than jitter) in reproducing the digital signal encoded on a CD? Is it vibration, something else?

And what may seem to be a dumber corollary question...
3) What effect does vibration have on the ability of the laser pickup to read data correctly? [looking for the technical answer]

This from a newbie trying to decide on a CDP/transport and wondering if build-quality should actually make a difference (Wadia 861 on a super-hard surface sounds better than on a table, wondering if rigid build-quality on Sony SCD-1 makes a difference or whether it could be built with plastic and have the same sound, and wondering whether what appears to be an ultra-rigid disc-clamping system made by TEAC reduces data errors)...

A big thank you in advance to all of those of you who contribute and make this forum interesting and informative to those of us just starting out...
t_bone

Showing 3 responses by kthomas

kthomas
911 posts
 
Some of this is a question I have tried to understand the answer to for a long time, to no avail. It sounds trite (or obvious) to say that the job of transport is to read the data on the CD and deliver it faithfully to the destination, in the case of music, the DAC. Given that a $50 CD drive in a computer can read a CD at much faster rates than is required for audio playback from a redbood CD, and deliver it provably bit-for-bit to another location, all while in a horrible electrical environment and, if you like, with the computer sitting on it's side, leads me to wonder why any of these issues exist in the audio component world. I see only one of two possibilities:

1. They don't and the performance differences people confer don't actually exist, or

2. They exist because truly bad engineering in the transport-to-DAC design that is fundamental to ALL audio component CD players. Assuming everyone who hears a difference isn't looney (and I assume they're not), there should be a technical explanation that is understandable in engineering terms to describe why the cheap computer CD can do it but the audio CD player can't. I've never read one that was convincing.

I think the far greater difference you'll find between a Wadia 861 and a Sony SCD-1 will be in the filtering algorithms they apply to the data stream. -Kirk

kthomas
911 posts
 
Aragain - You're definitely right that reading the bits from a CD is different than feeding them to a DAC with the proper timing, but feeding the DAC isn't a whole lot different than feeding the mechanics to write data to a hard drive, and that's basically a guaranteed, bit-for-bit correct operation with even relatively cheap hardware. I won't even pretend to understand the output side of a DAC and all the things that could make one sound different that the other, but the input side is a digital interface, and digital data is how I make my living. Again, I don't know a lot about how a DAC is physically constructed, but I don't know why it couldn't be constructed with an input buffer such that you aren't clocking the data directly into the sample space in the DAC that is going to be generated next directly. Feed it into a, say, 8K buffer that in turn feeds the DAC. The buffer could be built right in the DAC chip itself, and I can't imagine arguing that perfect transfer couldn't occur from one part of the chip to another.

Another way of asking the question is why, if I can ship data 100% reliably all the way around the world at data rates MUCH higher than is required for redbook CD playback and recreate the data perfectly at it's destination, can't I engineer a solution to read a CD and transfer that information 100% correctly to another chip in the same physical box? This is especially confusing because I can turn around and do an apparently analogous operation on another electronic device that is cheap and non-optimized.

I've often wondered if high-end transports employ filtering algorithms on the digital data stream they produce. It would be very possible to read the data from the CD, apply a filtering algorithm to the bit-stream, and produce a 16-bit/44.1Khz compatible stream that is intentionally different from what is on the disc. With well executed filtering, you could certainly change the sound, possibly in a way many would prefer. I've never read a reference that explicitly says somebody's transport does this, but I don't see any reason why somebody couldn't to produce a distinctive sound. -Kirk

kthomas
911 posts
 
It's the framing error on a UART model that I have in mind for this data transfer too. Maybe that's the wrong mental model, and when I get the right mental model it'll be clearer to me. Certainly, one problem with digital audio playback is that these connections don't have redundancy built in (ie, no retry logic). However, I can connect two computers up via async ports (ie, UARTS) and write a simple program to send on one side, receive on the other, and report any chip-level errors, let it run for days and not see a single error, all without retry logic, etc. So, it still seems very solvable, at least to my level of understanding of the problem set.

I guess another way I think of it is this - what if somebody re-engineered the concept of a CD transport / DAC combination such that the transport didn't attempt to play it back real-time, but rather read the CD "in" and reliably loaded it into memory on the DAC. This would be akin to reading a computer CD into memory and, since you'd be re-engineering the interface, could base it on different technology. Then, the "CD" would play back from within the DAC's memory. I would think the whole issue of jitter would be moot with such a setup, and the whole notion of an expensive transport and expensive interconnects would be moot as well.-Kirk

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