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Bits Are Bits, Right?

So I'm currently heading down the path of exploring which CD-Rs sound best in my CD player, along with what burn speeds sound best and what CD burners make the best CDs. I already know from my research that the more accurately the pits are placed on the CD (e.g. less jitter in the recorded data), the better chance I stand at getting the CD to sound good. There is a counter-argument to this idea that goes something like this: "Bits are bits and as long as the CD player can read them, the accuracy of the spacing doesn't matter because everything is thrown into a buffer which removes the effect of any jitter written into the data during burning." I know I don't agree with that logic, but for the life of me I can't remember the technical reasons. I know I used to know. Haha! 

So who here knows why buffers don't solve all of our problems in the digital realm? How come timing accuracy matters in the stages before the data buffer?
mkgus

Showing 2 responses by almarg

almarg's avatar
almarg
9,670 posts
 
Mkgus 2-21-2020
If it’s all the same data and the read errors are minimal, then what is happening before the buffer is having effect on the sound.

As I see it the reason is that what is happening before the buffer, which is dependent on the physical characteristics of the particular disc, is affecting circuitry that is after the buffer.  That is the basic point Kirkus was making in his 2011 post that I quoted above.  Those effects are not occurring via what he refers to as the "forward signal path," meaning the intended signal path that we tend to think of, but rather via coupling of noise via unintended pathways.  Those would potentially include grounds, power supplies, stray capacitances, or even the air.  Such effects will of course vary depending on the design of the specific component.  

Regards,
-- Al
 
almarg's avatar
almarg
9,670 posts
 
From a post by member Kirkus (who has a vast amount of hands-on experience with the internal workings of CD players) in this Audiogon thread from 2011:

Two big conceptual errors I see very commonly are the assumption that any intrinsic jitter related to retrieval of information off of a CD actually occurs through the forward signal/data path, and that any sonic artifact associated with parts upstream of the DAC must be classifiable as jitter.

In reality, CD players, transports, and DACs are a menagerie of true mixed-signal design problems, and there are a lot of different noises sources living in close proximity with susceptible circuit nodes. One oft-overlooked source is crosstalk from the disc servomechanism into other parts of the machine . . . analog circuitry, S/PDIF transmitters, PLL clock, etc., which can be dependent on the condition of the disc.

One easy way of measuring this on the test bench is to have two versions of the same test-tone CD, one pristine, the other scratched. A conventional distortion analyzer is used to null out the the tone(s), and then an FFT (or visual 'scope analysis) is used to analyze the residual. One would be surprised at some of the nasty things that sometimes come up out of the noise floor when the focus and tracking servos suddenly have to work really hard to read the disc.

Regards,
-- Al

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