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How does a Transport effect sound?

hi guys,

Been wondering about this: How does a CD Transport effect sound?  Isn't it just reading the disc and sending the 1s and 0s to the DAC.  Shouldn't every transport sound the same?

Thanks! 
leemaze
geoffkait's avatar
geoffkait
23,365 posts
 
Actually it’s not 1s and 0s. It’s not 1s and 0s until the data gets to the A to D converter. The laser is simply reading ON and OFF depending on whether the laser beam is hitting a pit or land. Actually the laser reading process is an analog process, not digital. Also, the length of the lands, which are variable, helps determine what the actually digital data is.

There are several problems that make the reading of the “data” on the CD imperfect. One is wobbling of the CD due to out of round condition, various types of vibration and/or not being absolutely level during play. Another is background scattered laser light that makes its way into the photodetector. The photodetector is kinda stupid and can’t tell the difference. Yes, I know what you’re thinking, “But the Reed Solomon Error program is supposed to take care of all that.”
audioengr
3,282 posts
 

The operative word here is JITTER.

Jitter is the variability in the timing of the bits coming out the end of the S/PDIF cable from the CD transport. At 44.1kHz sample-rate, the bits frequency is around 2.8MHz. ANY variability in the timing of these bits, one bit to the next bit, will cause the D/A conversion to be distorted. The output voltage will not be at the correct value at each point in time, creating analog distortion.

Jitter in a typical CD transport is caused by:

1) inaccurate reads of the pits because of imperfect disk and vibration in the disk while spinning - this can be improved by re-writing on a good CDROM disk and by applying treatments to the disk. Some older disk players used a platter like a turntable to eliminate the vibration.

2) jitter in the internal Master Clock oscillator - selected by the designer

3) power subsystem deficiencies that power the oscillator and associated circuits due to the design

4) imperfect S/PDIF output driver due to the design

5) imperfect S/PDIF cable - get the best one you can afford

When one refers to a CD player as a "Transport", it means they are using the digital output over a S/PDIF cable from the device, in conjunction with an external DAC or D/A converter.

Here is a typical jitter plot from a transport:

http://www.audiocircle.com/index.php?topic=154408.0

Here is how the S/PDIF cable adds jitter to the equation:

http://www.audiocircle.com/index.php?topic=154425.0

You can see from these measurements what a Synchro-Mesh reclocker can do to reduce jitter from ANY transport or server. None of the disk treatments are needed if a Synchro-Mesh is used, and it provides galvanic isolation, breaking ground loops. Also, see how good a $275 S/PDIF cable can be.

Steve N.

Empirical Audio

geoffkait's avatar
geoffkait
23,365 posts
 
Background scattered light has not been addressed by any manufacturers, at least not to any great extent. So there’s that. And magnetism and static electricity are still problems, there’s that, too. I suspect maybe “jitter” isn’t the do-all end-all for various problems involved with CD playback. There’s also the nagging question of why they can’t make the clear layer in CDs a little more transparent. 😬
rja's avatar
rja
2,371 posts
 
I don’t know all the variables involved but I do know from experience that CD transports can and do sound different. Bits is bits?
audioengr
3,282 posts
 
Background scattered light has not been addressed by any manufacturers, at least not to any great extent. So there’s that. And magnetism and static electricity are still problems, there’s that, too. I suspect maybe “jitter” isn’t the do-all end-all for various problems involved with CD playback. There’s also the nagging question of why they can’t make the clear layer in CDs a little more transparent.

All of these problems are addressed by two current solutions:

1) reclock the S/PDIF output from the transport

2) design a transport using a CDROM and a computer motherboard and memory.  Buffer in memory and spool it out over S/PDIF

And JITTER is the THE ONLY issue with the S/PDIF output on a transport, period!

I am a EE that has designed digital systems for 42 years and have designed dozens of digital audio interfaces including S/PDIF, AES/EBU, I2S, differential I2S, 6 generations of USB, Ethernet.  I know what I'm talking about.

There are obviously other issues that come up if you use the D/A inside the player and the analog outputs.

Steve N.

Empirical Audio

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