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Develop a metric for CD Transport and CD Player performance

mansr

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Unfortunately, the method does not work with CD players. The difference in rotation speed and thus resulting frequency makes Deltawave unable to work. With 1kHz dithered as an original signal, I get the stagecoach-wheel effect. From the player I get 999.84Hz. Funny.
That's a 160 ppm discrepancy, within the expected range for two unsynchronised crystal oscillators. It also has nothing to do with rotation. The disc motor is servo controlled to provide a reasonably constant readout rate. The data is then buffered, processed, and clocked out to the DAC by a crystal oscillator. You'll get similar results with any unsynchronised DAC/ADC pair.
 

Blumlein 88

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Jitter may be an interesting test because SPDIF is a "push" protocol. By this I mean the transport pushes the bytes to the DAC, instead of the DAC pulling them from the transport. This requires the downstream device (the DAC) to adapt to the transport's notion of the clock frequency. This adaptation is not a one-time thing but a continual process as the clock speeds can drift over time.

This means with SPDIF jitter can arise from either side: the transport sending bytes with uneven timing, or the receiving DAC having a not-sufficiently-smooth method to adapt to the transport's clock speed. If the clocks disagree by 1 part in 1 million, that's 1 sample every 23 seconds at 44-16 CD.

By comparison, USB "async" is a pull protocol. The DAC requests the data from the source, so the DAC is entirely responsible for data rate / clock speed. It obviates the need to adapt or sync clocks to each other. I believe this is why Amir sometimes measures more jitter with SPDIF (whether coax or toslink) than with USB.
All of that can happen, but I've not found it to occur in my limited testing. The transport seems to settle down into a steady speed, and consistent jitter, and whatever interaction with the receiving DAC also seems to reach a quick equilibrium. It isn't at all uncommon to have clocks in such devices disagree by a few ppm. Again I think this is an overblown worry.
 

Blumlein 88

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That's a 160 ppm discrepancy, within the expected range for two unsynchronised crystal oscillators. It also has nothing to do with rotation. The disc motor is servo controlled to provide a reasonably constant readout rate. The data is then buffered, processed, and clocked out to the DAC by a crystal oscillator. You'll get similar results with any unsynchronised DAC/ADC pair.
+1

As I mentioned earlier the rpm of the CD is varied and is kept so that a buffer is maintained at about a half full state for most of the earlier transports. And as mansr says a crystal clocks it out. Usually the isolation of the clock from the servos etc is quite good.

There have been other ways of doing that, I seem to recall a Mark Levinson that read a complete track into memory prior to playing. I don't think that made any sound difference however. As already said portable players often had a buffer of some 20-40 seconds to prevent skipping from shocks of portable use.
 

MRC01

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All of that can happen, but I've not found it to occur in my limited testing. The transport seems to settle down into a steady speed, and consistent jitter, and whatever interaction with the receiving DAC also seems to reach a quick equilibrium. It isn't at all uncommon to have clocks in such devices disagree by a few ppm. Again I think this is an overblown worry.
I mention it because in the past I encountered a USB DAC that ran in adaptive mode and would occasionally (like 1-4 times per hour) glitch/tic when playing.
In a similar vein, resampling is also a frequently overblown worry, but I have encountered devices that do it improperly and create audible artifacts.
We can call them the exceptions that prove the rule, but even so it's worth testing for it. One could even say that's what testing is for!
 

pma

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That's a 160 ppm discrepancy, within the expected range for two unsynchronised crystal oscillators. It also has nothing to do with rotation. The disc motor is servo controlled to provide a reasonably constant readout rate. The data is then buffered, processed, and clocked out to the DAC by a crystal oscillator. You'll get similar results with any unsynchronised DAC/ADC pair.
You are right. My momentary lapse.
 

lewdish

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I'd love to see if there's an actual difference between any of the old audiophile CD/DVD players that can be used as a transport and a dedicated CD transport. Recently I've been thinking of getting a transport cause they look cleaner, more compact, and more modern but wondering if there's really going to be any real value out of doing so when compared to putting discs in my blu-ray player.
 

Boes

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I'm trying to understand why people want to use a CD transport.
I think we can all agree a CD transport only makes sense if you're directly outputting the PCM from the disc without buffering. If you're buffering and re-clocking you might as well rip the CD.
In order to play back a CD at normal speed the linear velocity of the data on the disc needs to be constant. That's why the spindle motor has a higher RPM at the inside of the disc compared to the outside of the disc. There is a control loop in the CD player / transport that keeps adjusting the spindle motor's RPM in order to keep the bitstream from the CD constant against it's clock.
On top of that each CD has eccentricity (the whole in the middle is not actually in the middle and / or the CD is not perfectly clamped). This eccentricity causes linear velocity to go up and down each RPM. Put the internal PLL frequency of CD transport on a oscilloscope and you will see a nice sine wave over each revolution of the disc. The amplitude of the sine wave depends on the eccentricity of the disc.
Why in the world would anyone want to use that? And then we haven't even talked about scratches or finger prints.
People in this forum rightfully go bananas when someone suggest that an audiophile USB cable reduces the jitter. By using a CD transport you're inherently (and measurably) introducing jitter and fluctuations in the data/clock.
 
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mansr

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The exact readout rate from the disc is unimportant since every CD player/transport necessarily buffers the data and sends to the DAC/output under the control of a crystal oscillator. The rotation of the disc is controlled such that the average data rate into the decoding stage is within the bounds set by the buffer.
 

MRC01

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I'm trying to understand why people want to use a CD transport.
I think we can all agree a CD transport only makes sense if you're directly outputting the PCM from the disc without buffering. If you're buffering and re-clocking you might as well rip the CD.
...
Why in the world would anyone want to use that? And then we haven't even talked about scratches or finger prints.
Maybe you have a collection of CDs you want to listen to. The CD transport handles all that with buffering etc. The digital (SPDIF) output of a properly engineered CD transport will be indistinguishable from the digital output of the same CD data played from a non-optical in-memory player.
 

Prana Ferox

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This has been discussed to death here but:

- The CD is a common, portable, endurable medium. I can take a CD to my friend's house and play it there. I can play a CD in my car. I can play the CD in a 30-year-old stereo. I do not need to remotely care about compatibility, did I bring the right dongle etc.
- I pay for a CD with an upfront one-time fee and own it forever. It does not suffer from recurring fees and the licensing cannot expire.
- I have the option of ripping a CD onto other media if I want, or burning it to another disc, or copying to cassette, I own it. (Resale / sharing becomes legally iffy in places but that's not a medium limitation. In fact, the vibrant used market is a contributing factor.)
- The process chain for the digital stream already compensates for the mechanical concerns you mention, and that's also why we mock the audiophile nonsense - the correction is already built in
- The CD was designed to last for decades and is incredibly resistant to scratches, fingerprints etc. A CD is likely to outlive any specific spinning rust / solid state device you would copy the data to.
- As such, a CD player can and should return a bit-perfect stream on the digital end limited only by Red Book standards (DAC performance may vary.)

This should not be read as 'the CD is superior to X' but more 'the CD remains a viable non-obsolete alternative' in ways that, say, are not true for vinyl / cassettes / older physical media.
 

Boes

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The exact readout rate from the disc is unimportant since every CD player/transport necessarily buffers the data and sends to the DAC/output under the control of a crystal oscillator. The rotation of the disc is controlled such that the average data rate into the decoding stage is within the bounds set by the buffer.
Then I honestly don't understand why people spend thousands of dollars on CD transports. Any $20 cdrom drive can rip the CD bit perfect.
FYI: I still buy CDs from time to time, but rip them and play the flac files.
 
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somebodyelse

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Then I honestly don't understand why people spend thousands of dollars on CD transports. Any $20 cdrom drive can rip the CD bit perfect.
FYI: I still buy CDs from time to time, but rip them and play the flac files.
You could say the same about most hardware in this hobby...

There used to be sites that did in depth analysis of the performance of CD/DVD/BD drives, including their audio extraction capabilities. These included tests with test CDs with different defect types at different levels, simulating fingerprints, scratches etc. like the ones here. For a CD in decent condition, playing at single speed as it would be in a CD player, almost every drive would be perfect. The differences emerged in the presence of defects, with little correlation between price and performance. Sometimes it would change with a different firmware version on the same drive. I don't remember seeing similar tests of audio CD transports, but that's probably where the big difference will be.

If you go back to when DACs didn't handle jitter so well, it might have been a reason for an audible difference between transports. Looking at the jitter performance of most DACs tested recently it seems that time is long passed. The belief remains long after the reality behind it has changed.
 

Mart68

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Then I honestly don't understand why people spend thousands of dollars on CD transports. Any $20 cdrom drive can rip the CD bit perfect.
FYI: I still buy CDs from time to time, but rip them and play the flac files.
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People don't understand how digital audio works.

I have a CD transport, it's the only source I use. Don't like the slot loading or the looks so have been looking for an alternative, but nothing to do with the sound quality which is outstanding.
 

MRC01

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People don't understand how digital audio works.

I have a CD transport, it's the only source I use. Don't like the slot loading or the looks so have been looking for an alternative, but nothing to do with the sound quality which is outstanding.
I wish I could find a transport with a non-motorized manual loading door. That's one of the most common failure points of a transport. Back in the day, the Rega Planet CD player had such a manual door. I didn't like that player for other reasons, but I wish more players had a loading door like that.
 

Mart68

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I wish I could find a transport with a non-motorized manual loading door. That's one of the most common failure points of a transport. Back in the day, the Rega Planet CD player had such a manual door. I didn't like that player for other reasons, but I wish more players had a loading door like that.
There's CEC but they are expensive and have pointless belt drive system. Or the Project CD box but that's ugly as sin.
 

Blumlein 88

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There's CEC but they are expensive and have pointless belt drive system. Or the Project CD box but that's ugly as sin.
I have one of the very early CEC players. No telling how many hours on it. Still looks and works great. I did replace the belt once, but that was easy taking about 10 minutes. Otoh, you can use a cheap bluray player. If it breaks get another one.
 

Mart68

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I have one of the very early CEC players. No telling how many hours on it. Still looks and works great. I did replace the belt once, but that was easy taking about 10 minutes. Otoh, you can use a cheap bluray player. If it breaks get another one.
I've found cheap blu-ray players are mechanically noisy. You can hear them grinding in the quiet bits. I do have an Oppo that is silent but that's in use in the A/V system.

For a transport I was using a bottom of the barrel Panasonic DVD player I got for free but that started playing up so thought I'd push the boat out a bit and buy an Audiolab. Not keen on the looks or the slot loading but it does the job.

I want one of those old TEAC VRDS jobs with the white buttons, but they cost around a monkey used which is a lot to pay for old kit that could just up and die at any time.

CEC is quality equipment but more than I am willing to spend and the aesthetics are okay but don't really float my boat.
 

Mart68

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What a CD Transport should look like:




By comparison the Project CD Box is what I imagine all hi-fi equipment would look like if the Nazis had won the Second World War:

 

sq225917

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That audiolab of yours has a fancy buffer in it thought Martin, perfect for older dacs that might not have the best jitter correction.

I'll certainly not be selling my recapped 90s cdm transport. I should fetch shot of the spare unit at some point though.
 

Smitty2k1

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What a CD Transport should look like:




By comparison the Project CD Box is what I imagine all hi-fi equipment would look like if the Nazis had won the Second World War:

I want a nice desktop sized CD player that can sit in a little stack with my Topping DAC and amp. I also don't want to pay $300 for it.
 
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