Hopefully you know that the CD medium itself is very jittery.
First: When the disk is scanned, the speed does not have to be the correct speed.
Second: The thickness of the CD itself varies, witch makes it seem as if the disk is being read at varying speeds.
Third: The production tolerances of the edges of the optical holes, witch are stamped into the CD.
All this causes jitter and can't be avoided.
Doing jitter measurements on transports, this has to be taken into account. Witch means that jitter measurements of transports are pretty meaningless.
Lesson: Jitter in CD transports is unavoidable. But a properly designed DAC can reduce this jitter so that it doesn't affect the performance of the DAC at all.
Well, few thing about CD's I know.
I designed and built a CD transport from scratch and I have learned a lot, including that a CD transport can be far less jittery than a Windows PC.Always a good implemented DAC can help.
Regards,
Tibi
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IIRC, the CD spec says that the rotational speed of the disc is to be locked to a quartz clock. This clock is used to reconstruct the audio signal (and clock out the SPDIF bit stream) and a FIFO is used to take up any fluctuations in data rate, while a feedback loop adjusts the motor speed to keep the FIFO half full on average.
So, a CD transport should be better than many sound cards, as good as an asynchronous USB2.0 DAC like the Audio Widget. It is not like a record where mechanical issues translate directly to wow.
So, a CD transport should be better than many sound cards, as good as an asynchronous USB2.0 DAC like the Audio Widget. It is not like a record where mechanical issues translate directly to wow.
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Well... So are floppies...
Worked with an engineer who did this sort of thing back in the good old days, great fun...
Of cause the connection between the transport and DAC is a lowpass filter and therefore jitter increases.
I believe this is what an old-school telegrapher would have called bias distortion, and a new one data-dependent jitter. A long run of 1s or 0s will charge up the time constant of the filter, and since the rise time of the signal is finite, a change in bias voltage maps to a change in zero crossing time.
SPDIF receiver chips are supposed to minimize it by clocking the PLL only from parts of the data stream that have a known pattern. And besides, the Manchester coding minimises long runs of the same polarity. You have to try really hard to get it as bad as the 30ns quoted above as a threshold for jitter perception.
SPDIF receiver chips are supposed to minimize it by clocking the PLL only from parts of the data stream that have a known pattern. And besides, the Manchester coding minimises long runs of the same polarity. You have to try really hard to get it as bad as the 30ns quoted above as a threshold for jitter perception.
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Scopeboy already explained it, but I'll try to dumb it down as much as possible.
A low pass filter affects the point where a signal changes from 0 to 1.
Say a 0 is a voltage from 0 to 1 Volt and a 1 is a voltage from 4 to 5 Volt.
With a "perfect" square wave the part of the signal where it changes from 0 to 1 is very small. Lowpass this "perfect" square and the part of the signal where it changes from 0 to 1 increases. Resulting in more jitter after the low pass filter, in this case a cable or print track.
You don't have to dumb it down I am well aware of how digital signals are transmitted maybe I should have used a exclamation mark. It was more questioning how much jitter is going to be added! The low pass filter will round of the square wave edges(attenuate some of the high frequency content), shape is not that critical if the periodic switching point is the same distance apart, again its a basic digital interface, correct cable, minimal impedance mismatches will minimise these effects. Interestingly some audiophiles believe a longer SPDIF cable is better than a short one!
I'm sure that by "good sound" erin want to say bit accurate and low jitter.
Regards,
Tibi
In part yes, when listening to transports I have observed many differences in sound, but have no way to measure jitter. I can observe a SPDIF signal on my CRO, but that only tells me if the wave is square or rounded. It tells me very little so I just have to go by how it sounds.
I'm not an expert but my basic understanding is that reed solomon error correction means all drives are capable of (and usually generate) bit perfect output, assuming the media is not badly damaged.
As Tattoo discussed earlier, the focusing and control circuit works very hard to read the data correctly. I assume that the difference in sound between drives has something to do with the way the focusing and motor control circuit interacts with the SPDIF data output by the drive, by this I mean the grounding arrangement, and the way voltages are affected by the act of focussing and keeping the laser in the correct position to read the data off the disc.
There are people who tried different pucs on their CD players and report a difference in sound. (I tried this once when I had a CD player apart. Not a regular thing for me to do, but I did notice a difference to the sound) Some people add damping materials to the inside chassis of their CD players to reduce vibrations and have reported improved sound. It makes some sense that by making it easier for the laser to correctly track the disc will reduce extra work in focussing. Conversely, making the CD more suceptable to vibration and creating extra work for the error correction and focussing system should make the audio sound worse to varying degrees depending upon particular factors.
So if we consider that some circuit layouts are better than others, and the electro mechanical parts of the transport can be better or worse, then it makes some sense that the data is going to come out better on some and worse on others.
Hence my question regarding any good sounding currently manufactured drives, and thanks to tvicol for replying to me.
If people here equate good digital sound to low jitter and bit perfectness then we are most likely discussing the same thing.
I'll trust you on that since I have no way to test for this. This must come back to what I was saying about the laser controller circuit creating some noise in the circuit affecting the digital data.
I can only discuss jitter from my basic theoretical understanding, which is that jitter levels from almost any digital source are so low that no bits are actually put in the wrong place, and the digital word is reconstructed perfectly. So any negative effect upon sound must be caused by something else. Your proposal that radiated or induced noise is the cuase of "bad" sound sounds plausable.
Yes, there will be a noise contribution from the switching (and high current) circuits within the drive. Switching leads to 'ground bounce' and can appear in addition to the main contribution to CM noise which is coming from the SMPSU.
In general, CM noise is not radiated, its carried through the screen (or 0V) conductor of the cable carrying the digital data.
In general, CM noise is not radiated, its carried through the screen (or 0V) conductor of the cable carrying the digital data.
Again, I do maintain that it is a simple basic digital interface, disregarding noise, a bit perfect transmission from one drive or digital source should (and probably does) sound no different from another. Again I would expect jitter from a competent drive and correct cabling should be manageable by the DAC.
Noise is another issue, and there are many threads on controlling and filtering that. Interestingly Ethernet interface has a common mode choke as part of the magnetics....
Noise is another issue, and there are many threads on controlling and filtering that. Interestingly Ethernet interface has a common mode choke as part of the magnetics....
I'm one of these audiophiles who belive in what they hear and I'm always looking for explanations, or try to get a theoretical explanation myself.
Digital cable length (from UHF Magazine):
"When a transition is launched into the transmission line, it takes a period of time to propagate or transit to the other end. This propagation time is somewhat slower than the speed of light, usually around 2 nanoseconds per foot, but can be longer… When the transition reaches the end of the transmission line (in the DAC), a reflection can occur that propagates back to the driver in the transport. Small reflections can occur in even well matched systems. When the reflection reaches the driver, it can again be reflected back towards the DAC. This ping-pong effect can sustain itself for several bounces depending on the losses in the cable. It is not unusual to see 3 to 5 of these reflections before they finally decay away. So, how does this affect the jitter? When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter. If the rise-time is 25 nanoseconds and the cable length is 3 feet, then the propagation time is about 6 nanoseconds. Once the transition has arrived at the receiver, the reflection propagates back to the driver (6 nanoseconds) and then the driver reflects this back to the receiver (6 nanoseconds) = 12 nanoseconds). So, as seen at the receiver, 12 nanoseconds after the 25 nanosecond transition started, we have a reflection superimposing on the transition. This is right about the time that the receiver will try to sample the transition, right around 0 volts DC. Not good. Now if the cable had been 1.5 metres, the reflection would have arrived 18 nanoseconds after the 25 nanosecond transition started at the receiver. This is much better because the receiver has likely already sampled the transition by this time."
Regards,
Tibi
Depending on which optical connections are you referring.
If is TOS-link, than you have lot of jitter added by PIN diode. As you know some TOS-link's are bandwidth limited to 192K.
If is an AT&T ST fiber-link, than you are on the right spot, but your budget is on the "audiophile" side.
Personally, I always recommend standard S/PDIF using cable with BNC connectors.
Regards,
Tibi
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As marce keeps pointing out, jitter is manageable. But my point was that if electrical noise really was an issue, you should be able to determine that by observing the differences between using a toslink connection and an electric one.
And that is not a problem for me. 96K is more than enough for me, and my pet bat doesn't really like my choice of music anyway
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