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Old 19th November 2002, 04:40 PM   #171
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Quote:
Originally posted by dlharmon
1's and 0's are just that. It's either right or wrong. I have taken a huge file, burned it on a cd, and read it back. I ran a program to compare it with the original file and there were no incorrect bits. It would be interesting to buy 2 of a cd and read the data of and do a bit for bit comparison. I am guessing there would be few (0-10 per cd) bit errors even with a cheap cdrom.

Darrell Harmon
As others have pointed out in various threads, it's not quite that simple. Digital audio isn't just data; it is realtime data, where bit timing is every bit as important as bit correctness. Very small amounts of timing jitter can and do affect the final analogue signal. When you think about jitter, digital audio isn't really completely "digital" at all, in the sense that computers are digital. On a microscopic scale, there are analogue influences, via jitter.

I know this has all been said before, but I couldn't resist responding to this post. At one time I would have agreed that "digital is digital, bits are bits"... But as the members of this forum have shown me, it isn't this simple.
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Old 19th November 2002, 04:56 PM   #172
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Quote:
Originally posted by dlharmon
1's and 0's are just that. It's either right or wrong. I have taken a huge file, burned it on a cd, and read it back. I ran a program to compare it with the original file and there were no incorrect bits. It would be interesting to buy 2 of a cd and read the data of and do a bit for bit comparison. I am guessing there would be few (0-10 per cd) bit errors even with a cheap cdrom.

Darrell Harmon
Did you burn the file as data or as audio tracks? The formats
are quite different. I have tried ripping a pressed music CD
several times and compare the results. For that particular CD
I tried, all the resulting files differed slightly. I haven't bothered
to see it that is common behaviour for other CDs as well.
When bit errors occur on a data CD, it is possible to go back
and reread the block several times. For a music CD there is no
such possibility.
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Old 22nd September 2003, 06:10 PM   #173
Coulomb is offline Coulomb  England
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Well better late than never. I spoke to one of our optical engineers about the physics of this issue. We make very sensitive laser equipment that measures protiens through diffraction. Anyway he stipulated that there may be some benefit of flooding the CD surface with a green or blue incoherent light to improve the performance of the laser pickup. The benefit he stipulated was the reduction or elimination of white light reflections which are significantly noisier than the green or blue light off of a good source. The discriminator circuit can filter the green or blue light much easier than white light.

The net result maybe a lower noise floor from a technical perspective, how an Audio Fanatic may describe the effect is totally subjective of course.

Anthony
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Old 24th September 2003, 11:25 AM   #174
hifiZen is offline hifiZen  Canada
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Default just couldn't leave without adding my 2c...

Sorry to dredge this thread up again, but oh boy... Anyone familiar with the internal workings of optical drives can only laugh at this thread. Plus, it's been a while since I've posted a good rant.

Blue LEDs just aren't going to do anything, folks. The photodiodes are extremely small, and the entire reflected laser energy is focused on them, so the ratio of laser light to background light the PDs see is huge. Ever look at a CD player laser? It's bright, isn't it? Remember, you're observing it from a distance, therefore your eye is only receiving a miniscule fraction of the laser energy. Imagine gathering all that laser light and focusing it to a tiny point on your retina. Starting to understand the kind of ambient vs. laser light ratio we have here? Beyond that, these PDs are most sensitive to the red wavelength of the laser, less for other wavelengths. Shining a very bright light into the lens does not produce any measurable output (yes, I've tried it). Maybe I'll try it again tomorrow with a blue LED directly on the pickup lens.

More details of optical playback systems for those who are interested:

The photodiode outputs feed an analogue RF preamp, which does a few things with the signals to prepare various tracking, focus, and RF signals (lots of opamps, phase comparators, peak hold circuits and so on). These are then fed to high speed ADCs (low rez, like 8-bit or so), and processed by a DSP which handles control of the servo loops for focus, tracking and spindle speed. The spindle servo loop may or may not include a PLL (not all processors actually use one), but in general, the servo tries to control the spindle speed to keep the data flowing at a rate set by a crystal-derived reference. Once the EFM signal is demodulated into ones and zeros, the resulting data is put into a buffer for the read channel processor. If the EFM signal can be properly sliced and decoded into data, then this is the end of the line for EFM jitter. The EFM slicer does not care about a few ps of jitter. All it knows is that the eye pattern is good enough for it to distinguish ones from zeros. Thus, the only effects of consequence are those that impact the eye pattern to the point of causing or preventing errors.

Next, the read channel, which feeds on the buffered data. The read channel is generally a separate datapath from the servo controller, and is really just an elaborate state machine... clocked by a good ol crystal. It does not know about EFM jitter and such, as it's the servo processor's job to keep the buffer stocked appropriately with a steady stream of data bytes. The read channel handles de-interleaving (the data is scattered around the disc, and needs to be re-ordered to get the original sequence) and error detection / correction. The read channel also does interpolation of audio data to replace missing or bad samples in the case of errors. Perhaps if you screw up the data enough, you'll get lots of interpolated samples, which might sound 'smoother'(?), but this is an error, not the desired original bit-accurate data which audiophiles everywhere seem so obsessed about. If you want more interpolations, I can design a nice little FPGA to discard random data samples and fill in the blanks. I could make a fortune replacing audiophile-approved blue LEDs with a proven, reliable, and repeatable data 'enhancer'.

I hope at least some of you can now see how silly this is. Small amounts of photodiode noise and EFM jitter are irrelevant in the process of reading CD data, insofar as the eye pattern is still good enough to actually extract the data bits, not that the photodiodes are going to be affected much by ambient light anyway. If you want improved data recovery, you should focus on things which really will have an effect on the optical read process, such as mechanical vibration which keeps the focus and tracking servo loops busy (along with disc imperfections). These factors will have far greater impact on C1 and C2 errors than any blue light will.

Now, can we please put away all the green felt pens and blue LEDs? Or perhaps there's a phychologist amongst us who wants to claim that these are valid audiophile techniques (and which I might not entirely disagree with)!
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Old 25th September 2003, 05:37 AM   #175
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Default Another Expert Telling Us That We Are Wrong......

Chad,
Typical CD players have lousy power supplies and layouts, particularly earthing arrngements.
Mechanism servo currents (motors and F/T coils) cause modulation of the supplies and earth references.
This has the effect of causing modulation of the system master clock, and the result of this is jittered audio output.
It is all quite simple really.
If CD players had perfectly stable clocks then the Elso Kwak and Guido Tent oscillators would make no difference, but they do.
Suggest you learn more about real audio than is in your theory text books.

Eric.
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Old 25th September 2003, 07:41 AM   #176
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Arrow Re: Another Expert Telling Us That We Are Wrong......

Quote:
Originally posted by mrfeedback
Chad,
Typical CD players have lousy power supplies and layouts, particularly earthing arrngements.
Mechanism servo currents (motors and F/T coils) cause modulation of the supplies and earth references.
This has the effect of causing modulation of the system master clock, and the result of this is jittered audio output.
It is all quite simple really.
If CD players had perfectly stable clocks then the Elso Kwak and Guido Tent oscillators would make no difference, but they do.
Suggest you learn more about real audio than is in your theory text books.

Eric.
Hi Eric,
It is not only the supply but the way the oscillation with the crystal is generated. CMOS oscillators tend to have a lot more jitter than a simple Colpitts oscillator. This was documented in a Analog Devices application note.
The vast majority of the CDplayers use a CMOS oscillator very often on board of a larger chip like the digital filter or decoder.


On a rainy Sunday afternoon I tried red, blue and green LEDs as well as a black pen for the outher and innner edge. None of these mods brought any improvement to my ears.
I am looking forward to the Grunge-Buster mat. Well hopefully it brings something.....
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Old 25th September 2003, 08:40 AM   #177
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Hi Elso,
Yes I am aware that a crystal hanging off the side of a cmos dsp chip will be inherently unstable due the oscillator circuitry and also due to noise junk within the dsp chip itself, and that this is also further modulated by lousy power supplies and earthing arrangements.

I also understand that your external oscillator solution helps to overcome the above limitations in a standard cd player.
With your external clock module, I would expect that blue led's, green/black pens etc should make little if any difference to the jitter performance of your (modified) cd player.

The point that I am trying to get across to Chad is that in a standard cd player (with standard cmos oscillator arrangement) servo currents will influence the stability of the master clock and induce or at least influence the spectral nature of the jitter.

Doing just about anything like adding mass to the disc, extraneous light, vibration etc will affect the nature of the servo currents, and hence influence jitter in a standard cd player.

This is not explained to Chad in his theory books, but ought to be intuitive if a close study is made of circuit techniques and layout in a typical mass produced cd player.

We have many degreed 'experts' around here who frankly have no real world experience, and Chad's rant is clearly revealling of this - perhaps he now knows better than to shoot his mouth off in knowing company in future.

The likes of Peter and others are making comment based on their experience and experiments.
Peter is finely discriminating of the changes wrought in his system due to various tweaks and he is merely reporting the outcome.
That his systems seem to be very sensitive to minor changes leads me to look for deeper causes, and I suspect that the fact that the local street transformer is just outside his house is one of these influences.

Eric.
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