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How the "pits" are read

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Rodney Gold

In Reply to: Re: burn is indeed "deeper" posted by Oscillatus on January 03, 2003 at 02:07:33:

A pit is not a 0 and a land is not a one or anything like that at all , the laser actually reads troughs and non troughs , not a zillion "pits" , the data on a Cd is also not contiguous , it is interleaved and has error correction as well , the depth of a trough CANNOT actualy vary much as the reciver of the laser works when the pit is a 1/4 of the wavelength of the laser deep as it then "disrupts" it , you cannot go willy nilly and make deeper troughs etc. You might define the start of a trough better by sort of "slowing" and burning at this point or increasing power at the start, its actually called power ramping (I use it on my cutting lasers - where it slows down on curves or changes the power to less when doing 90 degree corners as it slows down at these corners)
There are also other conversions involved and the end result is not a series of ones and zeros or digital "pulses" , rather an analog sine like wave at about 3mhz - so the notion of an On/OFF and no grey area type thing is actually not really accurate.
The way a laser reads has all to do with reflectivity , speed and tracking issues and how the laser interprets or if indeed it CAN interpret the troughs or "less reflective" areas.
Rodney Gold

Follow Ups:

Re: How the "pits" are read - Oscillatus 15:18:49 01/03/03 (0)
I am aware of how it works at that level, and yes the 1/4 wavelength principle behind it means that the word "deeper" must stay in quotes, but I beg to differ on the last part of your description a little, and perhaps to add some. The 'sinewave' varies. A series of alternating 1's and 0's will give an even mark/apce ratio and hence approximate a sinewave, but a few 1's, followed by many zero's then some quick 1's and 0's will give a rather odd looking waveform. Now, the next thing that is done in the curcuitry, is to turn that wave into a digital waveform, a certain voltage level indicating a 1 and another indicating a 0. This is why I referred to reading 1's and 0's, not wanting to get into the nitty gritty of HOW they are read. Nevertherless, that is what the player is interested in.
Within limits, your pits and lands can get pretty sloppy and so long as detected waveform is at the correct level/state when sampled (let's not get into a lengthy discussion on oversampling and jitter etc etc) then the result is a 1 or a 0. So, within normal tolerance, I believe what I said is correct.
The proof of the pudding is in the eating, and I get no more errors at 16x than at 4x, and subjectively it sounds just as good. Of course a good burner might compensate for the increased speed (power ramping you called it), in fact, such techniques must be one of the reasons burners keep getting faster.
Re: How the "pits" are read - Mike McCall 08:50:57 01/03/03 (2)
Hi Rodney,
I have a disc made by the church I attend in their own studio. I can play it on the computer, and on a portable in my office. However, I am inable to play it at home on my DVD player as it skips. I exchanged the disc for another, but I only ended up with skipping in different places. What would cause a consistent problem such as this?
Thanks,
Mike
- Re: How the "pits" are read - Rodney Gold 09:45:09 01/03/03 (1)
  Either the reflectivity of the Disc , the Disc not having been burned to Red book standard , a bad glass master if they did a big volume run or the fact the DVD is faulty or perhaps intolerant to slightly out of spec recordings
  Its most likely badly recorded
  
  Rodney Gold
  - Re: How the "pits" are read - Mike McCall 15:23:00 01/03/03 (0)
    Thanks, Rodney. What's interesting is that this is the second disc with this problem, and they only made 1500. No other disc in my collection (somewhere between 400-700), has this problem. Yet, two of these do. Since I know the folks involved quite well, what should I tell them to do about the problem? How do they ensure that future recordings don't suffer from such problems?
    
    Thanks,
    Mike

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