OT: DVD recording/playback...

Disc brand is a definite variable. When I first started recording DVDs, I bought a big spindle of GQ (Great Quality - HAH!) blanks from Frys Electronics. Things that I recorded on them would be OK at the start, then start breaking up at the end. DVDs record from the inside to the outside, so the linear speed past the laser is faster at the end, thus more difficult to write and read reliably. Those GQ discs just wouldn't work reliably. Switched to using Memorex DVD-Rs, been working OK for me for a few years now, tossed the rest of the GQs. Tried +Rs once, but one of the players in the house won't recognize a

+R, so sticking with Memorex -Rs.

A little bit of googling might lead you to a website that rates the various brands of DVDs.

Hope this helps, Jerry

Disc brand is a definite variable. When I first started recording DVDs, I bought a big spindle of GQ (Great Quality - HAH!) blanks from Frys Electronics. Things that I recorded on them would be OK at the start, then start breaking up at the end. DVDs record from the inside to the outside, so the linear speed past the laser is faster at the end, thus more difficult to write and read reliably.

That isn't true, I think. CD and DVD use constant linear velocity (CLV) servos, rather than constant angular velocity (CAV). This means that as the laser tracks from the centre to the outside of the disc, the rotational speed of the disc is slowed, to maintain a constant writing or reading speed of the track, past the optical block.

Arfa

he

eed

If you can believe everything you read on the internet, I saw the following on the DVD Demystified website :

dfaq.html#1.1. Leads me to believe that there are both CLV and CAV drives out there.

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In order to maintain constant linear density, typical CD-ROM and DVD-
ROM drives spin the disc more slowly when reading or writing near the
outside where there is more physical surface in each track. (This is
called CLV, constant linear velocity.) Some faster drives keep the
rotational speed constant and use a buffer to deal with the
differences in data readout or writeout speed. (This is called CAV,
constant angular velocity.) In CAV drives, the data is read or written
fastest at the outside of the disc, which is why specifications often
list "max speed."

Jerry

If you can believe everything you read on the internet, I saw the following on the DVD Demystified website :

Leads me to believe that there are both CLV and CAV drives out there.

--

In order to maintain constant linear density, typical CD-ROM and DVD- ROM drives spin the disc more slowly when reading or writing near the outside where there is more physical surface in each track. (This is called CLV, constant linear velocity.) Some faster drives keep the rotational speed constant and use a buffer to deal with the differences in data readout or writeout speed. (This is called CAV, constant angular velocity.) In CAV drives, the data is read or written fastest at the outside of the disc, which is why specifications often list "max speed."

Jerry

That's interesting Jerry. I've never before seen any writeups on CD/DVD rotation control systems, which suggest that anything other than CLV servos are used. I suppose that if you keep the angular velocity the same, and vary the data transfer rate, then in terms of what's actually recorded, that amounts to the same thing, achieved by an alternate method. That being the case, then your original point about the writing speed at various places on the disc makes a lot more sense. Thanks for the info. I wonder what exactly is the thinking behind doing it in what seems to be a more complex way that requires a chunk of buffer memory, and why this should be (apparently) associated particularly with 'high speed' writers ? Time for some more research, I think !

Arfa

A CLV drive will need a significantly better motor and some care in the mechanical design to prevent resonances. A CAV drive can have much cheaper mechanicals and can handle random access. (A CLV drive has to diddle the motor speed on every seek till the tracking servo locks again - slow and clunky.)

Add in marketing factors like selling on buffer size and that a CAV drive claiming a particular access speed may only run that fast on AVERAGE across a maximum length disk, and I'd be surprised to find any new ROM drives that still do CLV. Recorders are a different matter, Its gotta be easyier to get a consistant result at CLV and I wouldn't be surprised to see both strategies in the same high end drive depending on recording speed selected and type of disk.

Yes, that seems to make a lot of sense. Maybe the reason that I have not come across this, is that most of my day to day experience with CD / DVD drives, is on 'entertainment' equipment, rather than computer based drives. Good to discover something new. I guess it's true that you never stop learning 'til the day you die. d;~}

Arfa

s

Well, CD-ROMs for PCs appear to use CAV since the disk always rotates at the same speed, except when it is having difficulties to read. But some cheap set-top DVD players use CLV. In one I repaired the motor was a standard DC brush motor and a double power op-amp driver chip. The controller set the speed by adjusting a voltage, and it could not only accelerate but also brake by taking away power generated in the motor. Without the braking it did play but would stall at the first seek to the end of the disk.

Well, CD-ROMs for PCs appear to use CAV since the disk always rotates at the same speed, except when it is having difficulties to read. But some cheap set-top DVD players use CLV. In one I repaired the motor was a standard DC brush motor and a double power op-amp driver chip. The controller set the speed by adjusting a voltage, and it could not only accelerate but also brake by taking away power generated in the motor. Without the braking it did play but would stall at the first seek to the end of the disk.

I think you'll find that pretty much all domestic entertainment DVD players / home cinema units, irrespective of their cost or pedigree use CLV servos.

Arfa

If digital disk players -- for whatever format -- were CAV, they'd have roughly only half the recording time they do. They don't need CAV, as the constant clock rate allows timing variations to be corrected.

As I understood the description given by another poster, William, Drives which use CAV servos for reasons that sound pretty valid (typically those fitted to computers), feed the data to the disk for recording, at a variable rather than constant rate, using a chunk of buffer memory to accomplish this. The net result is that the data is still recorded at a constant 'pitch' along the tracks, the same as it would have been if a constant data rate, and variable spin speed had been used. Thus, when the drive has finished recording the disk, its format will be exactly the same as any other CD / DVD. If you think about it, this would have to be the case, otherwise adherence to the standards laid down for CD / DVD, and compatibility with other players, would be compromised.

Presumably, on drives which used CAV servos, the reverse principle is used for playing back ? If so, it must all work out the same, otherwise, a commercial disk would not play in both your CAV computer drive, and the CLV drive in your home cinema unit.

Arfa

It could just be that the microprocessor in the drive detects the current bit rate and reads it accordingly. That way a disk can be written in in almost any method at almost any speed and still be read.

This compensates for buggy writers, component aging, etc.

Geoff.

--
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variable

data

Thanks for the clarification. I wondered why you said "CAV servos", rather than CAV.

As far as I know, CD and DVD writers record chunks of data at a constant spin rate. What you're suggesting is doable, but would get really messy having to continually change the data-writing rate. And it probably isn't necessary. See below.

Not necessarily. If each chunk of data represents only a few revolutions, the playback circuitry should have no trouble handling the slight change in data rate.

To put it another way... It isn't necessary for a digital optical disk to be written to at an absolutely constant data rate. The clocking and buffering circuits should be able to handle a change of a few percent.

To give a related example... I have a Sony 601 digital processor, the only one that can convert recordings made at 44.056kHz to S/PDIF format. An outboard DAC has no trouble with it, despite the fact it's the "wrong" frequency. Yes, it's only 0.1% slow, but the principle is the same.

CD/DVD/Blu-Ray are all CLV to achieve maximum storage capacity. The drive or player can do whatever the heck it pleases in reading them.

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Accepted, but the data rate or spin rate, would be significantly different between the start and end of the disk. If the (CAV) drive runs the disk at a constant speed, there will be a large difference in the data rate between the start and end, just as with a CLV drive, there is a large difference in rotational speed between the start and end. As far as accuracy of the writing speed goes, I would suggest that this is pretty tightly controlled, and is probably the reason that with high speed writers, the disk is run at a constant speed, and the data rate varied to match the writing position on the disk between outside and inside edges of the data area. It would be a lot easier to vary the data readout rate from the record buffer, than to try to maintain an accurate varying (high) rotation speed, particularly on drives that use a DC brushed motor, rather than the brushless DD types used by better drives.

As far as CAV drives reading data back, I was being general in assuming that the reverse principle was applied in that again, the disc is rotated at a constant speed, and the data is processed at a constant rate, by varying the readout speed of a chunk of buffer memory, rather than the drive switching over to CLV for reading.

As to your point about record rates, if the disc is spun at a constant rate, then the data must be presented at a variable writing rate, in order to maintain a constant data pitch along the recorded track. Messy or not, it is absolutely essential that this is done to preserve that constant pitch, which is part of the CD / DVD specification. The difference in the lengths of track on each rotation, between the start and end of the data area, is simply too great for the system to be able to cope with merely as a percentage error around the nominal data rate. This is why there is a significant difference between the spin rate at the start and end of a disc in a standard CLV home entertainment player, and a sophisticated servo to take care maintaining a correct spin rate.

Arfa

Arfa, variable sectors per track has been around for a long time. As your tracks move outward, you change the clock rate of the decoder circuit, which changes the number of sectors. Commodore was doing it with floppies over 25 years ago.

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Hmmm. I'm not sure how that stacks up with CD / DVD though, as with those media, there are no sectors as such, just constant length data frames, recorded to disc at a constant pitch. This being the case, I don't think that you would be able to decode by simply varying the clock rate of the decoder. I think, as was suggested by someone else, that you would have to run the data through a buffer memory, and vary the clocking of the MMU to arrive at a constant data rate input for the decoder ??

Arfa

They are "soft sectored", which has been the same for just about every disk drive made in the last 30 years. There were hard sectored hard drives, which disapeared in the 1970's and hard sectored floppies, but only one or two computers used them in smaller than 8 inch and they did not last long once soft sectored disks became cheap and common.

They work with a gap between sectors and a header and trailer.

It would be very easy to just "listen" for a header and adjust the clock speed until it decodes and then read until the correct sector header starts to come in before doing anything with the data. There is no need to buffer it and play around with the buffer.

You just have to look for a section of silence (gap) and then a particular bit pattern (sector header) before you start decoding.

The only cost is time, it may take a revolution or two to "sync up" or not depending upon how close the clock is to the data.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com  N3OWJ/4X1GM

Yes, agreed with hard and floppy drives, but I was not aware of there being any 'sectors' as such - hard or soft - on CD / DVD though. It's been a few years since I did the courses on CD and DVD, but my poor dim memory seems to recall that the data format 'belongs to itself' and does not borrow from other disc technology, hard or floppy. AFAIR, the data is pretty much continuously recorded - allbeit spread around between frames for error protection purposes - in constant length frames, and at a linear pitch. There is no 'silence' to listen for. Because the data is spread around between frames, it is necessary to have at least a small read buffer anyway, to allow reassembly of the data and error correction to take place.

Arfa