Writing nearer to the outer rim: Why problematic?

I’ve been noticing, and just read a statement by someone that writing near the outer rim of DVDs gets more problematic. I noticed that many of the media test show degradation near the end, and also in my own tests that is usually the case.

Isn’t the write speed equivalent across the whole disc? Wouldn’t the coating on the media be pretty much equal all over? So why is there so much degradation usually near the end? It seems this usually happens on lesser quality media. I see sometimes the beginning is pretty nice and clean and as soon as you get to like 80% everything quickly goes downhill sometimes leading to even data errors. Even when the disc is scanned at a constant speed all across!

Hope someone understands this phenomena and can explain it…

Tek.

AFAIK it’s because data concentration is higher the closer you get to the edge of the disc, so any marginal media will naturally have more problems there.

So you mean that even at the same 4x read speed, the edge of the disc spins faster relative to the laser and thus the data is being read at a higher rate, right?

Tek.

I thought all DVD data/pitts are distributed pretty much evenly on DVD. Also, DVD burner are 4x CLV (or Z-CLV at 8x). If i understand it correctly, CLV will spin the discs at higher RPM inside and slower RPM on outside to maintain Linear Velocity.

CAV, on the other hand, will maintain Angular Velocity which is basically RPM through out the discs. By spinning at the same rate, it results in laser head travelling over less CD surfact when inside track, but covering much more surface on outer track. Thus, during CAV, burning speed will climb up from maximum speed via CLV toward maximum burning speed.

IF burners are to blame for burning bad result on outer edge, it would be that Burner’s motor does not slow down to proper speed. But this is only true for CLV case.

I would think that it has more to do with either vibration of discs or dye on discs themselves. On outside track, slight vibration can create a big movement compared with inside track. Or maybe the process of manufacture discs are prone to have more flaws on dye toward edge more than inner portion.

you were saying…


Originally posted by simplelife
[B]I thought all DVD data/pitts are distributed pretty much evenly on DVD. Also, DVD burner are 4x CLV (or Z-CLV at 8x). If i understand it correctly, CLV will spin the discs at higher RPM inside and slower RPM on outside to maintain Linear Velocity.

CAV, on the other hand, will maintain Angular Velocity which is basically RPM through out the discs. By spinning at the same rate, it results in laser head travelling over less CD surfact when inside track, but covering much more surface on outer track. Thus, during CAV, burning speed will climb up from maximum speed via CLV toward maximum burning speed.

IF burners are to blame for burning bad result on outer edge, it would be that Burner’s motor does not slow down to proper speed. But this is only true for CLV case.

I would think that it has more to do with either vibration of discs or dye on discs themselves. On outside track, slight vibration can create a big movement compared with inside track. Or maybe the process of manufacture discs are prone to have more flaws on dye toward edge more than inner portion. [/B]

You’re partly right. The data density is equal throughout the disc at CLV, but is zonal at Z-CLV.
AS for vibration, the displacement (lateral movement) is the same on all parts of the disc. The inside can’t move less than the outside - they are connected.
2 primary reasons for outer edge errors are:

  1. crappy discs. Either warped, out of round or uneven dye distribution. You very rarely see this issue on TY.
  2. possible tracking error. There’s some logic to the notion that tracking error will increase exponentially as you get farther out on the disc. I couldn’t say for sure, but it would explain a few things. It’s also possible that the drive itself has some geometry that makes tracking worse on the outside.

Originally posted by rdgrimes
You’re partly right. The data density is equal throughout the disc at CLV, but is zonal at Z-CLV.

I thought Z-CLV is working just like CLV, but there is a slight pause for motor to basically change gear into faster speed. Motor now can spin disc at greater Linear Velocity. By doing so, it can cover more surface in the same amount of time, thus laser can burn more surface of discs with the same amount of time. Thus, resulting in higher burning speed zone. All this is done by maintaining same data density on discs.

If it is zonal, i think DVD player / DVD-rom will have a hard time reading different data density back from the disc.


AS for vibration, the displacement (lateral movement) is the same on all parts of the disc. The inside can’t move less than the outside - they are connected.

Sorry, i meant the discs wobble effect can be felt more on outer edge. Think about unbalance spin, discs are moved (up and down) much faster compare with inner part. Also, when spinning at lower speed, discs are less stable (tend to wobble more.)

I agree with you. I think it is more the way they producing blanks. Maybe the process can cause more flaws on dye toward outer edge more than the inner.

I think it is more the way they producing blanks. Maybe the process can cause more flaws on dye toward outer edge more than the inner.

It’s all these things. If the stamper is not perfectly flat and round, and perfectly even in thickness, the dye will be unevenly distributed when they spin it on. So you have a disc that’s out of balance, wobbles up and down, AND the dye is uneven.

On CDR’s you can sometimes see the defects after burning, they will be a patch of dye that’s a different shade, and may be associated with a band all the way around the disc caused by the laser trying to adjust for the change in dye. Haven’t seen this yet on DVD’s, but on the worst of them it might occur. The dye may wind up having a sort of “wave” in it running outward from the center like the spoke of a wheel.

Originally posted by rdgrimes
Haven’t seen this yet on DVD’s, but on the worst of them it might occur. The dye may wind up having a sort of “wave” in it running outward from the center like the spoke of a wheel.

Yeah, I burned a crummy Princo and now it looks like a vinyl record–has several rings throughout! :slight_smile: It was so bad, that data near the end could not be read…

Yes I believe the laser was trying to adjust every couple of minutes…

Tek.

P.S. Maybe I could take a photo of it?

Yeah my Verbatim DataLifePlus CD-RW 24X was like that. Makes the CD absolutely unreadable.