Anti-Skip CD Players, Why Not DVD's?

[muldoonthief]

@Doug_K

“Let It Go” is sweet relief after hearing “Fixer Upper” 10 times in a row. I haven’t seen the movie, but I swear “Fixer Upper” sounds like a rejected SNL skit being performed by a middle school drama class.

You haven’t lived until you’ve been in a tour bus full of 11&12 year old girls, all belting out “Let It Go” at the top of their lungs. Repeatedly.

[theR]

In addition to the completely correct answers already given, I’d also like to point out that some DVD players do handle scratches and debris better than others. Of the two DVD players in my house, one of them has trouble and will often freeze when playing scratched DVDs that the other player can play without any issue.

[Hail_Ants]

@E-DUB

You can use a portable DVD player to make a neat drive-in movie theater on a model railroad.

Not sure why but I find that the neatest idea for use of an old piece of tech I’ve ever heard!!

[BigT]

@Melbourne

CD’s use a continuous stream of music: if you miss one value, you can just skip it and go on to the next value.

DVD’s use compressed blocks. If you miss one value, you have to throw out the whole block. You can see the same thing on digital TV: when you change channels, you have to wait for the start of the next block, wait till the end of the block, decode it, then start showing it. That is why changing channels is so slow on digital TV.

But that’s not how anti-skip worked. It just built up a buffer, like Hampshire said. The OP is asking why DVD players don’t also build up a buffer.

The main reason I can come up with would be a lack of memory, with lack of speed being the next highest. But neither are too compelling now.

[campp]

@Hail_Ants

Not sure why but I find that the neatest idea for use of an old piece of tech I’ve ever heard!!

I’ve seen this done at a train show, and it was great. They were showing an old Godzilla movie.

[Musicat]

@Melbourne

CD’s use a continuous stream of music: if you miss one value, you can just skip it and go on to the next value.

The data on a CD or DVD is NOT a continuous stream of anything. It is encoded in data blocks with error correcting features.

An analog LP is a continuous stream, in contrast. The reason playing a CD seems like a continuous stream is because the software holds a small amount of data ready to be converted to sound and sent on to the listener. The listener receives a continuous stream after the blocks are verified and stored in a time sequence.

This is why, if a block cannot be decoded, the previous block (or 2 or 3) can be repeated with no noticeable degradation in the sound.

I don’t know the answer to the OP, and I see no reason why the same principle can’t be applied to DVDs, in fact, I always assumed it was. It’s a little more complicated for video – the coding uses several different compression algorithms compared to CD’s none – but there’s no reason why a frame or two, synced with the audio, can’t be substituted if an error can’t be corrected.

My guess is if the OP has players that balk at dirty or scratched DVDs, it’s because the recovery software has reached its limits. You probably aren’t aware of the corrections software makes in most cases, but it can only go so far.

DVD’s use compressed blocks.

So do CDs. Look into cross-interleaved Reed-Solomon coding sometime.

[Melbourne]

@Musicat

So do CDs. Look into cross-interleaved Reed-Solomon coding sometime.

CD’s do not use compression. If you loose a data point, you haven’t lost the subsequent differentially encoded data.

An unrecoverable error on a DVD creates an unrecoverable error on all subsequent data, until the next syncronisation point.

[RandMcnally]

@Alessan

My mistake - I was imagining a portable DVD player hanging from the ceiling by a piece of twine tied to a nail, which is about my level of handicraft competence. You can see how one of those swinging around at head level could be considered somewhat dodgy in terms of safety.

Heh. If my dad was the type to give us a dvd player to shut us up on a road trip* that’s the type of modifying he would do. So I definitely see why you would have pictured that.

*he’s more the "shut up because I told you and because you don’t want the belt.