Naked Science Forum

Non Life Sciences => Technology => Topic started by: Karsten on 28/11/2009 14:11:10

Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: Karsten on 28/11/2009 14:11:10
We have a DVD player. Occasionally we do not finish a movie and take out the disk. Other DVDs are watched before we reinsert the DVD that was not finished. Our DVD player seems to remember where we left off a long time before and goes right back to that place on the DVD to continue. Quite handy, however, I do not understand how the DVD player does this. First of all, it does not do this always. Secondly, it seems to be able to do even if it is completely disconnected from the power. Thirdly, it does not seem to matter if one or several other DVDs are played in the meantime.

How does this work?
Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: somewakko on 29/11/2009 03:10:30
Your DVD player definitely implemented this as a handy feature.

With CD's it was found that you could identify any CD based on a mathematical function called a hash applied to the table of contents. The basic idea is that an entire block of data can be transformed into a single large number.  The same number will always be obtained by the same input data, and it is very unlikely that the same number is generated from different input data.  This is not perfect. Occasionally with CD's there would be two commercially available CD's with the same exact hash. But duplicates are the rare exception.

Not knowing any specifics about your DVD, I'd guess they use a similar hash technology to identify the specific DVD you are watching. 

Then they simply must persist the identifier of the DVD and the location in a table in memory.

If this is persisted after removing power from your DVD player, the player may simply have enough capacitance (stored electricity) to prevent its memory from being erased during short power-off cycles.

If it is persisted over long power-off cycles, the DVD player must store that information in nonvolatile memory.  Nonvolatile memory does not require input electricity to maintain the data once it is written (such as a memory card from a digital camera) This memory is cheap enough today that it would probably be present to store your language preferences, subtitle, and other settings. It is likely the manufacturer chose to use some leftover memory to store a table of your last-watched scenes.

This seems like a great feature that some engineers thought-up, but the manufacturer never found a way to market. It's a surprise when you find this feature, and then can never find another player that behaves the same.
Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: ScientificBoyZClub on 29/11/2009 04:58:12
Perhaps, it fixes memory location of laser point position and Time of Exit your DVD.
may be it stores memory of every DVD you insert in to continue your time where you stopped it. Time and position are every important.

REALLY very good question BRO!
Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: derek.ifocus on 01/12/2009 08:57:27
I found this forum because I was searching for an answer to this very question.  I've been reading for hours!! Great stuff
Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: theory38 on 17/10/2010 18:44:53
My theory is that there must lye a memory table or some sort of indicator on the actual digital video disk that makes it possible for one to be able to return to a particular scene on the disk after it has been ejected from the player and inserted at a later time. This disk indicator must send a signal of some sort made available for a player to read and respond to. The player must also perhaps be equipped with this particular ability to read and respond to the disk signal, but to stress again it is the digital video disk that makes this memory feature possible.

And to further prove my theory I would like you to see for yourself,

the next time you watch a movie, I would like you stop it at what ever particular  scene or part you are viewing, then take out the disk, insert the disk in any other player and see then if you are returned to the scene you have left off at.

A very strange occurance i will share with you, I had been watching a DVD from it's beginning to about three quarters into the film on my home player, this player I was using was giving me problems, it was nothing new about how it was behaving, so I chose to watch the movie or the rest of the movie on my dvd drive bounded to my PC, It so happened that the movie began playing immediately upon insertion of the disk without my usual launching of which program for to be viewed with, and it continued off where i had viewed it last on my home player. 
Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: Lynda on 17/10/2010 19:46:36
This thread may have answered my puzzle over a DVD of a film I borrowed from our local library. 

In 2004 when I first bought my PC I decided to test the ability of the DVD drive  to play films (someone told me, when I first bought this computer, that it could).   

I therefore borrowed a copy of the DVD from our local library.   I had to stop it as there was a fault.    When I exchanged it for another copy of the same film the DVD drive knew where I had stopped the other copy!
Title: How does my DVD player remember where I stopped the DVD earlier?
Post by: bobwilliam on 21/10/2010 15:22:12
A very strange event, I will share with you, I have been watching DVD into the film from its start about three-quarters of the players in my home, I use this player for my problem, which is nothing new about it is how to express, so I choose to see movies or in my DVD drive is bounded to my computer movie rest Incidentally, the film starts playing immediately without the disk inserted, I usually launch the program how to look, and it continues to where I had to see it in my player the last time.
Title: Re: How does my DVD player remember where I stopped the DVD earlier?
Post by: dfavell on 15/01/2013 17:11:52
Yesterday I was watching a dvd with lots of episodes on it on my xbox.  I stopped the dvd after an episode, I then moved the dvd to another machine in my bedroom, that machine started playing the next episode in order.  The two machines are not connected in anyway.  ?
Title: Re: How does my DVD player remember where I stopped the DVD earlier?
Post by: evan_au on 15/01/2013 20:04:11
Another player resuming from the exact same spot on a CD/DVD sounds like a coincidence.

It is theoretically possible with sophisticated software on networked machines like XBox in the same house to monitor what everyone is watching/playing, and then resume from the same point on another machine - that would be helpful, but spooky!

The older AudioCassette/Videotape technology had a built-in mechanical memory for the last point played, and could be easily moved from one machine to another, and it would resume playing exactly where it left off!

Some reasons why the CD/DVD is not remembering:
DVD/CD media manufacture is now very cheap because the plastic disk and its reflective coating are "passive" - it just reflects the light that falls on it.

CDs & DVDs are also very robust, in that they can still be played despite a few scratches on the surface. This is due to:

So I suggest that the CD/DVD does not "remember" where it was last played, since that involves mechanical changes over a large area inside a solid lump of plastic. The memory is undoubtedly inside the DVD player/computer.
Title: Re: How does my DVD player remember where I stopped the DVD earlier?
Post by: evan_au on 16/01/2013 20:24:48
PS: With today's technology, you could easily embed a rewriteable RFID (Radio-Frequency IDentification) chip inside a CD/DVD/Blu-Ray.

This would probably double the manufacturing cost of the disk, but it could record information like who has been using this CD, where did they get up to, etc.

This information could be written by one player in some standard format when the disk was ejected, and read by another player when you insert the disk.

It is impractical to do this for existing generations of CD/DVD/Blu-Ray and their players, but it could be incorporated in the next generation of disk technology and its readers.

Now all we need is for some non-intrusive method of user-recognition, and find some way around all the privacy concerns.
[By the way, your smart-phone/tablet/PC/High-end Games machine already does all of these things...]