How do DVDs continue to work when other forms of data storage become obsolete?

[peter.ell]

I am very curious as to how it's possible for DVDs to even work. After all, DVDs have a single stream of bits encoded on them, right? So how in the world is it possible for that single stream of bits to simultaneously produce picture and multi-channel sound?

If there two streams of data are just inter-leavened, how can the 1's and 0's of picture be differentiated from those of audio, how can there be multiple tracks of audio at once, and how can all this data be reproduced simultaneously without delay or syncing problems since it isn't being read simultaneously?

This is a real conundrum, but I know the solution must be simple, otherwise DVDs wouldn't work! So what's going on? Thank you for the enlightenment!

 

[russ_watters]

I am very curious as to how it's possible for DVDs to even work. After all, DVDs have a single stream of bits encoded on them, right? So how in the world is it possible for that single stream of bits to simultaneously produce picture and multi-channel sound?

The data rate is just faster than what is needed to produce both.

If there two streams of data are just inter-leavened, how can the 1's and 0's of picture be differentiated from those of audio, how can there be multiple tracks of audio at once, and how can all this data be reproduced simultaneously without delay or syncing problems since it isn't being read simultaneously?

The data can be read much, much faster than our eyes and ears work. And it's buffered. It doesn't read straight from the disk to the tv, they read ahead and collect and process it before it needs to be displayed.

 

[Jiggy-Ninja]

I am very curious as to how it's possible for DVDs to even work. After all, DVDs have a single stream of bits encoded on them, right? So how in the world is it possible for that single stream of bits to simultaneously produce picture and multi-channel sound?

If there two streams of data are just inter-leavened, how can the 1's and 0's of picture be differentiated from those of audio, how can there be multiple tracks of audio at once, and how can all this data be reproduced simultaneously without delay or syncing problems since it isn't being read simultaneously?

This is a real conundrum, but I know the solution must be simple, otherwise DVDs wouldn't work! So what's going on? Thank you for the enlightenment!

The solution does not have to be simple, it just has to work. In fact, sometimes the simplest solutions have the most problems.

Optical media uses a reflective surface with holes in it to denote data. When the laser shines on it, it's either reflected by the surface or absorbed by a hole. As a simplification, a reflection would be 1, no reflection 0. The actual encoding system is a little different, but the concept is the same.

The term "burning" came from one way to make the holes. Blank CDs would be a solid reflective surface, and a high powered laser would literally burn holes in it to represent data.

The way a DVD player can tell the difference between audio and video data is because DVDs have a standard format. The standard means that the player can expect certain kinds of data to be in a certain place. If the data didn't follow the standard (for example, putting in a DVD-ROM from a video game) the player wouldn't be able to interpret it right, and it wouldn't play.

As for how it can read audio and video data at the same time, it doesn't. The data is read into a buffer, and the software works on the data in there to make the picture and sound. The read head is always a few steps ahead, reading data into the buffer.

The original CD players would read the data in more or less real-time, and were vulnerable to skipping if you bumped them too hard. That would make the disk wobble, throw off the laser, and the data would be bad. Then CD players with Electronic Skip Protection came out. It used a RAM buffer so that the read head was about 5 seconds ahead of the playback at all times. If it was bumped and skipped a second, it would still be 4 seconds ahead and you wouldn't notice any skip. The reading could then race back up ahead to stay 5 seconds ahead.

As russ said, all this reading and processing happens at a speed far, FAR too fast for our mortal minds to comprehend.

EDIT: Hard disks store files as a single stream of bits too. You could also ponder how the computer is able to read files off of them and all that jazz.

 

[MATLABdude]

It's a single stream, but a great deal can be done on that stream. A second for us is as much as 8 billion instructions for a computer (quad core running at 2 GHz), probably 40-200 million instructions for the little microprocessor inside the DVD player, and time enough for a bit of light to travel 3*10^8 m, 7.5 times around the Earth (that last bit is not really germane to this conversation, but just helps to put things in perspective).

So even stopping and starting (and updating the picture, the sound, updating the firmware, etc.) a microprocessor can do everything fast enough that you see everything as being seamless. It probably even has enough time to twiddle its virtual thumbs (so to speak). Very few of us notice the stops and starts in things that happen faster than 30 or so times a second (hence why you don't notice fluorescent lights flickering 60 times a second or the movie updating 30 times a second).

In a similar fashion, there are many optical fibers lying underneath the ocean carrying phone calls (and internet packets, and TV signals, etc.--sometimes in the same fiber) between North America and Europe. The number of these is dwarfed by the number of simultaneous connections. Yet in spite of having multiple machines routing packets and signals from many callers / computers on one continent to the other, you don't notice any delays while you're on the phone (or Skype, for that matter).

Even your local cell phone tower has a limited number of channels, yet can handle more (simultaneous) callers than channels--same idea: by sharing the same channel with multiple callers, you can fit in more callers. Bottom line: we're slow, and that's a good thing.

EDIT: Too slow on the draw...

 

[AtomicJoe]

I am very curious as to how it's possible for DVDs to even work. After all, DVDs have a single stream of bits encoded on them, right? So how in the world is it possible for that single stream of bits to simultaneously produce picture and multi-channel sound?

If there two streams of data are just inter-leavened, how can the 1's and 0's of picture be differentiated from those of audio, how can there be multiple tracks of audio at once, and how can all this data be reproduced simultaneously without delay or syncing problems since it isn't being read simultaneously?

This is a real conundrum, but I know the solution must be simple, otherwise DVDs wouldn't work! So what's going on? Thank you for the enlightenment!

You might have noticed if you get digital TV that there is a significant delay between that and a live analogue broadcast. This is because you have to wait a while (second or two) free you have enough to produce the first second or so of sound and picture.
So some data is being stored an recombined to give you your picture and sound signals.

It is relatively straightforward to identity which bit is which you basically send them out in the order you receive them, along with some control information to help you receive them should you lose track of where you were. You might for example package the info up in 'frames' and have a start of frame sequence.

Consider for example you could download, for example a video in a shorter time than you would need to watch it, so there is plenty of spare capacity in the data rates which can be achieved in transmission.

 

[AtomicJoe]

Just as an aside, do people still use CDs and DVDs?
I have pretty much given up on them, too much like hard work.
They also take up too much space, and have a tendency to lose their information for a number of reasons.

And finally I can't be bothered searching through a pile of CDs!

 

[DaveC426913]

Just as an aside, do people still use CDs and DVDs?
I have pretty much given up on them, too much like hard work.
They also take up too much space, and have a tendency to lose their information for a number of reasons.

And finally I can't be bothered searching through a pile of CDs!

Wait, so what do you use? Store them on your HD?

So, you say CDs "lose their data" but your hard drive doesn't?

 

[Jiggy-Ninja]

or the movie updating 30 times a second).

On that note, http://www.youtube.com/watch?v=lRidfW_l4vs". Seems at least tangentially relevant, so why not?

EDIT: Youtube tag doesn't seem to work. Maybe it's a permissions thing.

 

[davenn]

Wait, so what do you use? Store them on your HD?

So, you say CDs "lose their data" but your hard drive doesn't?

Well CD/DVD discs do have a limited life ...but heck, its a hell of a long time ...read some where some years back they were talking 80++ yrs The data would outlast the majority of people who recorded it ;)

I haven't seen any info on how long stored data on a HDD disc would last, tho I suspect the moving parts of the drive (motor etc ) would be likely to fail first after long unused storage.

The next interesting one that I haven't seen any info for is USB memory sticks...
how long will they hold data before it degrades, and what would the process of degradation be ?

Personally I use big HDD's ... As Atomic joe said CD/DVD discs are just a pain to use and store and VERY limited data storage space compared to HDD's and memory sticks.

1TB HDD's are now AU$70 and 2TB's $130 why would anyone even want to shag around with CD/DVD discs ??!

Dave

 

[davenn]

DaveC...

I guess with the HDD disc(s) fully enclosed it does have a big advantage over a CD/DVD disc as far as physical damage from scratches etc goes.

D

 

[DaveC426913]

DaveC...

I guess with the HDD disc(s) fully enclosed it does have a big advantage over a CD/DVD disc as far as physical damage from scratches etc goes.

D

Except that hard drives fail! So do USB sticks.

CDs fail one at time. Your hardrive/USB fails, you lose all your stuff in one shot.

I'll take permanent, archival storage over volatile storage any day.


Additonally, the common formats these days suck. As pointed out, you've lost half your music. It continually astonishes me that the 21st century is going to go down in history as the time when technological advancement caused quality of product to be set back 30 years. Why would people embrace this with open arms??

 

[Jiggy-Ninja]

Except that hard drives fail! So do USB sticks.

CDs fail one at time. Your hardrive/USB fails, you lose all your stuff in one shot.

I'll take permanent, archival storage over volatile storage any day.


Additonally, the common formats these days suck. As pointed out, you've lost half your music. It continually astonishes me that the 21st century is going to go down in history as the time when technological advancement caused quality of product to be set back 30 years. Why would people embrace this with open arms??

Hard drives are not volatile.

One of the great advantages* of digital information is that it is so easy to replicate and transmit it that backups are fairly trivial. That's why it's recommended to keep 3 good copies of your data.

* The RIAA and MPAA might argue how "advantageous" it really is though.

 

[sophiecentaur]

The thing with the shortest life is not the medium, it's the Recording System. Any data that you had 20 years ago, on its original medium is probably no longer playable on any equipment you have at home. Who has a compact cassette player or reel to reel recorder readily to hand? 5 1/4 inch floppies , 3 1/2 inch ones or minidiscs are often useless because the playout equipment is defunct / thrown away. All those piles of VCR cassettes in the attic. Will you ever watch them again? Their data is still watchably intact.