DVD Physical Format Explained
Author: Roupen Mouradian, Chief Technical Officer
There is much mystery about all the terms behind DVDs. How much storage space a DVD has, how data is stored on a DVD are among the topics that elude most of us but are critical to laying out your DVD.
Single Layer, Double Layer, Single Side, Double Side
The first thing we will look at is all the different physical structures the DVD can take. A DVD can vary in size depending on which one of these media types you decide to go with. You can have single or double layered DVDs and single or double sided DVDs.
For a single layer, single sided DVD, you are dealing with the smallest DVD size you will usually handle. The common industry term for this type of DVD is a "DVD-5". This DVD can hold 4.7 gigabytes of data. But this is done in human terms of base 10. In computer terms of base 2, however, this only equates to 4.37 gigabytes of space.
Your standard DVD burners can write to single layer single sided DVDs.
in general you would fit at most 90 minutes of footage on a DVD-5 before quality starts to deteriorate noticably with most MPEG-2 compressors.
The other option you will usually use is dual layer, single sided DVDs. These DVDs are referred to as "DVD-9".
In recent times DVD burners for these types of DVDs have been released as well. These DVDs can hold 8.54 gigabytes of data (7.95 gigabytes of binary data).
Though this type of storage still hasn't become very useful due to the high costs of media (a single dual layer writable DVD can cost you about $10).
Dual Layer DVDs work interestingly.
They write data to 2 different depths on the DVD.
Imagine a regular DVD sandwiched on top of another DVD.
Dual Layer DVDs essentially work like this.
Information is read from Layer 0 first and then, when the end of Layer 0 is reached, it moves on to Layer 1.
Lasers inside of a DVD reader are able to recognize these 2 standard depths of data. It is for this reason that a DVD-9 can hold double the information of a single layered DVD.
When DVD projects are presented to a replication house, each layer will usually be written to a seperate DLT.
A more detailed explanation of how data is written to multiple layers is discovered in our explanation of OTP vs. PTP further on in this tutorial.
Single and double sided DVDs are pretty straightforward. Both sides of the DVD can be written to in double sided DVDs. This will also double the space of your DVDs but it's not very practical. In fact, when you put a double sided DVD into a normal DVD player, when you want to access the other side's data, you have to open the DVD player and flip the DVD. This is an inconvenience in any scenario.
Rarely will you have so much video content that you will need to fit it on a double sided DVD.
Also rarely will you want your customers to have to flip the DVD when they reach a certain point in the DVD.
OTP vs. PTP
A term you will likely encounter while doing dual layer DVD projects is OTP and PTP. These terms stand for "Opposite Track Path" and "Parallel Track Path". This comes very key in the way the data is actually written to the DVD.
Picture how a DVD works. Data is written in circular lines coming out from the DVD's center.
So when a DVD laser is reading the data off of the DVD it is moving in circles coming out from the center of the DVD.
Now imagine you have a dual layer DVD and the laser reaches the end of the first layer (Layer 0).
PTP vs. OTP determine where the next bit of data lies.
In PTP, the next bit of data will lie where the first bit of data lies on the first layer (towards the center of the DVD).
There are pros and cons to this approach. On the positive side, PTP DVDs do not have any restriction on how big Layer 0 is versus how big Layer 1 is (more explained in the OTP section).
On the negative side, now the head of the laser has to move from the end of the DVD back to the beginning when it's attempting to switch layers.
What this will cause in the video is a slght pause while the head moves back towards the center of the DVD.
It is for this reason DVDs are typically made in OTP format. You will just have to be more careful with how you master your DVD as you will see.
OTP, or Opposite Track Path, arranges data differently from PTP. The first layer of the DVD (Layer 0) is written to the DVD the same way as in PTP.
However when the last bit of information is read off of the first layer, the change happens.
Instead of expecting the next bit of data to be at the beginning of the DVD, it is actually at the same exact spot as the final piece of data from the first layer.
From this point, the laser now starts moving back towards the inside of the DVD to get the sequential pieces of data.
The advantage to this approach is that the pause between switching layers is minimized. The laser head does not need to reposition between layer switches.
The disadvantage of this is that now you have to consider the size of Layer 0 versus Layer 1.
The reason is simple. Let's say you are writing to the DVD in rings. And on the first layer you write 10 rings of data. Now that you want to resume writing data and moving backwards you now only have 10 rings of "data space" to work with.
In parallel track path this doesn't come in to play because the position of the first bit on the second layer is always at the inside of the DVD.
When authoring DVDs, this problem is addressed by setting a "dual layer break point". This is the part of the movie that you want to stop writing to Layer 0 and start writing to Layer 1.
Though we will not go further into these concepts in this tutorial, you will see OTP and dual layer break points play a big part in your DVD-9 projects,
especially those where you push the full size limitations of a DVD-9 and finding a suitable break point in your movies becomes tricky.