A common question that clients always ask is “How much footage can I fit on a DVD?”. The glib answer is “As much as you want”, but it’s always good to try and back this up with some cold hard facts.
Often you will find on the packaging of blank media a duration of some sort. This is partly a throwback to the days of CD-Rs where the duration given was in fact how much Audio you could record to the disc. Audio has a fixed data rate and so there was always a direct relationship between the size of the disc (650Mb, 700Mb) and the amount of CD Audio you store (74 minutes, 80 minutes). With DVD VIDEO the data rate is variable and so when a blank DVD suggests a running time, this is usually based on a generic “quality” setting on set-top DVD Recorders – EP, SP, LP, HQ, SHQ etc. which are consumer speak for average data rates.
In actual fact, the easiest way to look at a DVD (or any kind of media) is as a ‘bit bucket’ – an object which can hold a certain amount of bits of Data. You have a certain amount of information you want to include on the disc and so you have to do a bit budget. It’s not different from doing any sort of budget.
There are probably lots of different ways to calculate a bit budget for a project, but the following was the way I was originally taught, using figures I was originally told. At some point, someone probably read them in either DVD Demystified or some other user guide…
A single-sided, single layer DVD (also known as a DVD-5) contains 36,096 Megabits of space (taking into account a little bit of headroom). Note that the value is Megabits not Megabytes. We use this measurement as we wish to calculate how many Megabits per second we have to describe our video.
A single-sided, dual layered DVD (DVD-9) contains 65,280 Megabits of space.
The first thing we need to do is add up the durations of all our footage (If you’ve only got a couple of moving menus, we ignore these as we’ll be rounding down figures for safety). Next, we need to convert this duration into seconds.
Let’s say we have 2 hours of footage to go on a DVD-5.
1. 120 minutes x 60 seconds = 7200 seconds of content.
Our audio is at a constant rate, so for example if we have 1 Dolby Digital audio track at 224Kbps:
2. 0.224 Megabits per second multiplied by 7200 seconds = 1612.8 Megabits just for the Audio.
3. 36096 Megabits minus 1612.8 Megabits = 34483.2 Megabits left over for our Video.
4. 34483.2 Megabits divided by 7200 seconds = 4.7893 Megabits per second.
So our average bit rate for our Video should be 4.7Mbps. If we were doing a CBR (Constant Bit Rate) encode that would be it, but you should get far better quality if you do a VBR (Variable Bit Rate) encode, which means that as long as the average across the length of the footage is 4.7Mbps, the encoder can go higher when it needs to compress really tricky video, it then needs to go lower at the easy bits to make savings in the budget.
For a VBR encode, you need to be able to tell your encoder what it’s peak or maximum bit rate is. The DVD VIDEO specification says that your combined Video and Audio cannot go any higher than 9.8Mbps, in which case your maximum Video bit rate would be 9.8Mbps minus your audio (0.224 Mbps) giving you 9.576Mbps for this example. Personally, I would never go above 9Mbps, and on a lot of titles will set my maximum to 8.5Mbps. The fact of the matter is you don’t know what kind of player you will be playing your disc back on, and some cheaper / older players might struggle if you max out your video encoding. Far better to have a safety margin than an issue with a title, because quoting the specification will fall on deaf ears at that point…
Remember that the data rate of your audio will vary depending on how many audio tracks you include, and what type of audio they produce. A number of encoders produce PCM audio by default which in an uncompressed state can take the best part of 1.7Mbps of space. Had we used PCM audio on the above job we would have given you an average video bit rate of 3.3Mbps.