DVD Quality, Less is More

By | 26th August 2011

When you make a DVD there are important artistic choices to be made about the content, menu design, navigation, first play item etc. Once you have all that in place you need to turn your attention to technical decisions.

Quality -v- quantity

One of the most important technical decisions is setting the target data rate for the MPEG encoding. The data rate value has an impact on playback quality. A higher data rate gives higher quality encoding providing that you stick within the guidelines for DVD authoring.
 DVDs can store a fixed amount of data. 4.38GB for single layer and 7.96GB for dual layer. A single layer disc can hold around two hours of reasonably good quality video. If you increase the target data rate to the theoretical DVD Video maximum of 9.8Mbit/s then the same disc will only hold about an hour of footage.

Too much of a good thing….

It’s very tempting, especially with short form content, to set the data rate (quality) to the highest setting. Some authoring software will make that decision for you if you do not over-ride the automatic options.

Procoder grab

If the combined data rate of the video and audio stream exceeds 9.8Mbit/s, even for a very short period, there is a good chance that there will be playback issues such as momentary pauses or stuttering as the player attempts to keep up.

Some poorly specified DVD players cannot sustain 9.8Mbit/s. Almost all players can manage 8Mbit/s.

Aim lower

The best way to avoid these risks is to set the data rate manually. Most good quality encoding software will let you have some control over the parameters. We recommend setting the video data rate no higher than 7Mbit/s and using AC3 digital audio at 224kbit/s. This will ensure that you have plenty of headroom without any risk of peaking close to the limit.

90% is enough

DVD Video discs are written from the centre towards the outside edge. Some players have problems reading data from the outer edge of the disc. Tests have shown that media quality and the “edge effect” account for the majority of DVD Video playback problems.

For this reason it is generally recommended that you should avoid filling any DVD much beyond 90% of it’s capacity. When space is an issue use a bitrate calculator to work out the optimal data-rate for the space available.

Divide and conquer to fit more on a DVD

You don’t need to use the same data rate for all content on a DVD Video disc. If there are less demanding chapters or sections (like extras) then you can apply different encoding strategies to increase the quality of the most important content.

Let’s take the example of a two hour DVD comprising of a 60 minute feature plus 60 minutes of extras. If we apply the 90% rule for a 4.38GB DVD then we have around 4GB (90% of capacity) to play with. 4GB divided by 120 minutes gives us a data rate of 4.3Mbit/s.

Once again a bitrate calculator helps to work out exactly what you can do to make optimal use of the available space. If you encode the main feature at a more healthy 5.5Mbit/s it will occupy around 2.3GB on the disc. The extras can be encoded at frugal but good-enough 3.6Mbit/s to take up take the remaining 1.7GB.

Why a 4.7GB DVD only holds 4.38GB

A standard single layer DVD-R disc holds 4,700,000 bytes of data.
Marketing people like to say that this is 4.7GB because it’s bigger and therefore sounds better.
4.7GB is a straight decimal division using 1000MB per GB. However, computers work in binary (divide by 1024 rather than 1000). This is why the capacity of a DVD-R is 4.38GB.

Resources

Bitrate calc at Videohelp.com does all you need for DVDs (requires Java).

Data rate article on Wikipedia with lots of useful information.

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