Squeeeeeeezing More Video onto a DVD - Part One

[Marc Peters]

The art of DVD encoding - Part One

Part One - Background

Following a trend of minimalist video editing applications, lower end DVD authoring packages produce DVDs with the minimum of user input. Although fantastic for the beginner, this design feature can to take far too much away from the more demanding end user. Two cases in point are opposite ends of an extreme: the user wanting to create the best possible image quality; and the user aiming to cram in as much video as possible on to a single DVD.

Despite these limitations, DVD authoring applications combined with more powerful MPEG2 encoders give far more flexibilty to the budding videographer. Be warned that your DVD authoring application should be capable of importing MPEGs – and author the DVD without re-encoding your expertly encoded video. This issue is addressed later with a list of applications meeting this criteria.

Before explaining the process of DVD creation it's worth explaining the fundamentals of DVDs. The typical commercial DVD contains over two hours of high quality video on a dual layered DVD. You may have noticed a slight "jump" in the middle of a film - this is where the laser jumps from reading the first layer of data to a second. Alas, this method is not available to the home user and DVDs created on home PCs are limited to the physical data limit of a single layered DVD (approximately 4.37Gb).

Update: Dual Layer DVDs are now widely available, but require a compatible dual layer burner to burn a Dual Layer disc. You can therefore fit two hours of maximum quality on a dual layer DVD.

So how does this affect the amount of video on a DVD? Well, video encoding coverts the picture into data and then compresses this data by using a technique of comparing one frame of video (frame A) to the next (frame B), discarding any data duplicated in frame B. More specifically, MPEG files have a "Group of Pictures" (GOP) structure, consisting of complete (intra) frames "I", predicted frames "P", and bidirectional frames "B". Predicted frames are based on past frames, bidirectional based on past and future frames. Secondly, the amount of data consumed every second by is known as the video bitrate: the higher the bitrate, the more data per second and the higher the quality - you get less video on a DVD the higher the quality of the picture.

The maximum bitrate under the DVD standard is 9800kbits/sec (this includes both video and audio). It’s important to adhere to this standard to ensure the created DVD plays in standalone players. At this constant maximum bitrate, you’ll get 60mins of video on your DVD. As we know, the lower the bitrate, the lower the image quality but the more video we can fit on a DVD. So if we reduce the bitrate by half, we can get 120mins of video on a disc. A maximum Constant or Variable bitate should be used to encode videos less than 60mins long - this will maximise video quality by ensuring no space is wasted on the DVD.

Notice the use of the word “constant” above. This is important: an alternative to constant bitrate (CBR) is variable bitrate (VBR). When you’re encoding more than 60mins, 2-pass VBR should be the preferred alternative. The encoder runs through the video twice, the first “pass” analyses the data, the second actually encodes the video, using a higher bitrate in scenes with high motion. This will then “smooth” your video and should achieve better quality video compared to CBR for videos greater than 60mins.

If you do the maths, you’ll see that these bitrates don’t quite add up – and that’s because the audio is treated separately to the video. Another DVD standard is PCM audio, this is uncompressed audio with a datarate of 1536kbits/sec. This may seem high – and it is. Although not part of the DVD standard, you can compress the audio to squeeze more video onto your disc! If we use MPEG layer II audio at a bitrate of 224kbits/sec, we can either slightly increase the video bitrate or fit an extra 12 more minutes of video onto the disc. AC3 audio would work in a similar vain.

So there you have it: we can alter the constant bitrate of the video to control either quality and the amount of video we fit on a DVD. We can also use a variable bitrate to maximise overall quality when using a lower average bitrate and we can then compress the audio to further control the amount of video we can fit into a DVD.

Part two: using TMPGenc to encode DVD compliant video…

[fruey]

This is a great introduction, leaving us all hungry for the second part.

If I could be so bold as to make a couple of comments, which I humbly hope will serve to enrich the excellent stuff above.

Not all encoders are created equal. Whilst the MPEG-2 format is a standard, there are different ways of making decsions about how to predict the next frames based on different motion detection algorithms, which themselves can be based just on luminosity or chrominance (colour) or a combination of both. Sometimes, for the sake of better performance, the length of time it takes to make these decisions is shortened which may sacrifice quality even in constant bitrate encoding situations. The actual settings available are numerous, if you take a look at all the advanced options (this applies especially to MPEG4 encoding, but that's another story for another thread).

The advantage of VBR encoding will depend a little on your encoder too. The better it is at predicting motion and putting this into the first pass cache file, the better the second pass decisions can be made about bitrate allocation. I wholly agree with Marc that two pass is the only way to go. It takes almost twice as long, but the results are well worth it.

Most DVD content I have analysed generally, for one language without subtitles, extra features, etc... will almost fit a 120 minute film on a single layer of the disc. Often the extra data above 4.3 Gig is actually the alternate soundtracks, extra features and subtitles. This shows that content on commercial DVDs is not simply encoded at CBR maximum quality, but rather optimised in the mastering process.

You can do almost everything a professional mastering studio would do for a top quality DVD release, but you may have to resort to separate tools for audio encoding, video compression, and DVD authoring (titles, menus, etc) in order to obtain the best results. The "complete perfect DVD authoring solution" does not necessarily exist as a single package. If you are a stickler for quality then you have a bit of learning to do, but this in itself is a fascinating pursuit. A number of free tools exist, if you have a bit of time to work out how to use them well (it may seem daunting and over complicated to start with). The commercial packages may leave you wanting, especially those that completely hide away advanced options, or which have defaults set to impress you with the speed that they can process your DVD, rather than with the final output quality.

[Marc Peters]

I totally agree with all encoders not being created equal. And it certainly pays to combine the best applications, specialising in different processes, to create the "perfect" DVD.

As for the second part. I'm affraid that my "real world" job has impossed on my hobby recently , so I haven't had the chance to sit down and complete the guide. However, I'm happy for people to add their thoughts on creating the perfect DVD - what applications, settings etc...

[VideoGeekWannabe]

... don't leave us hanging :( We need the rest!!

[Marc Peters]

Only a year and a half late

[VideoGeekWannabe]

Sorry, I think slowwwwwwwwwwly ...

[senu]

excellent write up mr peters ...where is part 2??? its seem to be travelling from your pc to the server at snail speed :(

[rpwall]

Excellent summary Marc- from your article I therefore assume that in order to maintain maximum quality, when burning to a single layer DVD the source material should not be longer than 60 mins. Above that and the quality drops.
I shall be burning using Adobe Premier Elements 2.

Thx Rich

[zeegurl]

We have video created with Adobe Premiere that we are placing onto an
interactive DVD (using Director). The files are being saved as
Quicktime and therefore we cannot predict the smoothness of playback
on any given laptop via the DVD player (they work seamlessly on hard
drives). Our goal is to run smoothly on a T-40 or better.

We have exported the video from Adobe Premiere directly to a DVD and
then copied the files from the DVD back to a hard drive where Director
can access them. What we have not yet found is the code from Director
that will read and play the DVD files via the interface.

Thank you for any assistance you may be able to provide.

[shaf]

I have recently completed my first video (3hr asian video..considering I had 5 mini DV's, I did well to ruduce the final video to 3hrs!)...I must admit, the most difficult stage is actually creating the DVD. I used Encore CS3 for this particular project...at first I attempted to export to encore from PPro CS3...this was a lengthy process. So what I did was to export movie (AVI), then imported the AVI video into encore, set the chapter markers, built an image of the DVD in a specific folder (.ISO file) and finally burnt the image on DVD using Nero...the quality is fantastic. I split the project onto 2 DVD's to be safe for the sake of quality (120min on disk 1 & 60min on disk 2). Really please with the end result