Archive for April, 2009
Video bitrate too high
I have lost count of the number of times this particular error message has been reported by authors using DVD Studio Pro, and how confusing it is to resolve.
By and large, the error reporting in Apple applications is fairly informative for most people, but like any other software there are some reports which just don’t make much sense, or inform you of what to do to fix the problem. Naturaly, when you see that your video bitrate is too high, you attempt to lower it and get stumped when no matter how low you go it still causes an error.
The maximum combined bitrate on a standard definition DVD, including all video, audio graphics and additional content, cannot exceed 10.08MBPS. The maximum rate for video alone is 9.8MBPS. There is no minimum bitrate above zero, as this is for you, the author, to decide given your footage and how it looks when it is encoded. However, the reduction of the video bitrate will also reduce your visual quality, and to an extent, raising the bitrate will help improve the visual quality (although there is a law of diminishing returns that you will get to). The lower the bitrate, the smaller the resulting file size and thus the more footage you can fit onto your disc. Raising the bitrate increases the file size and consequently you can fit less minutes onto your disc.
The trick is to balance the bitrate with the disc space available to get the best quality - in other words, to hit the ’sweet spot’. Since video is far more forgiving to compress, or rather your eyes are far more forgiving than your ears, it’s better to compromise on the video quality a little than it is the audio quality.
However, this is where the error message is misleading. Instead of decreasing your video bitrate, try re-encoding your audio. Yes, that’s right… re-encode your audio and convert it to AC3 (Dolby Digital) format. This is a much more efficient compression than you might realise, with the resulting files taking up a tenth of the space of the original, yet with almost no loss in audio quality. The bitrate for a stero based file can be as low as 192KBPS (yes, that’s kilobits), and you’ll still get a really good audio experience from that.
The best part is that the difference between AIFF and AC3 is that the bitrate of AIFF is around 1.5MBPS and AC3 can be vastly lower. In one step you’ll reduce the overall bitrate for the disc and give yourself more room for the video. You’ll also find your recorded discs are generally more compatible with a wider range of set top players, too.
There is no single bitrate setting that fits every situation, and each time you create digital footage to include on a DVD you need to look at it with a view to encoding it in the best way. The visual aspect of a disc is often the one that takes precedence for most people, yet it is the audio that will make or break your disc as an experience for the viewer.
When you make an AC3 file in Apple’s ‘Compressor’ you will find there is one setting that you have to change from default, and that is the dialog normalization value. It defaults to -27 but really ought to be set at -31. There are several really good reasons for this, not least of which is that you will almost certainly have spent time balancing the audio levels in your editor before encoding the video. If you don’t want any of that level setting to change for your audio, use -31.
So, the next time you see the dreaded ‘video bitrate too high’ error message, don’t change the video setting until you have looked at the audio more closely!
