So, in my previous article I spoke about how to calculate the bit rate for Video and Audio to get the most out of the space afforded to you on your single or dual layered DVD.
But what exactly does this data rate equate to in terms of picture quality?
The first thing to remember is that no matter what bit rate you give your footage, it will never look better than the source – the saying goes “garbage in, garbage out”, although sometimes more colourful terms are used. If you have shot your footage on a low quality camcorder, or worse VHS, then it doesn’t matter if you give it 9Mbps on a dual layered disc, it’s never going to look as something shot on a professional tape format or, better still, film. Ultimately, the best you can hope to achieve is an encode that is “transparent to master”.
The main strength in MPEG-2 compression for DVD comes from it’s use of Interpicture Compression, that is the differences between each picture. Because of the way MPEG-2 compression works, only every 12th or 15th picture is stored as an entire image. For the remaining pictures, only the differences between each image are stored.
In MPEG terms, you have what is known as a GOP or Group of Pictures:
I B B P B B P B B P B B P B B
The ‘I’ Frame is your only ‘whole’ picture of information, and provides your random access points (chapters etc). The remaining frames only contain information about the pixels that have changed.
The ‘P’ Frame is a ‘predicted’ picture, based on information from previous I and P frames.
The ‘B’ Frame is a ‘bi-directionally predicted’ picture, based on past and future frames.
Without going into too much detail, the ease of encoding your footage is down to the content – if there is a lot of fast or erratic motion – fire, water, smoke, hand held footage etc – then it is harder to compress as the changes between each picture are so much bigger and less predictable. If your footage is ’slower’ and ‘more sedate’ such as talking heads and landscapes then the changes between each frame are far less, and more predictable and so are easier to compress.
Taking this into account, if you have a low bit rate, then you are more likely to see ‘compression artifacts’ is hard to compress footage.
So, when you look at your footage and you look at your bit rate, think to yourself:
• Talking heads, not much motion at 4-5+ Mbps – I should be ok.
• Lots of fast moving action at 4-5 Mbps – That could look compresed.
• Lost of fast moving action at 6-7+Mbps – I should be ok.
• Anything less than 4Mbps – I might be trying to get too much on my DVD…
Tags: bit rate, DVD, DVD authoring, DVD Replication
