|Creating DVDs: Learning about Bit Rates and what they mean|
A Creative COW DVD Tutorial
Dynamic Media Group
DVD authoring can be daunting when you consider everything that is required to create professional results, and simple, consumer based marketing the likes of Apple's Shoot-Edit-Burn campaign won't tell you the pitfalls. In this article, David Chandler-Gick explains the concept of 'bit rates' for people wanting to learn about creating dvds.
So... You wanna make DVD's huh?
Think you're ready?
Yeah... I thought so too. Then I learned a valuable lesson: I wasn't.
DVD authoring can be daunting when you consider everything that is required to create professional results, and simple, consumer based marketing the likes of Apple's Shoot-Edit-Burn campaign won't tell you the pitfalls.
Like, let's say, a little thing called "encoding."
Something to consider is the Bit Rate... One of the reasons that I cannot stand the idea of the set-top DVD recorders for professional duplication / one off use is that they encode at fixed rates (Constant Bit Rate - CBR). For example, if I have a program that is 78 minutes, I must encode at the 2 hour setting under CBR.
That's like buying a 2 hour tape, except in this case, the result isn't just extra blank at the end, it's lower quality of the image and audio. (Rumor has it that a new model is coming with tighter adjustment. I'm just not aware of any yet.)
With Variable Bit Rate encoding (VBR), you can adjust the encoding of 78 minutes so it uses the best quality possible for the estimated space allowed (4.7gb (give or take))
A dual pass VBR encode allows you the added benefit of getting even better quality out of your material.
The way this works is that it scans your program and reads the data and determines what the bit rates should be based on the content.
For example, lets say you have a file size limitation of 10 for a 60 second spot. This is all rhetorical so work with me here. 10 is just a mythical number. A 30 second scene that is fast moving with a lot of detail and color changes may require a bit rate of 7 for an optimum image, but a solid black screen for 30 seconds can be encoded at a rate of 3 to achieve the desired results. That being the case, to encode a black screen at 7 is overkill. Correct?
In a single pass system, it uses a fixed bit rate based on your file size limits (10) - Well, 7 and 7 is 14... So, you have to compromise and encode each at 5 (5+5=10)
Now, in a dual pass system, it looks at the action scene, looks at the black still, determines that a data rate of 3 is enough to convey the black screen as intended, therefore leaving you 7 for the action scene. Result? Better quality overall...
I hope so.
According to Tyler Hawes, Creative Cow DVD Forum Moderator, better results can be expected with CBR as long as the data rate is upwards of 6MBps or better... Typically, this only allows you the space to fit about 90 minutes on a single DVD-R.
When you exceed that, then you'll have to rely on VBR.
However, there are advantages and disadvantages to VBR. For good clean results, a dual pass system is ideal, with a hardware encoder preferred over a software encoder. The biggest drawback is that this is expensive. $5k and up expensive... Another drawback, especially with software encoders, is they may not be 100% reliable.
Still understand? Good.
Okay. Now that you understand the above, know this: That's not the way it works at all... It's just the easiest way to convey the concept. So, when you go over to the Cow's DVD forum and you start hearing all about this and it looks nothing like what I've described, you'll know why.
Backyard Productions / Dynamic Media Group
Creative Cow Event Video Forum Host
Special Thanx to Creative Cow DVD Authoring Forum Gurus, Noah Kadner and Tyler Hawes, for their help in compiling this article.
Article © 2002 by David Chandler-Gick. This Publication © 2002 CreativeCOW.net. All rights reserved.
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