How is a Smaller File a Better One?

[happydog500]

For years I've captured in Constant Frame Rate. I read in my manual that if I click "Variable Frame Rate" I will, "generally get a better video."
On constant, an hour video (8000) goes right on a DVD-R. Obviously, if I lower the frame rate, say half at 4000, I get a less quality video. I can see the DVD-R is only half filled up compared to the 8000.
If I use the 8000fps setting with Variable Frame Rate checked, instead of the about 4GB DVD file, I get about a 2GB. When I burn it, I look and see all the space I'm wasting and think of the quality I'm losing.

How is a Variable frame rate better, but a much smaller video size with it? (same frame rate) On the surface, it doesn't make sense, but maybe there is a deeper explanation.

Thank you,
Chris.

[AlanHK]

How is a Variable frame rate better, but a much smaller video size with it? (same frame rate) On the surface, it doesn't make sense, but maybe there is a deeper explanation.

Depends what the "frame rate" setting means.

If it's the maximum, then obviously CBR willl give you more quality.
If it's average, then VBR is better.

However, if you're streaming to a recorder, then it's impossible to set "average rate".
That requires analysis of the entire file, which is why a VBR encode generally takes twice as long.

However (2) there is also "constant quality/variable rate", where you set a target quality and the rate is adjusted continuously to keep that.
That gives excellent results, but unpredictable file sizes: if there is a lot of finely detailed action, like motorsports, it will eat up space.
A sitcom with two people sitting and talking in a dim room much less.

So finally, the "rate" numbers for each of these methods aren't directly comparable in quality, and only the average will tell you the size of the resulting file. So if your variable rate is making a smaller, lower quality, file, increase the quality setting and see what happens. You should be able to hit a setting that gives the best results for the kind of recordings you make.

[edDV]

For years I've captured in Constant Frame Rate. I read in my manual that if I click "Variable Frame Rate" I will, "generally get a better video."

...

"Generally" is the key word.

CBR @ max rate (9570 kbps video + 224 Kbps audo = ~9800 Kbps aka 1 Hr mode) gives the "best" quality. Bit rate is floored for the full hour.

VBR @ lower average bit rates has potential to give better quality than CBR at the same average bit rate.

Lets take for example 90 minutes @ 6548 Kbps. CBR would record everything at 6548 Kbps.

VBR allows you to set average, max and low. Let's assume for this example you set max to 9570 Kbps and low to 1000 Kbps. Then the VBR algorithm will make judgements from the first analysis pass where to apply more or less bit rate based on the motion, contrast, noise or other in the various frames. Then on the second pass, it will allocate bit rate in such a way that it averages to 6548 Kbps thus fitting the DVD5 disc with 90 minutes.

This would have potential for the most demanding scenes to match CBR 1Hr mode for quality. VBR max can never exceed the max DVD bit rate ~9800 Kbps. But in practice, the VBR bit rate may never reach the selected max bit rate for say noisy shaky camcorder video. The more the noise and action, the more the VBR bit rate will be constrained to near the average setting so in that case, quality is much lower than 1 Hr CBR.

Second is the issue of algorithm quality and the bit rate acceleration that happens on slow to fast action transitions. VBR may generate artifacts that wouldn't be present with 1 Hr CBR.

Some VBR encoders only allow you to set the max VBR bit rate. In those cases, average rate will depend on the video analysis. If you spec too high, the file size may exceed the DVD size forcing you to start over.

So for "best" quality, spec CBR 1 Hr. I mostly use 1Hr mode for camcorder recordings. When longer record time is needed, I use double layer discs. I use VBR for TV captures.

How is a Variable frame rate better, but a much smaller video size with it? (same frame rate) On the surface, it doesn't make sense, but maybe there is a deeper explanation.

That would be an example of user setting = max VBR rate.

[happydog500]

Thank you for the replies,

I'm using my computer to record from my TV Tuner Card (http://www.youtube.com/watch?v=3hmkstMD84Y). Every once in a while I find a show I want to keep, so I record it to a DVD. My compro software doesn't have a max and min setting, just CBR and VBR, and the rate setting. Couldn't remember what "rate" to call it.

Chris.

[edDV]

Thank you for the replies,

I'm using my computer to record from my TV Tuner Card (http://www.youtube.com/watch?v=3hmkstMD84Y). Every once in a while I find a show I want to keep, so I record it to a DVD. My compro software doesn't have a max and min setting, just CBR and VBR, and the rate setting. Couldn't remember what "rate" to call it.

Chris.

Don't know that software. But it seems like max CBR is the quality setting.