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Bit rate Vs Sampling Rate


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#1 virendrakhanna

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Posted 26 March 2010 - 04:00 AM

Hi Friends,

I am not sure about is this right place to ask a simple (may be silly also...) question.

I am fighting between a lot of talk of higher bit rates that is 35 mbps,85 mbps and so on. list goes now 880 mbps where everybody seem to prefering for higer bit rate.

My question is does bit rate is playing important role in quality rather then sampling rate. People are not talking of 4:4:4 / 4:2:2 or even 4:2:0/4:1:1.

My feeling is sampling rate is important then bit rate for quality. Bit rate makes things faster but it is just a transfer rate, nothing with realtion to quality.

Please put light on this.
(I may be silly asking this question, but I am confused a lot)

Virendra Khanna
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New Delhi
INDIA
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#2 David Mullen ASC

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Posted 26 March 2010 - 10:25 AM

It depends, I don't think the difference between 4:2:2 versus 4:4:4 is all that extreme, you don't always need that extra resolution in the blue and red channels except for chromakey shots.

I think overall compression is a bigger issue within reason. I mean, in an extreme example, is uncompressed 4:1:1 or 4:2:0 better than horribly compressed 4:4:4? I don't know, probably is shot dependent. But in general, I'd rather have a 4:2:2 image with mild compression than a really compressed 4:4:4 image. But I'd avoid 4:1:1 or 4:2:0 whenever I can.
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#3 John Sprung

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Posted 26 March 2010 - 02:57 PM

Ah, the old 4:2:2 thing again.... All this stuff exists because of a couple things that the human visual system does. We see luminance with a lot more resolution than we see color. Better yet, given sharp luminance and fuzzy color, our brains automatically fit the color to the luminance -- up to a point. If the color wanders too far away, we see it.

For chroma key, we need full color resolution on the blue or green screen stuff, because we're using the color data to decide where to cut the matte lines. The BG that gets matted into the holes can originate in pretty much any color sampling structure. It's only the blue/green screen stuff that needs to be shot 4:4:4.

4:2:2 itself is an analog TV dinosaur. Back in component analog, luminance was given a bandwidth four times the color subcarrier frequency, while the color differences were each given twice the subcarrier frequency. That's where the numbers came from, and they really stuck. In the digital version, they're taken to mean that for four luminance samples along a scan line, we have two samples for each of the color differences.

Then along came memory. When we got to where a frame's worth of memory could be built into a camera rather than a big rack in the machine room of a post house, the constraint to adjacent samples along a line went away. Now we could look both horizontally and vertically. We could use the same color differences for two lines. This lead to a notation mutation: to indicate that color differences were taken every other line, people started using "4:2:0". In the old understanding, that would have meant that the blue record was completely discarded, which obviously wouldn't work at all.

A good thing, unfortunately, got done in a lot of bad ways. In theory , 4:2:0 should look exactly as good as 4:2:2, and use 25% fewer bits. But in practice, it's used in low end stuff, and got a bad reputation.

As for 4:2:2, now that we have memory, we should forget about it.




-- J.S.
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