[opendtv] Re: Image quality
- From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 13 Nov 2005 15:58:11 -0500
Tom Barry wrote:
>I get confused in going back and forth between computer and TV terms so
>I may be saying something stupid here. But in my own playing with
>various sharpening algorithms I always eventually run up against the
>point where sharpening or whatever starts to just amplify quantization
>or other noise. That is, it works well for high bit precise source but
>not very well for whatever 8-bit previously compressed source I might be
>playing with. I realize deconvolution is much more complex but the same
>principles likely apply.
>
>
Agreed.
>It appears you equate limited dynamic range with limited MTF at higher
>spatial frequencies. But that limited MTF can come from many sources
>and one of those is probably adjacent pixel averaging caused by your
>optical limitations. And it is that last factor we would try to correct
>if we got the signal in the camera before too much other noise was
>introduced.
>
>
It doesn't matter what reduces the level. Suppose we have an eight-bit
system in which peak white is level 255 and deepest black is level
zero. That's 48 dB of SNR plus any improvement associated with the form
of video. Let's ignore the improvement for the moment. If you need to
recover a white and a black line from half the levels, you've got 42 dB
of SNR to deal with. If a quarter of the levels, 36 dB.
You can add bits at the initial imager A/D to help with this. Many
cameras use 14-bit A/D converters, and there are valuable reasons why
they do so. But, if you're dealing with half of those levels, you're
still 6 dB down. Half of what's left is another 6 dB. Half of that is
another 6 dB, and so on. An MTF of 6.25% gives you 24 dB less SNR to
deal with -- and that was before the lens reduction of MTF. If that's
enough for the processing you want to do, great! If not, you may be
creating a white or black dot that weren't there in the first place.
TTFN,
Mark
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