[opendtv] Re: 625 video quality is good enough....
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 20 Oct 2004 10:37:52 -0400
At 4:45 PM +0100 10/19/04, Alan Roberts wrote:
>Don't teach me to suck eggs, these are exactly the points I've been
>hammering away at since 1987. Except your comment about frequency content.
>You'll find that the current crop of cameras will happily produce content at
>30MHz, I've measured it many times. The limit isn't the camera, it's the
>lens, put a Zeiss prime on an HD camera and you'll easily get 30MHz. If
>you're not seeing anything above 22-24MHz it's either because of filters of
>poor lenses, don't blame the cameras.
>
Yes, I know that you have been one of the objective participants in
this debate.
;-)
As for frequency content, I agree that the lens is a factor. perhaps
I did not state my position clearly enough.
There is no doubt, that under the right conditions you may find real
picture information in the frequencies all the way out to 30 MHz. The
problem is the other stuff that you will find above 22-24 MHz.
First, we must take into consideration MTF. Because these cameras are
NOT oversampling, the MTF at higher frequencies is WAY down. Any real
detail will be at very low contrast levels. When you view this
through the noise produced by the camera at these higher frequencies,
you might be able to perceive some detail on a good display.
Unfortunately, the consumer will never see this for two reasons.
First, most cameras do use roll-off filters that typically start
around 22-24 MHz, eliminating any info at frequencies above 25 MHz.
Sony does this with HD Cam, resampling to 1440 horizontal samples per
line prior to compression. The major reason they state for this is
the high levels of noise in the information above 22 MHz.
Second, There's no chance that these frequencies are going to make it
through an MPEG-2 emission encoder, except perhaps in the case where
the camera is locked down on a test chart. The noise reduction and
other pre-processing techniques used in current generation encoders
will in most cases roll off the high frequency detail prior to
encoding.
Let's talk about high frequency detail in HD when we get real
oversampling cameras with enough sensitivity to raise the noise floor
at these frequencies to acceptable levels.
Regards
Craig
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