[opendtv] Re: [oldvtrs] So Much for HDTV]

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 23 Aug 2008 11:03:10 -0400

At 6:11 PM -0400 8/22/08, Manfredi, Albert E wrote:
Right, exactly. Complaining about an artifact that has nothing to do
with MPEG compression is a different matter. What's the "shutter speed"
of a digital HD camera? If it's something on the order of 1/60th of a
second (or so), it's not going to freeze action very well. Which by the
way helps the MPEG process, because all edges are nicely blurred
already, as if prefiltered.

With a still camera, 1/60th of a second shutter speed is typically only
used for stationary subjects. It's usually considered to be the slowest
shutter speed adequate for hand-held photography, not to freeze rapid
motion. To freeze a runner going across the frame, I'd certainly be
looking at 1/400th at least, with a still camera.

Shutter speed and compression are tightly coupled. Virtually all modern cameras have the ability to shoot at higher shutter speeds; this feature is used in the coverage of sporting events all the time. The issue becomes one of creating strobing effects if the shutter speed is too high. You get nice sharp frames/fields, but as you reduce motion blur you start to get into strobing.

And as you sharpen up those frames you add more information for the encoder to deal with. It is difficult to say whether this adds stress, as more detail can help improve motion compensated prediction. But if you are already running in a bit starved mode, more detail is likely to make things worse and most likely will cause the in-loop low pass filters to kick in and remove the extra detail anyway.

I might add that interlace is a HUGE factor here. Shooting 1080i adds stress, simply because of the number of samples that must be encoded, and it also adds stress because of the need to deal with interlace. The same scenes shot in 720P would have less artifacts in a bit starved channel.

Hopefully this will be the last Olympics shot with interlace, but NBC is showing no signs of moving to 720P for high action content.


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