[opendtv] Re: Frames Per Second of 720P

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 24 Jul 2008 09:49:09 -0400

At 10:57 AM -0400 7/23/08, Manfredi, Albert E wrote:
 > You might conclude this, but you would be wrong.

Agreed. That was more or less rhetorical. I was merely suggesting that
using 60i instead of 30i has historical precedent. TV always used the
field rate in the past, so why should we suddenly become confused today?
Field rate and frame rate are the same for progressive, but not for

Don't confuse matters here Bert. THERE ARE NO FIELDS IN PROGRESSIVE.

Interlacing video fields is a form of video compression.

The fact that we referred to the NTSC/PAL/SECAM FIELD RATE rather than FRAME RATE made sense in an interlaced world. There really is no such thing as a FRAME in interlaced video since any combination of fields contains information from two temporal sampling periods.

This is even true for 24P film that is sampled into an interlaced system. While it is possible to pull the two fields out of the 3:2 sequence that represent a film frame, you do not get that frame back. You get a frame with reduced vertical resolution because of the filtering required to prevent interlace artifacts.

Things changed when we were able to create digital frame stores that could grab still images from video streams. Now we have the problem of deinterlacing to create a frame with information from only one sampling period. This then spilled over into the world of digital video compression, which works well with coherent frames, but chokes on interlace.

The fix for MPEG-2 is to treat each field as a frame when there is motion - if the motion is low, then two fields can be combined to create a frame for encoding, but we still have issues with the color difference signals since there is no de-interlacing in the MPEG-2 algorithm, and 4:2:0 emission encoding requires reduction of the color difference signals in both the H & V domains.

This is why I suggested that we correctly refer to the actual vertical resolution in an interlaced video stream.

1920 x 540 at 60i provides the most accurate description of what is really going on.

Now that CRTs are rapidly leaving the scene, and that progressive HDTV
cameras have become available, I don't see too much reason to continue
with interlaced. But again, it never seemed like an issue to me, because
all DTV standards out there can do just fine without interlacing. All
DTV receivers fielded to date can do just fine without interlaced
formats too. So this is not a big deal, as far as I can tell. At most,
production facilities would have to go through some sort of migration,
but that's it. And production facilities can migrate as they replace
older equipment, switching over at their own pace, no need for any major
industry-wide coordinated effort.

Tell this to all of the people who claim we MUST HAVE 1080 lines. 1080i is not going away any time soon. The main reason?

Millions of deployed decoders do not support 1080@xxxxx/60P.

Most of the world has let go of interlace. Unfortunately there is one relatively small group that has not. They are called broadcasters, and they do not wish to make the investment to switch from 59.94i to 60P. Can't blame them, as Bert is right about one thing. They can make the ATSC standard last until what we now call broadcasting is thrown onto the scrap heap of history.

By the way, modern production equipment could care less about the frame rate - just a setting in the preferences. IT is the broadcast facility that is hopelessly locked into 59.94i.


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