[opendtv] Re: Case for 720p60
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 21 Apr 2010 09:37:31 -0400
At 8:58 AM -0400 4/20/10, John Shutt wrote:
----- Original Message ----- From: "Craig Birkmaier" <craig@xxxxxxxxx>
In ATSC-land, where there are no MPEG-2 MP at ML restrictions, in
principle at least, broadcasters could begin to transmit SDTV as
480p24 or 480p60. So why don't they?
The only 480p format that could be used is square pixel 640 x 480
with a 4:3 aspect ratio. The ATSC does not support 854 x 480p or
704/720x480p. These formats are supported by MPEG-2 at various
levels, but there is no guarantee that all ATSC receivers will
display them, so broadcasters do not use them.
Craig,
I believe you need to re-read table 6.2, Compression Format
Restraints, contained in A/53 Part 4, 7 August 2009.
True, the ATSC has never supported 854x480P 16:9, but then there is
no production gear that supports this resolution, either. However,
the ATSC does support 704x480 16:9 progressively scanned at 23.976,
24, 29.97, 30, 59.94, and 60 frames per second.
You are correct John. I forgot that they did allow 704 x 480P. Square
pixel widescreen 480P has rarely been used in Broadcast TV; one
Japanese broadcaster (NTV) did use it for several years before HDTV
started to take off.
Another major reason that 854 x 480 did not take off was the short
sighted (or was it defensive) thinking of the folks driving the
ISO/MPEG MPEG-2 standard. I tend to believe it was a defensive tactic
because 854 x 480 looks ALMOST as good as HDTV. In their infinite
wisdom, however, the folks who drove the MPEG-2 standard set the
maximum pixel rate for Main Profile at Main Level (MP@ML) at about
10.4 Mpixels/sec, which is what you need for 720 x 480@60P. To
accommodate 854 x 480@60P you need to go to MP@HL, which was designed
for HDTV. I tried for several years to work in the ISO/MPEG process
to create a new Progressive Profile at 30 Mpixels/sec; the profile
would have only used progressive scanning to avoid the massive IP
that was created to support interlace in MPEG-2. Alas, I ran out of
sponsorship money before the work could be finished.
It is worth noting that square pixel 480P is now commonplace for web
video, although many source (including Apple) do not use this number
of horizontal samples to conserve storage bandwidth in their SDTV
offerings - iTunes movies are sold as 720 x up to 480p and 1280 x 720
- both drop lines for wider aspect ratio films; rentals of TV shows
are 640 x 480 with anamorphic squeeze for wide screen sources.
We seriously contemplated broadcasting 480p59.94 16:9 for our
subchannels that derive from HD source content. I even asked for
assistance on this reflector. Our main stumbling block was there is
no defined way to feed that format to our encoders. SDI doesn't
have enough bandwidth for 480p60, and HD-SDI doesn't included
704x480p60. Further, we tried Miranda, Ensemble Designs, and Ross,
but could find no vendor capable of supplying a 1080i30 or 720p60 to
480p60 downconverter. All downconverters wanted to output 480i30.
Yup. More confirmation that the industry was still trying to protect
interlaced SDTV, especially their investment in"601 Digital" which
begot SDI.
At the dawn of ATSC, Fox stood alone in choosing 480p59.94 for
broadcast, but they soon switched to 720p, probably because they had
the same problem finding suitable equipment for the format, and they
were losing the "genuine HD" P.R. war.
More the latter. There were solutions for 480P, which Fox was using.
Regards
Craig
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