From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Date: Fri, 7 Dec 2007 16:24:41 -0500
John Shutt wrote:
> Did you see the posting by Russ Wood?
Yes, but I didn't get why 23.976 is not the way this is always done.
Drop either 1 frame on 1001, or 1 frame in 1000, in a 24 fps stream, and
the resulting frame rate will be either
23.976023976023976023976023976024 or 23.976000000, respectively. So why
is this 23.976 frame rate a surprise?
> I'm talking about the extra headaches on the production side that
> those odd framerates cause. 23.976, 29.97, and 59.94 (all
> approximations, as they really are the whole number framerate
> divided by 1.001) are for "NTSC compatibility" and allows us to
> use NTSC blackburst for genlocking of our HD equipment. I admit
> that it makes things a bit easier to have a single reference
> in-house for all video.
I am actually trying to figure out if this is a case of someone being
overly persnickety when they specified the frame rate, to the third
decimal place instead of just the usual two, or whether specifying
23.976 *really* makes a difference.
But my general response on the ATSC oddball rates is only that the
alternative of NOT specifying those oddball frame rates are essentially
two. If ATSC only specified 24, 30, and 60, then any STB that must
convert ATSC to NTSC RF, where video and audio are sent together, would
have to:
1. Drop that one frame within the STB itself, or
2. Eliminate the color subcarrier and only transmit to NTSC RF in B&W.
The same applies to DBS and digital cable. So that's why I think they
are still being carried on in the digital spec.
> As a consumer of content, you should be fully insulated from
> these issues, just as I am insulated from network timestamp
> issues. You are absolutely right that as far as ATSC is
> concerned, it can work with any of those framerates.
True enough. But as an EE, I'd still like to know what the issue really
is.
Bert
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