[mythtv-users] Analog firewire recordings end early due to 25% fewer frames

David Brodbeck gull at gull.us
Wed Feb 4 20:28:43 UTC 2009

On Wed, February 4, 2009 6:35 am, Yeechang Lee wrote:
> David Brodbeck <gull at gull.us> says:
>> If they're really being delivered to the cable box as analog signals,
>> they can't be chopping out frames.  Otherwise an analog TV would fail to
>> sync to them.  NTSC equipment is only designed to run at one frame
>> rate.
> I can't explain what I've seen for the past 15 months in any other
> way. The framerate of the post-September 1997 analog-channel
> recordings upon playback, as well as comparing before and after
> elapsed times, file sizes, etc. are all consistent with an exactly 25%
> cut in the analog signal from the normal 29.97/30fps for 480i as of
> September 1997. The recordings all play at proper speed--just with
> noticeably more fuzziness and less-smooth pans than before--so I don't
> know exactly how the bandwidth throttling is done, but trust me when I
> say that it is obvious visually.

I'm not arguing with what you're seeing, I'm just saying it can't be for
the reason you say it is.  Analog signals are 29.97 or 30 fps, period, or
an analog TV won't sync to them.  I'd be willing to bet if you used an
analog capture card, or plugged an analog TV directly into the cable,
you'd find the analog channels are full framerate.

My guess is your box actually is getting the 2-99 channel signals from a
digital source.  Otherwise it would have to have an MPEG encoder in it
just to provide firewire output on those channels, which seems like a
needless expense.  It very well could be they've cut the frame rate on
that digital stream.

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