[Ardour-Users] "Extract LTC from audio and align video" menu option missing from "Transcode/Import Video file" popup

Chris Caudle chris at chriscaudle.org
Fri Apr 6 06:37:54 PDT 2018

> "Chris Caudle" <chris at chriscaudle.org> writes:
>> ...sample rate to 48k ...is the standard sample rate
>> for essentially all video formats.

On Fri, April 6, 2018 3:27 am, David Kastrup wrote:
> Which is interesting because 44100Hz was chosen because it had 294*3
> samples per 50Hz (half-)frame, 245*3 samples per 60Hz (half-)frame.

Yes, but that was because the old U-Matic ->black and white<- recorders
ran at exactly 60 fields per second/30 frames per second or 50 fields per
second/25 frames per second.
Once they went color the frame rate changed to 30/1.001 frames per second,
and the nice integer relation for black and white was no longer so nice.

The AES11 specification on recommended sample rates has a nice table in
section 4.5 laying out how samples pack into various frame rates. 
Hopefully this will format correct with various different font spacings if
I use tabs.  If not just pretend it lines up into a nice table:

sample rate	samples per video frame
		24fps		25FPS		30fps mono	30/1.001fps NTSC
32k		4000/3		1280		3200/3		16016/15
48k		2000		1920		1600		8008/5
44.1k		3675/2		1764		1470		147147/100

Well, that doesn't even look right on my mail editor when copying over
from my text editor.
But you get the idea, 48k has integer number of samples per video frame
for the historical 24fps film rate, the 25fps PAL rate, the 30fps black
and white rate, and is fairly simple integer divisor for NTSC.  44.1k does
not pack nicely into film 24fps, and gets into a nasty ratio for NTSC. 
Plus you have that ridiculously narrow transition band if you want your
anti-alias filters to go all the way to 20k Hz or very close to it.  48k
seems like a better choice for everything except the case where you want
to fit 70 minutes of audio on a 120mm disk  and you happen to have a bunch
of old monochrome records available to use as intermediate storage.

> But of course, with separately coded/packaged video and audio streams
> (and video being no longer strongly associated with PAL and NTSC formats
> and resolutions), the interest in clean ratios has declined.

You still have to figure out how to line up the  audio and video frames on
playback, and in production environments you have to generate audio sample
clocks and video frame lock clocks that are synchronized, so I think the
simpler ratios still make that easier to implement.

Chris Caudle

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