From: "Carl Hetherington" <cth@carlh.net>
To: "Leslie Hartmier" <leslieh1@shaw.ca>
Cc: dvdomatic@carlh.net
Sent: Sunday, April 28, 2013 5:43:04 AM
Subject: Re: [DVD-o-matic] A couple questions...

On Sun, 28 Apr 2013, Leslie Hartmier wrote:

> Good day!   One of the theatres I'm involved with is in need of a
> preview for a special booking we have upcoming, and the only one we had
> access to in a reasonable format was in 60 FPS, but since this preview
> is to be used with a DSS100/DSP100 which does not like anything but 24
> FPS, I was attempting to use DVD-o-matic 0.87 x64 Windows version (I
> have also tried earlier versions before now to the same effect) to
> change it to 24 FPS, and the outcome was that the audio sounded at least
> an octave lower.  As it is important for our managers to be able to do
> this with a minimum of fuss and perhaps as streamlined a process as
> possible, I was wondering if there was something I was doing wrong, or
> if there is a process to follow for such things.  I have found this to
> be an issue for me before now, but I was hoping that perhaps it was
> simply that I have done something in error.
>
> Also, I was noticing that when I'm converting some video into DCP,
> DVD-o-matic will change the audio (for 23.976 FPS or 24 FPS, 44.1kHz is
> adjusted to 48kHz (doesn't seem to sound odd), for 29.97 FPS to 24 FPS,
> 44.1 kHz is adjusted to 59940Hz (becomes a lower octave, which I cannot
> use, 30 FPS to 24 FPS takes the audio from 44.1kHz to 60kHz (can't use),
> and finally 59.94 FPS to 24 FPS changes the audio from 44.1kHz to
> 59940Hz (also cannot use - deeper again) ( I would have thought that the
> 60 FPS would be even higher than 30 FPS).)  Is there a reason that even
> if the video is at the frame rate I want, the sound still gets adjusted,
> and if I'm changing the frame rate, the audio resample goes up, but the
> sound becomes deeper? I would have thought the sound would becoming more
> shrill, if anything.

Hello Leslie,

What DVD-o-matic is doing with respect to the audio is trying to keep it
in sync.  Say for example you have a 29.97fps video which you are playing
at 24fps.  DVD-o-matic does not do any resampling of the video content, so
the projector will end up simply playing the frames from the 29.97fps
video at 24fps.  To be honest, I would expect this to look pretty bad
anyway...!

Since the video is being played slowly, so must the audio, otherwise
it will not stay in sync with the video.  To do this, DVD-o-matic
resamples that audio to a higher sample rate than the projector will use
to play it back.  Then, on playback, the audio will be slow to match the
slow video.

As far as I can see, this is your problem: you have a 60fps video.
DVD-o-matic should decide to play every other frame of it (getting it down
to 30fps without any serious problem) but then if you ask for 24fps output
it will have to play the video and audio slowly so that it hits 24fps.

I think there are two solutions to this problem:

1.  Re-sample (frame-rate-adjust) the video.  I don't think DVD-o-matic is
going to get into this minefield any time soon!

2.  Time-stretch the audio, rather than re-sampling it.  This is easier,
but still an inexact science.

Having said all that, the first question would be: does the video look ok
when you have run it though DVD-o-matic (ignoring the sound)?  Does it not
run much too slowly?

Best regards

Carl