[opendtv] Re: opendtv Digest V7 #147
- From: "Tom McMahon" <tlm@xxxxxxxxxx>
- To: "OpenDTV" <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 20 Jun 2010 00:46:27 +0000
Convolve all of what you say across all ages and culturers.
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-----Original Message-----
From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
Date: Sat, 19 Jun 2010 19:15:14
To: <opendtv@xxxxxxxxxxxxx>
Subject: [opendtv] Re: opendtv Digest V7 #147
M. Olivier HOUOT wrote:
> In audio, the samples are held and low pass
> filtered in the player. The output of the player is
> a signal that is mathematically identical to the
> original physical sound, at least within the limits
> of the 20 khz bandwidth designed in the system
> (neglecting any defects in the construction of the
> player, of course).
True, but it doesn't have to be that way. There were some of the early CD
players that oversampled at much higher rates than just 4X, and they got by
with no lowpass filter. Partly, of course, it was because the electronics of
the players themselves wouldn't reproduce the overtones.
I think that human hearing and vision are both sampled systems. If the sampling
rates used in the audio or video medium are adequately high, you don't need the
decoding to take place in the playback hardware, outside the human head? I
think that's the case for film.
> In cinema, the projector holds the sample (the
> individual frame) for 1/24 th of a second minus the
> time needed to move film, a black gap that is filled
> by POV (not needed in digital cinema).
Maybe we're saying the same thing, but I would describe it as the human is
doing the low-pass filtering of the 48 samples per second (I think each frame
is projected at least twice), and that the black gaps are ignored as they have
no information. If the movie were actually projected at 24 fps, or less, the
human would not be able to ignore the black gaps or the jumps between images,
and would perceive distinct images (judder). So my take on this is, persistance
of vision is the brain's sample and hold process.
> The low pass filter is the brain slow processing
> speed. Seen from behind this filter, the sampled
> and held output of the projector and the original
> continuous motion would appear as mathematically
> identical, at least within the 12 Hz banwidth that
> seem to be the limit of brain motion processing
> speed (if we accept that 24 frames per second is a
> good enough sampling rate).
It's not that simple, I don't think. The update rate required by human vision
seems to vary, depending on the speed of the motion.
Here's a simple experiment. Look at the second hand of a normal old fashioned
mechanical watch. You can see that the motion is not smooth. The second hand
moves at 5 updates per second and the updates are easily visible.
Then look at the second hand of a mechanical Rolex. They are updated at 10 Hz.
The motion appears almost smooth, with an occasional slight glitch, as if it's
right on the edge of what can be perceived.
But if the hand moves faster than a watch's second hand, it seems to require
higher update rates to achieve a smooth-looking display. Sometimes as high as
32 or 64 Hz, depending on the instrument. In movies too, I think it takes a lot
of cinematographer skill to depict fast motion without uncomfortable judder.
Enfin bref, I THINK that even if you interpret these effects as simple sampling
theory, the brain seems to react differently to different stimuli. My bet is,
the brain is smart enough to spend a lot less processing time on easy
situations (e.g. low sampling rate), and to assign more processing power to
difficult cases.
Bert
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