[Richard Dobson]

Radium wrote:
On Aug 19, 6:08 pm, Jerry Avins wrote:


Radium wrote:


This would be a start if I want to decrease the frequency of a video
signal without decreasing the playback speed.


Various compression schemes do that with varying degrees of resulting
quality.


I am talking about:

1. Decreasing the temporal frequency of the video signal without low-
pass filtering or decreasing the playback speed - an example of which
would be decreasing the rate at which a bird [in the movie] flaps its
wings. Hummingbirds flap their wings too fast for the human eye to
see. So the flap-rate of the wings could be decreased until the
flapping is visible to the human eye - without decreasing the playback
speed of the video. This decrease in flap-rate without slowing
playback is visually-analogous to decreasing the pitch of a recorded
sound without decreasing the playback speed. In this case, low-pass
filter would involve attenuating rapidly-changing images while
amplifying slowly-changing images -- I don't want this.


I confess I am jumping into a thread having just discovered it.

There are some mixed metaphors here. There is a video equivalent to
audio pitch shifting. think of the latter represetned in the frequency
domain (spectrum) - the peak correspindsing to the source partial moves
down (or up). the video equivalent is colour cycling or shifting. But
most simply, reds would be shifted to orange, green shifted to blue,
violet to ultra-violet (and hence llost to view). An alternatyive
stratgy is colour rotation using the artists colour wheel, where,
ideally, diametrically opposite colours are complementary. There is no
equivalent that I know of to colour complemenariness in audio.

I ~think~ I get what Radium wants - he wants to be able to modify a
recorded scene the way one can modify a CGI virtual scene, e.g. by
setting a slower wing flapping rate while leaving other parts of the
scene unchanged. As far as I know, computer vision and scene analysis is
nowhere near being able to do this. The only audio parallel I can think
of is wanting to pitch shift just one instrument in a polyphonic
texture, leaving other voices unchanged. With luck, some implementations
of Blind Source Separation can sometimes do this (they need the mixed
sounds to be very distinct - I have seen one example demonstrated at
DaFX); ths difficulties with video I would expect to be order of
magnitude greater.


Richard Dobson