[isw]

In article . com,
"Ron N." wrote:

On Aug 22, 8:31 pm, isw wrote:
In article om,
"Ron N." wrote:

MPEG-2 video compression already does a coarse
equivalent of time-domain pitch shifting via motion
estimation and compensation, e.g. it throws away
whole frames of video and repeats the spatial
components from previous frames, sometimes skipping
some new motion (leading to jerky patches of video
playback if the compression rate is lower than a
suitable information bandwidth).

Not really. MPEG-2 is frame rate conservative, end-to-end. In fact, the
output frame rate is required by the standard to be *identical* to the
input rate. That has to be the case for it to be able to handle NTSC or
PAL delivered to ordinary TV sets.

MPEG is frame rate conservative, but only the I frames are
actually sent as full images. The P and B frames are made
up out of some duplicated and possibly displaced contents of
other frames, plus some quantized portion of an error vector
depending on the compression rate. Thus the data bandwidth
required for an P and B frames is a fraction of that typically
required for the full image, as contained a nearby I frame.

Yup. Also, some of those frames are sent out of sequence (I or P
"anchor" frames must be present first, in order for the interpolated
frames to be recreated), but every frame has a representation of some
sort in the stream, every frame gets put in its proper place by the
decoder, and no frames are skipped.

Some pitch-shifters or time-stretchers also duplicate and
blend preceding and following periods of waveforms or
spectral frame contents.

Yes again. The difference is that with MPEG video the "duplicating and
blending" has zero effect on the frame rate (i.e., the temporal
resolution).

There is an interesting sort-of exception to frame rate conservation,
when film source is encoded at 24 FPS (actually about 23.98) and the
decoder performs 3-2 pulldown to deliver the NTSC-required 29.97 FPS,
but that's not germane to this discussion.

Isaac