[mythtv-users] tv: tv: Play in reverse (-1x)

David Schmidt david.schmidt.in.dallas at gmail.com
Thu Apr 12 14:09:54 UTC 2007


With some fancy coding, you *might* get better than one frame per
second and no audio, but I doubt on current consumer hardware you'll
get normal frames per second with full audio.

The problem is, the recordings are stored (usually) as MPEG.  MPEG
encodes like this to save space:

Full (compressed) image of frame x.
Differences (only) between frames x and x+1.
Differences between frames x+1 and x+2.
Differences between frames x+2 and x+2.
.
.
.
[you get the idea]
.
.
.
Full (compressed) image of frame y.
Differences between frames y and y+1.
.
.
.


So, in real time, Myth would need to:

- Read, decompress and display frame y (the easy part)
- Before time for the next frame (let's presume 1/30th second) and
decompress frame x, then apply all the differences for x+1, x+2, x+3,
x+4 ... x+29 to get to the image for x+29 which is also y-1 and
display the frame.   (The likely currently undoable on consumer
equipment of today part).
- Before time for the next frame, reverse apply the differences
between frames x+29 and x+28 and display (the possibly tricky but
probably doable part).


That's the technical problem.  I'm sure someone will correct me if
it's doable and just no one's done it yet....

On 4/12/07, David Fishburn <fishburn at ianywhere.com> wrote:
>
>
> > -----Original Message-----
> > From: mythtv-users-bounces at mythtv.org
> > [mailto:mythtv-users-bounces at mythtv.org] On Behalf Of David Fishburn
> > Sent: Friday, April 06, 2007 3:44 PM
> > To: 'Discussion about mythtv'
> > Subject: Re: tv: tv: Play in reverse (-1x)
> >
> >
> >
> > > -----Original Message-----
> > > From: mythtv-users-bounces at mythtv.org
> > > [mailto:mythtv-users-bounces at mythtv.org] On Behalf Of
> > Michael T. Dean
> > > Sent: Friday, April 06, 2007 8:59 AM
> > > To: Discussion about mythtv
> > > Subject: Re: tv: Play in reverse (-1x)
> > >
> > > On 04/05/2007 08:52 PM, David Fishburn wrote:
> > > > I found this web page:
> > > > http://www.mythtv.org/wiki/index.php/Telnet_socket
> > > >
> > > > Which indicates:
> > > > play speed -1x        - Playback at normal speed in reverse
> > > >
> > > >
> > > > Which is exactly what I want to do, except I can't figure
> > > out how to do it.
> > > >
> > > > I looked at the keybindings in MythWeb, using "a", "j",
> > "u", I can
> > > > modify forward time stretch but I can't figure out how to
> > get it in
> > > > reverse.  If I slow it down it eventually pauses instead of
> > > going negative (or in reverse).
> > >
> > > That's not time stretch (time warp, maybe?).  Hit '<' (a.k.a.
> > > sticky rewind--which can be enabled as the default rewind
> > mode).  Use
> > > '<' and '>' to adjust speed.  Also, see
> > >
> > http://www.gossamer-threads.com/lists/mythtv/commits/132808#132808 and
> > > http://www.gossamer-threads.com/lists/mythtv/dev/158632#158632 .
> >
> >
> > Thanks for the response.
> >
> > Following the dev posting you referenced above I created the
> > following entries in my settings table:
> > mysql> select * from settings where value like 'FFRewSpeed%';
> > +-------------+------+----------+
> > | value | data | hostname |
> > +-------------+------+----------+
> > FFRewSpeed0   1       NULL
> > FFRewSpeed1   5       NULL
> > FFRewSpeed2   20      NULL
> > FFRewSpeed3   60      NULL
> > FFRewSpeed4   180     NULL
> > FFRewSpeed5   360     NULL
> > FFRewSpeed6   720     NULL
> > FFRewSpeed7   1800    NULL
> > +-------------+------+----------+
> >
> > I thought setting the first entry to 1 would be 1x, in other
> > words realtime.
> > That way when I hit Rewind it would essentially be -1x.
> >
> > This raises 2 problems:
> >
> > 1.  It isn't -1x, it is -1 second.  So the play is choppy
> > jumping back 1 second at a time.
>
> ...
>
> So does this mean it is not possible to get real speed playback in reverse
> (or backwards), but just the 1 second jumps in rewind?
>
> Thanks,
> Dave
>
>
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