[mythtv-users] am i dropping frames?

Steve Smith st3v3.sm1th at gmail.com
Tue Jan 15 17:38:08 UTC 2008

On 15/01/2008, Steve Peters - Priority Electronics <
steve at priorityelectronics.com> wrote:
> -----Original Message-----
> From: mythtv-users-bounces at mythtv.org
> [mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Brian Wood
> Sent: Tuesday, January 15, 2008 8:45 AM
> To: Discussion about mythtv
> Subject: Re: [mythtv-users] am i dropping frames?
> Steve Peters - Priority Electronics wrote:
> >
> >
> >
> > That all makes perfect sense, but when I plug my cable line directly
> > into the tv, the sports programs are great quality. Also, if I up the
> > pvr-500 to the max bitrate, I think that would be enough to offset
> > this, but it doesn't. Does anyone else have this issue, or do most of
> > you have no problem with sports?
> Upping the bitrate gives diminishing returns after a certain point, see:
> http://www.mediachance.com/dvdlab/tutorial/bitrate.html
> There are other parameters besides bitrate, in addition to resolution
> there
> are a slew of parameters that can be set on the PVR's encoder, but
> remember
> that the PVR is a consumer-quality inexpensive device.
> But having said that, a PVR can yield very good results. If I look
> carefully
> I can see the artifacts produced by the card, but most viewers, not
> knowing
> what to look for, do not see a difference between the direct source and a
> Myth-reproduced picture.
> beww
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> Beww, so are you saying that this might be normal for myth recordings? Is
> everyone using mythtv stuck with this kind of playback problem? It's not
> the
> artifacts that are the problem, it's the lack of smooth playback that is
> the
> problem. I just don't see how any sports fan could put up with this. I go
> to
> other people's houses and watch sports there and I think to myself...I'd
> better not invite them over to my place to watch sports, cause that will
> make my mythtv setup look really bad.
> -Thanks
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It does sound to me like you are seeing the effects of deinterlacing. Quite
why bob doesn't work for you I'm not sure.

<Grandma sucking eggs warning!>
Basically the Sports programmes are probably sent interlaced. Over here in
PAL land this means that I would recieve a signal with:
   25 Frames per second (in NTSC land 30 FPS) each consisting of 2
fields.SoI'm actually receiving 50 Fields a second.
 Each field consists of "half" of the picture (frame) with one having the
odd lines and one the even.
You migh think that these 2 fields make up a frame from one instant in time,
but this is not usually the case (with SD anyway I don't know about HD). The
second field is actually recorded by the camera slightly later than the

So effectively what you recieve is a signal that is sort of at 50Fps  (in
NTSC land 60Fps) giving you lovely smoooth motion...
When you de-interlace down to 25 FPS you lose some of that smoothness...

This doesn't apply to movies shot on film as they are shot at 25Frames per
second full stop. You might say that the film camera is a "progressive scan"
type of device!
</Grandma sucking eggs warning!>

So the question really is... why is bob not working correctly?


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