[mythtv-users] Channels with poor quality create HUGE recordings

Fri Apr 15 16:33:03 UTC 2005

You might also want to check all the splitters in the house (including
on the outside) with the change over to digital cable the standards
required in these went up but I had to call the cable company back to
the house to have them change it out (they'll give you a few extra
ones and a line amp if you ask really nicely).  Also the conductor for
digital cable increased guage as well so any easy to access wires
should be changed out to reduce noise and lines that arent' being used
should be disconnected.  At least this is what I had to go through to
fix the problem.
BTW the effect on file size people were trying to explain above has to
do with the entropy of white noise being added to the signal.  It adds
a totally random factor to the stream and therefor can not be
compressed well at all.

Hope that helps,
Devan

On 4/15/05, Phil Buescher <alt.phil at gmail.com> wrote:
> Yeah, I was guessing the poor quality just caused larger recordings,
> and decided to focus a long detailed search last night to find out
> what would cause these quality problems.
> Near as I can tell, the lines are almost always due to either over the
> air ingress or simple RF/EM interference.
> 
> Plugging in a small TV set directly into the coax outside of my cable
> box, the channels are nearly crystal clear, but even at the shortest
> run of cable inside the house, the channels begin to flake out.  And
> at the furthestmost cable run (where my MythTV box is) the channels
> are much worse.
> 
> So it's looking like the cable runs are not as clean as I had hoped.
> I had pulled the cable through myself, and it must run alongside some
> power lines behind the walls somewhere.  I guess I need to re-route my
> coax and try running it another way.  Maybe I'll run a temporary cable
> outside of the house and in through a window just to see if that
> clears it up or not.  And if so, I'll at least know that's the
> problem.
> 
> Still, I would think if it was just EM/RF interference, an RF filter
> would have some effect!  Ah well.
> 
> If that doesn't solve it, then the only suggestion I found was to call
> the cable company and get an installer to come out, and if they can't
> fix it, insist that the problem be escalated, so that a special line
> testing technician will look into it for
> leakage/ingress/egress/interference - and all that other good jazz.
> 
> Thanks for your input - I'll certainly post whatever works for me,
> although it sounds like these diagonal lines can be cause by many
> things, all having to do with the simple fact that something is in the
> signal that shouldn't be there.
> At least now I know I can focus on quality issues instead of wondering
> what else might be causing it.
> 
> Phil
> 
> On 4/14/05, Joe Votour <joevph at yahoo.com> wrote:
> > Phil,
> >
> > It's because of the cable noise and its effect on the
> > recording codec (in your case, RTJPEG, since you're
> > using a software encoder).
> >
> > In order to save space on the hard disk, MythTV (in
> > software) will compress the video and audio, since
> > storing every frame in full detail would be enormous.
> > Therefore, you'd start off with a full-frame image,
> > and then you'd probably store the changes to that
> > image for a period of time, and then another full
> > image (this is basically how MPEG-2 works, although
> > it's a bit more complicated than that - I imagine that
> > RTJPEG uses something similar).
> >
> > The noise screws up the deltas - since the noise
> > causes picture distortion, you end up with more
> > changes between frames, thus increasing the filesize.
> >
> > The only way to fix this is to fix your cable signal,
> > which may or may not be possible.  I'm in a similar
> > situation, and Comcast won't touch it, nor will my
> > landlord (as they don't want to be digging up the
> > ground or going through the walls to figure out where
> > the problem is).
> >
> > -- Joe
> >
> > --- Phil Buescher <alt.phil at gmail.com> wrote:
> > > I've been running Myth for a couple years now.  I'm
> > > just running it
> > > with a software based encoder Happauge WinTV-Radio.
> > >
> > > I moved to a new town just recently, and a different
> > > cable company, of
> > > course - I have Comcast now.
> > > Anyway, onto the problem...
> > >
> > > Certain channels come in pretty fuzzy, with bad
> > > quality, showing
> > > diagonal lines on the screen.  Most channels come in
> > > just fine and
> > > look great.  These channels come in all fuzzy on a
> > > normal TV as well.
> > >
> > > Here's the big problem though - on those channels
> > > with the poor
> > > quality, the recording sizes are abnormally HUGE!
> > > We're talking about 3-4 times larger than the
> > > recordings should be.
> > > For instance, a normal hour-long show will take up
> > > 0.74 gigs, but the
> > > ones on the flaky channels will take up anywhere
> > > from 2.5 to 3.5 gigs!
> > >  It's really crazy!
> > >
> > > I didn't change anything besides my tvlisting feeds
> > > on my MythTV box,
> > > so I know it isn't something I did.  I didn't change
> > > my recording
> > > format, I didn't change my setup, and this is
> > > certainly a per-channel
> > > problem.  Only two channels I know of have this poor
> > > quality and HUGE
> > > recordings problem.
> > >
> > > I did get basic cable, nothing digital, no satellite
> > > or anything goofy
> > > - just the basic standard cable package with no need
> > > for a set top
> > > box.
> > >
> > > Could this be that these channels are actually HDTV,
> > > and that's why
> > > they're taking up so much room and come in all fuzzy
> > > on tuners that
> > > aren't HDTV-ready?  Or can it just be that channels
> > > with this poor
> > > quality problem simply take up a ton more room to
> > > record?  I have DSL,
> > > but could I be receiving some interferance that has
> > > to do with cable
> > > Internet?  I don't know - I'm totally guessing
> > > because I have no clue
> > > anymore.
> > >
> > > I really need some help, as the two channels we
> > > record the most shows
> > > off (shows that are on while we're at work), are the
> > > ones that have
> > > this problem - and even with over 300 gigs of HD
> > > space, I'm running
> > > out rather fast.
> > >
> > > I really don't want to have to re-transcode/mencode
> > > the shows after
> > > recording them, since that's a pain - any idea what
> > > the problem is?
> > > The cable installer thought the poor quality was due
> > > to bad wiring,
> > > but between him and I, we replaced every single run
> > > of cable in and
> > > outside of the house with high-quality wiring and
> > > ends.  The installer
> > > even replaced the cable running from the
> > > neighborhood box that runs to
> > > our house.  We never did figure out why these
> > > channels come in fuzzy.
> > > He left, proclaiming the problem fixed - and it
> > > wasn't fixed, he just
> > > checked one of the channels that come in just fine.
> > > I even bought an
> > > expensive coax tester, and all the runs are totally
> > > clean.
> > >
> > > I tried putting a signal booster in front of my
> > > MythTV box, but that
> > > had no effect.  I also tried an RF filter, to no
> > > effect.  I'm totally
> > > stumped and frustrated.
> > >
> > > Please help - any suggestions or insights are
> > > appreciated.
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-- 
Thanks,
Devan Lippman <devan at lippman.net>