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

Fri Apr 15 16:03:07 UTC 2005

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.
> > > _______________________________________________
> > > mythtv-users mailing list
> > > mythtv-users at mythtv.org
> > >
> > http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
> > >
> >
> > __________________________________
> > Do you Yahoo!?
> > Yahoo! Small Business - Try our new resources site!
> > http://smallbusiness.yahoo.com/resources/
> >
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
> 

Are you using the correct type of coax cable?  RG6, 75 ohms.  If a
short run shows a noticeable signal degradation I would guess an
impedance mismatch between the cables.

-- 
Tim
www.magicitx.com