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

[Jim Kusznir]

Hi:

I'm not an expert here, but if you're using software encoding (e.g.,
compression), I think the noise is your problem.  Video compression
generally works by taking a starting frame, then saving the differences
from one frame to the next for a few frames, then including a new
complete frame, etc.  In MPEG, the full frames are called I-frames and
the difference frames are called P-frames (if my memory is correct; its
been a while since I've read up on this).

Now noise is generally random.  This means that each frame is going to
be a fair bit different from the last just due to the noise.  Since each
frame has more differences, the P-frames are bigger (assuming the
bitrate is not constrained; if it is (like the PVR series does), then
you'll see an increase in mpeg artifacts and other signs of poor
encoding or insufficient bitrate).  This will result in larger recordings.

If I recall correctly, this was a major consern of the digital sat
providers in the early days; they went through great lengths to clean up
analog video before encoding it in an attempt to get better compression.

I would strongly recommend contacting Comcast and placing a trouble
call.  They're supposed to proide clear pictures on all channels, even
in analog, and will often do this at their expense.  I know out here
they were very responsive when I called up with signal problems after
adding another capture card (and splitter).

Alternatively, you can see if you can constrain the bitrate on your
encoder/transcoder settings.

--Jim

Phil Buescher 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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