[mythtv-users] Dirty haze on certain channels

jr jraymyth at gmail.com
Sun Feb 20 22:16:14 UTC 2011


On Sat, Feb 19, 2011 at 2:42 AM, Joseph Fry <joe at thefrys.com> wrote:
>
>> >> When recording on the HDHR it works well, except on the CW and
>> >> occasionally on Fox, where I see a sort of dirty film over the
>> >> picture.  Especially, on the particularly dark scenes.  Its looks like
>> >> someone wiped the screen with a dirty cloth leaving a dirty haze.  It
>> >> last for a few seconds, disappears, then happens again.  Is there a
>> >> name for this phenomenon?  The terms "dirty," "film," and "haze" do
>> >> not return much.
>> >
>> > Ghosting of previous images?  If so, it sounds like someone (CW/Fox or
>> > your local broadcaster or your local re-broadcaster (cable co)) is
>> > encoding with a bad encoder (or bad encoding options).
>> >
>>  I am not sure. Its like at some point during the scene, the recorder
>> makes a light charcoal rubbing of the scene on transparent paper, then
>> removes the paper, waits a few seconds and does it again.  We just
>> finished watching two episodes of first season Supernatural that I
>> ripped from my DVD, and saw the same effect through MythVideo, if less
>> often.
>
> I see the exact problem your referring to and I just call it smearing.
>
> Essentially what happens is that at each key frame you get a nice clean
> image. Then the subsequent I-frames are either too low bandwidth to maintain
> the clarity of the image and/or they use encoding settings that do not
> maintain contrast in the dark regions of the image and it starts to degrade.
>  It continues to get worse and worse until finally the next key frame comes
> along and clears it up.
>
> The good news is, its easy to fix.  The bad news is, there is nothing you
> can do to fix it.  This is simply an encoding issue at your cable company.
>  They are notorious for compressing video to the point of being unwatchable
> so that they can add one more channel of infomercials.
>
> I highly recommend getting a good antenna and pulling you local stations in
> OTA (over the air). Broadcasters don't try and shove 10 channels on a single
> multiplex like the cable companies do, and they typically put out a much
> higher quality signal.
>
> DVD encodes can suffer the same fate if the person encoding the video isn't
> careful to prevent it.  Most commonly you would see this in dark scenes
> where there is high motion.  Most compression algorithms take advantage of
> our limited perception of contrast in very dark regions.  The downside is
> that scenes that are mostly dark and depend upon contrast in the dark areas
> get muddied up pretty bad.  My bet is that your DVD's were encoded at the
> lowest possible bitrate so that they could get the entire season on the
> fewest number of disks, it's not surprising that you would see quality
> suffer.
>
>
>

Hello Joseph,

Thanks.  That makes sense as to why it happens.  But why do I see
smearing on the recordings, but not on the TV Tuner?

jr


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