[mythtv-users] PVR-500 picture quality
beww at beww.org
Mon Feb 1 14:17:11 UTC 2010
On Sunday 31 January 2010 09:09:31 pm Mw wrote:
> I see a significant drop in quality with my 500. Specifically, diagonal
> banding from lower left to upper right. It may be related to the type B
> Philips tuners on it. The very early PVR-350 I was using was almost spot
> on. Hardly any artifacts at all.
Diagonal "banding", which has been mentioned by several users, is commonly
referred to as "beats". This is caused by a steady carrier falling within the
video passband of the analog channel. The unwanted signal can be caused by
several things, usually heterodyne "beat products" caused by the mixing of 2
or more signals. This can occur in the cable TV amplifiers (trunk amps,
bridgers or line extenders), in an amplifier at the customer's premises or in
the front end of the tuner in the PVR itself. The "LNA" in the PVR-500 is also
a possible source of such beats.
Whatever device is causing the beats will usually be very sensitive to the
level of the signals presented to it.
Such problems can also be caused by the cable system not maintaining their
levels accurately, audio carriers might be running at a higher levels than
they should be, special carriers used by "sniffer" systems often cause such
> > The PVRs use a consumer grade encoder, which I would expect to behave
> > this way, the degradation is not enough to bother me, considering the
> > source is analog CATV, which is not all that great to start with.
> Noise in the source from the headend can greatly impact the quality of
> the PVRs. Typically, the analog SD from my Comcast feed is pretty noisy.
> The encoders on my 500 seem to be very sensitive to this noise and
> compression artifacts are obvious.
Noise does not generally compress well, and noisy signals will cause problems
in many ways. Analog cable signals, especially far out on a cascade, are
generally pretty bad, and the encoders can't tell the difference between signal
and noise, and try to reproduce the undesired noise, usually making things
> Every now and then, one of the HE techs isn't paying attention and the
> signal becomes really, really clear. Almost as clear as unscrambled 480i
> digital. The artifacts are hardly noticeable on these recordings but
> the banding remains. Eventually, the technician will fiddle with the mux
> chain until the signal gets crappy again.
It might be a HE tech, but there are many other factors. Cable attenuation
varies with temperature, and the systems used to compensate these changes are
imperfect, so the levels you receive will vary with temperature, time of day
and even the amount of cloudiness (sunshine on a black jacketed cable causes
its temperature to change independent of the ambient air temperature). It's
hard to pin down precisely, there are just too many variables.
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