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Rich West wrote:
<blockquote cite="mid45DDF51E.1050306@wesmo.com" type="cite">
<pre wrap="">Brian Wood wrote:
</pre>
<blockquote type="cite">
<pre wrap="">On Feb 22, 2007, at 11:17 AM, <a class="moz-txt-link-abbreviated" href="mailto:ryanlists79@gmail.com">ryanlists79@gmail.com</a> wrote:
</pre>
<blockquote type="cite">
<pre wrap="">I've been trying to figure this out for the last few weeks, and am
pretty frustrated. On certain channels, mainly 3 (ABC), 4 (NBC), 43
(Cartoon Network) & 69 (Spike), I am having what appear to be
reception
issues. Basically I have small "bars" rolling across the screen. On
channel 69, they are color, red and a semi-green, and sometimes
roll up
and down, and sometimes they don't roll at all. On channel 3 they are
just darker & lighter bars, almost a change in brightness, and scroll
from the bottom left corner to the upper right corner. On channel 4,
they are similar to channel 3, but smaller, a bit more subtle, and
scroll directly up. And channel 43 is an even more subtle version of
channel 4.
You can see screen shots from channels 3 & 69 on my flickr site:
<a class="moz-txt-link-freetext" href="http://www.flickr.com/photos/83544026@N00/">http://www.flickr.com/photos/83544026@N00/</a>
</pre>
</blockquote>
<pre wrap="">What you have are called "beats" in the vernacular. They are caused
by a carrier mixing or "beating" with the visual carrier causing
hetrodyne products that fall within the baseband video frequency range.
The fact that they are more or less steady indicates that the
interfering carrier is more or less CW (ie" not modulated), and more
or less "on" a multiple/submultiple of the horizontal scan frequency
or, possibly, the vertical scan rate. The source is almost certainly
from your computer somehow. If the interfering beats are moving
slowly it indicates that the beat product is merely "close" to a
multiple/submultiple of the scan frequency, and as the beat gets
farther and farther away from such multiple (moving faster and
faster) it eventually gets unrecognizable as a "beat" and rather just
results in a general fuzziness or lack of detail in the baseband video.
If they are absolutely stationary then they are right on some
multiple or sub-multiple of the horizontal scan frequency. You could
theoretically count the number of bars and thus calculate the precise
frequency of the interfering beat (actually there would be 2 possible
solutions, depending on whether the interfering carrier was above or
below the scan frequency). There's no real point in doing this
because it would not help you in eliminating the problem.
The beats *could* be third-order products happening in the RF realm,
and being enhanced by cheap tuner on the PVR card. You could try
attenuating the signal going into the card. The bottom line here is
that expecting broadcast-quality performance from a $100 card is
pretty much the height of optimism.
But what you're interested in is what you can do about it. From the
behavior you describe it sounds like it's happening in the recording
itself, you could prove that by playing the mpeg back on another
machine.
You could try moving cards and/or cables around inside your case.
Video cards are a notorious source of crap so you could try having
the backend make a recording with no video card in the machine (set
it up then remove the card and re-boot and let it run headless). You
could try some sort of shielding, check that all mobo screws are
tight, make sure everything's grounded well.
Even power supplies can generate crap that's within the video
bandwidth ( essentially 0 - 5 Mhz). Cheap switcher supplies
especially are prone to this.
But in the end there may be nothing you can do short of a better-
designed PC case and/or motherboard combination. Even well-designed
professional broadcast gear is prone to this sort of thing if the
designer is not extremely careful.
But at least you now know *why* you are having problems :-)
All this should be obvious to any decent engineer after a 1 second
glance at your pictures, but I guess I have to keep reminding myself
that few here on this list worked in TV broadcasting for 35 years :-)
Obviously what I've said here is a gross oversimplification but I
hope it makes the point clear.
</pre>
</blockquote>
<pre wrap=""><!---->
Yes, the bars are interference from something at or around the same
frequency. Finding out what it is can be a bit of a pain with little
reward since it might not be possible to fix the problem without some
additional investment or reconfiguration of hardware.
Of course, there is the simple question that should be asked: do you get
this same problem with the cable plugged directly in to your television
without the MythTV box in the loop (and powered off or moved away from
the cable line)?
If the problem goes away, then the problem definitely lies within your
MythTV box and you can focus on the stuff you have in there, such as the
quality of the power supply, the proximity of the capture card to other
devices like the video card, the processor, and case fans, and the
grounding of the motherboard.
If the problem remains, then it is safe to say it is not your MythTV
box. Unfortunately, if this is the case, there is very little you can
do without contacting the cable company. You are limited to replacing
the cable that you are using to connect the TV to the wall jack by using
a thicker (higher quality) coax cable (you can get lengths of this for
free from the cable company).
-Rich
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</pre>
</blockquote>
Actually, I only experience the problem when recording on the mythtv
box, not on the TV. With the mythtv box running, I can disconnect the
coax cable from the mythtv box and plug it into the TV and everything
displays perfectly.<br>
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