[mythtv-users] Channel Reception Issues

[Rich West]

Brian Wood wrote:
> On Feb 22, 2007, at 11:17 AM, ryanlists79 at gmail.com wrote:
>
>   
>> 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:
>> http://www.flickr.com/photos/83544026@N00/
>>
>>     
>
> 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.

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