[mythtv-users] Channel Reception Issues

[ryanlists79 at gmail.com]

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.
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
>
>   
well crap.... :)  At least I have a better idea of what I'm fighting.  
I'll try the "headless" idea, but I think the last time I did this, the 
machine never actually booted.  Of course at that point, I could have 
had other problems. 

Anyway, I'll give it a try and post my results.  Thanks for the info.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mythtv.org/pipermail/mythtv-users/attachments/20070222/a461a28e/attachment.htm