[mythtv-users] Electrical power in my cable line? Is that bad?

Brian Wood beww at beww.org
Fri Sep 7 16:43:18 UTC 2007

David Brodbeck wrote:
> On Sep 6, 2007, at 8:15 PM, Keith Pyle wrote:
>> Brian Wood wrote:
>>> Such problems can show up as AC hum in the video, usually as a double
>>> bar slowly rising through the picture (assuming NTSC).
>> I see a horizontal distortion on many recordings (see attachment,  
>> just below the rating box).  Is this a manifestation of the AC hum  
>> Brian mentioned?  I only see this on recordings captured with my  
>> PVR-500, not on televisions viewing the signal directly.
> It certainly looks like a hum bar.
> I have this problem with my system sometimes, but I only see it on  
> *some* of the channels, which makes me suspect it's actually  
> happening at the cable head-end.

Hum can be induced into a cable system in many ways, some of which can
result in the problem being more apparent on some channels than others.

For example: mixing of the 60 Hz. AC power carried in the coax with the
RF signal can result in sidebands of the RF carrier at plus and minus 60
Hz. This can be caused by several types of non-linear performance
ranging from amplifier problems to corroded connectors. The strength of
these sidebands (or, actually, the ratio of the SB power to the carrier
power) will differ based on several factors, including frequency.

Most often such system-induced hum is more apparent at the lower
frequencies. Note that with channel mapping, used a lot these days, the
lower channel numbers do not always correspond with lower frequencies.

Of course head-end problems with modulators, processors, scramblers or
the link can cause channel-dependent problems, as you pointed out.

Of course, all of these problems were caused by the SD people forcing
all of the broadcasters and cable operators (at gunpoint I believe) to
do things their way, and suppressing all discussion of alternative
technologies :-)


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