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

Brian Wood beww at beww.org
Fri Sep 7 13:21:24 UTC 2007

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.

What I see in your pic certainly looks like it could be a hum bar. If it
is, it should be moving up the screen slowly.

The reason 60 cycle AC hum moves up the screen is that the NTSC frame
rate is not really 60/sec, but 59.something (sub-multiple of 3.579545
color subcarrier which, if it were 3.600000 would result in a precise 60
Hz. frame rate).

Hum bars are usually pairs, representing 120 Hz. due to a bad full-wave
rectifier circuit or the like. Single hum bars represent 60 Hz, and can
be caused by just about any non-linear device in the cable system (even
corroded connectors).

The fact that you only see it on PVR recordings might indicate a
grounding problem with your computer, a power supply problem in the
computer or "something else".

> I don't have a STB, but Time Warner did install a powered amplifier
> before I set up the PVR.  The outlets in use are properly wired
> (polarity and ground).

If you don't see the problem on a regular TV I'd look into computer
PSU/grounding problems first.

The FCC spec for AC hum in a CATV system is 5% (actually the spec is for
"low frequency AM components", but AC hum is by far the most common such
product). 5% hum is actually enough to be quite annoying and most cable
systems run far below that spec.

One trick that sometimes helps if the problem is ground differential
with the computer is to isolate the PVR's ground by using a pair of
300/75 ohm baluns ("transformers") back to back in the RF feed to the
card. This assumes that you have enough signal level to survive the loss
of the two devices. You also have to watch out as the 300-ohm "twinlead"
section between the two baluns can cause signal leakage and/in off-air


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