Sunday, December 7, 2008

TVI - collecting data

We've got some minor TVI and RFI problems in our house that I'll need to tackle at some point.

Our home entertainment system consists of:
  • A Sony Trinitron 27-inch color television (CRT-type)
  • A Sony DVD player
  • A Pioneer stereo receiver
  • A TiVo Series 2 DVR
  • Conventional (non-digital) cable TV
  • The cable enters the house and goes to a "T" connector. One branch goes to the cable input of the TV, the other goes to our TiVo.
  • During operation on almost any HF band, there is some TVI, but only when the TV is connected to the TiVo. If I switch the TV to its cable TV input, there is no TVI.
  • The interference usually manifests itself by changing the picture to black-and-white, or reducing the color. There may be some crosshatching.
  • Interestingly, this is most pronounced when running PSK-31 and no data is being transmitted. When just keying down for a few seconds in CW mode, there is a brief flash of "color dispearance" and the set seems to recover. I'm not sure why, but transients (key-down, or AFSK shift) seem to be involved.
  • It happens even when I reduce the power output to 5 watts, although it is more pronounced at higher output levels.
  • It's worst on 20 meters, but occurs on at least 80, 40, 20, and 15 meters.
  • The TVI happens even if I am transmitting on the 40m vertical in the front yard.
  • We also saw TVI when I was using a temporary 160M longwire fed from the feedpoint of the 40M vertical.
  • The amateur antenna feedlines (coaxial cable) for the G5RVjr and temporary 20M vertical run underneath the house through the crawlspace, and exit at the back of the house right where the cable TV lines enter.
  • The feedline for the 40M vertical also runs through the crawlspace, but exits on the east side of the house, and comes no closer than 25 feet from the TV/cable lines.
The fact that the TVI is present when using the 40M vertical suggests that the cause isn't feedline radiation, or you'd expect that the TVI would be worse on the antennas at the rear of the property.

I could also swear that the TVI problem got worse when I pulled our old TiVo out of the system (it's been powered off for a while now) and I hooked our new one into the audio/video inputs used by the old one.

My best guesses:
  • The signal is being picked up by a cable connected to the TiVo, and the TiVo is the TVI victim,
  • The TiVo has poor shielding, and is getting overloaded.
  • The signal is being picked up on a cable from the TiVo to the TV
I'll do some experiments with disconnecting cables and see if I can narrow things down. Since the TiVo is also an RFI source, I suspect I'm going to have to expend some effort on it.

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