karn (10/27/82)
Regarding Marc Majka's comment on the rules concerning interference: The rules are NOT the same for terminal -> TV interference as they are for amateur -> TV interference. The FCC has a very simple criterion for deciding blame in an interference case. If the generator of the interfering RF energy has a license from the Commission, and is radiating RF only within the assigned frequency and power limits specified in the license, than he/she is blameless. What makes this confusing is that TVI can sometimes be the fault either of the receiver or the transmitter. If a source is producing spurious signals on an unlicensed frequency that are causing interference, then the transmitter is at fault. Examples of this are harmonic radiation from an amateur or CB station falling in VHF TV channels, or radiation from a computer terminal. On the other hand, if the interference is due to the receiver's inability to reject signals to which it is not designed to respond, then the RECEIVER is at fault, and the transmitter operator is under no obligation to shut down. An example of this is RF rectification by audio equipment, or front-end blocking of a TV set by a nearby transmitter's fundamental. Since computer terminals are not licensed as radio transmitters, they are ALWAYS be "to blame" in event of interference. Legally, such a terminal would have to be fixed (at the expense of the owner) or shut off. There is almost always a technical solution to any interference problem. I would suggest that in cases of terminal-to-TV interference that the following be tried: 1. Move the terminal to a different outlet. Much RF interference is radiated or conducted through AC power lines. 2. Try a AC line RFI filter ON THE TERMINAL. Filters on the TV set power cord would probably be ineffective in this case, since TV receivers are designed to receive weak signals on their antennas, not their AC power cords. Filters on the TV power cord are really useful only in cases of stray fundamental pickup from strong nearby transmitters. A RFI filter on the TV's antenna would be equally useless, since the terminal is emitting RF within the TV channels that the set is designed to receive; any filter removing the interference would remove the desired TV signal as well. 3. If you have a separate modem, try one of the new DB-25 feed-through RFI suppressors on the RS-232 line. I've found them to help in some cases. Remember to ground the suppressor's shield. 4. If you feel adventurous, and have the skills, try shielding the terminal case. Most terminal cases are made from molded structural foam which is transparent to RF. Be sure to connect the shield to a good ground. 5. Get a terminal (and modem) that meet the FCC Class B Computing Device RFI limits. The Heath H-19A meets them. I also know that the DEC DF-03 (212-type) modem meets the Class B specs; I don't know how many other modems do as well. Good luck! Phil Karn, KA9Q