Headend system interference

Thanks for setting this forum up Chris.

I'm wondering how the other headend operators are dealing with the "new" digital TV transmissions.
My local stations have added two channels that are messing up two of the satellite channels on the motel systems that I work on. At first, I was putting a BPF on the adjacent channel amps to keep the digital channels out of the system, with some relief. But to really get rid of the problem, I have abandoned the digital channels altogether. I suppose the problem is different in each market due to the various channel assignments.

Anyone else have any solutions?

Interference from digital broadcast transmissions usually takes one of two forms. It may be that a UHF broadcast signal is leaking into the distribution wiring, or it may come from "inadvertent conversion" if it leaks into a UHF to VHF heterodyne converter and appears as noise on an adjacent channel.

If you have a channel 26 to 3 converter and digital transmissions now occur on channel 27, as is common in my market (Washington, D.C.), then the channel 27 digital signal gets inadvertently converted to channel 4 and makes the channel 4 picture look like the picture tube's surface was wiped with sandpaper. You don't have a snowball's chance in Hades of eliminating this with bandpass filters. Your options are 1) move any UHF/VHF conversion plagued by an upper adjacent digital signal to a place where the adjacent target channel is vacant (converting channel 26 to 6 will dump the interference on the FM radio band, converting the 26 to 13 will dump it onto cable channel 23, which may already be vacant, etc.), or 2) you can demodulate and remodulate the UHF signal or 3) you can downconvert it using more expensive, SAW-filtered heterodyne processors that will filter off the upper adjacent digital input signal adequately. The advantage of using the more expensive SAW-filtered heterodyne processor over a demodulator-modulator pair is that the SAW filtered processor retains closed captioning, MTS stereo and SAP (second audio program).

UHF channels 14-43 tend to place interference on channels 65-94, respectively. UHF channels 44 and up place interference on channels beginning with 100. You just add 51 to channels 14-43 or 56 to channels 44 and up to see where the interference will be, and make sure channels afflicted by this signal ingress are not used.

If this does not address the problem your are experiencing, give me your channel line-up and I'll try to evaluate it further.

I haven't had the conversion problem yet, since most of my systems use the demod/mod scheme to convert the UHF channels.
The problems here in the Tampa area are thus:
Local analog ch 13 has added a ch-12 digital broadcast
Local analog ch 8 has added a ch-7 digital broadcast

Most of my systems use a simple SCA(single channel amp) or strip-amp to amplify the off-air VHF signals. Since these SCAs don't have a good input filter they are picking up some of the adj-ch digital transmission and amplifing it along with the intended analog on-ch signal, which produces a grainy look on the existing ch7 or ch12 program from the satellite.
Turning up the RF level on the ch7 or ch12 modulator only helps a little.
Putting a BPF-8 or 13 (band pass filter) on the input of the SCA helps at many of the sites, but isn't entirely satisfactory, and the Digital signal levels are eventually going to go up to full strength, which will make things worse.
When they first started the digital transmissions, the local cable company (Time Warner) had a big problem with the interference, and the broadcasters agreed to turn down the transmitter levels until the cable company could relocate channels or whatever.
The solution that works the best, but is slightly more expensive, is to just move the programs to other unused channels by adding a couple of modulators. The motels don't like it too much, since they have to print new in-room TV channel line-up cards. Many of them have decided to just live with the grainyness.

Have you tried demodulating, and then remodulating to the same channel? Have you tried using an agile, heterodyne channel processor, like a Holland AP-40/60, a Pico PFAP, or a Drake 450TCP? I have had good luck with all of these, but I have always been filtering off adjacent upper channels, whereas you are trying to filter off adjacent lower channels, which might be more difficult.

In either case, make sure that the input signal is padded to near the lower end of the recommended input range.

Both of the channels you are trying to distribute are available via DBS satellite. I believe the going rate for free-to-guest for the entire hotel is $20/month for all available locals. You can remodulate those signals to their natural, VHF channels.

If, for some reason, you absolutely have to distribute NBC-8 and Fox-13 off-channel to clean them up, you could use cheap titlers to display messages like "Local channel 13 Fox affiliate WTVT has been relocated to channel 27 to assure you of the finest quality viewing." I think a SIMA titler that sells for $150 or less could be programmed to serve that purpose. Or you could sell the hotel something better, like a Video Messinger, and display the channel mapping on the header and let them use the rest of the page for advertising.

Thanks for the demod-remod idea. I haven't tried that yet.
It would be slightly more expensive than just abandoning the channel with the interference and moving that program to another channel, but at least the motel wouldn't have to print up new channel line-up cards.