digital reception ota

splitsound

New member question.
Does anyone know if digital ota reception will totally eliminate the co-channel and adjacent channel interference associated with analog ota reception? Living on Cape Cod can be quite an experience with ota reception. It can be a day to day change of signal quality, and stations received, just like our weather.

tigerbangs

OK, where on the Cape are you? once you get past Hyannis, reception of Boston can be a challenge: get much farther out that that, and Providence becomes a challenge, too. Yes, you should experience much better reception with digital than you did with analog, but remember that most of the digital stations are on UHF, and require a deep-fringe UHF antenna, a rotator and a preamplifier.

WBZ-DT's signal coverage: (Boston CBS)
http://www.fcc.gov/fcc-bin/FMTV-serv...DT1127010.html

WJAR-DT's signal coverage (Providence NBC)
http://www.fcc.gov/fcc-bin/FMTV-serv...DS1073865.html

__________________
Real men don't use indoor antennas; neither do real women!

splitsound

I am in Dennis on the north side on Scargo Hill. Reception can vary from excellent to fair at times, all dependent on the weather. I haven't switched yet to HD, but that will happen soon.
I recently installed a Ant. Direct DB-8 with a PA 16 low noise UHF only pre amp. I have for years now used a Winegard SC820 with an AC9820 pre amp, I removed the uhf section of the antenna do to lost of elements and put in its place a couple of RS Multi element corner reflector antennas. Use the latter for the Boston area reception and have a seperate U/V combo ant for Providence.
Still looking for an answer on the co-channel and adj. channel interference with digital reception?

otaota

DTV is slightly better in some aspects, but I wouldn't say it eliminates the problems.



Digital TV will not show any visible signs of co-channel and adjacent channel interference like you have with analog TV (e.g., snow, ghosts, super-imposed images, herring-bone, etc.). If a digital signal has sufficient signal to noise ratio, then it will decode properly and you'll get a clean looking picture. If the signal is marginally receivable, then you might see some MPEG blocking artifacts or occasional "blue screens" as the signal fades in and out of receivability. In general, the transition between a very clean picture and no picture at all happens over a very small range of signal levels. Digital TV is mostly an all-or-nothing proposition.

In the sense that the picture is clean down to the level of no longer being able to receive a digital channel, you might say that digital signals handle interference better, but that does not mean it eliminates the interference problem.

If you are receiving two different transmissions on the same channel at nearly the same power, you still have a co-channel interference problem. If the signal levels are close to each other at your location, then there's a good chance you won't be able to receive either channel. Digital channels need a theoretical minimum of about 15.2 dB signal to noise ratio over any interfering signal. In reality, you'll need much more than that to have stable/reliable reception all year round.



There are two kinds of adjacent channel interference that can be a problem: 1) adjacent channel rejection, and 2) receiver dynamic range.

Adjacent channel rejection is generally handled better by digital signals. In the analog world, your screen will actually show signals bleeding over from a neighboring channel as visible artifacts. In the case of a digital signal, you will get a nice picture without artifacts as long as the signal remains decodable.

Receiver dynamic range is an equal problem for both analog and digital signals. When an adjacent channel is much much much stronger than your target channel (e.g., perhaps living very close to one transmitter and far away from others), the automatic gain control in the receiver's circuitry cannot accommodate both channels simultaneously and therefore the weaker channel gets lost in the noise. The quality of the receiver design will determine how well it handles this scenario, but there's no inherent advantage for analog vs. digital signals.



One way to combat co-channel and adjacent channel interference is to use directional antennas. If the interfering channel is very far off axis relative to the desired channel, then a deep-fringe directional antenna will help out a lot by giving you about 20 dB worth of channel selectivity or more.

If the interfering channel is not that far off axis (less than 30 degree separation), then you may need to combine two antennas and play tricks with tweaking their combined antenna pattern. The combined antenna trick can create deep nulls in the antenna pattern so that you get high gain on your desired channel while getting very low gain in the direction of the interfering channel.

__________________
Chuck

tigerbangs

What Otaota said: absolutely true! You really don't have any very strong TV signals on the Cape, so signal overload usually isn't a problem. Just like where I live in New London, though, since you are practically on the ocean, lots of stations skip in, especially in the spring and fall, and they can play havoc with your normal reception. I have regularly seen skip from Maryland, Philadelphia and Virginia in the summertime, and sometimes it is so strong that it obliterates some otherwise good local stations. I suspect that you have the same issue on the Cape.

You will find that digital signals, because of their very design, suffer far less adjacent channel interference that analog stations do. As for co-channel interference, it's much harder to identify in digital because you won't see anything unusual on the TV screen: you just will see an intermittent signal or no picture at all.

If you are serious about analog reception, I strongly suggest replacing your current VHF antenna with a Wade-Delhi VIP-306 or a VIP-307, either of which will do a LOT better combating off-axis competing signals that the compromised Radio Shack or Winegard antennas will do. The Wade antennas will also do a lot better on high-band VHF stations like channels 7, 10 and 12 than will the other antennas. I hope that you are using a good all-channel preamplifier like a Channel Master Titan 7777, too!

BTW: you would be MUCH better off using a highly directional antenna array with a rotator than using multiple antennas if co-channel problems are your issue.

__________________
Real men don't use indoor antennas; neither do real women!

splitsound

Chuck & Tiger,
Thanks for the information, hopefully in a few weeks I shall have gone digital and will let you know my results. Until then I will be monitoring any related post from others on the subject.
Thanks again.

splitsound

Well I took the plunge to-day, all I can say is "WOW". This HD is fantastic. Picked up a Westi SK32H590D and I was totally surprised at the quality right out of the box.
I had installed a DB-8 antenna and a PA-16 UHF low noise pre-amp (Antenna's Direct) on the side of my house a couple of weeks ago and was ready for the ultimate test to-day.
I actually had monitored signal levels for the past 2 weeks and was not happy with the original location I chose. I had decided to reinstall it at a location that I had been using for my original analog install for the past 35 years. That location was only about 8' away and 6' lower that the orig UHF position. This turned out to be a better position for reception. Out here on the Cape every bit of signal level is important. So far reception is outstanding, now all we have to do is see if digital is as affected by weather changes as analog. I can see that viewing the same channel, analog verses digital, that the digital signal level seems to be the stronger. I'am hooked on digital.
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