[Naturephoto]

I recently purchased a Zenith HDV420 and a Radio Shack conventional antenna, which I mounted in the attic. HDTV reception was marginal, and I couldn't orient the antenna to receive both the standard commercial channels (98 degrees, according to antennaweb) and PBS (128 degrees). Figuring that a less directional antenna with an amplifier might help, I purchased the new Radio Shack amplified antenna, installed it in the attic alongside the existing antenna, and connected it both through a combiner to the HDV420. Now I get a pretty strong signal on all HDTV channels (though occasional dropouts on some channels). However, I notice that on PBS HDTV broadcasts, when there is a lot of movement in the scene, the picture pixellates. When the scene is relatively stable, the pixellation stops. Is this due to some distortion induced by the Radio Shack amplifier? If I replace it with a CM 4221, recommended by some members of this forum, will I have the same problems of inability to receive the stations at 98 and 128 degrees? And if so, how can I address these problems? TIA for any assistance.

 

[jckessler]

This is likely not a reception problem.


Does your PBS multicast?

 

[qpham]

Naturefoto - I think that is problem with PBS, I live in South California, I do have this problem as well as anyone else.

 

[Naturephoto]

To respond to jckessler, yes, my local PBS station, WETA, broadcasts on 26-1, 26-2, 26-3, and 26-4. 26-1 is HDTV, and the others either mirror the analog broadcasts or program educational or children's programming. Thanks for your input. I'm relieved to find it's not just me!

 

[Steely]

Naturephoto,


Was your amplified Radio Shack antenna a directional? Can you post the model number? I am going to be moving into our new house next monday and would like to experiment with attic mounted antennas.


Thanks,


Dan.

 

[AZGuy]

Don't rule out a reception problem. If you have marginal signal-to-noise or corruption of the signal due to other reasons, you may experience excessive errors with moving images which causes pixillation. Still images do not require transfer of as much data and the receiver can correct most of the errors. With moving images there are higher data transfers and more demands on the error correction processing. Often an antenna amplifer can go into saturation from other nearby stations, or out-of-band sources if not filtered properly, and this can cause distortion and intermodulation. Sometimes an amplifier causes more problems than it fixes.

 

[jckessler]

He's got a PBS-HD with 3 SD subchannels. If he wasn't seeing pixelation on moving scenes, I'd be shocked.


While I guess it's possible that reception is an issue in this case, I'd have to think that it's extremely unlikely.

 

[AZGuy]

Yes, maximum usage of the data channel - which also makes it more critical to have enough headroom on the receiving end to minimize transmission errors. Unless of course they just don't give a damn on the transmitting end and like to send out garbage, which of course would not surprise me based on local experience with OTA DTV.

 

[arxaw]

Quote:

Originally posted by Naturephoto ... Is this due to some distortion induced by the Radio Shack amplifier? If I replace it with a CM 4221, recommended by some members of this forum, will I have the same problems of inability to receive the stations at 98 and 128 degrees?...

The 4221 will usually receive stations ~60° apart. I can receive stations that are 130° apart with a 4221, but it's outdoors. But if you have multipath problems, the 4221 may not be a good choice. And there may be better antennas than the 4221 for attic use, according to others in this forum.


Attic installations, cheap noisy RS amps and combining multiple antennas with splitter/combiners can all cause more problems than can be diagnosed here.

 

[Naturephoto]

To respond to Steely, the model number of the Radio Shack amplified antenna (supposedly designed for HDTV, but curiously, the spec sheet included nothing on distortion), is 15-2185, and it is rated for 20 or 21 dB amplification, I forget which. It costs about $70.00. Is it directional? Sort of: it looks like the swept wing on a Boeing 707, and all antennas are directional to some extent, but I figured this one would be less so than high-gain fringe-antenna designs.


I have checked the major network HD shows, and none of them show the pixellation on moving images that I encounter with PBS/WETA. So I think jckessler nailed it with his point about WETA trying to cram too much data into one of four subchannels. It's too bad: their program material is gorgeous and when the picture stabilizes, it's magnificent.


And yes -- I have lurked here long enough to have read that an exterior mount is better and multiple antennas and combiners are anathema, etc., etc. All I want is a setup that works well enough and I seem to have achieved that, compromises notwithstanding. Many thanks for everyone's advice.

 

[AZGuy]

Yes, the pixelation could be due to problems on the transmitter end. But there is also the possibility that you have one or more signals from other closer transmitters, digital or analog, that is putting the antenna amplifier into gain compression (overload) and actually degrading the quality of the signal you want to amplify. The stronger signal will still come through - although with some distortion - but the data can still be fully recovered. The amplifier is not selective unless you trap out the higher power signals before the amp, or have a narrow passband to amplify only the desired station. The best solution is a directional antenna for the problem signal with only enough gain to overcome cable losses. It might be worth trying a directional antenna with less preamp gain, or confirm the pixelation with someone who has good signal reception. Maybe even contact the engineer wih the station, many will repond to emails.

 

[LCH]

Quote:

Originally posted by Naturephoto To respond to Steely, the model number of the Radio Shack amplified antenna (supposedly designed for HDTV, but curiously, the spec sheet included nothing on distortion), is 15-2185, and it is rated for 20 or 21 dB amplification, I forget which. It costs about $70.00. Is it directional? Sort of: it looks like the swept wing on a Boeing 707, and all antennas are directional to some extent, but I figured this one would be less so than high-gain fringe-antenna designs...

Alan -


I own this antenna (as well as one I just bought to replace it ). It's actually made by these folks. As you can see, it's multi-directional. Not quite omni, not quite uni...


It's VHF/UHF and I asked a question of their tech. support. Got this answer:


The disk leading edge is best for UHF, and the angle between the "wings" is best for VHF. So you would point whichever side is appropriate toward your chosen source. I had some luck with it. Mine was mounted outdoors, about 25 feet in the air. I'm in No. Va also.


But - I had ghosting problems. Strong signals that fluctuated up and down. No good. Also had reception problems with certain channels that should be simple to get. My current opinion is that it simply lacks the surface area required to do the job.


I just got my new CM 3021 4 Bay bowtie. It's standing on a short mast on my deck (maybe 15 feet off the ground). It doesn't have the same clear shot the flying disc had, but it's drawing everything in great. I can't wait for this god-forsaken rain to stop so I can place it properly.

 

[Naturephoto]

Well, LCH, far be it from me to contradict antennacraft tech support, but I ran some practical tests, moving the RS amplified antenna around until I got the best signal, and orienting the disk side toward the signal was a disaster. I got no signal or a weak signal on almost every UHF channel. By contrast, orienting it more conventionally, with a line bisecting the "V" pointing toward the signal source, yielded a useable signal on every digital channel receivable from my area.


Just the same, I really don't want to use an amp if I don't have to, so I also order a CM 4221 and will install that and see if it does the trick, and in particular, if it makes any difference in the pixellation problem.


Alan

Quote:

Originally posted by LCH Alan - I own this antenna (as well as one I just bought to replace it ). It's actually made by these folks. As you can see, it's multi-directional. Not quite omni, not quite uni... It's VHF/UHF and I asked a question of their tech. support. Got this answer: The disk leading edge is best for UHF, and the angle between the "wings" is best for VHF. So you would point whichever side is appropriate toward your chosen source. I had some luck with it. Mine was mounted outdoors, about 25 feet in the air. I'm in No. Va also. But - I had ghosting problems. Strong signals that fluctuated up and down. No good. Also had reception problems with certain channels that should be simple to get. My current opinion is that it simply lacks the surface area required to do the job. I just got my new CM 3021 4 Bay bowtie. It's standing on a short mast on my deck (maybe 15 feet off the ground). It doesn't have the same clear shot the flying disc had, but it's drawing everything in great. I can't wait for this god-forsaken rain to stop so I can place it properly.