[mythtv-users] OT: OTA CBS in Chicago?

[Mw]

Dewey Smolka wrote:
> On Tue, Mar 31, 2009 at 12:35 PM, David Brodbeck <gull at gull.us> wrote:
>> Justin Johnson wrote:
>>> WBBM-TV in Chicago is broadcast on digital channel 3, make sure it's being
>>> included in your scans.
>> And make sure your antenna can pick up VHF.  A lot of "digital" TV antennas
>> are UHF-only.  At only five miles away you shouldn't need much of an
>> antenna, though.  An old set of VHF rabbit ears would probably work to test.
> 
> I'll have to check this when I get home tonight.
> 
> I last tested on Sunday. Using the Windows utility, I get a strong
> signal on VHF channel numbers -- WBBM's channel 2, for example, gives
> me around 90 percent signal strength but no lock and no reception
> through the HDHR.
> 
> I'll take a closer look this evening and see what I can find.

Hi Dewey. I believe your situation is similar to mine (and about 9 
million others.) VHF-LO in Chicagoland has been a problem since I can 
remember.

One word: Multipath.

In the analog world, low broadcast frequency + high-powered transmission 
+ close receiver proximity = severe ghosting. More specifically, the 
receiving antenna is not only getting the signal directly from the 
Hancock tower, but also fairly strong and discrete signals that bounce 
off of nearby structures. Those reflected signals, of course, arrive at 
the receiving antenna slightly later than the direct signal.

How does this translates to the digital world? Multipath conditions are 
addressed by digital receivers by utilizing a circuit which attempts to 
select the direct signal and mask the reflected. This 2005 FCC analysis 
has a far more accurate description of what happens behind the scenes:

<quote>
With digital (ATSC) television, multipath does not cause ghost-like 
displaced images on the screen, though the term “ghost” is still used to 
describe multipath propagation.  Instead, a weak ghost may have no 
effect on the picture at all.  A somewhat stronger ghost may cause 
picture impairments such as blockiness or freeze frames.  An even 
stronger ghost can completely prevent the television from decoding the 
digital data necessary to produce a picture and sound.  Consequently, 
all ATSC television receivers contain a circuit called an equalizer, the 
function of which is to adaptively cancel ghosts.  If the equalizer 
reduces the amplitudes of all but one signal path to a sufficiently low 
level, the picture will be displayed with no impairment at all.  If the 
cancellation is insufficient, the TV may fail to produce a picture even 
when signal level is very strong. </quote>
http://www.fcc.gov/oet/info/documents/reports/TR-05-1017-ATSC-reception-testing.doc

I am at 18 miles NNW, exhibit severe ghosting in analog on CBS2 and 
routine pixelization and/or complete loss-of-signal (lock) with an HDHR 
and a 2006 Zenith plasma ATSC tuner.

The solution, for now, is to use an antenna with a /very/ high 
front-to-rear rejection ratio. This generally means a directional Yagi 
such as the Winegard HD7210P (VHF/UHF.) WG and Antennacraft have some 
yagis tuned for VHF-LO as well. Example rooftop install here:
http://www.mythtv.org/wiki/Image:Rooftop_Antenna-26.jpg

Or you could just wait until June and the problems will go away after 
you re-scan and CBS will be broadcasting on 12.

Interesting reading for Chicago viewers:
http://www.satelliteguys.us/hd-over-air-ota/103303-ota-digital-tv-where-cbs.html
http://www.avsforum.com/avs-vb/showthread.php?p=9948641&&#post9948641

--
meatwad

"Standards and practices are a way of keeping good and funny ideas away 
from you, the television viewer!"