Tuesday, December 11, 2012

Granny Box Mystery...Solved!

 
Are over-the-air ATSC digital-to-analog converter boxes now failing with age? Maybe...maybe not!

At work, I oversee over a dozen granny boxes used to receive and convert local over-the-air digital TV channels for viewing on our in-house analog cable TV system. Reception of three channels was problematic, often locking up and freezing at will. The models we use are the most inexpensive available marketed under the brand names 'Micro Gem' and 'Access HD' sold for around $29.95. I tried swapping converters, testing converters for input sensitivity and analyzing the output of the wall-wart power supplies grasping for any clue. What was making decoding these specific channels so unreliable?

I discovered that these converters run fairly hot, especially when perched upon other pieces of gear in equipment racks. One tiny electrolytic capacitor, directly adjacent to the bright metal tuner module, sometimes dries out and limits the sensitivity of the converters. I have replaced a few of those under-rated caps, with good results, but this fix did not cure my endless lock-ups. A listing in The Worldwide TV-FM DX Association's bulletin, VHF-UHF Digest, showed a new WCBS translator on the air on RF Channel 22 from Plainview, Long Island. I could easily see this signal, so I changed antennas and switched to Channel 22 instead looking at the WCBS main transmitter on Channel 33. The results were immediately fruitful: WCBS reception became stable as a rock. Still, the mystery continued!

My troublesome channels are all in the New York City market: WNBC (RF channel 28,) WCBS (RF channel 33,) and WWOR (RF channel 38.) All three broadcast from the iconic Empire State Building and all three broadcast two virtual channels per transmitter. After some pondering and research, I decided to take a look at these transmissions with a spectrum analyzer. All three signals had a distinct and horrifying signature. A competent digital TV signal should have a nice square waveform with an obvious pilot carrier on the leading edge. My troublemakers looked like black diamond ski slopes! Obviously, these would be pretty challenging for my granny boxes, too!

What is the common thread? WNBC, WCBS and WWOR share a master antenna mounted high atop the Empire State Building. A friend described the design as "a bunch of panel antennas strung together with Heliax jumpers." Since I don't have access or design knowledge of this installation, it's very hard for me to comment. One thing for sure, the result is a mess! I can only wonder why no one has reacted to such an anomaly. (WNYW, New York's Fox affiliate, on RF Channel 44, looks crispy and nice using their own antenna also mounted on ESB.)

This is an interesting case. Thinking out loud, I know that ATSC DTV transmitters have sample loops that monitor various aspects of their broadcast signals. The transmitters themselves are most likely producing competent waveforms but, in this case, the antenna system is wildly altering the linearity of the signal equalization presented to the public. Digital TV transmitters pre-distort themselves to insure output signals are tight and square. Obviously, nothing compensates for severe antenna problems. It would take an enormous amount of EQ to correct these bumps! You would think that a glaring problem like this would cause warning flags due to excessive reflected power and lowered efficiency. Maybe not?

Although I have cured my WCBS reception by reverting to receiving another transmitter, I have no recourse for WNBC and WWOR. The closest alternative NBC broadcaster is WVIT Hartford which also uses Channel 33. WWOR is a MyTV affiliate. It features local content, most notably New York sports coverage that cannot be replicated by another station. I guess I will just have to live with the signals available. My granny boxes are not malfunctioning! I just can't resolve rollercoaster digital TV! Is anyone watching?




2 comments:

Anonymous said...

Merry Christmas Kaaaaaaaaaaaaarl!

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