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It Ain't Over Till it's Over

KFMB-DT Sign-on Delayed

By Gary Stigall, CSTE
Chairman, Chapter 36
Staff Engineer, KFMB TV/DT

It's been encouraging to speak with so many callers eager to view the new CBS HDTV product, but disappointing to have to tell them we're experiencing "technical difficulties" that prevent KFMB-DT channel 55 from signing on. I asked Rick Bosscher, KFMB RF Supervisor, to answer a few questions about the problems.

GS: The Comark Advantage Digital channel 55 transmitter has been in place since October. Why haven't we seen it on the air except for sporadic tests?

RB: After the transmitter was basically running we wanted to see if we could induce failure in the antenna system and if we had any other problems such as tower reflections, etc. That way we would have time to schedule in tower work which can be more difficult to do. The antenna appeared to function very well but our 'RF Mask' compliance was not consistent and a lot 'iffy'. We also had some control issues that also made it difficult to justify remaining on the antenna. Since then, we've heated a lot of water in the test load to shake the system down. The FCC rules state that the DTV occupied bandwidth RF Mask shoulders that extend each side from channel edge out to +500 kHz must be at least 47db down in level. Since the implementation of high powered DTV, most transmitter manufacturers have had a real challenge in attempting to achieve that spec. in the field when the IOT Klystrode tube is pushed up near its rated limit (like ours). Some manufacturers claim success and consider a plant fully commissioned when a proof-of-performance shows 47.5db (or less) has been measured. The problem is that this value would provide virtually no safety margin in the event that the $44,000 IOT amplifier tube sags a little in performance headroom farther down its road to retirement perhaps in 2 or 3 years. The transmitter has a 1 year warranty, right? So we have targeted a value of 48 or 48.5db to be more conservative and thats a real job currently for the system. Incidently, (for you techies) the HP89441V Vector Signal Analyzer test equipment (another $50K+ worth of DTV monitoring gear) will easily display changes as small as 0.001 decibel at Channel 55.

GS: You've had a factory service rep on-site for sometime now.

KFMB-DT building interior. Click to view large version
KFMB-DT transmitter.
Click to view larger version (200k).

RB: Well, the transmitter manufacturer has been very good about addressing the needs at KFMB. They have identified the KFMB commitment to provide the correct environment for their hardware. Additionally, the transmitter model that we purchased is the on cutting edge of new transmitter technology currently available and is already showing the genius in its design by the Thomcast/Comark group. Their contract field engineer having commissioned his share of transmitters has spent considerable time at our facility making sure that EVERTHING is correct. Due to their complex nature, high power transmitters have personalities just like people and this serial number is no exception. Once tamed, configured, and fully outfitted, this system will be managed by us (or if need be, the manufacturer in Massachusetts) remotedly via ethernet LAN. This system is truly awesome in its potential and we feel a little extra time spent now in getting it right will be in everyones best interest down the road.

GS: Does the factory have a clue as to what's causing the problem?

KFMB-DT antenna on side
Channel 8 antenna on top, channel 55 antenna on side, ENG dish on bottom. Photos by Rick Bosscher. Click to view large photo.

RB: Yes, and they are working with their vendors to eliminate and/or minimize the stacking of tolerances within some sub-assemblies that correct digitally for distortions that normally exist in high power UHF amplifiers. We've been, in some cases, able to cherry-pick some of these items so as to optimize the results with high power at Channel 55. However, we're not done yet and there are some exciting changes to come that we will be able to share in. But if you were to ask me if I would consider buying this same kind of transmitter again knowing what we know now, I would still say absolutely Yes.

GS: You have until May 1 to comply with the current CP extension, right?

RB: Yes, but if everything goes well it will be much sooner than that, by far. It's just like with other aspects in life--you give a time by when and some new external uncontrolled factor sticks its foot in the door. February? Maybe, and then maybe not.