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LOCAL CALLER: HOW COME WHAM DOESN'T COME IN WELL ANY MORE?

S

Savage

A recent caller on WHAM Rochester's Bob Lonsberry Show complained she was having trouble getting the station recently (I can't recall where she was calling from, but it was from outside of the native county.) The host responded - paraphrasing here - "You know, I've had that problem myself recently! I'll have to ask the powers that be what's going on."

Of course, most readers here know that WHAM's facility is as good as it gets in US medium-wave radio: 50kw nondirectional unlimited hours on 1180 kHz. It's WBZ vs. KDKA (and WYSL) all over again. There's a reason Citadel turned off IBOC at night. Not even 50kw can overcome IBOC skywave.

It's been a topic of discussion around here how IBOC is killing even this giant signal regionally, with the advent of nighttime AM-HD on WWVA. The upper-sideband hash is terrible. I can hear it at the WYSL site 25 miles from WHAM's transmitter. Nighttime reception in metro Ontario County has been severely impacted.

I have also heard complaints that WHAM's daytime coverage seems impaired since they started using IBOC. I think there's a lot of merit to the theory that the robust COFDM sideband noise is throttling back receivers which are reasonably wideband and which have fairly active AGC. Combined with the analog bandwidth limitation these conditions result in less-than-terrific reception above the noise floor.
 
Savage said:
I have also heard complaints that WHAM's daytime coverage seems impaired since they started using IBOC. I think there's a lot of merit to the theory that the robust COFDM sideband noise is throttling back receivers which are reasonably wideband and which have fairly active AGC. Combined with the analog bandwidth limitation these conditions result in less-than-terrific reception above the noise floor.
Click to expand...

When I listen to WHAM (and other AM IBOC stations) on the factory-installed Visteon radio in my 11 year old truck, the daytime noise floor is significantly higher than on analog-only signals, and I do find this annoying.

Those of you who've studied the AM system will recall that the tertiary digital sidebands are actually transmitted on-channel within +/- 5 kHz of the analog carrier (yes, directly under the analog sidebands) with the upper group of digital subcarriers inverted in phase from the lower group. So in theory, the tertiary noise should cancel out in an analog receiver's envelope detector, but only if the front end is properly tuned and the amplitude response across the IF passband is symmetrical. See Figure 2 of this document for a diagram of the hybrid AM spectrum:

http://www.hd-radio-home.com/hd-radio-technology.html

However, there are several real-world problems:

1) IF filters used in consumer receivers aren't perfectly symmetrical.

2) As a "digitally tuned" receiver ages, the crystal reference in its synthesized local oscillator can drift, causing the tuning to be offset by 1 or 2 kHz -- and there goes the symmetry even if the filters are good. After a few years, most car radios develop this problem.

3) Needless to day, "analog tuned" receivers are often set slightly off center by the user, which also introduces asymmetry.
 
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