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NAB Says FCC Should Act Now to Allow All-Digital on AM

R

rbc

The NAB says there’s no need for further industry testing before the FCC can allow AM stations individually to switch to all-digital operation on a voluntary basis.

It also opposes any notification period, instead favoring a simple immediate approach. It sees no need to put a carrier frequency tolerance standard in place. And it hopes the FCC will recommit to IBOC rather than reopen discussion of other formats.

In comments filed with the FCC, the broadcast association has reiterated its support for allowing all-digital on the AM band in the United States for those stations that want to adopt it.
[...]
https://www.radioworld.com/tech-and-gear/digital-radio/nab-says-fcc-should-act-now-to-allow-all-digital-on-am
 
bulwnkl said:
What is the down side to no carrier frequency tolerance standard?
Click to expand...

Mainly it's a burden, because one would have to shut off modulation to conduct a center carrier frequency check to confirm compliance. With analog it's easy, because one can easily see the center of the carrier, since much of the energy with AM is in the sidebands. All modern AM transmitters that can do MA3 can easily comply with the +- 1hz frequency tolerance. This rule is a left-over from the old rock-bound (crystal oscillator) AM transmitters, which maintained their frequency stability by use of a heating element. In the old days, if a heater went out, or very low, or high frequencies made it into the modulator, the frequency could drift off several cycles.

Those proposing this technical rule are claiming it's to protect adjacent stations on either side of the MA3 station. The reality is; the carrier drifting off center even two cycles, isn't going to effect an adjacent station any more or less than if dead on frequency. That's because the modulation is no longer peak modulation, but is RMS, or continuous from one side of the mask, to the other.

What's more important to set as a standard is output power measurement. With analog AM it's measured at the antenna base as current, or for directional patterns; measured at the common feed point to the array. These analog power measurements are made at quiet passages, or silence. Since digital is modulating the carrier all the time, no matter low loud or quiet the audio, some sort of new power measurement standard needs to be created that can be used across the board specifically for digitally modulated at Medium Wave frequencies.
 
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