An odd proposal to save AM

Created Wednesday 24 April 2024

This is an odd proposal, I know, I even said so in the title, but please hear me out, because this involves tweaks to all of the services that provide domestic broadcasting. I'll begin with an overview of the problem, then describe the modifications to TV, FM and AM, in that order, that would be needed to make this all fall in place. I'll finish up with the quality and compatibility requirements that need to be placed on stations and new radios. Part of this will work like the All Channel Receiver Act, which required new TVs in 1962 to include UHF capability, and which was amended in 2005-2007 to require new TVs to handle digital ATSC broadcasts.

References

Throughout this, I will make reference to a number of existing standards that are part of the existing body of broadcast standards, but let me take a moment and identify each.

Under normal circumstances, a basic AM signal's RF bandwidth is twice the highest frequency in the passband. This could be anywhere from 3-10 kHz, making a signal that is 6-20 kHz, give or take.

For FM, it's more complex, with the RF bandwidth being roughly the highest frequency in the passband plus the maximum deviation, summed and then doubled. For instance, a standard monaural signal will carry sounds up to 15 kHz, and the deviation is 75 kHz. Adding these together gives you 90, and when you double that, the bandwidth is potentially 180 kHz. This oversimplifies things, though, because using less deviation at higher audio frequencies can yield lower bandwidth, even if the full deviation is available at lower frequencies. The easiest way to proceed, therefore, is just to take it as read that a conventional FM broadcast station is 200 kHz wide.

The main reason I lay out the explanation of bandwidth on FM is that my proposal below includes the possibility of a "narrow FM" station that will have an RF footprint of only 50 kHz for limited applications.

AMAX is a quality standard for AM broadcasters and receivers. Many broadcasters adhere to it as a matter of pride, but disturbingly few receivers do, even unofficially. The standard requires an audio passband of 40-7500 Hz for both broadcasters and receivers (though portable receivers are allowed to have a more relaxed 40-6500 Hz passband) and can go as wide as 20-10,000 Hz. Receivers need to be equipped with an impulse noise filter and (again, with the exception of portables) must have an external antenna connection. Finally, there is a pre-emphasis and de-emphasis curve. An AMAX compliant signal is up to 20.4 kHz wide.

CQUAM (or C-QUAM) stands for Compatible QUadratue Amplitude Modulation. It is a standard for stereo audio in analog AM. It is independent of whether or not a station or radio is AMAX compliant. It uses a complex chain of quadrature amplitude modulation, a 25 Hz pilot tone, and a stage of conventional amplitude modulation to produce a signal that is both 100% compatible with conventional AM monaural receivers, and easily and reliably rendered in stereo by properly equipped radios.

MPX stands for MultiPleX, and is the standard used in most of the world for stereo audio on the FM broadcast band. It can be thought of either as rapidly switching the audio to either the left or right speaker, or as a dual-sideband, supressed carrier L-R signal added at 38 kHz. There is a 19 kHz pilot tone that synchronizes it if viewed as switching channels.

SCA stands for Secondary Channel Authority and is a system for adding low-quality extra channels of audio to an FM broadcast, on a carrier at either 67 kHz or 92 kHz.

RDS is Radio Data System, and is a system for adding text to an FM broadcast.

HD Radio is two systems, really, but they're somewhat similar. One adds a digital audio program to an AM broadcast; the other adds multiple digital audio programs to an FM broadcast. A station broadcasting in HD Radio can either broadcast a fully digital signal, in which case only an HD Radio receiver can pick it up, or it can broadcast a hybrid-digital signal, in which a conventional AM or FM signal is flanked by a digital signal. On the AM band, HD Radio broadcasts are 30 kHz wide; on the FM band, they are 400 kHz wide. HD Radio is a proprietary system owned by iBiquity, but has become the accepted standard for digital audio broadcasts in the US.

dbx is a dynamic range compandering scheme, in which a dynamic range compressor is applied at the transmitter, and a dynamic range expander at the receiver. It has history in analog broadcasting found in the MTS standard for stereo audio that was used in analog television, where it was used to reduce noise in the L-R channel.

The problem

I call this "the problem" in singular, but realistically, there are actually four.

The first problem is that most analog AM radios are objectively awful at what they do. The audio passband is narrower than necessary, often (inexplicably) lacking bass response, or (less inexplicably) sounding muddy. They lack sensitivity, sometimes so badly that a 1000-watt station a scant few miles away is very noisy (this is one of my personal complaints about the radio in my car), and, despite options available to make them stereo, they rarely are. All of this is solvable without changing the specifications, but we can do even better yet with some tweaks to the standards.

The second problem is that the braodcast radio bands are full. The AM broadcast band is sparse during the day, but becomes quite full during night-time hours, when long-range propagation becomes easy. The FM broadcast band, even in some areas of modest population, can easily have every possible position filled. On both of these bands, the problem is exacerbated by the use of HD Radio, which has the unfortunate side-effect of increasing an AM station's footprint to 30 kHz (versus 6-20 for analog-only stations) and an FM station's footprint to 400 kHz (versus typically 200 kHz).

The third problem is that when the AM broadcast band is fully populated, radio signals overlap one another, resulting in interference. Anyone with good enough hearing can listen to the AM broadcast band at night, and hear the tell-tale 10 kHz whistle that results from stations on immediately adjacent channels both being within range.

The fouth and final problem is that the VHF-low band for television is underutilized. This is not as large of a problem as the others, but it seems inappropriate to allow a portion of the spectrum to go fallow while others are overcrowded.

The solution, part I: TV

This part is easy. As I mentioned above, the VHF-low television band is barely used anymore. While its range is potentially tremendous, it doesn't work well in urban environments, and requires larger receiving antennas than end-users are likely to find acceptable. This plan doesn't decommission the whole range, but it does involve eliminating TV channels 5 and 6. Channel 5 occupies the space from 76-82 MHz and channel 6 is 82-88. The astute may notice that this adjoins the bottom of the FM broadcast band, which is useful.

In places where there are TV stations that are still using channels 5 or 6, there can be a period of time for them to reposition, but also there shouldn't be any cases where both channels are in use in the same place. This will become relevant in a moment.

The solution, part II: FM

With channels 5 and 6 decommisioned from TV, the FM dial can now be expanded so that it runs from 76 to 108 rather than the current 88 to 108.

Some readers may observe that the current FM band actually starts at 87.8, with the first channel being 87.9. While this is true, it is sufficient for the discussion to ignore the one channel of the FM broadcast band that overlaps TV channel 6, as it is only available in limited locations under the current plan. Anyway, there are no radio stations in the US currently assigned this frequency, so the point is moot.

Radios that cover this entire range of 76-108 already exist, especially those oriented towards an audience of international travelers. This is because the FM band isn't 88-108 the world over. In Japan, for instance, it is 76-95. Most notably, Brazil has made the change I have just described, of decommissioning TV channels 5 and 6, and expanding the FM dial to go from 76-108, and I would propose that we should follow suit.

During the transitional period, there may be some locations where there is a TV station on channel 5 or 6 either locally, or nearby whose contours must be protected until they are able to migrate to another channel. In case of a local channel 5, the FM dial will simply start at 82; in cases of a local channel 6, it will just skip 82 to 88. Once the TV migration is complete, the FM band will occupy the full range from 76 to 108 MHz.

The solution, part III: AM

This was all about saving AM, right? Okay, so here we go. The first thing we do is that we offer AM stations that want to get out of AM, an opportunity to do so. Brazil did this as well. The first stations to be offered slots on the newly-opened FM spectrum would be existing AM stations that don't currently simulcast on FM. Next would be stations that simulcast using a low-powered translator (who would now be able to operate at full power in the new allocation instead), and finally, everybody else (including new stations). Stations that have existing FM simulcast allocations can be given the opportunity to darken their AM operations and move exclusively to FM.

Wait. I'm saving AM by letting it go dark?

Not quite. In a nutshell, the idea here is to let stations that no longer wish to broadcast AM to make a graceful exit from the band. This is to make room for what must happen next, which is to remove channels from the band so that they do not overlap.

Under the existing AM band plan, the frequencies 530 and 1610 are reserved for traveler information stations. These stations have a narrow bandwidth, and a resulting low sound quality, which is completely fit for purpose. These frequencies are reserved for broadcasting vital information to travelers about delays, weather conditions, hazards, or other things that might be useful to know, such as information about local major attractions or events.

Under my proposed band plan, these two frequencies would remain allocated to TIS stations. With only two such frequencies allocated, though, there are locations where the stations suffer from interference from other nearby TIS stations on the same frequency. To remedy this, the TIS allocation would be expanded so that it also includes 540, 550, 1620 and 1630, bringing the total number of allocated frquencies to six.

for the remainder of the band, the frequency step will be moved to 30 kHz. The first main-line broadcaster in the new band plan would be 570, then 600, 630, 660, etc. Existing AM stations that wish to continue broadcasting on the AM band would be required to move to one of the new slots within a few years time. By default, they would move to one of the two nearest stations from their current allocation. Which one would be dependent on the quality tier the intend to operate.

So what is a quality tier? So glad you asked.

Quality tiers

In the AM band, channels would be grouped into three quality tiers: TIS, AMAX and HD Radio. A stations's frequency will identify which tier it is on:

In the FM expansion band, channels would be grouped into two quality tiers, with an optional third:

If the narrowband FM tier is implemented, the expansion band would consist of 75 new channels:

On the other hand, if the option is to not implement the narrowbwnd FM tier, then the analog-only tier would fully occupy the first three and last three MHz of the expansion band, giving us 45 new channels:

In the mean time, the AM broadcast band would be reduced from its current 118 channels to 43 channels:

Broadcaster standards

Broadcasters are expected to meet minimum quality standards, according to the tier in which their signal appears, as follows:

Radio standards

Radios are expected to meet minimum quality and compatibility standards as follows:

Conclusion

It is my belief that if we were to implement all of these specifications, the quality of the AM listening experience would improve to a degree to make the band viable, while also improving the avaialbility of FM broadcast channels, and putting back to use frequencies that are currently allocated to television broadcasting, but grossly under-utilized.