Digital is the world today, analog is on it’s way out. NTSC (Not The Same Color) television is being replaced by HDTV, and now AM and FM radio stations are being digitized. There are big advantages. Digital transmission allows for error correction and redundancy eliminating many of the annoying reception problems of analog service.
We’ve all experienced the swish-swish-swish of a marginal FM station signal or the buzz on an AM station as we drive under power lines. Digital promises to eliminate these things.
Digital also promises to improve sound quality giving us digital AM signals that approximate the quality of good FM reception, and FM signals that approximate CD quality.
On FM, digital will make additional channels available allowing the targeting of niche broadcasting markets.
There is a dark side. In the United States, AM channels are spaced 10 KHz apart allowing for 5 Khz upper and lower side bands without splattering into adjacent channels. However, current FCC regulations actually allow modulation up to 10 KHz, but sidebands past 10.2 KHz must be attenuated at least 25 DB from the main carrier. The reality is that there isn’t much energy content above that in most program material anyway.
Digital transmission is allowed to go out to 15 KHz. The analog audio occupies 0-5 KHz, and the digital portion of the signal occupies 5 KHz – 15 KHz. The ramification of this is that adjacent channels are going to be totally clobbered. Under the old rules, the sideband distant from the adjacent channel was reasonably protected, so with a narrow receiver you could still receive a station next to a stronger adjacent station. With digital, both sidebands will be clobbered and the adjacent channel will no longer be receivable.
Analog signals decode gracefully, as the signal gets weaker, the signal to noise ratio gets worse, but the signal may be receivable over thousands of miles at night. With digital either there is enough signal to decode, or there isn’t. This means that signals will either be receivable cleanly or not at all. The effect of night time fading may be to have signals come and go entirely.
Ah, but you still have the analog signal! Yep, except it will probably be buried by the digital transmission of a nearby digital station on an adjacent channel. I’m afraid they’re trying to kill one of my hobbies (DXing). It’s hard to say how it will play out. Surprisingly, TV DXers have had a fair degree of success with digital television.
To prevent adjacent channel interference, AM stations are presently only allowed to transmit digital during the day. I’m sure that this restriction will be lifted if digital AM proves to be popular.
Digital FM is less problematic because FM sidebands are wide enough to allow the digital signal to fit within the existing spectral footprint. With frequency modulation, sidebands are created not only at the sum and difference of the modulating frequency but also at harmonics of the modulating frequency. However, it is possible to choose a modulation index that limits significant sideband energy content to a desired bandwidth. Digital transmission may require discontinuing the use of certain subcarrier frequencies that would overlap the digital signal but the digital signal provides additional channels that can replace the functionality of the subcarriers.
The amount energy that goes into the digital subcarriers is very small so digital FM should not substantially detract from the analog signal and therefore should not negatively impact long distance reception.
A trend that seems to be accompanying digital conversion is power increases. I feel the FCC’s rules regarding FM broadcast really need to be revised to accommodate the realities of modern receivers. The FCC generally considers the 1mv contour an FM stations service area and does not protect beyond that contour. But many modern receivers have sensitivities below 5uV (mv = millivolt 1/1000th of a volt, uV = microvolt, 1/1,000,000 of a volt). I found one receiver with an advertised sensitivity of .25 uV. Even the cheapies tend to be better than 50uv these days.
In this region, what tends to be the reception limiting factor is not signal strength but multi path interference. The frequency of an FM station is being continuously modulated by the program content. This means that a 100.1 Mhz FM station might be 100.099 Mhz one moment and then a millisecond later 100.101 Mhz. The first signal arrives directly at the antenna, and then a signal reflected off a conductive object arrives at the antenna a millisecond later, and the two signals create a “beat” signal of 2 Khz. The end result of this is that you here this static distortion of the original signal that renders it very unpleasant to listen to.
More power doesn’t help with this problem, if often seems to make it worse. Modern FM signals which have a stereo pilot, stereo sidebands, and a variety of SCA subcarriers make the problem worse because all of those signals are modulating the frequency as well. Digital should improve this situation.
It is my belief that current FM stations are in large part overpowered. If you aren’t able to receive a distant station it is only rarely because that station is underpowered. More often it’s because a closer adjacent station is overpowered and overmodulated.
Given the reality of energy supplies being tight, this would seem to be an area where a lot of energy could be saved. I believe the FCC should update the signal level requirement to reflect modern receiver sensitivities, replace 1mv with 50uv or so, reducing transmitter power accordingly. There are 9000 FM stations in the United States, their total power output approximates that of a medium sized nuclear reactor. If contours were reduced from 1mv to 50uv, that would reduce
While increasing power generally does not help with multi-path interference, but does create more interference problems for people listening to other services, I believe the rules regarding antenna height above terrain verses power should be re-worked to favor higher towers over higher power. If the rules were re-worked to give higher towers an advantage over higher power then more broadcasters would opt for big towers and lower power.
This would be good for everyone concerned. It would be good for the broadcasters because they would get increased functional coverage area for only a capital expense while their operational expenses would be reduced. It would be good for the consumer because they would get a cleaner more multipath free signal. It would be good for the planet because less energy would be wasted.
In areas where the land is flat, not mountainous and hilly, and the antenna is not located at an extreme height relative to the terrain, a combination of more bays providing a higher gain and vertical directivity, that is to say power is concentrated in the horizontal plane, and lower transmitter power (to compensate for the higher antenna gain) can result in equivalent signal levels at a distance but lower signal levels near the transmitter resulting in less interference and more economical operation.
On the AM bands I am also seeing power increases left and right, KJR from 5 Kw to 50 Kw, same for KOL, KRKO also is getting a power increase later this year. With AM, if we still had clear channels, these power levels would be useful. Under present circumstances I’m not sure it really accomplishes anything except to better overload receivers in the local vicinity and cause more co-channel interference.