Monday, October 18, 2010

AM Radio Bandwidth (Part 1)

It is a simple engineering argument over AM audio frequency response. But the topic is so conceptually above the laypersons head, that very questionable decisions are being made in an arena where regulation should be making the final decision. I've kept the specifics out of the preamble and I'll begin in the arcana of AM history.

The problem is nearly as old as radio itself.  It's called "splatter" which is short for Spectral Splatter. This is when the broadcast includes noise at frequencies other than the frequency of the carrier wave. If there was only one radio station this wouldn't need to exist, but we have 14,000. So much like the suburbs, where your lawn ends, another neighbors lawn begins.  Because the goal is to have as many choices as possible, we want to use the existing bandwidth efficiently.  But you cant put your ficus bush on your neighbors property.  Radio is much like this where stations are squeezed in together such that splatter will occur on adjacent channels and not vacant space. Essentially, there is no vacant space. This situation requires a referee (the FCC) and a lot of regulations. In radio, FCC regulations require radio signals be contained in a particular frequency band. This is defined by a "spectral mask". OK, new word. It's also called a channel mask or transmission mask.
"...a mathematically-defined set of lines applied to the levels of radio transmissions. The spectral mask is generally intended to reduce adjacent-channel interference by limiting excessive radiation at frequencies beyond the necessary bandwidth. Attenuation of these spurious emissions is usually done with a band-pass filter, tuned to allow through the correct center frequency of the carrier wave, as well as all necessary sidebands."
The key phrase there is "all necessary sidebands."  This is the topic over which engineers have been arguing. the carrier wave is what you tune the radio to.  If you're listening to 100.1 FM, on a graph 100.1 FM is just a line, or a point. Data takes up space.  In the most rudimentary sense this is referred to as bandwidth. Here is a picture of an FM HD signal to help visualize the relationship. The sidebands are mirror images and the carrier wave a dividing line. How far away from this center point the side bands can be is the point of contention. Enter the NRSC.
Right now AM bandwidth is fixed at 10 kHz as per the NRSC standards that were set November 20th 1986 read it here. The ruling was sort of late to the AM radio game. FM radio had already overtaken AM radio by the early 1980s at least in sheer numbers. FM had them on fidelity, and bandwidth  was partially why. I'll quote the December 1976 issue of Popular Mechanics to summarize the situation:
"Most of the inexpensive portable or table radios... are too insensitive to pick up any but the strongest signals clearly, are plagued by interference, and are limited by tiny speakers that produce only tinny sound. Even the AM sections of component high-fidelity tuners and receivers are frequently cheap, poorly designed circuits... the fewer listeners who can hear the difference at home between  good and bad AM broadcasts, the less motivation AM stations have to clean up and improve their signal."
AM stations are spaced 10 kHz apart. that sounds fine except that the FCC allowed AM stations to broadcast sidebands on some stations up to 30 kHz wide!  That's 15khz to each side  of course. the math is obvious, broadcasting more than 20kHz increased the odds of interference significantly. But there was a second problem. Most AM radios tuned much more narrowly than 30 kHz.  they did so for 2 reasons. First is was cheaper, second it reduced interference by avoiding second adjacent stations. But that also meant not receiving the stations high frequencies. It made everything sound muddy.

Stations fought back. They used an audio process called "pre-emphasis" to boost high frequencies.  It's wasn't a radical new technology.  The RIAA equalization curve on 33 rpm and 45 rpm vinyl records used pre-emphasis. it can also be used in digital processing to reduce bit errors. the downside was that in the already narrow world of AM bandwidth, it caused even more interference. Makers of consumer radio tuners narrowed bandwidth even further to reduce that interference. By the 1980s the end result of this downward spiral is that most AM radio tuners reproduced 4 kHz of bandwidth. That's only a slim margin better than the audio quality of a land line telephone (3.4 kHz.) Consumers were driven away from AM toward FM. It was about then that the FCC OK'd AM stereo. It was the perfect storm.

The NRSC tried to salvage a radio service from this nightmare.  The NRSC studied the problem and came out with a simple compromise: the 10 kHz steep audio roll-off.  ...more in part 2