Digital devices come in thousands of types, from PCs and laptops to iPods and watches. Some, like a Wi-Fi laptop or Bluetooth earpiece, deliberately transmit radio signals to communicate with other devices nearby. But even those without built-in transmitters, like a CD player or digital alarm clock, still act as sources of radio waves. Digital circuits work by turning electric currents on and off very quickly, millions of times per second. That is all it takes to emit radio energy.
The stray radio signals from digital devices can interfere with radio communications. To minimize that risk, the FCC imposes rules on most digital devices that limit their radio-frequency emissions.
The FCC regulations establish two different sets of emission limits, according to where the equipment is intended to be used, and by whom. "Class A" limits apply to digital devices marketed exclusively for use in commercial, industrial, or business environments. "Class B" limits apply to all other kinds, including any digital product marketed to consumers or for use in a residential setting. The difference is significant. Class B maximum emissions are lower than Class A by a factor of ten.
The limits were adopted in the late 1970s, shortly after the appearance of "home computers." That was no coincidence. Antenna-driven analog TV is highly sensitive to interference, and early small computers emitted copious quantities of radio noise, often causing jagged bars on any TV screen nearby. Some computer models — we won’t name names — could wipe out "Mork & Mindy" over an entire apartment building. Other systems were also disrupted. In one famous incident, a defective restaurant video game shut down police communications for miles around.
To limit the interference threat, the FCC capped emissions from nearly all digital devices, imposing more stringent limits on Class B (residential) equipment. The primary goal of Class B was to protect residential TV reception, on the reasonable assumption that people do most of their TV-watching in residences.
The rules worked. By the mid- to late 1980s, when most of the pre-regulation computers had been junked, TV interference from digital devices was rare, despite a near-exponential proliferation of computers over those same years.
But times have changed. Is the Class B limit still necessary?
A year from next February, the last of the analog TV stations will go dark. The only TV signals coming in over the air will be digital. And digital TV is inherently less sensitive to interference than analog. This is true for digital media generally, not just TV. Even a small scratch on a vinyl analog LP causes an annoying repetitive "pop," but the same scratch on a digital audio CD would probably be inaudible.
Analog TV is particularly vulnerable to interference in "fringe" areas — on the outskirts of the transmitter’s useful range — where the weak incoming signal is easily overpowered by stray radio-frequency emissions. Digital TV does not have this problem because it does not work at all under weak-signal conditions. In fringe areas, the picture is either perfect or absent — never weak and snowy. In theory, there may be a fringe region where a little interference to digital TV could make the difference between a good picture and none at all, but in practice that region will be very small, if it exists at all.
Considering the relative resistance of digital TV to interference, it is reasonable to ask whether Class B protection will still be necessary after analog TV has joined vinyl records in the technology attic.
In setting up the Class B limits, the FCC also mentioned broadcast radio and two-way radios, along with TV, as services that warrant protection. But unlike analog TV, FM radio is inherently resistant to most types of interference — and its recent adjunct, FM digital "HD Radio," is even more so. Two-way radios work well in commercial environments, where Class A digital devices are common, suggesting that the stricter Class B limits may be unnecessary to protect them. The only other broadcast service – AM radio — is highly interference prone, much like analog TV (and for the same technical reasons), but the frequencies assigned to AM radio are outside the range used by modern digital devices.
Whether or not we still need Class B emissions limits may turn on three other questions. First, can a digital TV receiver tolerate nearby digital devices operating at Class A levels? If not, then Class B (or something like it) will remain necessary. Second, would relaxing the Class B limits make life significantly easier for manufacturers of digital consumer products? Otherwise, the effort required to change the rules just isn’t worth it. And third, would reduced emissions limits open the way for devices that are not practical in Class B regime?
I welcome comment from anyone with views on any of these questions, or with other observations.