Digital Signals Spark Static From AM Radio

Carl Bialik writes "Digital radio is touted as broadcast radio's golden ticket, but the transition to digital broadcasts is creating static and interference for many smaller AM stations that are still analog-only, the Wall Street Journal reports: 'The AM stations most affected are those whose neighboring stations -- nearby on the dial -- add a digital signal.' The WSJ adds, 'For some small AM operators, it adds insult to injury that the only company licensing the digital broadcast technology is one backed by the small stations' deep-pocketed competitors.' Critics question why the FCC only approved the technology from that big radio-backed company, Ibiquity."

Comments

Modulation Theory 101

[wowbagger]

I design radio test equipment for a living - so welcome to Modulation Theory 101.

Amplitude modulation, or more correctly double-sideband non-suppressed carrier amplitude modulation (FCC emission type A3E), results in an RF spectrum that is twice as wide as the highest frequency component of the modulating signal. In other words, if the signal you are modulating has as its highest frequency component 3kHz (normal voice signals), then the resulting AM signal will occupy 6 kHz of RF spectrum - from 3kHz below the nominal carrier frequency to 3 kHz above the nominal carrier.

Now, we have to consider the concept of "receiver bandwidth". A properly designed radio receiver will only pick up signals within a given frequency difference of where it is tuned (the "tuned frequency" or TF) - this is the receiver bandwidth (sometimes referred to as "IF bandwidth" since in modern superheterodyne receivers it is the bandwidth of the narrowest intermediate frequency section that determines the overall receiver bandwidth).

Now, consider the case of 2 radio stations spaced such that their carriers are 10 kHz apart - the normal spacing for AM radio stations. Assume your radio is tuned to one of the stations. If your radio has a receiver bandwidth of 20 kHz (in other words receiving signals from 10 kHz below tuned frequency to 10 kHz above tuned frequency), you would hear the station you *weren't* tuned to as a 10 kHz whine on your radio (the carrier of the other station, 10 kHz off your tuned frequency), plus the audio of the other station inverted in frequency (low tones become high tones and vise versa).

So, your radio has to have a narrow filter to receive only those signals within 5 kHz of tuned frequency (total 10 kHz). Now, a perfect "brick wall" filter would allow, say, 4 kHz through, but stop 4.00001 kHz dead. Now, filters are not perfect, and so if your filter allows signals from, say, 4 kHz away from TF, then it will not totally block signals until they are, say, 6 kHz from TF. So, radios are designed to allow signals +/- 3kHz from TF in (receiver bandwidth of 6kHz), and block signals more than 5 kHz from TF.

OK, now, how do we add any new signals to the A3E signal so that we can put the digital signal in place? We cannot place those signals within 3kHz of the carrier without going to a LOT of trouble, otherwise analog radios will "hear" the digital signal as noise. So what IBOC does is exploit that "no man's land" from 3kHz away from carrier to 5kHz away from carrier to put the digital signal in. Now, your old analog radio will still "hear" these signals to an extent, but between the attenuation of the receiver bandwidth and the attenuation of the audio chain, this noise will not be very perceptible.

HOWEVER - remember how I said there were no perfect "brick wall" filters? Well, that applies to transmitters too. The transmitter may be putting signal into the 3 kHz to 5 kHz region, but it will put some unwanted signals beyond 5kHz (they will just be very weak compared to the desired signals) - and that means into the frequency band of the next guy on the dial. However, if the next guy is far enough away in space, your signals that are in his band (which are already weak) will be weakened further by distance, and won't be perceptible by the other guy's listeners. Also, your signals that are in the 3kHz to 5kHz area will be weakened by distance, and attenuated by the receiver's filters, and so they, too, won't be very objectionable.

Except in the fringes between where your spatial region stops and his starts. That's what is happening here - if you are close to "the other guy" you won't hear the interference, but if you are far from him, and close to the digital station - you get noise where there was none before.

Add to this the fact that the stations that are going digital are the stations with money - and how do you get money? By having lots of listeners. How do you get lots of listeners? Among other things, by having lots of POWER <Tim Allen Grunt>. The little station