So, what is the difference between AM, FM, and HD® radio?
AM, or Amplitude Modulation, was the mainstay of radio from its inception on through the 1960s. With AM, the transmitter frequency remains constant. The program audio changes the power output of the transmitter instantaneously. The transmitter is usually set up so that with no audio present, the transmitter puts out a little more than half of its licensed power. It’s done this way so that when the loudest audio is being broadcast, the negative peaks don’t drive the transmitter all the way to zero output when the positive peaks hit 100%. As mentioned in an earlier post, the relatively low frequency of AM allows it to bounce off the ionosphere and be heard at great distances, especially at night. The main disadvantage, though, is that AM is quite susceptible to environmental interference. Any electrical noise sources, such as power lines and especially lightning, are indistinguishable from an AM broadcast to an AM radio. If there is a component of the noise that falls onto the frequency of the station being tuned in, that component will be played along with the desired audio. AM is still going strong even though many of the music formats have moved to FM.
In Part 3, we talked quite a bit about FM Radio, so we’ll only touch on it here. Since FM radios are only sensitive to a frequency change within a specific range, most environmental noise is ignored. That’s the main reason that FM can be so quiet, even with a thunderstorm going on. As an aside, FM transmitter sites are often vulnerable to lightning. Since FM relies on line-of-sight transmission, FM transmit antennas are mounted on tall towers or atop mountains…sometimes both. Being high enough up to “see” a long way, makes the tower a great lightning rod. KPBX is located on a relatively short tower (less than 100 feet), but located on a tall mountain (Mica Peak, almost a mile above sea level.)
The newest kid on the block is HD® Radio. HD® Radio is the trademark of iBiquity Digital Corporation for their method of encoding digital data in what’s called the In-Band, On-Channel (IBOC) method. This is a fairly recent addition to the industry. HD® Radio technology can be applied to both AM and FM stations but so far, very few AM stations have adopted it. I think there’s only one in Spokane (KQNT). All three of Spokane Public Radio’s stations are transmitting in HD. We also are broadcasting in HD from KIBX in Bonners Ferry, ID.
For FM IBOC transmission, two identical sets of digital information are transmitted, one on either edge of the FM analog envelope. You’ve probably heard this digital data buzz when you tune your radio slightly off dead center. This digital information is transmitted at either 1/100th or 1/25th the power of the analog, depending on the transmitter and how it’s set up. Having the twin sets of data allows some error correction to be done in the receiver.
The main advantages of HD® Radio are improved audio quality and, most importantly, the ability to multicast. KPBX makes great use of this by broadcasting KSFC’s program as KPBX-HD2 and KPBZ’s program as KPBX-HD3. Licensing requirements state that HD1 must always mirror the program of the parent analog station.
There are a few disadvantages of IBOC transmission.
• IBOC requires higher cost transmitters and lowered efficiency if the same transmitter broadcasts both the analog and digital signals. KPBX and KSFC both use separate transmitters and antennas for analog and digital transmission. KPBZ, being relatively low power, uses a single transmitter and antenna to broadcast both signals.
• The digital encoding takes a long time, somewhere between 8 to 10 seconds, depending on how old and slow the equipment is. This requires us to delay our analog signal so that it matches up perfectly with the digital signal. This is done so that your program doesn’t jump back and forth in time when you’re listening near the edge of the digital signal’s effective range. Your radio will be “blending” back and forth between the digital and analog signals. Since there’s no backup analog for the HD2 and HD3 channels, they just drop out when the digital signal is too low.
• A third disadvantage is that transmitting the digital carriers adds some noise (buzz) that is detectable by some older analog radios that have a wide IF bandwidth. Most of the newer radios don’t have this problem.
So, we’re doing the experiment. More and more cars are offering HD® Radio as standard equipment. We’re hoping that more and more listeners will enjoy it in the car, at home, and at work.
Next time….Getting the signal from the studio to the transmitter.