Radio Engineering 101

Paul Thurst, writing in the excellent Engineering Radio blog, reveals a talent for explaining the ineffable in radio engineering to the typical neophyte. Here he gives a graphic example of the damage HD radio does to adjacent channels. As he notes, engineers know exactly what the following picture shows:

Then, for those of us engineering-challenged readers, he explains what exactly this shows:

On the left hand side of the screen is the signal strength scale. Each vertical division is 10 dB. This is not absolute signal strength, it is referenced to -20 dBm. However, it gives a good relative signal strength for both the analog carrier and the IBOC carriers. The analog carrier is centered on the screen, it slopes upward like a steep mountain, peaking at -50 dBm relative. The IBOC carriers are on either side of the analog carrier, they are flat, approximately 75 KHz wide and peak approximately 20 dBm below the analog carrier (-20 dBc). For some reason, likely the bandwidth and/or impedance match between the antenna, high level combiner and the two transmitters, the left IBOC carrier is actually peaking around -14 dBc.

The span, as noted on bottom right hand side of the screen is 500 Khz. Each horizontal division is 50 KHz. The entire span of the measurable signal is eight horizontal divisions, thus 400 KHz.

As noted above, the allocated channel bandwidth is 200 KHz, thus this station is exceeding it’s allocated bandwidth by 100%. This is allowed under CFR 73.404, which is titled “Interim hybrid IBOC DAB operation.”

IBOC proponents will make the argument the FM radios work on something called “The capture effect,” which is to say that if two signals are on or close to the same frequency, only the stronger signal will be demodulated. Thus, the IBOC carriers have no effect on the adjacent channels that they are interfering with so long as the adjacent signal is stronger than the IBOC carrier. The argument is further carried forward by assuming that with a stations protected contour (60 dBu in most cases), the IBOC carrier will never exceed that analog carrier.

That is not necessarily true, especially in areas where terrain (and buildings, underpasses, unintentional directionality in transmitting antenna, etc) can attenuate signals close in causing the IBOC signal to become equal to or stronger than the adjacent analog signal. This effect causes picket fencing. Lower powered FM stations; class A, LPFM, etc, are especially vulnerable to this effect.

Further, even in areas where the analog carrier is stronger than the IBOC carrier, the noise floor has been moved from -100 dBm or so to -70 dBm, which is a 1,000 times greater. To assume that raising the noise floor by 1,000 times will have no effect is, as they used to say in the Navy, making an ASS out of U and ME. Mostly you, in this case. This effects the receiver by making it less sensitive, it will also add noise to the demodulated signal as the elevated noise floor will show up as background hiss. Even further still, higher IBOC carrier levels, as authorized by the FCC in January of 2010 can interfere with the stations own analog carrier.

Got it? There’ll be a quiz next week. Paul then quotes some iBiquity and FCC swag about “enhanced sound fidelity, improved reception, and new data services,” in addition to this: “IBOC was designed to bring the benefits of digital audio broadcasting to analog radio while preventing interference to the host analog station and stations on the same channel and adjacent channels.” Then the pièce de resistance:

Few if any of those goals have been met. As far as the forward/backward compatible thing, it just isn’t so unless a person actually owns an HD Radio. As noted in previous posts, few consumers have seen fit to purchase an HD Radio, nor have car manufacture’s taken to installing them en mass in new cars, so there is no forward compatibility. Instead, we have FM radio stations interfere with each other and themselves in an attempt to “modernize” the audio broadcasting business. This is a bigger problem for small, community radio stations that can neither afford to install the expensive, proprietary HD Radio system, or broadcast quality receivable signals with an adjacent HD Radio signal raising the noise floor by 1,000 times or more.

I can think of no other greater threat to free over the air broadcasting than HD Radio and the degradation of AM and FM services that comes with it. The consumer has shown that they don’t care. If given the choice between free over the air broadcasting that has mediocre programming and is full of interference, and some type of paid internet streaming service that sounds reasonable with good programming, they’ll go for the latter.

In short, some cobbed together digital modulation scheme is the last thing that radio needs right now.

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2 Responses

  1. The number of listeners that turn off the radio due to interference outweighs the few that actually listen to digital radio.

  2. IBOC is a monster. This is a dangerous balancing-act between destroying radio altogether, versus forcing this crap down our throats. My guess is that listeners have no clue what is going on, and will just tune away from any interference. Listeners have obviously lost reception of the smaller, adjacent channel stations, and are being forced to listen to the local HD Radio stations, only.

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