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HD-only FM Boosters

N

nd2023

Banned
Could there be FM boosters that only repeat the IBOC part of a station's broadcast so that HD doesn't drop out in the area of the booster? The booster could only broadcast one sideband so that it won't interfere with HD reception where the main and booster signals overlap.

I know that it's economically unfeasible to operate a booster just to serve 1 more person with an HD radio, but let's assume that HD is widely adopted and only focus on the technical aspects.
 
They did a few tests on this SFN idea which boost the digital only and for digital listeners it was seamless transition between booster and parent station. I think it still interfered with the analog signal when you were within 40 meters of it. Good suggestion about boosting one side band only instead of both the USB and LSB so as to lessen the interference impact. NPR labs also suggested using SFN in some of their IBOC reports but didn't mention using one sideband though.
 
briankay said:
They did a few tests on this SFN idea which boost the digital only and for digital listeners it was seamless transition between booster and parent station. I think it still interfered with the analog signal when you were within 40 meters of it. Good suggestion about boosting one side band only instead of both the USB and LSB so as to lessen the interference impact. NPR labs also suggested using SFN in some of their IBOC reports but didn't mention using one sideband though.
Click to expand...

SFN is a good idea, but the hybrid digital system we're stuck with (indefinitely) will make it very difficult to implement without affecting analog FM reception.

You mention a test in which analog interference was limited to "40 meters", but if this was indeed the extent, then I doubt it did much to improve digital coverage. Here's why:

Let's conservatively assume a line-of-sight path from booster to receiver, and also assume that the analog signal would be adversely affected if the digital field strength from the booster (first-adjacent interference on both sides) exceeds analog by 6 dB. This is 12 dB worse than the FCC allocation standard which applies to first-adjacent FM stations.

At a radius of 80 meters from the booster, the digital and analog field strengths should be nearly equal ("inverse square law" says the received power drops 6 dB each time distance is doubled)

So at a radius of 160 meters, the digital will be 6 dB less than analog.

At 320 meters, the digital field will have fallen to 12 dB less than the analog, and at 640 meters, it will be down 18 dB.

Therefore, at just 1 km (0.6 mile) from the booster, the power received from the booster will be less than the -20 dBc digital signal coming from the station's main site. At such a low level (as necessary to keep analog from getting trashed), this is not likely to make a significant improvement in digital reception.
 
So in short, HD only boosters would be useless if radios are mostly analog except perhaps in cities where a kilometer covers a lot of people.

But if every radio were an HD radio, an HD only booster would be useful, because the analog signal wouldn't matter if the HD signal is strong enough to interfere with the analog: the HD would simply be decoded.
 
So let's say the parent station is 103.5 is a class B at 20 miles away and the local digital only HD booster (no analog) is 2 watts digital total power (each of the 10 IBOC carriers will be 0.2 watts in strength) and is 200 meters away and is broadcasting on the upper sideband in the ranging from 103.6 to 103.7 MHz . Are you saying in a typical auto FM tuner assuming line of sight to both transmitters that the parent analog will not be listenable over a large area or just in the immediate vicinity of the digital booster?

Of course the booster would typically used where there is terrain shielding of the parent station which would be the reason for the poorer digital reception in the first place. Radios with poor selectivity may suffer further out. But should technical advances be stopped just to facilitate the receivers with the worst selectivity?
 
briankay said:
So let's say the parent station is 103.5 is a class B at 20 miles away and the local digital only HD booster (no analog) is 2 watts digital total power (each of the 10 IBOC carriers will be 0.2 watts in strength) and is 200 meters away and is broadcasting on the upper sideband in the ranging from 103.6 to 103.7 MHz . Are you saying in a typical auto FM tuner assuming line of sight to both transmitters that the parent analog will not be listenable over a large area or just in the immediate vicinity of the digital booster?
Click to expand...

What I'm saying is: In the immediate vicinity of the digital booster, in order for it to avoid significant interference to the analog signal (which is transmitted from the main site only), its power would need to be reduced to such a low value that it would be of little help in improving digital coverage. In the example you cited, it would only improve coverage within a radius of 1 km.

Of course the booster would typically used where there is terrain shielding of the parent station which would be the reason for the poorer digital reception in the first place. Radios with poor selectivity may suffer further out. But should technical advances be stopped just to facilitate the receivers with the worst selectivity?
Click to expand...

This boils down to a value judgement. Would the "technical advances" of the digital booster offset the loss of interference-free analog coverage near the booster? What do you think?
 
If there are enough HD radios (over 75% of radios are HD) in the market then yes, digital boosters will help since on an HD radio, if the IBUZ from the booster is strong enough to bleed to the analog, then the radio will be in HD mode.
 
Nick said:
If there are enough HD radios (over 75% of radios are HD) in the market then yes, digital boosters will help since on an HD radio, if the IBUZ from the booster is strong enough to bleed to the analog, then the radio will be in HD mode.
Click to expand...

Circular firing squad comes to mind. Where did you get that 75% number?
 
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