Digital Hash Impacting Adjacent Frequencies?

[louis_p]

When scanning the FM dial a few weeks ago here in Vancouver, I noticed that an existing station started duplicating their programming on another frequency. The station is CKYE-FM (known on air as RED) which broadcasts a signal on 93.1 covering Metro Vancouver. The station started broadcasting the same programming on 89.1 with an overlapping signal in Surrey (10 miles away).

CKYE claims that this is necessary because KISM-FM in Bellingham, WA which broadcasts on 92.9 enabled HD Radio which interferes with adjacent frequencies. I haven’t noticed any interference on 93.1 from where I live but 92.9 also transmits a strong signal into Vancouver. There must be some impact for CKYE to go through the trouble and capital required to fire up a new transmitter.

As crowded as the FM dial is here with local stations and adjacent signals spilling over from Victoria and Washington state, I can’t imagine how much more crowded the dial would become if stations had to power up additional transmitters to maintain their protected coverage area simply because a neighbouring station enabled HD.

I don’t have a HD-capable radio but I’ve tried to listen for the digital interference on my analogue radio but I haven’t noticed anything.

I’m surprised that the FCC/CRTC would allow a station to encroach on another stations protected contour just to accommodate a limited HD listening audience.

Here’s the decision from the CRTC:
https://crtc.gc.ca/eng/archive/2016/2016-464.pdf

 

[DaveBayArea]

What they say is true. HD radio is not really "in band on channel". It uses adjacent-channel frequencies to transmit digital carriers. Initially those carriers were 20 db down and barely met the mask for spurious radiation set forth in 73.317 by the standards of the day. At that point a small percentage of stations experienced interference issues, but they were not significant. Lab tests had shown that HD coverage at -20 db was about equivalent to analog. In the real world, however, that didn't happen. So the HD injection level was raised to -14 db, and even -10 in some cases to increase the range of the digital signal. They came up with the creative argument that each individual carrier meets the requirements of 73.317 and you have to measure it with a spectrum analyzer set to 1 khz bandwidth. Essentially what this did was to increase the noise floor on the FM band, and now pretty much every market has adjacent channel reception problems for some stations when listening in cars. Even so, the whole concept still works fairly well. It only falls apart when signals are attenuated or enhanced due to terrain. That's definitely true in Vancouver, and I suspect that the terrain is causing significant interference in certain areas, and that is why they requested the additional transmitter. More on the actual HD transmission system here:

https://www.nrscstandards.org/standards-and-guidelines/documents/standards/nrsc-5-d/nrsc-5-d.pdf

The spectrum characteristics are detailed on page 34 of that document. In the case of KISM I can see where the digital carriers on 93.1 would be strong enough to cause interference.

Dave B.

 

[rbc]

That's one of the hangups with manufacturers churning out analog radios. It just lengthens the amount of time that stations are encouraged to run hybrid digital. Once stations start to go all digital, that should help stations stay in their channel, reduce interference and make the spectrum management easier.

 

[DaveBayArea]

That's one of the hangups with manufacturers churning out analog radios. It just lengthens the amount of time that stations are encouraged to run hybrid digital. Once stations start to go all digital, that should help stations stay in their channel, reduce interference and make the spectrum management easier.

True for AM. Not for FM. I'm not sure why, but the spec for all-digital FM shows INCREASED power in the sidebands, with the power density down by 20 db at +/- 100 KHz of the assigned frequency. I did a double-take, and maybe it's a misprint. But that's what it shows.

Dave B.

 

[rbc]

True for AM. Not for FM. I'm not sure why, but the spec for all-digital FM shows INCREASED power in the sidebands, with the power density down by 20 db at +/- 100 KHz of the assigned frequency. I did a double-take, and maybe it's a misprint. But that's what it shows.

Dave B.

Well, I think an advantage of the FM band is that HD sub-channels will replace translators over time, as HD radios are deployed. As translators are decommissioned, that ought to help with the interference.