FM Self Interference with HD power above -20dbc

[BobOnTheJob]

I have only 1 station to base this on, but a station 30 miles from me upped their HD power to -14dbc several weeks ago and indeed the HD coverage increased to perhaps the 50dbu circle on a car radio. But...when the HD signal drops, there appears to be a substantial amount of spitting noise in the analog signal. In the 40-50dbu region, it's often so bad that I can't listen to it--and I'm a DXer with a high tolerance for interference. While I doubt that the station in question gets any revenue from beyond the 50dbu circle, their talk shows get a load of callers from there. The analog sounds fine in the local area where the money is. Has anyone else observed interference to the analog signal where HD power has increased from -20dbc? And wasn't the -20 figure selected because that was determined to be the highest level at with no self interference would be expected to occur?

 

[Tom Wells]

I live in a very high airplane activity zone. I live directly under the approches to O'Hare, and the drive to work
puts me about 20 miles NW. I haven't kept up with who in Chicago has upped digital power,
but there is so much multipath from airplane traffic, that with iboc the the entire FM band has gone "spitty",
exactly as you describe. I commented a long time ago about it and was attacked by the usual supporters of iboc.

I liken it to the beautiful city grade solid FM signal now dropping out in spits and spurts just like it did when
I was living-working 45-50 miles out of the city, back in the 70s and 80s.

Sounds to me just having having imported fringe noise characteristics to all but the most very local listeners.

 

[jhardis]

wasn't the -20 figure selected because that was determined to be the highest level at with no self interference would be expected to occur?

There is also an issue of interference to the first adjacent channels, since the sidebands lie in the first adjacent channels. When NPR ran their AICCS study to determine how powerful the sidebands could be, this is what they were testing.

- Jonathan

 

[rbrucecarter5]

The HD FM self interference problem has been widely discussed on DX and audiophile technical forums. The extra 6 dB only makes a bad situation even worse.