HD noise on the FM band?

[Robert Bass]

There's a debate over in the HD board about noise being heard on the FM band from stations that are broadcasting in HD mode. I've heard of HD noise on the AM band, but is it also on the FM band?

R

 

[w9wi]

> There's a debate over in the HD board about noise being
> heard on the FM band from stations that are broadcasting in
> HD mode. I've heard of HD noise on the AM band, but is it
> also on the FM band?

Yes.

On FM it only extends into the first adjacent channel. (unlike AM where it extends into both the first and second)

You can hear it by comparing the sound on the first adjacent frequencies between a station that's running HD and one that isn't. On radios with decent selectivity (factory-installed car radios are your best choice), on a station without HD you should hear "sideband splash" - a VERY distorted version of the program audio - on the first adjacent frequency. On a station with HD you should hear noise, as if the channel was completely empty.

 

[westlife]

> You can hear it by comparing the sound on the first adjacent
> frequencies between a station that's running HD and one that
> isn't. On radios with decent selectivity (factory-installed
> car radios are your best choice), on a station without HD
> you should hear "sideband splash" - a VERY distorted version
> of the program audio - on the first adjacent frequency. On
> a station with HD you should hear noise, as if the channel
> was completely empty.

This is what makes FM IBOC noise rather insidious, and is likely why some people claim it doesn't exist: because it sounds very similar to the static hiss you get from an FM radio tuned to a completely blank channel.

In many parts of the country, a very selective car radio should be able to tune in a listenable signal on nearly every channel across the entire FM band -- except those channels which are adjacent to a nearby IBOC signal, which will sound like static. But that "static" isn't from no signal being received; it's from FM IBOC "hash" being received, drowning out whatever distant station(s) would've otherwise been listenable on that channel.
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[tpt]

The "hiss" has a different sound to it on my Ford car radio than empty channel hiss.

Here in Ohio we usually do have something on every channel, but some days the band dies, and you can hear the typical empty channel hiss. IBAC hiss is much louder.

 

[midatlanticengin]

> The "hiss" has a different sound to it on my Ford car radio
> than empty channel hiss.
>
> Here in Ohio we usually do have something on every channel,
> but some days the band dies, and you can hear the typical
> empty channel hiss. IBAC hiss is much louder.
>
Yes indeed it does! The hiss is slightly higher in pitch when compared to the white noise that you hear on an open FM channel.
Also depending on the quality of the radio and how close it is to a multiple number of high powered transmitters depends on how well the radio is able to reject this "noise" so as to not cause a problem with adjacent channel reception.

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-Mid-Atlantic Engineering Services of Utica NY-
http://www.geocities.com/midatlanticengineeringservice
--------------------------------------------------
Echolink RF node 56703/145.240,VHF repeater</P>

 

[tpt]

On WOSU I found that it carries out to around the 54 dbu contour

 

[TXengineer]

> On WOSU I found that it carries out to around the 54 dbu
> contour
>

Good selective car stereos can filter it out however.
I live right underneath a Class B FM running IBOC.
(8.9kW 93.1 MHZ line of site) and with my car stereo I can still get a listenable signal on 92.9 out of Boston, MA., 90 miles away without
much problem. Same thing with the Denon I have in the house.
However my father's car stereo can't seem to eliminate the noise from 93.1's side band.

So I believe radio design does do alot to clean it up.

And I'll be IBOC ready in two days when my new radio arrives. =)

 

[rbrucecarter]

VERY noticable. A good FM tuner does not produce much noise of its own. A good test is to disconnect the antenna, measure the noise level on a channel you know to be empty - then hook up an antenna. If you get more noise, you are listening to Jupiter or other sources of natural noise.

In the case of IBOC - three matched 110 kHz ceramic filters in the IF do little to alleviate adjacent channel IBOC noise. That is because the station is actually transmitting the equivalent of noise, and the tuner's IF is unable to reject a man-made signal.

Using a good yagi pointed at the distant station (or putting the local IBOC in the null) raises the received power of the distant station in relation to the IBOC sideband and can alleviate the problem to an extent. But IBOC is bad news for first adjacents any way you look at it.