FM 87.7 Signal Question

[JayDavis]

I drive a lot between Dallas and Austin. I've noticed as I'm driving down, there is an FM station
around Waco or even a tad south of Waco on FM 87.7 and it basically plays the audio from the NBC
Television Network. There is a NBC TV Station in the area, KCEN-TV and they are right on the highway
of I-35. Yet, at the top of the hour, aren't all radio stations supposed to give their station ID.

Is this just a weird radio occurence that I am picking up the audio portion of the NBC TV station?

Just curious.

 

[Gridlock Joe]

Not a weird coincidence. You are actually listening to the audio portion of Channel 6, which is broadcast at 87.75 MHz.

 

[radi0chik]

Not a weird coincidence. You are actually listening to the audio portion of Channel 6, which is broadcast at 87.75 MHz.

Wow, LTNS Joe! But yes Joe is correct, theres a TV channel 6 down there which is an NBC affiliate. TV stations don't have to ID "out loud" - they're allowed to put up a graphic or superimpose their call letters/city of license on the screen - so if you were JUST listening to the audio, you might never "hear" it ID, because it's hard to hear graphics

 

[JayDavis]

Do other TV stations put out their audio on a FM signal? Why would a TV station do that and if they are broadcasting on a radio airwave channel, wouldn't they still have to give a station ID at the top of every hour?

 

[jd]

Do other TV stations put out their audio on a FM signal? Why would a TV station do that and if they are broadcasting on a radio airwave channel, wouldn't they still have to give a station ID at the top of every hour?

Every TV station on channel 6 broadcasts their audio on 87.75 (give or take a little either way). It isn't really a separate FM signal; it's where the audio is assigned within the entire channel width, which in the case of channel 6 is between 82 and 88 mHz. The FM band in the U.S. technically starts at 88.1 mHz (although there are a couple of exceptions with non-commercial stations operating on 87.9). Channel 6 TV audio is the only one close to the regular FM band, and it just so happens that your radio is tunable down to 87.7.

 

[radi0chik]

I think you're missing the point. 87.7 is NOT the FM band. It's BELOW the FM band. Broadcast band for FM starts at 88.1 (minus the 3 class D stations on 87.9 in the US) for all practical purposes. You're actually hearing the tv transmitter on the frequency it's licensed to broadcast on. Any other part of the US that there's a channel 6 licensed, you'll hear their audio too. It's all on that frequency. None of the other tv stations have audio carriers that a normal broadcast radio can pick up.
See the info under North America's entry in Wikipedia about VHF frequencies:

http://en.wikipedia.org/wiki/Very_high_frequency

 

[arnoldevns]

Of course this will go away when the digital TV transition takes place in February of 2009.

 

[JayDavis]

You know what? I'm getting old! I should have remember that fact about FM radio only going down
to 88.1 from my days at Mass Communications at Texas Tech.

Senior moment, I guess. Thanks to everyone for sharing the answer.

Now here is another question. Why would my radio even go down that low if 88.1 is the lowest number
on the FM dial for radio?

 

[TheRover]

Do other TV stations put out their audio on a FM signal? Why would a TV station do that and if they are broadcasting on a radio airwave channel, wouldn't they still have to give a station ID at the top of every hour?

Every TV station on channel 6 broadcasts their audio on 87.75 (give or take a little either way). It isn't really a separate FM signal; it's where the audio is assigned within the entire channel width, which in the case of channel 6 is between 82 and 88 mHz. The FM band in the U.S. technically starts at 88.1 mHz (although there are a couple of exceptions with non-commercial stations operating on 87.9). Channel 6 TV audio is the only one close to the regular FM band, and it just so happens that your radio is tunable down to 87.7.

My Accurian Radio Shack Digital HD Tabletop begins tuning at 87.7 and goes to 107.9.

 

[fifthestate55]

Yep CH.6's audio wasa prime reason why Baylor's FM is around the 107 range. They were afraid of it clashing. At times around 87.7 if you are close to an airport ,you can hear air traffic controllers.

 

[PTBoardOp94]

Why would my radio even go down that low if 88.1 is the lowest number on the FM dial for radio?

Here's some circuitous (hah) logic:
So people can listen to the audio from Ch. 6.

 

[JayDavis]

Why would my radio even go down that low if 88.1 is the lowest number on the FM dial for radio?

Here's some circuitous (hah) logic:
So people can listen to the audio from Ch. 6.

Nah !!

Although I have played audio Jeopardy while driving in that area.

 

[CW]

Yep CH.6's audio wasa prime reason why Baylor's FM is around the 107 range. They were afraid of it clashing. At times around 87.7 if you are close to an airport ,you can hear air traffic controllers.

HUH???? Aircraft band is 108-136MHz....and is AM....kinda hard to hear that on a FM receiver.....the Local Oscillator in a FM radio is HIGH side of the signal (RF channel +10.7MHz) so a radio tuned to 107 or so would hear an image around 21MHz higher...or 128MHz....but tuned to 87.7?? No way! Baylor's FM was allocated by the FCC...there are many places where FMs at 88.xx are in a Channel 6 TV area....so your statement above leaves a lot to be desired.

 

[CW]

Every TV station on channel 6 broadcasts their audio on 87.75 (give or take a little either way). It isn't really a separate FM signal; it's where the audio is assigned within the entire channel width, which in the case of channel 6 is between 82 and 88 mHz. The FM band in the U.S. technically starts at 88.1 mHz (although there are a couple of exceptions with non-commercial stations operating on 87.9). Channel 6 TV audio is the only one close to the regular FM band, and it just so happens that your radio is tunable down to 87.7.

Ahh actually, Analog TV audio IS a separate FM signal....it is always 4.5MHz above the video signal...but it is a totally separate exciter in a lot of cases with older transmitters. Some newer ones combine at low level and then push the video and audio carriers through linear amp stages...though this is not as efficient as separate video and aural carriers then combined at high level RF levels. The FM frequencies are actually channeled in the FCC rules. Channel 200 is 87.9, 201 is 88.1 and so on to Channel 300 which is 107.9. As you stated, 87.9 has only been assigned to two stations that I know of in the US..and that was because of circumstances where they had no other place to go from their previous channel.
Japan's FM band extends down to 77MHz iirc.....and who knows...ours MAY if they do away with TV 2 thru 6 after the DTV transistion and the remaining 5 stations per lowband channels nationwide get off the things!! Actually 82-88MHz would be perfect for LPFMs to operate....and translators...AHH that would get the God Radio operators behind that idea big time..hear Praise the Lord, Pass the Loot over most of your analog TVs on channel 6

 

[thathoustonradiogeek]

Japan's FM band extends down to 77MHz iirc.....and who knows...ours MAY if they do away with TV 2 thru 6 after the DTV transistion and the remaining 5 stations per lowband channels nationwide get off the things!! Actually 82-88MHz would be perfect for LPFMs to operate....

That'd be so cool. I'd love to witness such an event in US broadcasting history.

Maybe the channels could be renumbered again. We could have channel 1 back!
(I'm serious)

Praise the Lord, Pass the Loot over most of your analog TVs on channel 6

Sing that to the tune of "Get Your Kicks on Route 66".

 

[Lancer]

I used to have my XM fm modulator set on 87.7, and I would have to change the frequency if I went from Dallas to San Antonio. Same place.