W9WI.com/"FrankenFMs" and the FCC regulations

WARNING: the author is an engineer, not a lawyer. Anyone planning on actually operating a channel 6 LPTV as a FM radio station needs to consult a communications lawyer. My interpretation of the FCC regulations may not agree with the interpretation of Commission staff; obviously, it's what the staff believes that matters!

"FrankenFMs" are stations licensed as channel 6 low-power TV stations, but operating as FM radio outlets. The first such station was KZND-LP (now KNIK-LP) in Anchorage, Alaska; today, there are several such stations operating throughout the United States.

This kind of operation is possible because TV channel 6 directly borders the FM broadcasting band. TV-6 analog audio is broadcast at 87.75MHz (+/-10KHz). Since the FM broadcasting band extends down to 87.5MHz in several countries, many receivers sold in the U.S. will tune 87.7 or lower.

There are also some important differences between the technical standards established in the rules for TV sound, and those for FM radio. Importantly, **some of these rules do not apply to low-power stations**.

(also importantly, "low-power" is a specific class of license. Some "low-power" stations are actually more powerful than some "full-power" operations.)

There are no longer "full-power" analog TV stations in the U.S.. However, the technical regulations applicable to such stations are still published, and still referenced by the low-power rules.


FCC regulation 73.603 establishes the boundaries of TV channel 6 as 82-88MHz. 73.682(a)(2) then establishes the visual frequency of channel 6 as 83.25MHz. ("nominally 1.25 MHz above the lower boundary of the channel.") Finally, 73.682(a)(3) establishes the *aural* frequency of channel 6 as 87.75MHz. ("shall be 4.5 MHz higher than the visual carrier frequency.")

For low-power ("LPTV") stations, regulation 74.761 requires the visual frequency be within 0.02% or 0.002% of the "assigned visual carrier frequency". It then also requires the aural carrier be 4.5MHz higher. For a channel 6 LPTV station, with an assigned visual carrier frequency of 83.25MHz, this again requires the aural frequency be 87.75MHz.

(for both low-power and full-power stations, some stations are required to operate on visual frequencies offset by 10KHz from the "nominal" 83.25MHz frequency. In these cases, the aural frequency is also offset 10KHz in the same direction.)


73.614(b)(1) and (b)(2) establish a maximum visual power for channel 6 "full-power" analog stations of 20dBk, or 100,000 watts. (0dBk is one kilowatt)

74.735(a)(1) limits the maximum power of a channel 6 analog LPTV to 3,000 watts. This regulation doesn't specify whether that limit applies to the visual transmitter, or the aural -- it would appear to apply to both.

The aural power of an analog station is generally lower than the visual power. (less power is required to transmit the narrower audio signal) 73.682(a)(15) limits the aural power of a full-power analog station to 22% of visual power. (any figure less than 22% was permissible. 10% was VERY common, but 15% or 20% was not particularly unusual.)

There is no mention of aural power limits in the LPTV rules in Part 74. Regulation 74.780 is a list of full-power rules which are also applicable to LPTV stations. However, 73.682 is not on this list. This means that (in my opinion) there is no limit on the aural power of a LPTV station, except for the overall 3kw limit applicable to both aural and visual signals.


Regulation 73.1570(b)(3) defines 100% modulation of TV sound as +/-25KHz. (and prohibits the modulation from exceeding this level on frequent peaks) This figure is 1/3 of the +/-75KHz figure specified for FM radio in 73.1570(b)(2). As a result, TV sound as received on an FM radio would appear to be only 33% modulated -- very soft and subject to noise.

However, 73.1570 is also not on the list of full-power rules which are also applicable to LPTV stations. This would appear to make it legal to operate the aural transmitter of a LPTV station at +/-75KHz modulation -- 100% modulation as received on an FM radio. There is no other regulation in the LPTV rules limiting this modulation.


The stereo systems defined for FM and TV are different.

The FM stereo system is defined in 73.322. This regulation establishes the pilot frequency as 19KHz, the subcarrier frequency as 38KHz, and does *not* define any companding of the L-R signal.

The TV stereo system is not defined in the rules. The FCC prohibits the use of 15.734KHz as the pilot frequency unless the BTSC standard is used. (regulation 73.682) The BTSC standard also specifies the subcarrier frequency as twice the pilot (~31.5KHz) and defines DBX companding of L-R. An FM radio will not decode a BTSC TV stereo signal in stereo. (it will however reproduce it OK in mono.)

Note that the BTSC standard is only required if the pilot frequency is 15.734KHz. This means that another standard may be used if the pilot frequency is 19KHz -- it means that it's legal to use the FM stereo system on TV.

Again, 73.682 is not incorporated into the LPTV rules -- so even the regulation requiring use of BTSC if a 15.734KHz pilot is used does not apply to LPTVs.


At one time, the regulations required the program material on the aural transmitter be related to that on the visual. Over the years this regulation resulted in some disputes, when stations wished to transmit a visual news "scroll" accompanied by background music, or to use their aural transmitter for one channel of a stereo simulcast with a co-owned FM station. (FM stereo, using only one FM station to transmit both channels, has only existed since 1961.)

This regulation has since been repealed. 73.653 states: "The aural and visual transmitters may be operated independently of each other or, if operated simultaneously, may be used with different and unrelated program material."

The current wording certainly seems to legalize the broadcast of a text "scroll" accompanied by unrelated music. (it also would legalize the use of the TV sound transmitter as one channel of a stereo simulcast, although with single-transmitter FM stereo on the books for over 50 years, there would be no point!)

It would also appear to allow the transmission of *only* an aural signal -- it would seem to be legal to not transmit any picture at all.

However, after receiving several complaints, the FCC required KZND to transmit a visual signal. Existing "FrankenFMs" do so -- transmitting identification slides, or nature scenes, or other unrelated video.


LPTV stations which, as of November 28, 1999, broadcast at least 18 hours a day, and which broadcast at least 3 hours a week of local programming, qualified for Class A status under 73.6001. These stations cannot be bumped from their channel by full-power stations. (except as necessary for the DTV transition.....)

Regulation 73.6024 incorporates 73.682 into the Class A rules. This means Class A stations have the same aural power limit applicable to full-power stations: the aural power may not exceed 22% of visual. (660 watts, for a station of maximum power on channel 6)

73.1001 states that anything in Subpart H - 73.1000 through 73.4280 - is applicable to Class A stations. This includes 73.1570, the rule that limits TV sound to +/-25KHz deviation.

These two requirements would suggest that Class A stations are not suitable for "FrankenFM" service -- their power is limited to 22% of that of a "regular" LPTV, and their modulation is limited to 33%.

I count seven authorized analog Class A stations on channel 6. Strangely, one is a station believed to be operating as a "FrankenFM".


As you might guess, digital TV signals cannot be received by analog radios. Digital "HD" radios are now available. However, the "HD Radio" standard is VERY different from the digital TV standard. Digital TV signals cannot be received on HD Radio receivers.

Full-power TV stations were required to cease analog transmission after June 12, 2009. After that date, full-power TV sound could no longer be received on FM radios.

Low-power and Class A stations, on the other hand, were allowed to continue broadcasting in analog. As a result, such stations on channel 6 can still be received on FM radios.

74.731(l) will sunset this operation. After September 1, 2015, low-power and Class A stations will not be allowed to continue analog broadcasts. They will be required to convert to ATSC digital transmissions -- which cannot be received on a radio -- thus bringing the "FrankenFM" era to a close.


Some engineers have suggested it might be possible to simultaneously broadcast an ATSC digital signal *and* an analog audio signal compatible with FM radios, at the same time, on the same channel. This would allow "FrankenFM" operation to continue past 2015.

Two stations are known to have tried such broadcasts. Full-power WRGB Schenectady, New York operated an analog simulcast of their DTV audio on 87.9MHz briefly in 2009. This transmission has long since ceased. It's not clear whether those transmissions were authorized by the FCC.

WNYZ-LP New York City also operated a dual-mode signal for some time. Given the number of full-power regulations that aren't applicable to LPTVs -- and the fact that 74.731 has not yet taken effect -- I strongly suspect WNYZ's operation was legal.

It would seem unlikely such a dual-mode operation would *work*. At least not on TV receivers. It would be difficult but very possible to filter out enough of the digital TV spectrum to open a hole for transmission of an analog FM audio signal. Most FM radios could probably reject the adjacent digital TV signal and deliver a useful FM audio signal.

However, TV receivers are expecting the digital TV signal to occupy the entire 82-88MHz channel. These receivers will pick up the powerful analog FM signal at the top of the channel -- and will, I would think, find it impossible to decode the TV signal through the interference.

That said... I have heard from a NYC TV engineer who says he *did* successfully decode WNYZ-LP's digital TV broadcasts on channel 6, while the analog FM audio was on the air.

So, dual-mode transmission might possibly work. Except that I'm pretty confident it won't be legal after 2015. From 74.731(l): "...may no longer operate any facility in analog (NTSC) mode." It would seem to me this wording would preclude any analog transmission, even if aural only.


By my reading, it is currently legal for a LPTV station assigned to channel 6 to transmit an analog aural signal compatible with FM stereo radios, and to do so at an aural power of up to 3,000 watts. (as long as interference with other stations doesn't result) By my reading, it will continue to be legal to do so until September 1, 2015. At that time, the "FrankenFM" service will go away, barring any changes in the rules. (IMHO very unlikely)

© D. Smith W9WI 2012

Page created 2012-Jun-20