Analog TV transmissions

Under the original NTSC-M analog standards, a TV station's audio and video components are broadcast separately, with the audio transmitted as an FM signal. In 1945, TV channel 6 was assigned use of 82-88 MHz,^[10] with the channel's audio located at a center frequency of 87.75 MHz. That same year the standard FM broadcasting band was reassigned to 80 channels from 88.1 to 105.9 MHz, which was soon expanded to 100 channels ending at 107.9 MHz (channels 201–300).^[11][12] One additional FM channel, at 87.9 MHz (channel 200), was added in 1978.^[13]

The location of TV channel 6's audio just below the FM band meant that many consumer FM radios could readily pick up the sound from channel 6 stations. Full-power stations sometimes promoted this as a valuable feature for commuters and in emergency situations, although the primary audience remained TV viewers. A few specialty "TV Sound" receivers were sold that could pick up audio from the VHF band TV stations, which were sometimes marketed to office workers who wanted to listen to their favorite daytime TV programs.^[14] Also, some Japanese and Soviet receivers could tune further down the VHF low band, making it possible to receive audio from VHF channel 5 at 81.75 MHz using Japanese band receivers that tuned to 76 MHz, while Soviet OIRT band receivers included audio reception of VHF channels 4 (71.75 MHz) and 3 (65.75 MHz).

In 1982, licensing began of a new classification of "Low Power Television" stations (LPTV).^[15] Eventually a small number of channel 6 LPTV stations determined it was more profitable to operate as de facto radio stations, although this had not been anticipated by the FCC. On August 1, 2002 KZND-LP, a channel 6 LPTV station in Anchorage, Alaska, began promoting itself as a musical format radio station on 87.7 MHz.^[16] Competing local conventional radio stations quickly challenged this as a misuse of a TV station authorization, but the FCC determined that as long as KZNP transmitted sufficient visual features, which did not have to be coordinated with the sound, the station was in compliance with regulations. Moreover, as a TV station it was exempt from having to follow any radio station rules.^[17] In early 2008 Monitoring Times magazine reported the existence of three channel 6 de facto radio stations in addition to KZND: KSFV-LP in Los Angeles; K06NC in Kauai, Hawaii; and WNYZ-LP in New York City.^[18] A Radio World review later that year identified a total of eight ongoing examples.^[19] In late 2019, InsideRadio identified 28 active stations.^[20]

While operating using analog transmissions, television video commonly consisted of minimal offerings, such as still frames, test patterns, automated weather conditions and news, or silent films. Audio for both television and FM reception was identical. (TV signals potentially could have carried separate audio through their Second audio program (SAP) feeds, although none did so as of 2019.) With the mandatory 2009 digital television transition in the United States, full power TV stations were required to switch from analog to ATSC 1.0 digital transmissions. This meant that their audio could no longer be picked up by FM radios, as the ATSC 1.0 format is incompatible with both FM's analog and digital in-band on-channel HD Radio standards.

Digital TV transmissions

Attempts to have analog FM signals coexist with the original ATSC 1.0 digital standards proved unsuccessful. In 2009 WRGB, a full-power channel 6 station in Schenectady, New York, was ordered by the FCC to cease experimentation.^[21][22] In 2012 Venture Technologies Group, which owns several channel 6 low power TV stations in major markets, applied to install modified versions of the ATSC 1.0 standard in order to add an FM signal for channel 6 LPTV stations KFMP-LP in Lubbock, Texas and WBPA-LP in Pittsburgh, Pennsylvania. However, the FCC denied this request, stating that, among other deficiencies, this "proposal is likely to increase the interference potential to co-channel DTV operations".^[23]

A newer, and currently optional, digital TV transmission standard, "NextGen TV" ATSC 3.0, was determined to have more potential, and Venture Technologies developed a revised approach for allowing an analog FM audio subcarrier to coexist with an ATSC 3.0 digital TV signal.^[1][24] The company cited the FCC rules, which provides that "DTV broadcast stations are permitted to offer services of any nature, consistent with the public interest, convenience, and necessity, on an ancillary or supplementary basis", and in general affords broadcasters broad permission "to offer services of any nature" as long as they "do not derogate DTV broadcast stations' obligations" to transmit at least one over-the-air video program signal at no direct charge to viewers,^[25] a distinction that Venture says allows a digital television signal to incorporate an FM analog subcarrier.^[26]

The basic ATSC 3.0 standard specifies a full 6 MHz channel for the digital signal, but running a hybrid DTV/FM service reduces the DTV transmission bandwidth slightly, shifting the center frequency of the digital signal about 160 kHz below the channel center.^[27] To ensure the analog FM signal does not interfere with the ATSC 3.0 DTV signal, a modified combiner and filtering system must be used prior to feeding the broadcast signals to the antenna.^[28]

A switchover to digital transmissions was not immediately required for low power TV stations, and some channel 6 stations retained their analog transmitters in order to function primarily as radio stations. The FCC notified these stations that eventually all low power TV stations would be required to convert to digital transmissions, which, after a series of extensions, was set for July 13, 2021.^[29] On July 6, 2021 an FCC Public Notice reiterated that "After 11:59 p.m. local time on July 13, 2021, LPTV/translator stations may no longer operate any facility in analog mode and all analog licenses shall automatically cancel at that time, without any affirmative action by the Commission."^[30] (A waiver, delaying the required change until January 10, 2022, was issued for 15 Alaska translators, but a low-power Alaska station broadcasting on channel 6, KNIK-LP in Anchorage, was not included in this waiver.)^[31] The prohibition of LPTV analog transmissions resulted in the elimination of an estimated 28 de facto radio operations,^[32] although some of these stations eventually returned after installing upgraded ATSC 3.0 transmitters.

Venture's KBKF-LD in San Jose, California began transmitting an FM subcarrier using the ATSC 3.0 TV standard in February 2021. On June 10, 2021 the FCC issued a six-month Special Temporary Authority (STA) grant allowing KBKF-LD to include analog FM broadcasts on 87.75 MHz using this dual transmission approach.^[33] A second Venture station, WRME-LD in Chicago, was also provisionally authorized to use the new standard, until January 10, 2022.^[32]

STA applications for ATSC 3.0/FM operation by KXDP-LD in Denver,^[34] WMTO-LD in Norfolk, Virginia^[35] and WTBS-LD in Atlanta^[36] were approved July 16, 2021, for the period through January 15, 2022. On July 27, 2021 an STA was issued for KZNO-LD in Big Bear Lake, California, expiring on January 27, 2022.^[37]