TV Channel 6 was the ONLY channel where this trick worked because Channel 6 was the only television broadcast channel adjacent to the FM broadcast band. The signal it transmitted at 87.75 MHz was not a licensed FM radio station; it was part of the television broadcast station licensed to use TV Channel 6.
If by "it", you mean simultaneous transmission of a TV audio signal on an FM carrier in the FM broadcast band, sure it can be done. Any TV station can apply for an FM license for that purpose. And if it can prove to the FCC that such an authorization is in the "public interest, convenience, and necessity," it might even be able to get it.
But "public interest, convenience, and necessity" is a pretty high standard. The applicant would have to prove (this is the _bureaucratic_ reason) that such use of an FM broadcast channel would be a better use for the channel than a separately-programmed FM broadcast station.
Furthermore, the number of available FM channels is limited. At the very minimum, signals should be separated by 0.4 MHz (alternate FM channels); this imposes a maximum of 50 channels in a given market. Adjacent- and co-channel interference to or from distant stations imposes further restrictions. These factors severely restrict what can "_physically/technically_" be done.
Finally there's a _financial_ reason. Do you have any idea what an FM broadcast license is worth in a major market like Philadelphia? Offhand, I don't, but can tell you that major-market FM stations typically trade for prices in the millions . That little piece of paper that says FM BROADCAST LICENSE is often worth more that the entire physical assets of the station.
So even assuming that a TV station licensee could (a) prove to the FCC that an FM station carrying its audio signal would be the best "public interest, convenience, and necessity" use of an FM channel, and (b) come up with enough money to purchase a licensed station (or repurpose a co-owned sister FM), would it be a good investment?
Empirical evidence suggests that it's not.
Maybe after multichannel HD radio becomes widespread, it might be economically possible for an FM station to carry its sister TV station's audio on one of its digital subchannels. But I wouldn't count on it.
A related issue concerns the consumer electronics industry. If a market actually existed for a TV-audio-over-FM service, radio manufacturers could certainly incorporate TV tuners into FM radios. Some high-end multiband receivers incorporate such capability, but most consumer-market FM radios/tuners/receivers don't.
In other words, empirical evidence suggests that the consumer electronics industry doesn't think TV-audio-over-FM would be a good investment either.
Perhaps they understand the technical and financial realities of that box. Some evidence of the value of Philadelphia FM broadcast licenses can be found in PirateJim's "Philly FM Radio History Page 2" at which cites the following:
- "In January 1996, WWDB was purchased by Mercury Broadcasting for $48 million."
- "Today ... WBEB remains as one of the last independently-owned stations in a top market, with an estimated value of over $100 million."
- "In 1997, WIOQ's license was transferred from EZ to American Radio Systems through a $655 million exchange in stock."