The FCC has denied a creative proposal to get an additional DTV subchannel on cable.
Must-carry rules allow a full-power digital TV station to require local cable systems to carry one of their program streams. The station may select which stream is entitled to carriage. However, if the station broadcasts more than one stream, only one is required to be carried.
(in many cases, stations negotiate with cable to obtain carriage for additional streams. These agreements are business contracts, not enforced by the FCC.)
ION (formerly known as "Pax" and licensee of most TV stations with the combination "PX" in their callsigns) proposed to split their licenses. They proposed to license a second station on each of their channels, and assign that license to a firm called "Urban Television LLC". The existing ION station and the new Urban station on each channel would share time, with ION using the channel part of the day and Urban using it the rest of the day.
This kind of thing has happened in the past. Growing up in Milwaukee, I remember occasionally DXing two stations sharing channel 10 in Lansing, Michigan. Michigan State University's station WMSB broadcast educational programs during the day; commercial station WILX-TV broadcast NBC and local programs in the afternoon and at night. A new timesharing situation is likely to develop in Sacramento, California in the next year or two, as two non-commercial licensees were found equally qualified, and both will receive licenses to operate on channel 43 there.
What's different in the ION case is the length of each time slot. In the WMSB/WILX case, WMSB used the channel continuously for several hours; it then signed off for the day, allowing WILX to use it continuously for the rest of the night. In ION's case, each station would operate for a few milliseconds. It would transmit one of ION's video packets, then one of Urban's audio packets, then one of Urban's video packets, then an Urban metadata packet, then an ION metadata packet, etc., etc.....
And to some degree, the FCC has on its own motion proposed doing just this. A station operating above channel 37 would give up its own channel and would share time on the transmitter of some other station operating below 37. Both stations would be entitled to a slot on cable/satellite; each would be individually responsible for what airs on their subchannel.
But in ION's case, the Commission isn't buying it. They labeled the scheme as "...a division of spectrum...", not a division of time -- and denied the license assignment.