HD Radio is being successfully implemented for FM/VHF with a popular option of multicasting. Multicasting splits the HD bandwidth providing at least 2 program channels in the 96kb/s HD Radio channel.
However, HD Radio for AM/MW has an adjacent channel interference problem that shows itself as a hiss or hash in the adjacent channels and sometimes under the HD Radio station's analog audio.
Many owners are considering using a multicast channel on their HDFM for the AM programming. This would be Other Band On Channel Two or OBOC2. Also EV-DO or Evolution Data Optimized high-speed wireless broadband networks are being deployed nationwide for future wireless access to information and entertainment. An EV-DO “radio” could use these networks for retransmission of AM analog programming once the receivers are mass-produced. AM programming would be available on wireless phones and EV-DO enabled laptops as well. Qualcomm's Mediaflo and/or DVB-H could also be used.
My idea enables an AM station to transmit a low speed data channel containing address and ID information. This will direct a HDFM or EV-DO radio to tune to the multicast or stream that's retransmitting the AM station's programming.
In the 1950s, early 1960s, AT&T used time assignment speech interpolation (TASI) on its submarine cables across the Atlantic and Pacific. When people speak, the intervals in their speech are a waste of bandwidth. With TASI, AT&T used all the capacity of their cables to send information. The intervals (measured in microseconds) when there is no audio are filled with other audio or data. This is just an early form of statistical multiplexing.
Listening to most radio, a second or two of dead air usually exists in every minute of programming with the exception of music. During those times of near zero AM modulation, the carrier could be phase modulated with low speed data. Envelope detectors should see nothing. Existing radios would ignore the low speed data, but a new OBOC2 and/or EV-DO receiver could recover this data from the AM broadcast and use the data to find the OBOC2 or EV-DO retransmission.
At present, HD Radio secondary program channels mute if impaired so returning to analog AM would be better than nothing. Also under this arrangement, a 5kHz upper audio limit would no longer be required so the AM section of the receiver could be wider to soften the difference between digital and analog.
When a listener tunes from one station to another, the radio would revert to analog AM and there would be a delay of a few seconds to a few minutes so it can collect the instruction data for tuning to the multicast or stream. Of course if a memory button was used or the radio was simply turned off and on it would quickly find the multicast or stream. The radio's display may show the AM analog frequency.
The encoder at the AM station would need to include an audio input (analog, AES, or IP) and an audio output for the audio input of the transmitter, an RF output to the transmitter's RF input. This would supply the AM carrier with the periodic phase modulation that happens during program audio silence.
Internally, the encoder would sense the silence in the program, time the period of silence, delay the audio for a very short time and insert as much data as possible for phase modulation. Additional delay could be included for HD Radio latency.
The receiver would use a phase detector for it's decoder and the data would be captured and retained so as to be complete and useful.
Since Ibiquity is already collecting license fees for their IP rights, the encoder for the AM station could be license free as technically HD Radio is not directly being used by the AM broadcaster.
Some stations today use C-QUAM AM Stereo and I don't know much about this except that this idea could be used with the only artifact being a flickering AM Stereo LED or icon during the data intervals. The 25Hz pilot would not be sent during the data transmission and thus a C-QUAM radio would become a mono only radio during the interval. This could serve as an indication that OBOC2 and/or EV-DO is available to a new receiver.
Anyone wishing to build the encoders and/or receivers may do so. If anyone has a way to improve this idea, please do. I like HD Radio FM/VHF especially the idea of a 48/48kb/s multicast split. Also I would like to see MPEG spatial coding used for parametric stereo/surround, possibly as a system with 43kb/s for coded audio and a 5kb/s spatial coding rate if this could work.
At the SBE meeting I suggested DPSK for the PM probably with significant sidebands restricted to no more than 1 to 1.5 kHz on either side of the carrier or center frequency, but the equipment manufacturers and NRSC have a lot of engineers smarter than me that can figure this out.
All I'm looking for some way to make the AM IBOC hiss go away. WBBM at 780 turned on its HD Radio and blew away WEW 770. I like both stations and I'm trying to find a solution.