High density FM subcarrier modulation with standardized network layer

DETAILED DESCRIPTION As illustrated in FIG.
1a, in an FM subcarrier system, a modulated subcarrier is linearly combined with an FM stereo signal to form a composite multiplex signal.
Because this signal has not yet been used to FM modulate a carrier, it is still at baseband (0 to 100 KHz).
FIG.
1b illustrates the spectrum of the baseband FM stereo signal.
This stereo signal comprises a left (L) and a right (R) audio signal forming L+R and a L-R sidebands as well as a phase reference tone at 19K Hz.
As can be seen from this spectrum, all of the baseband FM stereo signal exists below 53K Hz.
However, each "station" on the FM dial is separated by 200K Hz.
Based on the well-known bandwidth of FM broadcasting, a full 100 KHz of baseband information may be transmitted.
As can be seen from FIG.
1b, the spectrum remaining after inclusion of the FM stereo signal is available to carry additional information.
The FCC regulates the spectral mask and injection level restrictions of modulated subcarrier signals that may occupy this sideband area.
The resulting composite multiplex signal, having an FM stereo signal existing below 53K Hz and a modulated subcarrier between 53K Hz and 99K Hz, is used within the FM exciter to produce the familiar FM radio signals available to the public.
For example, the FM exciter could translate this composite signal (transformed by FM modulation) to 89.
3 M Hz.
This signal would be at 89.
3 on the FM dial.
A listener having an ordinary FM radio receiver tuned to 89.
3 would only hear the FM stereo signal used by the commercial station--the FM reception would be unaffected by the modulated subcarrier due to the design of the FM radio receiver.
However, a user having a receiver configured to demodulate the subcarrier would receive whatever information has been carried in the sideband channel.
As discussed earlier, a number of formats have been developed to exploit the available sideband bandwidth, including DARC.
The data payload in DARC is organized around a block comprising 22 information bytes (176 bits)