Method and apparatus for AM compatible digital broadcasting

A method of modulating electrical signals, said method comprising the steps of:

• providing an amplitude modulated signal in a first frequency band;
• providing a first plurality of orthogonal quadrature amplitude modulated carriers in said first frequency band;
• and
• providing second and third groups of quadrature amplitude modulated carriers in second and third frequency bands, said second and third frequency bands encompassing frequencies above and below frequencies encompassed by said first frequency band, respectively.

A signal transmission method, said method comprising the steps of:

• amplitude modulating a first carrier signal by an analog signal to generate a first signal having a first frequency spectrum;
• generating a plurality of digitally modulated carrier signals for transmission, said signals being within a frequency range which encompasses the frequency spectrum of the first signal, each of said digitally modulated carrier signals being digitally modulated by a portion of a digital signal, wherein a first group of said digitally modulated carrier signals overlap said first frequency spectrum and are modulated in-quadrature with said first carrier signal, and wherein second and third groups of said digitally modulated carrier signals lie outside of said first frequency spectrum and are modulated both in-phase and in-quadrature with said first carrier signal;
• and
• simultaneously transmitting said first signal and said plurality of digitally modulated carrier signals.

A radio frequency transmitter comprising:

• means for amplitude modulating a first carrier signal by an analog signal to generate a first radio frequency signal having a frequency spectrum;
• means for generating a plurality of digitally modulated carrier signals within a frequency range which encompasses the frequency spectrum of the first radio frequency signal, each of said digitally modulated carrier signals being digitally modulated by a portion of a digital signal, wherein a first group of said digitally modulated carrier signals overlap the frequency spectrum of the first radio frequency signal and are modulated in-quadrature with said first carrier signal, and wherein second and third groups of said digitally modulated carrier signals lie outside of the frequency spectrum of said first signal and are modulated both in-phase and in-quadrature with said first carrier signal;
• and
• means for simultaneously transmitting said first radio frequency signal and said plurality of digitally modulated carrier signals.

A radio frequency receiver comprising:

• means for receiving both analog and digital portions of a composite radio frequency waveform, wherein the waveform includes a first signal having a first carrier amplitude modulated by an analog signal, and a plurality of digitally modulated carrier signals within a frequency range which encompasses the frequency spectrum of the first signal, each of said digitally modulated carrier signals being amplitude modulated by a portion of a digital signal, wherein a first group of said digitally modulated carrier signals overlapping the frequency spectrum of the first signal are modulated in-quadrature with said first carrier signal, and wherein second and third groups of said digitally modulated carrier signals lie outside of the frequency spectrum of the first signal and are modulated both in-phase and in-quadrature with said first carrier signal;
• means for detecting said analog signal on said first amplitude modulated carrier;
• and
• means for detecting said digital signal on said digitally modulated carriers.