Method and apparatus for digital audio broadcasting and reception

• US 5,278,826 A
• Filed: 04/11/1991
• Issued: 01/11/1994
• Est. Priority Date: 04/11/1991
• Status: Expired due to Term

• Current Assignee: iBiquity Digital Corporation (Adeia Inc.)
• Original Assignee: USA Digital Radio, Inc. (Adeia Inc.)
• Inventors: Murphy, John L., Rich, Mark J.
• Primary Examiner(s): Olms, Douglas W.
• Assistant Examiner(s): Nguyen, Chau T.
• Application Number:
  US07/683,945
• Time in Patent Office: 1,006 Days
• Field of Search: 370/76, 370/110.4, 370/69.1, 370/18, 370/20, 370/71, 370/124, 370/73, 370/74, 375/1, 375/54, 375/101, 455/6.1, 455/6.3, 381/4
• US Class Current: 370/343
• CPC Class Codes: H04H 20/31 using in-band signals, e.g. subsonic or cue signal
• CPC Class Codes: H04H 20/95 characterised by a specific format, e.g. an encoded audio stream
• CPC Class Codes: H04H 2201/183 FM digital or hybrid
• CPC Class Codes: H04H 40/18 Arrangements characterised by circuits or components specially adapted for receiving
• CPC Class Codes: H04H 40/54 generating subcarriers
• CPC Class Codes: H04H 60/07 characterised by processes or methods for the generation
• CPC Class Codes: H04J 1/05 using digital techniques
• CPC Class Codes: H04L 27/2602 Signal structure

Abstract

A radio broadcasting system is provided for transmitting and receiving through free space a composite signal consisting of a frequency modulated (fm) analog signal and a multicarrier modulated digital signal which is especially adapted to be resistive to multipath degradation. The fm signal and the digital multicarrier modulated signal are fully coherent. Further according to the invention, the digital signal comprises a plurality of carriers having a maximum amplitude at least 20 dB below the unmodulated fm signal and preferably, 30 dB below the analog signal. The multicarrier modulated signal is phase locked according to the invention to the recovered analog fm pilot tone at 19 kHz in the composite baseband spectrum of the fm signal which is at least 20 dB above the multicarrier modulated signal, which enables rapid and reliable acquisition of signal for coherent detection. In a specific embodiment, the multicarrier modulated signal is a synthesized vector-modulated signal which is a quadrature phase shift keyed (QPSK) modulated set of synthesized carriers each occupying 9.5 kHz of spectrum replicated twenty-one times within a 199.5 kHz bandwidth with no more than two bits per vector. The broadcast system is designed to be used in support of compressed digital audio programming material. In a demodulation process according to the invention, a demodulator is operative to phase lock to the recovered high-amplitude analog pilot tone, to coherently demodulate the digital signal, and to format the recovered data stream for source decoding.

Claims

1. A system for broadcast transmission and reception of an analog signal and digitized program material, including means for producing an analog frequency modulated (fm) signal from a composite baseband signal, and a transmitter for transmission of at least said analog fm signal, wherein said composite baseband signal includes a single unmodulated pilot tone, said broadcast system comprising:

a transmitter system comprising;

first signal processing means for generating an equivalent of a plurality of equivalent tones which are spaced equally at a baseband between 0 Hz and a selected maximum frequency, each of said equivalent tones being at a frequency which is an integer multiple of a base frequency tone and each one of said equivalent tones being constrained to be phase coherent with said unmodulated pilot tone by means of a master clock, said first signal processing being means coupled to accept said digitized program material for quadrature phase shift key (QPSK) modulating each one of said equivalent tones to distribute said digitized program material over substantially all of said equivalent tones in order to produce a multitone QPSK modulated signal at a selected intermediate frequency carrier;

means for linearly combining said analog fm signal and said multitone QPSK modulated signal, said multitone QPSK modulated signal having a magnitude when combined with said analog fm signal of at least 100 times less than the magnitude of said analog fm signal, to produce a combined information signal; and

means for broadcasting said combined information through atmospheric free-space; and

a plurality of receivers, each receiver comprising;

means for acquiring a replica of said combined information signal through atmospheric free-space, said atmospheric free-space being subject to multipath such that said replica is inclusive of transmission errors;

means coupled to said acquiring means for splitting said replica of said combined information signal into a first signal path and a second signal path;

frequency (fm) demodulating means in said first signal path for extracting a replica of said unmodulated pilot tone;

means in said second signal path for separating a replica of said multitone QPSK modulated signal substantially from said combined information signal; and

signal processing means in said second signal path coupled to receive said replica of said unmodulated pilot tone and said replica of said multitone QPSK modulated signal for coherently demodulating said replica of said multitone QPSK modulated signal to extract said digitized program material.

2. The system according to claim 1 wherein said transmitter system further includes means for interleaving in time said digitized program material to temporally spread said program material and wherein said receiver further includes means for deinterleaving in time said digitized program material with correction in order to enhance resistance to selected short-interval-dependent data errors.

3. The system according to claim 1 wherein said analog fm signal and said multitone QPSK modulated signal are sufficiently close to one another in frequency to overlap in spectrum, wherein said separating means in said receiver further includes:

means for cancelling said replica of said analog fm signal in said second signal path in order to permit reuse of common spectrum.

4. The system according to claim 1 wherein said signal processing means comprises a digital signal processing means employing a fast Fourier transform.

5. A method for broadcast transmission and reception of an analog signal and digitized program material, employing means for producing an analog frequency modulated (fm) signal from a composite baseband signal, and a transmitter for transmission of at least said analog fm signal, wherein said composite baseband signal includes a single unmodulated pilot tone, said method comprising the steps of:

generating in first signal processing means an equivalent of a plurality of equivalent tones which are spaced equally at a baseband between 0 Hz and a selected maximum frequency, each of said equivalent ones being at a frequency which is an integer multiple of a base frequency tone and each one of said equivalent tones being constrained to be phase coherent with said unmodulated pilot tone by means of a master clock, and quadrature phase shift key (QPSK) modulating each one of said equivalent tones with said digitized program material over substantially all of said equivalent tones in order to produce a multitone QPSK modulated signal at a selected intermediate frequency carrier;

linearly combining said analog fm signal and said multitone QPSK modulated signal, said multitone QPSK modulated signal having a magnitude when combined with said analog fm signal of at least 100 times less than the magnitude of said analog fm signal, to produce a combined information signal;

broadcasting said combined information through atmospheric free-space;

acquiring a replica of said combined information signal through atmospheric free-space, said atmospheric free-space being subject to multipath such that said replica is inclusive of transmission errors;

splitting said replica of said combined information signal into a first signal path and a second signal path;

extracting a replica of said unmodulated pilot tone from said first signal path, said unmodulated pilot tone having an amplitude at least 100 times greater than said multitone QPSK modulated signal;

separating a replica of said multitone QPSK modulated signal substantially from said combined information signal in said second signal path; and

coherently demodulating said replica of said multitone QPSK modulated signal to extract said digitized program material upon reference to said unmodulated pilot tone at a signal level at least 100 times greater than said digitized program material in said replica of said combined information signal, said output program material being a substantial duplicate of said digitized program material.

6. The method according to claim 5 wherein said demodulating step includes applying a fast Fourier transform to said replica of said multitone QPSK modulated signal.

7. A transmitter for a system for broadcast transmission and reception of an analog signal and digitized program material, including means for producing an analog frequency modulated (fm) signal from a composite baseband signal, and a transmitter for transmission of at least said analog fm signal, wherein said composite baseband signal includes a single unmodulated pilot tone, said transmitter system comprising:

first signal processing means for generating an equivalent of a plurality of equivalent tones which are spaced equally at a baseband between 0 Hz and a selected maximum frequency, each of said equivalent tones being at a frequency which is an integer multiple of a base frequency tone and each one of said equivalent tones being constrained to be phase coherent with said unmodulated pilot tone, said second signal processing being means coupled to accept a digital representation of a program signal for quadrature phase shift key (QPSK) modulating each of said equivalent tones with said digital representation of said program signal thereby to distribute said digital representation of said program signal over substantially all of said equivalent tones in order to produce a multitone QPSK modulated signal at a selected intermediate frequency carrier;

means for linearly combining said analog fm signal and said multitone QPSK modulated signal, said multitone modulated signal having a magnitude when combined with said analog fm signal of at least 100 times less than the magnitude of said analog fm signal, to produce a combined information signal; and

means for broadcasting said combined information through atmospheric free-space.

8. A receiver for a system for broadcast transmission and reception of a combined information signal comprising an analog signal and digitized program material in a multitone QPSK modulated signal, including means for producing an analog frequency modulated (fm) signal from a composite baseband signal, and a transmitter for transmission of at least said analog fm signal, wherein said composite baseband signal includes a single unmodulated pilot tone, wherein said combined information signal comprises said analog fm signal and said multitone QPSK modulated signal, and wherein said analog fm signal and said multitone QPSK modulated signal are sufficiently close to one another in frequency to overlap in spectrum, said receiver comprising:

means for acquiring a replica of said combined information signal through atmospheric free-space, said atmospheric free-space being subject to multipath such that said replica is inclusive of transmission errors;

means coupled to said acquiring means for splitting said replica of said combined information signal into a first signal path and a second signal path;

frequency (fm) demodulating means in said first signal path for extracting a replica of said unmodulated pilot tone, said replica of said unmodulated pilot tone being of a signal level at least 100 times greater than said multitone QPSK modulated signal;

means in said second signal path for separating a replica of said multitone QPSK modulated signal substantially from said combined information signal, including means for cancelling said replica of said analog fm signal in said second signal path in order to permit reuse of common spectrum; and

signal processing means in said second signal path coupled to receive said replica of said unmodulated pilot tone and said replica of said multitone QPSK modulated signal for coherently demodulating said replica of said multitone QPSK modulated to extract said digitized program material.

9. The receiver according to claim 8 wherein said signal processing means comprises a digital signal processing means employing a fast Fourier transform.

10. A method for transmitting, in a broadcast transmission and reception system, an analog signal and digitized program material, including an analog frequency modulated (fm) signal from a composite baseband signal, wherein said composite baseband signal includes a single unmodulated pilot tone, said method comprising the steps of:

generating in first signal processing means an equivalent of a plurality of equivalent tones which are spaced equally at a baseband between 0 Hz and a selected maximum frequency, each of said equivalent tones being at a frequency which is an integer multiple of a base frequency tone and each one of said equivalent tones being constrained to be phase coherent with said unmodulated pilot tone, and quadrature phase shift key (QPSK) modulating each of said equivalent tones with said digitized program material thereby to distribute said digitized program material over substantially all of said equivalent tones in order to produce a multitone QPSK modulated signal at a selected intermediate frequency carrier;

linearly combining said analog fm signal and said multitone QPSK modulated signal, said multitone QPSK modulated signal having a magnitude when combined with said analog fm signal of at least 100 times less than the magnitude of said analog fm signal, to produce a combined information signal; and

broadcasting said combined information through atmospheric free-space.

11. A method for receiving, in a broadcast transmission and reception system, a combined information signal of an analog signal and digitized program material in a multitone QPSK modulated signal, including an analog frequency modulated (fm) signal of a composite baseband signal, wherein said composite baseband signal includes a single unmodulated pilot tone, wherein said combined information signal comprises and analog fm signal and said multitone QPSK modulated signal, and wherein said analog fm signal and said multitone QPSK modulated signal are sufficiently close to one another in frequency to overlap in spectrum, said method comprising the steps of:

acquiring a replica of said combined information signal through atmospheric free-space, said atmospheric free-space being subject to multipath such that said replica is inclusive of transmission errors;

splitting said replica of said combined information signal into a first signal path and a second signal path;

extracting a replica of said unmodulated pilot tone from said first signal the, said unmodulated pilot tone having an amplitude at least 100 times greater than said multitone QPSK modulated signal;

separating a replica of said multitone QPSK modulated signal substantially from said combined information signal in said second signal path, including cancelling said replica of said analog fm signal in said second signal path in order to permit reuse of common spectrum; and

coherently demodulating said replica of said multitone QPSK modulated signal in said second signal path upon reference to said replica of said unmodulated pilot tone to extract said digitized program material.

12. The method according to claim 11 wherein said demodulating step includes applying a fast Fourier transform to said replica of said multitone QPSK modulated signal.