Method and apparatus for AM compatible digital broadcasting

A broadcasting method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel is provided by: broadcasting an amplitude modulated radio frequency signal having a first frequency spectrum, wherein the amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal; and simultaneously broadcasting a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum, each of the digitally modulated carrier signals being modulated by a portion of a digital program signal, wherein a first group of the digitally modulated carrier signals lying within the first frequency spectrum are modulated in-quadrature with the first carrier signal, and wherein second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Transmitters and receivers which operate in accordance with the above method are also provided.

BACKGROUND OF THE INVENTION 
This invention relates to radio broadcasting and, more particularly, to 
methods of and apparatus for broadcasting digitally modulated signals and 
analog amplitude modulated signals within the same frequency channel 
assignment. 
There has been increasing interest in the possibility of broadcasting 
digitally encoded audio signals to provide improved audio fidelity. 
Several approaches have been suggested, including out-of-band techniques 
in which the digital radio signals would be broadcast in a specially 
designated frequency band, and in-band techniques in which the radio 
frequency signals would be broadcast within vacant slots between adjacent 
channels in the existing broadcast band (interstitial approach) or within 
the same frequency channel allocations currently used by commercial 
broadcasters (in-band on-channel approach). The in-band approach may be 
implemented without the need for additional frequency coordination and 
with relatively minor changes to existing transmitting equipment. Of 
course, any digital audio broadcasting (DAB) technique should not degrade 
reception by conventional analog receiver circuits. 
In-band approaches to digital audio broadcasting have thus far only been 
proposed in the FM band (88 MHz to 108 MHz), since the bandwidth of AM 
channels is quite narrow. However, the use of digital audio broadcasting 
in the AM band (530 kHz to 1700 kHz) would provide AM broadcasting 
stations with a means to compete with high quality portable audio sources 
such as cassette tapes and compact disc players. It would therefore be 
desirable to extend the in-band on-channel (IBOC) approach to AM 
broadcasting frequencies to provide enhanced fidelity through digital 
signalling without affecting reception by existing analog AM receivers. 
SUMMARY OF THE INVENTION 
The broadcasting method of this invention utilizes a composite waveform 
comprising: an amplitude modulated radio frequency signal, wherein the 
amplitude modulated radio frequency signal includes a first carrier 
amplitude modulated by an analog signal; and a plurality of digitally 
modulated carrier signals within a frequency range which includes the 
frequency spectrum of the amplitude modulated radio frequency signal, each 
of the digitally modulated carrier signals being digitally modulated by a 
portion of a digital signal, wherein a first group of the digitally 
modulated carrier signals overlap the frequency spectrum of the amplitude 
modulated radio frequency signal and are modulated in-quadrature with the 
first carrier signal, and wherein second and third groups of the digitally 
modulated carrier signals lie outside of the frequency spectrum of the 
analog modulated radio frequency signal and are modulated both in-phase 
and in-quadrature with the first carrier signal. 
The invention also encompasses a radio frequency transmitter comprising: 
means for transmitting a composite radio frequency signal, having an 
amplitude modulated signal including 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 amplitude modulated signal, each of the digitally modulated carrier 
signals being digitally modulated by a portion of a digital signal, 
wherein a first group of the digitally modulated carrier signals overlap 
the frequency spectrum of the analog modulated signal and are modulated 
in-quadrature with the first carrier signal, and wherein second and third 
groups of the digitally modulated carrier signals lie outside of the 
frequency spectrum of the analog modulated signal and are modulated both 
in-phase and in-quadrature with the first carrier signal. 
Transmitters which broadcast signals in accordance with this invention use 
a method of modulating electrical signals 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 the first frequency band; and providing second and third 
groups of quadrature amplitude modulated carriers in second and third 
frequency bands, with the second and third frequency bands encompassing 
frequencies above and below frequencies encompassed by the first frequency 
band, respectively. 
The invention further encompasses 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 amplitude modulated radio 
frequency signal, each of the digitally modulated carrier signals being 
digitally modulated by a portion of a digital signal, wherein a first 
group of the digitally modulated carrier signals overlap the frequency 
spectrum of the first signal and are modulated in-quadrature with the 
first carrier, and wherein second and third groups of the digitally 
modulated carrier signals lie outside of the frequency spectrum of the 
first signal and are modulated both in-phase and in-quadrature with the 
first carrier; means for detecting the analog signal on the first carrier; 
and means for detecting the digital signal on the digitally modulated 
carriers. 
This invention provides an in-band on-channel broadcasting method by which 
digital representations of audio programming material, or other digital 
data, can be transmitted within an existing AM broadcast channel without 
adversely affecting existing analog AM receivers and with relatively minor 
modifications to existing AM transmitting equipment. Transmitters and 
receivers that transmit and receive signals in accordance with the 
broadcasting method are also encompassed by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention provides a method of simultaneously broadcasting both an 
analog amplitude modulated signal and a digital signal on the same channel 
assignment as the existing analog AM broadcasting allocation. When this 
technique is applied to AM radio broadcasts, the broadcasting can be done 
in the same frequency band and at the same carrier frequencies that are 
currently allocated for AM broadcasting. The technique of broadcasting the 
digital signal in the same channel as an analog AM signal is called 
in-band on-channel (IBOC) broadcasting. The need to prevent mutual 
interference places restrictions on the digital waveform that is placed 
beneath the analog AM spectrum. This broadcasting is accomplished by 
transmitting a digital waveform by way of a plurality of carriers, some of 
which are modulated in-quadrature with the analog AM signal and are 
positioned within the spectral region where the standard AM broadcasting 
signal has significant energy. The remaining digital carriers are 
modulated both in-phase and in-quadrature with the analog AM signal and 
are positioned in the same channel as the analog AM signal, but in 
spectral regions where the analog AM signal does not have significant 
energy. There are various methods for producing orthogonally related 
signals. The specific method employed to ensure this orthogonality 
condition is not a part of this invention. In the United States, the 
emissions of AM broadcasting stations are restricted in accordance with 
Federal Communications Commission (FCC) regulations to lie within a signal 
level mask defined such that: emissions 10.2 kHz to 20 kHz removed from 
the analog carrier must be attenuated at least 25 dB below the unmodulated 
analog carrier level, emissions 20 kHz to 30 kHz removed from the analog 
carrier must be attenuated at least 35 dB below the unmodulated analog 
carrier level, and emissions 30 kHz to 60 kHz removed from the analog 
carrier must be attenuated at least [35 dB+1 dB/kHz] below the unmodulated 
analog carrier level. 
FIG. 1 shows the spectrum of an AM digital audio broadcasting signal having 
carriers positioned in accordance with the present invention. Curve 10 
represents the standard broadcasting amplitude modulated carrier signal, 
wherein the carrier has a frequency of f.sub.0. The FCC emissions mask is 
represented by item number 12. Recent advances in source coding, such as 
the German Institut Fur Rundfunktechnik MUSICAM (Masking-pattern Adapted 
Subband Coding And Multiplexing) algorithm, have shown that enhanced audio 
quality for stereo program material can be achieved by broadcasting 
digital signals at rates as low as 96 kilobits per second (kbps). 
Waveforms which support this data rate can be inserted within the FCC 
emissions mask presently allocated for AM stations by employing bandwidth 
efficient modulation techniques. 
The digitally modulated carriers in this invention are generated via 
orthogonal frequency division multiplexing (OFDM). This format enables the 
spectra of these carriers to be overlapped without any intervening guard 
bands, thereby optimizing spectral utilization. However, a guard interval 
can be used in the time domain to compensate for signal timing jitter. The 
OFDM modulation technique is extremely beneficial for successful DAB 
operation since bandwidth is a premium commodity in the AM band. An 
additional advantage is that there is no need to isolate the DAB digital 
carriers from each other via filtering in either the transmitter or 
receiver since the orthogonality condition of OFDM minimizes such 
interference. 
The OFDM waveform is composed of a series of data carriers spaced at 500 
Hz. This produces enhanced spectral containment and enables the AM DAB 
waveform to extend extremely close to the edge of the FCC emissions mask, 
yet remain compliant. An additional feature of this approach is that the 
amplitude of each carrier can be tailored to boost signal power in areas 
where high interference levels are anticipated, such as locations close to 
the carrier frequencies of interferers. This strategy produces an optimal 
allocation of signal energy and thereby maximizes the potential AM DAB 
coverage region. 
In this invention, the composite analog and digital DAB waveform includes a 
plurality of modulated carriers which are fully compliant with the FCC 
emissions mask. In the preferred embodiment of this invention, 76 
carriers, spaced f.sub.1 =500 Hz apart, are used to carry the digital 
information. A first group of thirty four of the digitally modulated 
carriers are positioned within a frequency band extending from (f.sub.0 
-17 f.sub.1) to (f.sub.0 +17 f.sub.1), as illustrated by the envelope 
labeled 14 in FIG. 1. Most of these signals are placed 30 to 40 dB lower 
than the level of the unmodulated AM carrier signal in order to minimize 
crosstalk with the analog AM signal. Crosstalk is further reduced by 
encoding this digital information in a manner that guarantees 
orthogonality with the analog AM waveform. This type of encoding is called 
complementary encoding (i.e. complementary BPSK, complementary QPSK, or 
complementary 32 QAM) more fully described and claimed in copending 
application Ser. No. 08/368,061 filed Jan. 3, 1995. Complementary BPSK 
modulation is employed on the innermost digital carrier pair at f.sub.0 
.+-.f.sub.1 to facilitate timing recovery via a Costas loop. These 
carriers are set at a level of -25 dBc. Eighteen carriers in this first 
group located at f.sub.0 -10 f.sub.1 to f.sub.0 .multidot.2 f.sub.1 and 
f.sub.0 +2 f.sub.1 to f.sub.0 +10 f.sub.1 are modulated using 
complementary QPSK and have a level of -39.7 dBc. The final fourteen 
carriers in the first group are located at f.sub.0 -17 f.sub.1 to f.sub.0 
-11 f.sub.1 and f.sub.0 +11 f.sub.1 to f.sub.0 +17 f.sub.1. These carriers 
are modulated using complementary 32 QAM and have a level of -30 dBC. 
Additional groups of quadrature amplitude modulated digital signals are 
placed outside the first group. The need for these digital waveforms to be 
in-quadrature with the analog signal is eliminated by restricting the 
analog AM signal bandwidth. This is not anticipated to be an unreasonable 
requirement since the ceramic IF filters typically found in analog AM 
receivers limit the audio response to 3.5 kHz. All of the carriers in a 
second and a third group, encompassed by envelopes 16 and 18 respectively, 
are modulated using 32 QAM. The carriers located at f.sub.0 -19 f.sub.1, 
f.sub.0 -18 f.sub.1, f.sub.0 +18 f.sub.1, and f.sub.0 +19 f.sub.1 have a 
level of 28 dBC. The carriers at f.sub.0 -39 f.sub.1 to f.sub.0 -34 
f.sub.1, f.sub.0 -32 f.sub.1 to f.sub.0 -23 f.sub.1, f.sub.0 -21 f.sub.1, 
f.sub.0 +21 f.sub.1, f.sub.0 +23 f.sub.1 to f.sub.0 +32 f.sub.1, and 
f.sub.0 +34 f.sub.1 to f.sub.0 +39 f.sub.1 have a level of -31 dBc. The 
remaining carriers at f.sub.0 -33 f.sub.1 and f.sub.0 -22 f.sub.1 and 
f.sub.0 +22 f.sub.1 and f.sub.0 +33 f.sub.1 have a level of -32 dBc. 
The OFDM carriers are spaced at f.sub.1 =500 Hz. However, because a 
time-domain guard band is used, the symbol rate for each carrier is 
f.sub.r =128*200 500/132 symbols per second. The pair of complementary 
BPSK carriers has 1 bit per symbol, resulting in a bit rate of 18 f.sub.r. 
The nine pairs of complementary QPSK carriers each contain 2 bits per 
symbol, resulting in a total of 18 f.sub.r bits per second. The seven 
pairs of complementary 32 QAM carriers each have 5 bits per symbol, 
resulting in 35 f.sub.r bits per second. The 42 individual 32 QAM carriers 
each carry 5 bits per symbol, resulting in 210 f.sub.r bits per second. 
The total data rate for all OFDM carriers is 264 f.sub.r, or 128 k bits 
per second. 
The occupied bandwidth of the entire composite AM DAB signal is 40 kHz, as 
measured to the outermost first nulls of the digital waveform. This 
spectrum falls within the central 40 kHz portion of the FCC emissions 
mask. The OFDM sidelobes that extend beyond frequencies outside f.sub.0 
.+-.20 kHz, fall below the -35 dBc portion of the emissions mask without 
any additional filtering since the OFDM sidelobe spacing is only f.sub.1 
=500 Hz. 
There are vacant OFDM slots at .+-.20 f.sub.1 and .+-.40 f.sub.1. This 
provides additional immunity to first and second adjacent channel 
interference since the predominant AM signal component occurs at the 
carrier frequency. Likewise, the AM DAB spectrum is virtually unoccupied 
outside of f.sub.0 .+-.20 kHz to ensure a degree of protection against 
second adjacent channel interferers. 
FIG. 2 is a block diagram of a transmitter constructed in accordance with 
this invention. An analog program signal (which in this example includes 
right and left stereo portions) that is to be transmitted is impressed 
onto input terminals 28 and 28'. The left and right channels are combined 
in summation point 29 and then fed through an analog audio processor 30 to 
increase the average analog AM modulation from 30% to 85%, which extends 
the coverage region considerably. Such processors are commonplace at 
analog AM radio stations throughout the world. That signal is passed 
through a low pass filter 31 having a sharp cutoff characteristic, to 
produce a filtered monaural analog program signal on line 32. Filter 31 
may, for example, have a cutoff frequency of 6 kHz and 40 dB attenuation 
beyond 6.5 kHz. 
For those applications in which the analog and digital portions of 
transmitted signal will be used to convey the same program material, a 
digital source encoder 34, which may conform to the ISO MPEG Layer 2A, 
converts the right and left analog program signals to a 96 kbps joint 
stereo digital signal on line 36. A forward error correction encoder and 
interleaver circuit 38 improves data integrity over channels corrupted 
with impulsive noise and interference, producing a 128 kbps digital signal 
on line 40. For those instances where the digital signal to be transmitted 
is not a digital version of the analog program signal a data port 42 is 
provided to receive the digital signal. An ancillary data source 44 is 
also provided for those instances in which the digital version of the 
analog program signal, or a digital signal supplied to port 42, is to be 
supplemented by including additional data. 
Data parser 46 receives the digital data and produces a plurality of 
outputs on lines 48. The signals on pairs of lines 48 from the data parser 
46 constitute complex coefficients that are in turn applied to an inverse 
Fast Fourier Transform (FFT) algorithm in block 50, which generates the 
baseband in-phase, I, and quadrature, Q, components of the data signal, on 
lines 52 and 54 respectively. The processed baseband analog AM signal is 
converted to a digital signal by analog-to-digital converter 60 and 
combined with the in-phase portion of the digital DAB waveform at 
summation point 62 to produce a composite signal on line 64. The composite 
signal on line 64 is converted to an analog signal by digital-to-analog 
converter 66, filtered by low pass filter 68, and passed to a mixer 70 
where it is combined with a radio frequency signal produced on line 72 by 
a local oscillator 74. The quadrature signal on line 54 is converted to an 
analog signal by analog-to-digital converter 76 and filtered by low pass 
filter 78 to produce a filtered signal which is combined in a second mixer 
80, with a signal on line 82. The signal on line 72 is phase shifted as 
illustrated in block 84 to produce the signal on line 82. The outputs of 
mixers 70 and 80 are delivered on lines 86 and 88 to a summation point 90 
to produce a composite waveform on line 92. The spurious mixing products 
are muted by bandpass filter 94, and the resulting DAB signal is 
subsequently amplified by a power amplifier 96 for delivery to a 
transmitting antenna 98. 
FIG. 3 is a block diagram of the data parser 46 of FIG. 2. The data parser 
includes a serial-to-parallel converter 100 which receives a serial 
digital signal, as illustrated by the input line 40, and produces a 
plurality of outputs in the form of digital signals on a plurality of 
groups of lines as illustrated by groups 102-1 to 102-N. Each group of 
lines feeds a QAM encoder, such as encoders 106-1 to 106-N to produce an 
in-phase output signal I.sub.1 -I.sub.N and a quadrature output signal 
Q.sub.1 -Q.sub.N. In a practical application there may be, for example, 5 
lines per group and 76 QAM encoders. In addition, some QAM encoders may 
use BPSK or QPSK. 
FIG. 4 is a block diagram of a receiver constructed to receive digital and 
analog signals broadcast in accordance with this invention. An antenna 110 
receives the composite waveform containing the digital and analog signals 
and passes the signal to conventional input stages 112, which may include 
a radio frequency preselector, an amplifier, a mixer and a local 
oscillator. An intermediate frequency signal is produced by the input 
stages on line 114. This intermediate frequency signal is passed through 
an automatic gain control circuit 116 to an I/Q signal generator 118. The 
I/Q signal generator produces an in-phase signal on line 120 and a 
quadrature signal on line 122. The in-phase channel output on line 120 is 
input to an analog-to-digital converter 124. Similarly, the quadrature 
channel output on line 122 is input to another analog-to-digital converter 
126. Feedback signals on lines 128 and 130 are input to digital-to-analog 
converters 132 and 134, respectively. The digital-to-analog converters 
outputs on line 136 and 138 are used to control the automatic gain control 
circuit 116. The signal on line 120 includes the analog AM signal which is 
separated out as illustrated by block 140 and passed to an output stage 
142 and subsequently to a speaker 144 or other output device. 
A band reject filter 146 filters the in-phase components on line 128 to 
eliminate the energy of the analog AM signal and to provide a filtered 
signal on line 148. A fast Fourier transform circuit 150 receives the 
digital signals on lines 148 and 152, and produces output signals on lines 
154. These output signals are passed to an equalizer 156 and to a data 
rate filter and data decoder 158. The output of the data decoder is sent 
to a deinterleaving circuit and forward error correction decoder 164 in 
order to improve data integrity. The output of the deinterleaver/forward 
error correcting circuit is passed to a source decoder 166. The output of 
the source decoder is converted to an analog signal by a digital-to-analog 
converter 160 to produce a signal on line 162 which goes to the output 
stage 142. 
The present invention utilizes an AM DAB waveform that minimizes the 
magnitude of changes necessary to convert existing AM radio stations to 
DAB because the bandwidth is completely within the FCC emissions mask for 
AM transmission. Therefore, it is expected that broadcasters can retain 
their existing transmit antennas. Their feed networks may need to be 
updated, however, since group delay variation in the channel needs to be 
reasonably constant to minimize intersymbol interference for the digital 
signal, a consideration that was less critical for analog AM 
transmissions. It is suspected that existing analog AM transmitters can be 
retained, provided that the power amplifier is operated in a reasonably 
linear mode. The primary hardware alteration would be to replace the low 
level carrier input with an AM DAB exciter. This module generates both the 
analog and digital portions of the AM DAB modulation and the transmitter 
therefore functions primarily as a linear amplifier. 
Although the present invention has been described in terms of an AM digital 
audio broadcasting system, it should be understood that the technique 
could be applied to any system that transmits digital signals along with 
analog amplitude modulated signals. Furthermore, it should be understood 
that the information sent by the digital signal can be different from the 
information sent by the analog amplitude modulated signal. Therefore the 
methods of this invention can be used to transmit data of various types, 
such as traffic or weather information, video signals or military 
communication signals, in combination with an amplitude modulated signal. 
Potential application areas include amplitude modulated military 
communications, and television signals in which the video information is 
amplitude modulated.