Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: COMPATIBLE AM BROADCAST/DATA TRANSMISSION SYSTEM
BACKGROUND OF THE INVENTION
While the invention is subject to a wide range of
applications, it is especially suitable for use in a system
for transmitting data concurrently with the transmission of
music and voice programs using the same transmitting and
antenna structure as a conventional amplitude modulation
(AM) broadcast station.
There have been a number of methods proposed for
transmitting data along with an AM broadcast signal. Most
of these methods transmit data at relatively slow speeds.
Generally, the data is transmitted by phase or frequency
modulating the carrier and then this angular modulated wave
is amplitude modulated by the normal music and voice program
material. The resulting composite modulated wave can then
be demodulated with an envelope demodulator to extract the
normal program material. Since the envelope demodulator is
insensitive to the phase of the composite wave, listeners
are unaware of the data modulation. Indeed, secret
transmissions have been reported to have been made with such
a system during World War II.
However, the rate of information flow through such
systems have generally been very slow. If higher data rates
~5 are attempted, the bandwidth of the composite wave will be
noticeably wider than normal AM broadcast signals because
each sideband generated by the phase or frequency modulation
is then surrounded by sidebands produced by the ampli-tude
modulation process.
There are two basic types of interference that are
pertinent to the instant invention.
The first is self interference, specifically
interference to those wishing to receive the normal
broadcast program on the one hand and interference to data
reception on the other.
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The second type of interference is interference to
listeners of other s-tations, both adjacent or co-channel
stations.
Considering first the self interference and, more
specifically, interference to the normal broadcast program
listeners, it is important that the data signal not be
detectable.
The instant invention accomplishes substantially
interference-free operation by a number of mechanisms.
First of all, and in common with the prior art, the
modulation for the data is substantially a form of angular
modulation; i.e., quadrature modulation. While quadrature
modulation includes an in-phase (envelope) component which
can be detectable by envelope detectors, the amplitude is
small. For example, if each of the quadrature modulation
sidebands is restricted, to say 10% of the carrier
amplitude, the resulting envelope modulation is
approximately 1%. It must be stressed, however, that errors
in receiving tuning, multipath conditions, etc. can convert
the quadrature sidebands to larger în-phase components.
Fortunately, under most conditions such problems will not
cause any difficulty.
In one embodiment of this invention, it is seen
that means are provided for controlling the amplitude of the
~5 quadrature modulation sidebands as a function of the program
amplitude modulation. Thus, when the normal program is
absent, the data quadrature modulation sidebands are reduced
to zero amplitude. However, as the amplitude modulation
increases, the radiated level of the data sidebands is
increased so that, for one embodiment of the invention, the
quadrature modulation sidebands are always at least
approximately 15 db below the level of the program amplitude
modulation sidebands. This provides a masking effect for
listeners to the normal broadcast program in addition to the
isolation provided by quadrature modulation and, for all
practical purposes, the data sidebands do not interfere,
under normal conditions, with the broadcast channel.
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This invention may be used -to transmit both
monophonic and stereophonic broadcas-t program material. All
proposed methods of transmitting stereo require both
in-phase and quadrature modulation components. In the
stereo systems, the L-R components produce angular
modulation. Thus, the demodulation means for such stereo
signals is responsive to angular modulation and would be
subject to interference by the data quadrature modulation
components In at least one presently operating AM stereo
system, the ISB system, as described in U.S. Patents
3,908,090 and 4,373,115 uses a mixed highs (i.e., where
stereo separation is substantially reduced or eliminated
above a frequency, say, in the order of 6 to 8 kHz) method
of operation is provided. At some frequency, generally 6 to
7 kHz, the stereophonic separation is reduced substantially.
Accordingly, the sensitivity of the receiver to frequencies
above 6 or 7 kHz to angular modula-tion can be grea-tly
decreased without altering the stereo performance.
In order to maintain the low interference
characteristic for stereo reception of the amplitude
modulated signal, the data is transmitted preferably in the
frequency range where the "mixed highs" technique is
functioning. Accordingly, the data is quadrature sidebands
standard at approximately 8.5 kHz from the carrier. A
typical range of operation would be 7.500 Hz to 9.500 Hz.
By the use of the mixed highs approach the amount
of interference suffered by data signal receivers is also
minimized because the broadcast material has little or no
angular modulation at the frequencies to which the data
receiver must respond. The data receiver transmission
system would best use modulation techniques that can produce
low data error counts even when subject to relatively poor
signal-to-noise and interference situations. It is also~ of
course, possible to use various error correcting codes or at
least error sensing codes plus redundancy to further
decrease data error counts.
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The second type of interference, i.e., interference
to adjacent channels may be maintained within acceptable
levels by always maintaining the data sidebands well below
the level of the AM broadcast signal.
Another feature of the invention improves reception
of the data signal. When the data speed is reduced, i.e.,
when the broadcast modulation level is low, the data
modulation is reduced. This reduced data signal level will
accordingly reduce the signal-to-noise ratio of the received
data signal. The bandwidth of the data receiver channel
need not be as wide as during periods of high data flow.
Therefore, it is possible and desirable to reduce the data
channel bandwidth as a function of the data signal
transmitted level. This variable bandwidth filtering means
may be used either at IF or at baseband. In other words,
the filtering means can be reduced in bandwidth during low
speed data transmission periods so as to improve the signal-
to-noise ratio and reduce the error count. Alternatively,
the lowpass filter, which would normally be part of the PSK
demodulation 422, can be made to vary its cutoff as a
function of the data rate. An effective method for
controlling the bandwldth is to derive a control voltage
from the received program audio level. This feature is
further described below.
There are a number of means for producing dc
controlled bandwidth filters. Recently, an excellent
technique called switch capacitor filters has been developed
which allows variable bandwidth filters to be implemented
with integrated circuits. A variable frequency clock is
used to change the cutoff frequencies of such filters. For
example, the National Semiconductor Corporation of Santa
Clara, California, introduced the MF10 universal dual switch
capacitor filter. Generally, such filters are used at audio
frequencies and can be configured as bandpass or lowpass
filters. Thus, those skilled in the art have a number of
variable bandwidth filter means, including RF and IF filter
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means, from which they may choose a filter which best serves
their specific design requirements.
For a better understanding oF the present
invention, together with other and further objects thereof,
reference is made to the following description, taken in
conjunction with the accompanying drawings, and its scope
will be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objective features and
characteristics of the present invention will be apparent
from the following specification, description, and
accompanying drawings relating to typical embodiments
thereof.
FIG. 1 is a block diagram of one form of
transmitter using the invention. This embodiment
illustrates the use of phase shift keying but it will be
understood by those skilled in the art that other forms of
data transmission might be used, such as, FSK, as well as
other engineering design choices.
FIG. 2 shows the two blocks that must be
substituted in FIG. 1 when frequency shift keying is used
for the data transmission rather than phase shift keying
system provided for in FIG. 1.
FIG. 3 is a sketch of a typical spectrum signature
for the wave produced by a transmission system shown in
FIG. 1.
FIG. 4 is a block diagram of a receiver suitable
for receiving the signal produced by the transmitter shown
in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a block drawing showing one embodiment of
the subject invention. Block 102 is a source of
stereophonic signal such as the circuitry shown in U.S.
Patents 3,218,393 or 3,908,090 or 4,373,115. It includes an
envelope modulator so that the IF wave out of block 102 is a
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complete stereophonic signal including the L~R component.
The preferred form of AM Stereo wave is the lndependent
sideband wave, although the system disclosed herein may be
adapted to other forms of AM Stereo such as forrns of
quadrature modulation proposed by the Harris and Motorola*
Corporation or the AM/PM system as proposed by Magnavox.*
The IF stereo wave, which in one embodiment is a 1.4 MHz
carrier wave, is fed to summation circuit 104.
The invention may also be used to transmit a data
lû signal with a monophonic signal. For monophonic
transmission operation L may be made equal to R, the input
signals to the AM stereo generator 102. However, if the
station continuously transmits a monophonic signal, block
102 may be deleted and a simple amplitude modulation wave
generator 100 be substituted. In this case, switch 103 is
thrown to the position connecting AM generator 100. In the
following discussion stereophonic transmission is
considered, although it will be understood by those skilled
in the art that monophonic transmission can be similarly
used.
The L and R audio inputs to the stereo generator
are also fed to a summation circuit 106 which produces an
L+R output. This output is fed to level detector 108. In
the monophonic case when block 100 is used, switch 107 is
thrown so that the mono signal source feeds level detector
108. The combination of blocks 106 and 108 are used to
generate a control that varies the amount of data signal
combined with the stereo wave transmitted. This amount must
be carefully controlled so that listeners to normal
broadcast programs are not disturbed by the data signal.
Therefore, it is important that when there are pauses or
weak L~R modulation segments the level of the data signal be
suitably attenuated so as to avoid interfering with
broadcast listeners.
The control signal from level detector 108 controls
attenuator 116 which controls the level of the data signal
which is combined with the stereo wave in block 104. The
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level detected control signal is also fed to the data source
so as to cause the flow of data to be controlled as a
function of the power in the transmitted da-ta siynal. At
one extreme, when the amplitude of the data signal is
maximum because the L+R level exceeds a certain amplitude,
the data rate can be maximum. At the other extreme, when
the L~R is absent or below a certain level so that no data
signal can be transmitted, then the data stream must be
stopped.
The output of the data source is fed to difference
circuit 110, which in turn feeds phase shift keying
modulator 112. In order to provide the best feedback
effect, the modulator must be a linear phase modulator. A
phase locked loop can be used as a phase modulator, for
example, the output of block 112 typically would be a 8.5
kHz and could be phase shift keyed in any one of a number of
PSK methods well known to communication system designers~
For example, a four phase signal using differential phase
detector may be used.
The output of modulator 112 feeds balanced
modulator 114 which is also fed an IF carrier component at a
phase that will ensure the double sideband components that
are produced in balanced modulator 114 will be in quadrature
with the IF carrier component of the stereo wave fed to
summation circuit 104. It is desirable to cause the data
sideband components to be in quadrature with the carrier so
as to ensure minimum in-terference to listeners to the AM
broadcast program. Block 118 can be adjusted to provide
this quadrature relationship.
The double sideband suppressed carrier wave output,
which for the example discussed above, are at a frequency of
IF +8.5 kHz, is fed to attenuator 116. Attenuator 116
adjusts the level of the FSK data sidebands so that they
support the data transmission without interfering with
normal broadcast reception. The output of attenuator 116 is
fed to summation circuit 104.
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The output of the summatlon circuit 104 is the
complete AM Stereo plus data wave, which must then be
converted to the proper carrier frequency and amplitude so
as to be suitable to be used with an external transmitter in
order to produce the desired combined stereo and data waves
at a suitable power level.
A sample of this signal is fed to a circuit for
demodulating the data wave so as to provide negative
feedback for minimizing errors in the data message. This
sample is fed to a product demodulator which is also fed a
quadrature carrier component which can be accurately
adjusted in phase by variable phase shift block 122. The
resulting audio is fed to a BPF 124 that selects the audio
PSK wave which in this example is centered at 8.5 kHz. This
filtered PSK is then fed to PSK demodulator 126.
The PSK demodulator 126 should be of the same type
as used in a typical data signal receiver. It will be
apparent to those skilled in the art that FSK operation will
require an FSK demodulator to be used in block 126.
Examples of phase shift keying demodulators (as well as FSK
demodulators) including differential phase detectors (as
well as phase shift modulators) are treated in "Data
Transmission", W. R. Bennett and J. R. Davey, McGraw-Hill
1965 and elsewhere.
The output of the PSK demodulator 126 is fed
through a feedback network so as to maintain stability and
finally to difference circuit 110 to complete the negative
feedback path. The negative feedback is helpful in
maintaining low error counts eventhough a certain amount of
interference can be expected from stereo components falling
within the data channel bandwidth.
The combined stereo and data IF wave is then fed to
the "Flatterer" option circuit 130 for minimizing asymmetry
in transmitter antennas. Such a circuit was originally
disclosed in U.S. Patent No. 4,194,154. This circuit should
be used at stations where the transmitting antenna can be
expected to significantly disturb the quadrature between the
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data channel sidebands and the carrier. If the data
sidebands are shifted from their quadrature relationship
with the transmitted carrier the data signal can be expected
to cause somewhat more interference and be heard by
listeners to the main broadcast signal. This problem should
not be of concern to stations with wideband symmetrical
frequency response antenna system and therefore block 130 is
shown dotted and is to be considered optional. For further
details of the antenna compensation circuit and its
operation, please consult U.S. Patent No. 4,194,154.
The output of the antenna compensation circuit
feeds limiter 132 and product demodulator 134 which prepares
the wave for use in an Envelope Elimination and Restortion,
EER, system as disclosed in U.S. Patent No. 2,666,133 and a
number of publications; including, Kahn "Comparison of
Linear Single-Sideband Transmitters with Envelope
Elimination and Restortion Single-Sideband Transmitter",
Proc. IRE, Volume 44, p-p 1706-1712; Dec. 1956.
Limiter 132 serves the purpose of removing envelope
modulation so as to isolate the angular modulation. The
input and output of limiter 132 are multiplied together so
as to envelope demodulate the output of flatterer 130. The
resulting audio wave is fed to adjustable time delay 136
which in turn feeds audio to the audio input of an
associated amplitude modulation transmitter.
The angular modulated wave from the limiter 132
feeds time delay circuit 138 which in turn feeds frequency
translator 140. The frequency translator is also fed by a
final carrier frequency wave generated in oscillator 142.
The output of oscillator 142 is phase modulated in modulator
146 by the stereo pilot wave which in the preferred example
is 15 Hz wave generated in oscillator 144.
The RF output from frequency translator 14û is
amplified in amplifier 148 to a suitable level to excite the
associated transmitter, where a high powered combined stereo
and data signal is produced.
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FIG. 2 shows how -to modify the phase shift keying
data transmission system o-F FIG. 1 for use with frequency
shift keying (FSK) data transmission.
An FSK modulator 212 is substituted for the PSK
modulator of FIG. 1. This produces a frequency shift keyed
wave which in turn is fed to balanced modulator 11~. The
frequency shift keyed wave produced should be a true FSK
wave, not a two tone wave so that when the circuit is part
of the feedback system the corrections for keying distortion
by interference from the program broadcas-t material can be
compensated.
Similarly, block 226, in FIG. 2 is substituted for
PSK demodulator 126. A phase locked loop circuit can be
used for such frequency demodulation. The subject of
frequency shift keying is well known and many standard
communications provide full information describing such
circuitry.
A suitable frequency shift would be 1,000 Hz and,
for example, the mark Frequency could be, for example, 8,000
Hz and the space frequency 9,000 Hz for the transmission of
data at a rate up to 1200 bits/second. In some respects
frequency shift keying is more rugged than phase shift
keying. However, under favourable conditions phase shift
key;ng has a lower error count.
It is expected that this invention will be applied
to both types of data transmission keying systems.
It is noteworthy that the overall sys-tem is most
compatible with a frequency separation type stereo such as
the Independent Sideband AM Stereo system. Some phase
separation systems, such as the system proposed by Motorola,*
which have relatively poor spectral characteristics can
cause splatter into the data channel, increasing error
count. Furthermore, having L or R-only program segments
will cause the receiver carrier to shift in phase causing
data errors. Nevertheless, embodiments of the present
invention can be used with phase separation AM Stereo
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systems and the claims are not limited to -Frequency
separation AM Stereo systems.
The output level of attenuator 116 should be set so
that when the data signal is combined wi-th the broadcast
signal, the peak phase modulation of the resulting wave
caused by the data signal is approximately +10 which will
limit peak distorticn of the broadcast signal to
approximately 5%. Since these figures are peak, the average
distortion is to be expected to be significantly less.
lû Also, it is noteworthy, that the distortion drops rapidly
with the program percentage of modulation. Indeed, a drop
from 100% modulation to 90% reduces the peak distortion -to
approximately 2.5%. This distortion could, of course, be
eliminated completely if the data signal was combined with
lS the broadcast signal in a conventional multiplication
process rather than the linear summation process. The
penalty would be a significant widening in spectrum
occupancy of the combined signal.
Phase shift keying systems generally have a lower
error count than FSK. However, PSK can be disturbed by
phase modulation of the carrier caused by stereophonic
modulation of the main channel. Also, carrier phase error
caused by the data signal can be disturbing to the phase
separation stereo systems, such as the Motorola*system, that
rely on the phase relationship between the carrier and
sideband components to transmit the L-R stereo information.
Fortunately, the problem is much less significant in the ISB
AM stereo system because stereo separation is not a function
of the relative phase of the carrier and sidebands.
In FIG. 1, the BPF 124 in the data feedback pa-th is
made to vary by using the control voltage from block 108 to
vary the bandwidth of filter 124. This is the same type of
arrangement as will be used in the receiver shown in FIG. 4.
A very important feature of the invention is that
the transmission speed of the data signal adapts to the
level of the normal broadcast signal's program level.
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This feature allows relatively high average levels
of data flow to be achieved while maintaining low levels of
perceived interference. To implement this feature 9 the flow
of data is controlled as a function of the broadcast program
level. Those skilled in the data transmission and handling
arts will be aware of means for storing data at one rate and
recalling it at a variable rate. For example, an endless
loop which records the data at one speed, stores the
recorded tape, and then takes tape out of storage and
playbacks the tape at a variable rate as a function of the
level of the broadcast signal may be used. In U.S. Patent
No. 3,341,883, Mr. Paul R. Jones discloses means that may
readily be adapted to store and recall data for use in this
invention. One skilled in the art of designing equipment
using semiconductor storage circuits will be able to readily
implement the storage and recall means without recourse to
tape mechanisms. A clock signal can be recorded along with
the data signal and its frequency will then vary directly
with the playback tape speed in synchronisrn with the data
flow.
Accordingly, the clock signal can be used to
synchronize the received data signal.
Another means for achieving synchronization of the
data receiver with the data transmitter is to use a return
to zero (RTZ) polar binary signal.
This type of data signal contains symbol timing
information. As pointed out in the above referenced Bennett
and Davey book, such signals are self-clocking. Each
information bearing keying element is surrounded by a zero
signal, therefore, the data signal can be fully recovered
without providing additional clock information.
As the main program level drops, the speed of data
flow is reduced and when the main broadcast signal's
modulation is very low or absent 9 the data flow actually
stops. At this time the amplitude of the radiated RF data
signal is caused to drop to a very low amplitude or zero.
In one arrangement, the full character being transmitted is
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transmitted prior to any pauses due -to low modulation
levels. In order to accomplish this, a minimum data speed
must be used; for example, say 200 bits/second. If 8 bits
words are used the maximum data -tail would be 40 ms, which
is a reasonable data tail length, to be masked by the decay
waves of speech and music.
FIG. 4 is a blocl< diagram of a receiver sui-table
for recovering phase shift keying data signals of the type
generated by the apparatus of FIG. 1. An antenna, 402,
which may be a small ferrite rod antenna feeds an RF
amplifier, 404, operating at the carrier frequency of the
station to be used. This amplifier, in turn, feeds a mixer
406. A crystal oscillator comprising the oscillator and a
quartz crystal 408 provides the proper injection frequency
for mixer 406.
The resulting stable IF wave is fed to amplifier
412. The output of this amplifier feeds a carrier bandpass
filter which may be a narrow band crystal filter, for
example, or it may be a phase locked loop operating as a
narrow band filter.
The effective bandwidth of the filter should be
quite small so as to remove sideband components and
attenuate the pilot modulation which 9 for one system of
stereo broadcasting, is 15 Hz. The output of the filter,
414, feeds a phase shifter which shifts the carrier phase by
90 degrees.
The output of the phase shifter, 416, feeds a mixer
circuit 418 which may be a balanced mixer. Also feeding the
mixer is a sample of the IF output wave from block 412. The
data signal at the output of mixer 418 is selected by
bandpass filter 420 whose bandwidth is adjustable and should
be wide enough to pass at least first order sideband
signalling components. The output of the bandpass filter
feeds phase shift keying demodulator 422. Of course, a
similar receiver could be used for FSK reception and a
suitable demodulator would be substituted for block 422.
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Another sample of -the IF output of amplifier 412
feeds envelope demodulator 424, the dc componen-t from the
envelope detector filtered by capacitor 426, resistor 428 of
capacitor 430 produces a suitable AVC voltage for
controlling the gains of the RF stage 4û4 and the IF stage
412. The audio output of envelope 424 is amplified in
amplifier 432 which can feed an audio output line if it is
desired to utilize the program signal to listen to voice or
music transmissions. The output audio wave is rectified or
detected by a level detector 434.
This level detector provides control voltage to
control the bandwidth of bandpass filter 420. When the
level is low the data rate is reduced at the transmitter end
and therefore, the bandwidth of the filter can be reduced,
improving the signal-to-noise ratio.
Conversely, at higher modulation levels when the
data rate is maximized, the bandpass filter 420 must have a
wide bandwidth so as to pass the keying information. At
this time, of course, the transmitted data level is
increased providing sufficien-t signal level to support the
higher speed data transmission.
Those skilled in the receiver art will recognize
that it is also practical to make a data receiver according
to this invention that does not use an intermediate
frequency but to do the required amplification and filtering
prior to demodulation of the data wave at the radio
frequency transmitted. Thus, receiver types that are not of
the superheterodyne type may be used.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended claims.
