CA1237782A - Compatible am broadcast/data transmission system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A system for transmitting a composite signal comprising a data transmission signal component and an AM broadcast signal component.
The broadcast signal component may be monophonic or stereophonic.
The level of the data signal component is made a function of the modulation level so that the data signal is masked by the program modulation and, therefore, AM radio listeners will not be disturbed by the data signal. The rate of data transmission is reduced as the level of the data signal is reduced. The data signal is in quadra-ture with the AM carrier so as to minimize detection of the data signal by an envelope demodulator. Suitable data receivers are also disclosed.

Description

I
BACKGROUND OF THE INVENTION

While the invention is subject to a wide range of applique-lions, it is especially suitable for use in a system for trays-milting 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 transmit-tying 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 some posit modulated wave can then he demodulated with an envelope de dilator to extract the normal program material Since the envelope demodulator is insensitive to the phase of the combo-site 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 Rio However, the rate of information flow through such systems have generally been very slow. If higher data rates are attempt-Ed 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 side-bands produced by the amplitude modulation process There are two basic types of interference that are pertinent to the instant invention.

~;~3';17~2 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.
The second type of interference is interference to listeners of other stations, 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, multi path conditions, etc. can convert the quadrature sidebands to larger in-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 quadrature modulation sidebands as a function of the program lZ3'77~

amplitude modulation. This, when the normal program is absent, the data quadrature modulation sidebands are reduced to zero amplitude. However, as the amplitude modulation - pa -Z 3'~'~'6 increases, the radiated level of thy data sidebands is increased so that, for one embodiment of the invention, the quadrature duration sidebands are always at least approximately 15 dub below the level of the program amplitude modulation sidebands. This pro-vises a masking effect for listeners to the normal broadcast pro-gram in addition to the isolation provided by quadrature modulation and, for all practical purposes, the data sidebands do not inter-lore, under normal conditions, with the broadcast channel.
This invention may be used to transmit both monophonic and stereophonic broadcast program material All proposed methods of transmitting stereo require both in-phase and quadrature mod-elation 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 come pennants. In at least one presently operating AM Stereo system, the IS system, as described in Us SO Patents 3,908,090 and 4,373,115 uses a mixed highs (ivy, 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 modulation can be greatly decreased without altering the stereo performance.
yin 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 quad-a _ 3 to nature sidebands standard at approximately 8.5 kHz from the courier typical range of operation would be 7.500 Ho to 9~500 Ho.
By the use of the mixed highs approach the amount of inter-furriness suffered by data signal receivers is also minimized be-cause 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 tech-piques 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 feast error sensing codas plus redundancy to further decrease data error counts The second type of interference, i.e., interference to ad-jacent channels may be maintained within acceptable levels by always maintaining the data sidebands well below the level of the AM broadcast signal.

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A general object of the present invention is to provide a system for transmitting data concurrently with normal broadcast programs over a standard AM broadcast station.
A further object is to achieve such concurrent data module-lion without disturbing listeners to the normal broadcast programs A still further object is to provide data transmission with-out causing significant additional interference to other broad-cast stations.
An additional object is to permit higher speed data trays-mission with low error rates.
Another object is to provide suitable data receivers for use with such a system.

SUMMARY OF THE INVENTION

The present invention combines a data transmission signal with an Amplitude Modulated (AM) carrier signal. The two signals are sum mated linearly rather than multiplied together so that the overall spectrum is not significantly widened. The data signal comprises two sidebands - an upper and a lower-sideband component whose sum is in quadrature with the carrier. One embodiment of this invention uses a base band phase shift keying (SKYE signal with a frequency of 8.5 kHz. This frequency is above the frequency range where current Independent Sideband AM Stereo broadcast signals generally limit stereo separation; i.e., 6 kHz in one model, and 7.5 kHz in a second model of AM Stereo exciter. The frequency is also well within the normal occupied bandwidth of AM broadcast signals.
Since the data signal is in quadrature with the carrier and the LO or envelope modulation component of the broadcast stereo signal occupying the same spectrum space, the system makes good use of the station's authorized bandwidth.
In addition to quadrature relation of the data signal minimizing interference, the invention takes advantage of the 1~3'~7~;~

"masking" phenomenon. This is a phenomenon whereby, under certain conditions, listener's threshold of hearing to one sound is raised by the presence of a second sound. Details concerning "masking" is treated by Fletcher in "Speech and Hearing in Communications", D. Van Nostrand, 1953.
In order to make effective use of masking, the level of the data signal is maintained below that of the sidebands representing the broadcast program signal. Thus, the data signal is made a function of the broadcast program level.
The rate of data transmission is reduced as the data signal level is reduced.
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 base band. 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. Alter-natively, the Lopez filter, which would normally be part of the ASK demodulation 422, can be made to vary its cutoff as a function of the data rate. An effective method for controlling the band-width 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 do 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 Corpora-lion 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 band pass or Lopez filters. Thus, those skilled in the art have a number of variable bandwidth filter means, including RF and IF filter means, from which they may choose a filter which best serves their specific design requirements.
For a better understanding of the present invention, to-getter 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.

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BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objective features and characters-tics of the present invention will be apparent from the follow-in 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, FISK, 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 trays-mission 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.

I.

1~3~7782 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. Relents 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 complete stereophonic signal including the LO component. The preferred form of AM Stereo wave is the index pendant sideband wave, although the system disclosed herein may be adapted to other forms of AM Stereo such as forms 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 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 gent orator 100 be substituted. In this case, switch 103 is thrown to the position connecting AM generator 100. In the following discus-soon stereophonic transmission is considered, although it will be understood by those swilled in the art that monophonic transmission can be similarly used.

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The L and R audio inputs Lo the stereo generator are also fed to a station circuit 106 which produces an LO output.
This output is fed to level detector 108. In the monophonic case when bleakly 100 is used, switch 107 is thrown so that the moo 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 trays-milted. This amount must be carefully controlled so that list-enters to normal broadcast programs are not disturbed by the data signal. Therefore, it is important that when there are pauses or weak LO 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 alien-valor 116 which controls the level of the data signal which is combined with the stereo wave in block 104. The 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 data signal. At one extreme, when the amplitude of the data signal is maximum because the LO level exceeds a con-lain amplitude, the data rate can be maximum. At the other ox-tree when the LO is absolute or l~clow a certain level so that no data signal can be transmitted, then the data stream must be stopped.

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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 PI 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 Id an IF carrier component at a phase that will insure 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 Quadram lure with the carrier so as to ensure minimum interference to listeners to the AM broadcast program. Block 118 can be adjusted to provide this quadrature rclationsl~ip.
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 FISK data sidebands so that they Support the data transmit-soon 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 summation 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 demodulate in 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 come potent 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 ASK wave which in this example is centered at 8.5 kHz. This filtered ASK is then fed to PUS
demodulator 126.

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The ASK 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 FISK operation will require a FISK demodulator to be used in block 126. Examples of phase shift keying demodulators (as well as FISK demodulators) include in differential phase detectors (as well as phase shift modulators) are treated in "Data Transmission", W. R. Bennett and J. R. Davy, McGraw-~lill 1965 and elsewhere.
The output of the ASK 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 even though a certain amount of interference can be expected from stereo components falling within the data channel bandwidth.

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The combined stereo and data IF wave is then fed to the "Flatterer" option circuit 13û for minimizing asymmetry in transmitter antennas. Such a circuit was originally disclosed in US. Patent #4,194,154. This circuit should be used at stations where the transmitting antenna can be expected to significantly disturb the quadrature between the 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 sideband 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 US. Patent ~t4,194,154.
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 Retortion, HER, system as disclosed in US. Patent #2,666,133 and a number of publications; including, Kahn "Comparison of Linear Single-Sideband Transmitters with Envelope Elimination and Retortion Single-Sideband transmitter". Pro. IRE, Volume 44, p-p 17û6-1712; Dec. 1956.

Limiter 132 serves Lye purpose of removing envelope modulation so as to isolate the annular 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 duration transmitter.
The angular modulated wave from the limiter 132 feeds time delay circuit 138 which in turn feeds frequency translator 14~.
The frequency translator is also fed by a final carrier frequency wave generated in oscillator 142. The output of oscillator 142 is phase dilated in modulator 146 by the stereo pilot wave which in the preferred example is 15 Ho wave generated in oscillator 144.
The RF output from frequency translator 140 is amplified in amplifier 148 to a suitable level to excite the associated trays-miller, where a high powered combined stereo and data signal is produced.
FIG 2 shows how to modify the phase shift keying data trays-mission system of FIG. l for use with frequency shift keying (FISK) data transmission.
An FISK modulator 212 is substituted for the ASK modulator of FIG. l. This produces a frequency shift keyed wave which in turn is fed to balanced modulator 114. The frequency shift keyed wave produced should be a true FISK wave, not a two tone wave so that when the circuit is purl of the feedback system the corrections for keying .lisLor~ion by interference from the program broadcast material can lo compensated.

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Similarly, block 226, is FIG. 2 is substituted for ASK
demodulator I 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 Lull information describing such circuitry.
A suitable frequency shift would be 1,000 Ho and for ox-ample, the mark frequency could be, for example, 8,000 Ho and the space frequency 9,000 Ho for the transmission of data at a rate up to 1200 bits/second. In some respects frequency shift keying is more rugged Han phase shift keying. However, under favorable conditions phase shift keying 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 system 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, cmbodimcnLs of the present invention can be used with phase separation AM Stereo systems and the claims arrant limited to frequency separation AM Stereo systems.

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The output level of attenuator 116 should be set so that when the data signal is combined with the broadcast signal, the peak phase modulation of the resulting wave caused by the data signal is approximately +10 which will limit peak distortion of the broadcast signal to approximately 5%. Since these figures are peak, the average distortion is to be expected to be signify-gently less. 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 elm-inated completely if the data signal was combined with 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 FISK. However, ASK can be disturbed by phase modulation of the carrier caused by stereophonic modulation of the main channel.
Also, any carrier phase error caused by the data signal can be dlsturbinc3 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.

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information. Fortunately, the problem is much less significant in the IS AM stereo system because stereo separation is not a function of the relative phase of the carrier and sidebands.
In Figure 1, the BPF 124 in the data feedback path is made to vary by using the control voltage from block 10~ to vary the band width of filter 124. This is the same type of arrangement as will be used in the receiver shown in Figure 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.
This feature allows relatively high average levels of data flow to be achieved while maintaining low levels of perceived interference. To implement this feature, 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, and 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 US. Patent 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.

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A clock signal can be recorded along with the data signal and its frequency will then vary directly with the playback tape speed in synchronism 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 no-sever with the data transmitter is to use a return to zero (RITZ) polar binary signal.
This type of data signal contains symbol timing information.
As pointed out in the above referenced Bennett and Davy 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, the data flow actually stops. At this time the amplitude of the radiated RF data signal is caused to drop to a very low am-plotted or zero. In one arrangement, the full character being transmitted is transmitted prior to any pauses due to low modulation levels. In older to accomplish this, a minimum data speed must be used; for example, say 200 bits/sec. If 8 bits words are used the mammal data toil would be I my, which is a reasonable data tail length, to be masked by the decay waves of speech and music.

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FIG. 4 is a bloat< diagram of a receiver suitable for recovering phase shift lying data sigllals Or the type generated by the apparatus of FIG. 1. I 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 issue an-plificr feeds a carrier ban(lpass 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 smell so as to remove 5 sideband components and attenuate the pilot mod-elation which, for one system of stereo 'broadcasting, is 15 Ho.
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 'l18 which may he 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 lay hand pass filter 420 whose bandwidth is adjustable and should be wide enough to pass at least first order sideband signaling components. The output of the band-pass filter- feds phase shift keying demodulator 422. Of course, a similar receiver could lo usual for FISK reception and a suitable den~dulator Waldo lo substituted ion lock 422.

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Another sample of the IF output of amplifier 412 feeds envelope dedulator I the do component from the envelope detector lit-toned by capacitor 426, resistor 428 of capacitor 430 produces a suitable ARC voltage for controlling the gains of the RF stage 'lo 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 Jo utilize the program signal to listen t o 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 band pass ~iltcr 420. When the level is low the data rate is reduce-l at the transmitter end and therefore, the band-idol the filter can be reduce improving the signal-to-noise ratio.
Conversely, at higher modulation levels when the data rate is maximized 9 the band pass filter 420 must have a wide bandwidth so as to pass the keying information. At this time, of course, the transmitted dale level is increased providing sufficient signal level to support the higher speed data transmission.
'rose skilled in the receiver art will recognize that it is also practice Lo make a data receiver according to this invention that dyes nil use an inLermcdiaLe freq-lellcy 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 superhetero(3yne type may be used.

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While there have been described what are believed to be the preferred embodiments of -the invention, those skilled in the art will luckiness that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the true scope of the invention.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A transmission system for transmitting amplitude modulated waves, suitable for reception by conventional amplitude modulation broadcast receivers, simultaneously with the transmission of data signals, comprising:
(a) means for generating an amplitude modulated wave fed by a source of program material and a carrier wave source, (b) data modulation means fed by a source of data said data modulator producing a wave in the audio range, (c) a quadrature modulator fed by the source of carrier waves and by the data modulation means to produce a pair of quadrature modulation sidebands above and below the carrier frequency, (d) means for combining the waves produced by (a) and (c) means, (e) means, for increasing the power of the combined waves produced in (d) means without introducing significant additional spectrum products.

2. The transmission system of Claim 1 wherein the (a) means is a stereophonic generator fed by a stereo source of program material.

3. The transmission system of Claim 1 wherein the (a) means is a monophonic generator fed by a mono source of program material.

4. The transmission system of Claim 1 wherein subsequent to the (d) combining means tube amplitude-frequency and phase-frequency characteristics of the combined wave is altered by network means so as to compensate for the amplitude-frequency and phase-frequency characteristic of the antenna the combined data and amplitude modulated wave is fed.

5. A transmission system for transmitting amplitude modulated waves, suitable for reception by conventional amplitude modulation broadcast receivers, simultaneously with the transmission of data signals, comprising;
(a) means for generating an amplitude modulated wave fed by a source of program material and a carrier wave source, (b) data modulation means fed by a source of data said data modulator producing a wave in the audio range, (c) means for measuring the level of the program material modulation, (d) means for controlling the data transmission speed as a function of the program level, (e) a quadrature modulator fed by the carrier wave source and by the data modulation means to produce a pair of quadrature modulation sidebands above and below carrier frequency, (f) means for controlling the level of quadrature modulation sidebands as a function of the program level as measured in (c), (g) means for combining the above produced amplitude modulated wave and the data modulated wave, and (h) means for deriving a wave of suitable power and carrier frequency from (g) means.

6. The transmission system of Claim 5 wherein the (a) means is a stereophonic generator fed by a stereo source of program material.

7. The transmission system of Claim 5 wherein the (a) means is a monophonic generator fed by a monophonic source of program material.

8. The transmission system of Claim 1 wherein a sample of the combined wave produced in (d) means is fed to a simulated data receiver and the demodulated output of the simulated receiver is fed to a difference circuit where it is subtracted from the data input signal to produce a negative feedback term and accordingly reduce distortion in the data signal.

9. A transmission system as claimed in Claim 1 wherein said means for combining the wave produced by (a) and (c) means linearly combines the waves.

10. A transmission system as claimed in Claim 1 whereon said sidebands are modulated from said source of data and envelope elimination and restoration type means fed by the (d) means to be used to produce an angular modulation wave and an envelope function wave suitable for adapting a conventional AM transmitter so as to produce a combined program/data signal.