Subcarrier injection system and method using adaptive level-shifted minimum shift keying

Abstract

A system for transmitting a main channel signal, a first subcarrier signal, and a second subcarrier signal includes a control signal generator for producing a control signal in response to the amplitude of the main channel signal and the first subcarrier signal, a modulator coupled to the control signal generator and generating the second subcarrier signal, a voltage controlled amplifier for providing the second subcarrier signal at an injection level that varies with the control signal, and a transmitter for transmitting the second subcarrier signal at the varying injection level.

Description

BACKGROUND AND FIELD OF THE INVENTION

This invention relates generally to broadcasting systems, and specifically to a system and method for transmitting data on a subcarrier while transmitting program material on a main channel.

Many radio broadcast systems are known to exist in which digital data are transmitted along with audio program material. For example, the United States Radio Broadcast Data System ("RBDS") Standard, published by the National Radio Systems Committee and sponsored by the Electronics Industry Association and the National Association of Broadcasters, describes a system for broadcasting a variety of program-related information on a subcarrier of a standard FM broadcast channel. The RBDS standard teaches a system for transmitting station identification and location information, as well as time, traffic and miscellaneous other information.

U.S. Pat. No. 5,491,838 to Takahisa et al., the contents of which are incorporated herein by reference, discloses a system that automatically recognizes program material being broadcast and transmits associated data related to such program material. For instance, if a musical piece is being broadcast, data concerning the composer and performers of the piece are also broadcast.

Numerous systems are known to permit transmission of data on a subcarrier while transmitting main program material on a main portion of a broadcast channel.

One such system is known as Level-controlled Minimum Shift Keying, or L-MSK. An L-MSK system is described, for example, in Yamada, et al., NHK's High Capacity FM Subcarrier System, NAB 1993 BROADCAST ENGINEERING CONFERENCE PROCEEDINGS, pp. 415 et seq., the contents of which are incorporated herein by reference.

As described in part therein, FM multiplex broadcasting allows digital signals to be transmitted along with composite stereo audio signals by frequency division multiplexing. A composite stereo audio signal includes a summed left and right channel or "L+R" monophonic signal that is transmitted as a baseband signal, as well as a difference or "L-R" stereophonic signal that is multiplexed using a first subcarrier centered at 38 kHz modulated on the broadcast channel. The digital signals are multiplexed for transmission on a second subcarrier, and such signals are generally maintained in frequency regions from 53 kHz to 100 kHz, modulated on the broadcast channel.

In conventional systems, a problem arises in that spurious frequency components from the multiplexed signal may extend beyond the desired frequency range and cause crosstalk with stereo audio information in the L-R signal on the 38 kHz subcarrier. To prevent such crosstalk interference from being objectionable, the level of injection of the multiplexed data signal on the second subcarrier is kept relatively low. Conversely, spurious frequency components from the L-R signal may also extend into the range of the multiplexed data signal, causing crosstalk interference to the data signal as well.

Unfortunately, low levels of injection (whether for the data signal or the L-R signal) result in marginal signal to noise ratios for received signals. In practice, it is found that the well-known phenomenon of multipath interference exacerbates this problem significantly, causing the L-R stereo audio signal and the multiplexed data signal to interfere with one another more drastically than would otherwise be the case.

In an L-MSK system, the level of injection for the multiplexed data subcarrier is varied as the L-R signal modulation varies. When there is very little modulation in the L-R signal, crosstalk from the multiplexed data signal can be quite noticeable to listeners. In this situation, however, there is relatively little risk of objectionable crosstalk interference to the data signal caused by the L-R signal. Therefore, the L-MSK system reduces the level of injection of the multiplexed data signal during such periods.

On the other hand, when the modulation of the L-R signal is great, the risk of noticeable crosstalk interference to the L-R signal caused by the multiplexed data signal is significantly reduced. At the same time, however, the risk that the L-R signal will interfere with the multiplexed data signal increases. Accordingly, the L-MSK system increases the level of injection of the multiplexed data signal during such periods.

In the system discussed in the above-referenced Yamada, et al. article, an injection level for the data signal of 4% of the .+-.75 kHz deviation of the FM carrier is used when the L-R signal is unmodulated, and an injection level of 10% of the .+-.75 kHz deviation of the FM carrier is used when the L-R signal is filly modulated. The term "injection level" refers to a measure of the amount of modulation of a frequency-modulated subcarrier, expressed as a percentage of the maximum overall signal deviation. In the case of current FM radio broadcasting standards, the specified maximum overall signal deviation is typically .+-.75 kHz, so an injection level of 10% refers to a subcarrier modulation level that will cause deviation of the overall carrier of .+-.7.5 kHz.

In practice, it is found that conventional L-MSK systems can provide reduced data error rates compared with systems not using L-MSK. It would be desirable, however, to have a system that exhibits improved performance over that possible with conventional L-MSK.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a subcarrier injection system and method that exhibits improved performance over conventional L-MSK systems by using not only a first subcarrier signal (e.g., the L-R audio signal), but also a baseband monophonic signal (e.g., the L+R audio signal) to adaptively control the injection level of a second subcarrier signal (e.g., a data signal).

In accordance with the present invention, a system for transmitting a main channel signal, a first subcarrier signal, and a second subcarrier signal includes a control signal generator for producing a control signal in response to the amplitude of the main channel signal and the first subcarrier signal, a modulator coupled to the control signal generator and generating the second subcarrier signal at an injection level that varies with the control signal, and a transmitter for transmitting the second subcarrier signal.

In another aspect of the invention, the main channel signal is a monophonic audio signal and the first subcarrier signal is a stereo difference audio signal.

In still another aspect of the invention, the modulator is coupled to the control signal generator by a voltage controlled amplifier (VCA), and the control signal corresponds to the sum of the amplitudes of a main channel (L+R) audio signal and a difference (L-R) audio signal.

Also in accordance with the invention, a system for transmitting a main channel audio signal, a difference subcarrier audio signal, and a second subcarrier signal includes a control signal generator that produces a control signal in response to an amplitude of the main channel audio signal, a modulator with VCA level control that generates the second subcarrier signal at an injection level varying in response to the control signal, and a transmitter for transmitting the second subcarrier signal.

Further in accordance with the invention, a method of transmitting a main channel signal, a first subcarrier signal, and a second subcarrier signal includes producing a control signal in response to amplitudes of the main channel and first subcarrier signals, generating the second subcarrier signal at an injection level that varies with the control signal, and transmitting the main channel signal, the first subcarrier signal, and the second subcarrier signal.

The features and advantages described in the specification are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system (100) for transmission of audio and a data subcarrier signal, in accordance with the present invention.

FIG. 2 is a graph showing change in injection level of a multiplexed data signal (172) with changes in a control signal (150), in accordance with the present invention.

FIG. 3 is a block diagram of a system (300) for transmission of audio and a data subcarrier signal using program time delay, in accordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The figures depict a preferred embodiment of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.

Referring now to FIG. 1, there is shown a transmission system 100 in accordance with the present invention. Known L-MSK systems provide advantages over fixed-injection subcarrier schemes, but only in situations where the L-R audio difference signal varies dynamically. There are numerous situations in modern broadcasting where monophonic signals are transmitted, for instance during newscasts or "talk" programs. In these situations, conventional L-MSK systems do not provide any advantage over other systems.

In such monophonic situations, no modulation is occurring in the first subcarrier responsible for transmission of the L-R audio difference signal. Thus, no interference to a signal in a second subcarrier, such as a multiplexed data signal, from the L-R signal occurs. If the broadcast station has turned off the 19 kHz pilot tone that is used to indicate stereo transmission, receivers will not attempt to decode any received L-R information, so no interference to the L-R signal from the multiplexed data signal will be noticed either. However, some crosstalk interference still may occur between the main composite audio channel in the baseband, i.e., the monophonic or "L+R" audio signal, and the multiplexed data signal. Particularly during periods when multipath interference is present, such interference can be noticeable in both the main channel and in the subcarrier used for the multiplexed data signal.

In short, transmission system 100 reduces the deleterious effects of such interference by adaptively increasing the injection level of the multiplexed data signal when the amplitude of the main channel signal increases.

The operation of the transmission system 100 is described in greater detail by discussion of the component parts illustrated in FIG. 1. In a preferred embodiment, transmission system 100 includes left and right audio sources 110, 111; sum and difference amplifiers 120 and 121 producing L+R and L-R signals 130, 131, respectively; amplitude detectors 140, 141; comparator 145; control shaping circuit 146 producing control signal 151; voltage controlled amplifier 150; 1.216 MHz voltage controlled oscillator (VCO) 161; divide-by-16 divider 162; divide-by-76 divider 164; divide-by-64 divider 165; 19 kHz phase-locked-loop (PLL) circuit 167; 76 kHz phase-locked-loop (PLL) circuit 162; data source 160; modulated oscillator 170; bandpass filter 171; stereo generator 175; summer 176; transmitter 180; and antenna 181. Except as otherwise discussed herein, the subsystems of transmission system 100 are implemented in a conventional manner using known circuitry.

Summing amplifier 120 operates conventionally on input from left and right audio sources 110, 111 to produce L+R signal 130. Difference amplifier 121 also operates conventionally on the same input to produce L-R signal 131. In an alternate embodiment, the L+R and L-R signals 130, 131 may be available from the operation of existing conventional circuitry, thus obviating the need for amplifiers 120 and 121. Each of the signals 130, 131 is applied as input to a corresponding amplitude detector 140, 141.

Each amplitude detector 140, 141 uses conventional rectification and low-pass filtering circuitry to produce a signal indicative of the time-averaged absolute value of the signal applied to it. In a preferred embodiment, the time response of amplitude detectors 140 and 141 is non-linear and program-dependent. Specifically, the time response exhibits a dual time response characterized by providing a 2.5 ms rise time from minimum injection level to maximum injection level and 5 ms fall time from maximum injection level to minimum injection level.

The amplitude signals produced as output by amplitude detectors 140 and 141 are applied as inputs to comparator 145, and the resulting signal is applied as input to control shaping circuit 146. Control shaping circuit 146 produces and sets bounds for a control signal 151 that, in a preferred embodiment, ranges in amplitude from 1.6 volts when neither the L+R signal 130 or the L-R signal 131 is modulated to 2.0 volts when both the L+R signal 130 and the L-R signal 131 are fully modulated. Thus, shaping circuit 146, operating in conjunction with amplifiers 120 and 121, amplitude detectors 140 and 141, as well as comparator 145, provide a control signal generator subsystem.

The signals from left and right audio sources 110, 111 are also applied as input to a conventional stereo generator 175, which produces a conventional composite stereo signal applied to summer 176, as well as a conventional 19 kHz pilot tone.

In order to ensure that no undesirable heterodyning or beat artifacts are introduced in system 100, several signals used in the operation of system 100 are maintained in phase synchronization with one another. Specifically, a 1.216 MHz voltage controlled Oscillator 161 produces a 1.216 MHz master timing signal, phase synchronized with the 19 kHz pilot tone produced by stereo generator 175 as discussed below. A divide-by-64 divider 165 accepts as input the 1.216 MHz signal and produces therefrom a 19 kHz signal which is applied, together with the 19 kHz pilot tone from stereo generator 165, to a 19 kHz PLL circuit 167. The output of PLL circuit 167 is fed back as a correction signal to VCO 161 to maintain the frequency stability of the 1.216 MHz signal. A divide-by-16 divider 163 also receives as input the 1.216 MHz output signal from VCO 161 and produces therefrom a 19 kHz signal that is used as a reference signal for a 76 kHz PLL circuit 162.

A divide-by-76 divider 164 also receives as input the 1.216 MHz signal from VCO 161, and produces therefrom a 16 kHz bit clock signal, which is applied to data source 160. Accordingly data source supplies data at a 16 kbps rate to a direct FM input of modulated oscillator 170, which frequency-modulates the data on a 76 kHz subcarrier. A feedback loop is provided from modulated oscillator 170 to 76 kHz PLL circuit 162, so that PLL circuit 162 can then provide modulated oscillator 170 with a correction signal, in a conventional manner. Using this configuration, all signals that are nominally related by some harmonic relationship are maintained as phase synchronous throughout system 100.

Data source 160 is, in a preferred embodiment, a conventional source of digital data, producing a data signal suitable for subcarrier transmission, for instance using known minimum shift keying techniques in which a "0" value is represented as a signal of one frequency and a "1" value is represented as a signal of another frequency. In a preferred embodiment, the data are provided in a known version of frequency shift keying format called minimum shift keying (MSK), also sometimes referred to as fast frequency shift keying (FFSK). Minimum shift keying uses a frequency shift in hertz that is exactly one half of the corresponding signaling rate in baud, thereby resulting in a modulation index of 0.5. In a preferred embodiment, a data rate of 16 kbps is used, resulting in a frequency shift of 8 kHz. In an alternate embodiment, data source 160 can provide digital data in other formats or other types of data, such as analog audio data.

As mentioned above, modulator 170 is configured in a conventional manner to provide a modulated subcarrier for transmission of the signal from data source 160. Using the frequency shift example mentioned above, modulator produces a nominal subcarrier frequency of 76 kHz down-shifted to 72 kHz to represent a logical zero and up-shifted to 80 kHz to represent a logical one.

The output of modulator 170 is applied as an input to voltage controlled amplifier 150, the gain of which varies based on control signal 151.

The output of voltage controlled amplifier 150 is applied to a bandpass filter 171 that attenuates any frequency components outside of a desired passband. In a preferred embodiment, bandpass filter provides a passband centered at 76 kHz and having 3 dB cutoff points at approximately 70 kHz and 82 kHz.

The output of bandpass filter 171 is a multiplexed data signal 172. This signal is summed with the conventional composite stereo audio signal produced by stereo generator 175 by summer 176. The output of summer 176 is applied to transmitter 180 for conventional FM broadcast transmission thereof from antenna 181.

The configuration illustrated in FIG. 1 is based on an assumption that stereo generator 175 and FM transmitter 180 provide conventional audio processing and FM exciter circuitry. It should be recognized that, depending on the conventional circuitry used to implement certain components of system 100, there may be variations from the circuitry illustrated in FIG. 1. Thus, FIG. 1 is merely illustrative of one possible implementation in accordance with the present invention.

Referring now also to FIG. 2, there is shown a graph illustrating a transfer function between control signal 150 and the injection level of multiplexed data signal 172 in a preferred embodiment. As illustrated in FIG. 2, when the value of control signal 150 is at a minimum value, indicating no modulation from either the L+R signal 130 or the L-R signal 131, the injection level of multiplexed data signal 172 is set to be 4%. As the control signal increases from its minimum value to a first threshold value, the injection level stays constant at 4%. Increases in the control signal value beyond the threshold cause the injection level to begin rising, until a maximum injection level of 10% is reached when the control signal 150 is at a second threshold. Increases in the control signal beyond this threshold have no further effect on the injection level. In this manner, the injection level varies, within bounds, as the L+R signal 130 and the L-R signal 131 vary. It should be recognized that other transfer functions could also be used, whether linear, exponential, hysteretic, or otherwise, as desired in any particular application.

In a preferred embodiment, the injection level varies based on the modulation levels of either the sum (L+R) or difference (L-R) signal, depending upon which one is "controlling" in the following manner. In a preferred embodiment, the first threshold (i.e., 4% injection level) is used when both:

a) the audio sum (L+R) signal is at or below a 10% modulation level; and

b) the difference (L-R) signal is at or below a 2.5% modulation level.

The second threshold (i.e., 10% injection level) is used when either:

a) the audio sum (L+R) signal rises to at least a 20% modulation level; or

b) the difference (L-R) signal rises to at least a 5% modulation level. In any other event, the following process (expressed here as pseudocode) is applied by amplifiers 120, 121; amplitude detectors 140, 141; comparator 145 and control shaping circuit 146 to determine the desired injection level:

Factor.sub.L+R =(Modulation.sub.-- Level.sub.L+R -10%)/10%

Factor.sub.L-R =(Modulation.sub.-- Level.sub.L-R -2.5%)/2.5%

Controlling.sub.-- Factor=Maximum(Factor.sub.L+R, Factor.sub.L-R)

Injection.sub.-- Level=4%+(Controlling.sub.-- Factor*6%)

In the embodiment shown in FIG. 1, this process may be achieved simply by appropriate scaling of gain in amplifiers 120, 121, as will be evident to those skilled in circuit design. It should also be evident that numerous other circuit configurations could also be used to implement the mapping from the (L+R) and (L-R) modulation levels to the desired injection level as set forth in the pseudocode above. Once this mapping is determined, known characteristics of VCA 150 can then readily be used to determine the corresponding level of control signal 151 required to provide such injection level.

It also should be recognized that in some applications, it may be desirable to ignore entirely the signal on the first subcarrier, i.e., the L-R signal 131 in the system illustrated in FIG. 1. For example, if the first subcarrier is used not for audio difference information but instead for digital data, crosstalk interference between the first and second subcarriers may be less important than interference with the main channel audio.

Referring now to FIG. 3, there is shown an alternative embodiment illustrating processing that may be used either in conjunction with, or instead of, the adaptive techniques discussed above. System 300 is similar to system 100, but further includes a digital delay circuit 310 interposed in the left and right channel audio feeds. The purpose of this delay is to allow the control signal generation circuitry of system 300 and data source 151 to operate with a priori knowledge of the audio modulation levels to be expected, and to adaptively adjust injection levels or send data accordingly.

It is well know that the usable coverage area for subcarrier transmission is reduced when injection is reduced. Therefore, receivers that are able to obtain generally error-free data from transmitter 180 when the multiplexed data signal is injected at a 10% level may not enjoy error-free data when the data signal is injected at only 4%.

As shown in FIG. 3, control signal 151 is applied not only to VCA 150, but also to data source 160. In the embodiment illustrated in FIG. 3, data source 160 includes sufficient processing capability to determine when the value of control signal 151 is such that data being sent for transmission will be subject to a relatively low injection level. In one embodiment, whenever this happens, data source 160 re-sends such data upon determining, from the value of control signal 151, that greater injection levels are again available. In another embodiment, data source 160 re-sends such data in these circumstances only if the portion of data being transmitted has at least a predetermined priority level. In still another embodiment, decisions as to what data to transmit are constantly made based on the upcoming audio modulation levels and the various priorities of data to be sent; important data blocks are sent during times when high injection levels are provided, while less important data blocks are sent at other times. In many environments using current-generation data sources, such decision-making requires some time, therefore necessitating the addition of digital delay 310 so that system 300 can match appropriate data with appropriate injection levels.

Similarly, if the operation of the control signal generation subsystem uses components that cannot determine the control signal 151 in real time, digital delay 310 allows the control system to apply the appropriate control signal level at exactly the right time so as to achieve the results discussed herein.

It should be noted that in some program recognition schemes, such as those discussed in the above-referenced U.S. patent, program material is automatically recognized, so upcoming modulation levels can be predicted based on a priori information about the transmitted audio. Information from such systems can then be used instead of digital delay 310 if desired.

From the above description, it will be apparent that the invention disclosed herein provides a novel and advantageous improved subcarrier injection system, in which adaptive techniques are used to increase a subcarrier injection level based at least in part on the amplitude of a main channel signal. The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. It should also be recognized that the invention could also be used in different applications than FM subcarrier data transmission. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.

Claims

1. A system for transmitting an applied main channel signal, an applied first subcarrier signal, and an applied second subcarrier signal, the applied second subcarrier signal not being derived from the applied main channel signal, the system comprising:

a control signal generator for producing a control signal varying over more than two values in response to a first overall amplitude corresponding to said applied main channel signal and a second overall amplitude corresponding to said applied first subcarrier signal;

a modulator operatively coupled to said control signal generator, the modulator accepting as input said control signal and generating as output said applied second sub carrier signal at an injection level, the injection level varying in response to said control signal; and

a transmitter operatively coupled to said modulator and accepting as input said applied second subcarrier signal for transmission.

2. A system as in claim 1, wherein said applied main channel signal is an applied monophonic audio signal and said first subcarrier signal is an applied stereo difference audio signal.

3. A system as in claim 1, wherein said control signal generator and the modulator are coupled by a voltage controlled amplifier and wherein said control signal corresponds to a selected one of the first overall amplitude and the second overall amplitude, selection being responsive to comparison of the first overall amplitude and the second overall amplitude.

4. A system as in claim 1, wherein said control signal generator and the modulator are coupled by a voltage controlled amplifier and wherein said control signal corresponds to a first detected overall amplitude of an applied monophonic audio signal and a second detected overall amplitude of an applied stereo difference audio signal, the applied monophonic audio signal corresponding to the applied main channel signal and the applied stereo difference audio signal corresponding to the applied first subcarrier signal.

5. A system as in claim 1, wherein the applied second subcarrier signal corresponds to an applied digital data signal.

6. A system for transmitting an applied monophonic audio signal and an applied second subcarrier signal, the applied second subcarrier signal not being derived from the applied monophonic audio signal, the system comprising:

a control signal generator for producing a control signal varying over more than two values in response to a first overall amplitude corresponding to the applied monophonic audio signal;

a modulator operatively coupled to said control signal generator, the modulator accepting as input said control signal and generating as output said applied second subcarrier signal at an injection level;

an amplifier for coupling said control signal generator and said modulator, the amplifier varying the injection level in response to said control signal and producing an adjusted level version of the applied second subcarrier signal; and

a transmitter operatively coupled to said amplifier and accepting as input said adjusted level version of said applied second subcarrier signal for transmission.

7. A method of transmitting an applied main channel signal, an applied first subcarrier signal, and an applied second subcarrier signal, the applied second subcarrier signal not being derived from the applied main channel signal, the method comprising:

producing a control signal varying over more than two values in response to a first overall amplitude corresponding to the applied main channel signal and a second overall amplitude corresponding to said applied first subcarrier signal;

generating said applied second subcarrier signal at an injection level, the injection level varying in response to said control signal; and

transmitting said applied main channel signal, said applied first subcarrier signal, and said applied second subcarrier signal.

8. A method as in claim 7, wherein said applied main channel signal is an applied monophonic audio signal and said applied first subcarrier signal is an applied stereo difference audio signal.

9. A method as in claim 7, wherein said control signal corresponds to a selected one of a first detected overall amplitude of an applied monophonic audio signal and a second detected overall amplitude of an applied stereo difference audio signal, selection being responsive to comparison of the first overall amplitude and the second overall amplitude, the applied monophonic audio signal corresponding to the applied main channel signal and the applied stereo difference audio signal corresponding to the applied first subcarrier signal.

10. A method as in claim 7, wherein the applied second subcarrier signal corresponds to an applied digital data signal.

11. A method as in claim 7, wherein said control signal is further responsive to a priori information about said applied main channel signal and said applied first subcarrier signal.

12. A system for transmitting an applied main channel signal, an applied first subcarrier signal, and an applied second subcarrier signal, the applied second subcarrier signal not being derived from the applied main channel signal, the system comprising:

means for obtaining a priori information about said applied main channel signal and said applied first subcarrier signal;

a control signal generator, for producing a control signal varying over more than two values in response to a first overall amplitude corresponding to said a priori information about said applied main channel signal and a second overall amplitude corresponding to said a priori information about said applied first subcarrier signal;

a modulator operatively coupled to said control signal generator, the modulator accepting as input said control signal and generating as output said second subcarrier signal at an injection level, the injection level varying in response to said control signal; and

a transmitter operatively coupled to said modulator and accepting as input said applied second subcarrier signal for transmission.