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United States Patent |
5,699,433
|
Moore
,   et al.
|
December 16, 1997
|
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.
Inventors:
|
Moore; Philip (San Leandro, CA);
Hasegawa; Koyo (Tokyo, JP);
Takahisa; Tsutomu (Santa Clara, CA)
|
Assignee:
|
Digital DJ Inc. (Milpitas, CA)
|
Appl. No.:
|
614505 |
Filed:
|
March 13, 1996 |
Intern'l Class: |
H04H 005/00 |
Field of Search: |
381/14,3,4,13,1,6
|
References Cited
U.S. Patent Documents
4633495 | Dec., 1986 | Schotz | 381/3.
|
4698842 | Oct., 1987 | Mackie et al. | 381/1.
|
5349386 | Sep., 1994 | Borchardt et al. | 381/14.
|
Other References
Osamu Yamada, et al., NHK's High Capacity FM Subcarrier System, NAB 1993
Broadcast Engineering Conference Proceedings, pp. 415-423.
|
Primary Examiner: Kuntz; Curtis
Assistant Examiner: Chang; Vivian
Attorney, Agent or Firm: Fenwick & West LLP
Claims
What is claimed is:
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.
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.
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