Note: Descriptions are shown in the official language in which they were submitted.
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COMBINED ANALOG/DIGITAL DATA TRANSMISSION SYSTEM
Cross Reference to Related Application
This application claims priority based upon United States provisional patent
application Serial No. 60/085,820 filed May 18, 1998.
Technical Field
The invention relates generally to data transmission systems and, more
particularly, to a data transmission system which enables digital signals such
as
data and control signals to be transmitted together with analog signals such
as
voice and/or music information.
Background
A variety of techniques may be used to represent information. Two such
techniques are the use of analog signals and digital data signals. Analog
signals
are used most often to represent audio, video or other types of information
characterized by a continuously variable amplitude. Digital data signals, on
the
other hand, are used to represent information using two discrete states. As a
result
of this distinction, devices such as televisions and stereo speakers which
reproduce
information from analog signals remained apart from devices such as computers
which reproduce information from digital data signals.
In recent years, these seemingly disparate technologies have begun to
converge, particularly in multi-media applications such as a digital computer
equipped with a sound reproduction system or a stereo system equipped to
handle
digital control signals originating at the audio signal source. Similarly,
many
televisions can generate teletext using digital data originating at the analog
video
source. The digital data used by televisions to generate a teletext display is
typically injected into the horizontal and/or vertical blanking pulses
contained in all
analog video signals, a portion of the video signal where no video data is
carried.
As a result, it has been a relatively simple task to equip a television with a
special
decoder which extracts digital data out of this portion of the analog video
signal.
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Analog audio signals, on the other hand, lack the blanking pulse which has
enabled
the injection of digital data into analog video signals. Accordingly, to
transmit an
analog audio and digital data simultaneously, a voltage signal source has been
used
to superimpose a data signal across the two wires of the cable pair carrying
the
analog audio signal. By modulating the data signal onto a carrier frequency
which
lies above the upper bound of the audio frequency band, typically, around 20
KHz,
the superimposed digital data signal will have no effect on the analog audio
signal.
However, in a typical audio reproduction system, the driving impedance at
the power amplifier is very low, in effect, a short circuit while the load
impedance at
the speaker is often as low as 4 ohms. As a result, a series of inductors are
mandatory at each end of the system that allow the lower frequency audio
signals
to pass with little attenuation while presenting a high impedance to the
carrier
frequency at which the digital data has been modulated. The problem associated
with this arrangement is that the amplifier's damping factor will hardly be
preserved unless the coils for the inductors have an extremely low impedance,
ideally below 0.1 ohms, at audio frequencies. In order to achieve this, the
inductors
must have relatively heavy gauge wire and relatively large, high permeability
cores. The physical size of the cores has to be big enough to prevent
saturation with
audio frequency current surges of over 1 Ampere, e.g., often 5 to 10 Amperes.
This
presents a difficult design problem both for size of the circuit and the cost
of the
components.
It is, therefore, an object of the disclosure to provide an efficient design
of a
mixed signal communication system, suitable for use in audio and other
applications, which, like the afore described data transmission systems, carry
digital data and/or control signals on the same pair of wires used to carry
analog
signals.
Summary
In one embodiment, a mixed signal communication system for transmitting
digital signals on the same pair of wires which carry analog signals is
proposed. An
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inexpensive transmitter circuit adapted to transmit the digital signals is
disclosed
to use a transformer for performing current mode transmission. The current
mode
transmission of the digital signals eliminates design difficulties
conventionally
associated with voltage mode transmission. Further, a low-cost digital signal
generator, such as a switch circuit or a serial pulse stream generator, can be
integrated to output a desired digital signal. And in order to accommodate
situations where a DC power is not available at or around the transmission
portion,
a DC power extraction circuit adds the feature of obtaining the power from the
same transmission wires and supplying the power needed for the digital signal
generator or other components of the transmitter portion. At the receiver
portion of
the system, a current sense circuit is also installed to download the
transmitted mix
signal from the transmission wires, and send the signal to a decoding circuit
to
output digital signals originally encoded at the transmitter portion. In
another
embodiment, the transmitter and receiver portion of the system can be designed
in
such a way that the whole system is capable of exchanging digital information
bi-
directionally.
One example application is an analog audio system with an audio and data
transmission capability, such as a personal computer or home stereo, and an
output
device such as a speaker. In this example, it is desired to not only send the
analog
audio signals from the audio system to the speakers, but to transmit data
signals
back and forth using the same pair of wires. The present invention facilitates
this
desire in an efficient and economical manner.
Brief Description of the Drawings
Fig. 1 is a circuit diagram of a transmitter portion of a mixed signal
communication system constructed in accordance with one embodiment of the
present invention.
Fig. 2 is a circuit diagram of a receiver portion of the mixed signal
communication system of Fig. 1.
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Fig. 3 is a circuit diagram of a transmitter portion of an alternate
embodiment of the mixed signal communication system of Figs. Z-2.
Fig. 3A is a power extraction circuit in accordance with the teachings of one
embodiment of the present invention.
Fig. 3B is a detailed switch circuit in accordance with the teachings of one
embodiment of the present invention.
Fig. 3C is an oscillator circuit in accordance with the teachings of one
embodiment of the present invention.
Fig. 3D is a transmitter circuit in accordance with the teachings of one
embodiment of the present invention.
Fig. 4 is a circuit diagram of a receiver portion of the mixed signal
communication system of Fig. 3.
Fig. 5 is a block diagram of a bidirectional mixed signal communication
system constructed in accordance with the teachings of one embodiment of the
present invention.
Fig. 6 is an expanded block diagram of a first transceiver of the
bidirectional
mixed signal communication system of Fig. 5.
Fig. 7 is an expanded block diagram of a second transceiver of the
bidirectional mixed signal communication system of Fig. 5.
Fig. 8 is a circuit diagram of line interface units used in Figs. 6 and 7.
Detailed Description
Turning now to the drawings, in Fig. 1, the reference numeral 10 illustrates
a data transmitter portion of a mixed signal communication system. As
disclosed
herein, the mixed signal communication system is uni-directional, i.e., analog
and
digital information are transmitted to respective target devices for use
thereby.
However, the present invention is equally suitable for use in bi-directional
systems
such as those embodiments of the invention to be described with respect to the
figures below. Similarly, while the mixed signal communication system is
disclosed
as transmitting an analog audio signal to an audio signal reproduction system,
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again, the present disclosure is equally suitable for use in conjunction with
other
types of analog and/or digital information signals, for example, an analog
video
signal such as that received at an outlet for a cable TV distribution system.
As may now be seen, the data transmitter 10 includes an audio signal
generator 22 and a digital signal generator 24 for generating analog audio and
digital signals, respectively. The audio signal generator 22 may be physically
incorporated into the data transmission portion 10 of the combined
analog/digital
data transmission system or, as illustrated herein, be externally located,
relative to
the data transmission portion 10, and placed across the line of the data
transmitter
portion 10 by coupling the signal output lines of the audio signal generator
22 to
terminals 12 and 14 of the data transmitter 10.
As previously mentioned, the present disclosure is directed to a system and
method for adding digital information, such as data or control signals, to an
analog
signal. The resulting signal carries the digital signals to the data receiver
portion
where the digital signals are extracted by a digital data output device such
as a
tone detector. Furthermore, because the digital information is added to the
analog
audio signal in the form of a varying current, difficulties traditionally
associated
with the addition of digital information to an analog audio signal using a
superimposed voltage signal are avoided.
As previously set forth, the digital signal is generated by the digital signal
generator 24. As is well known in the art, the digital signal is a binary
signal
which, by varying between logical "0" and logical "1" states, conveys
information to
a device. It should be clearly understood that, while Fig. 1 shows the digital
signal
as being produced by the digital signal generator 24, it is fully contemplated
that
the digital signal may be produced by a processor subsystem of a personal
computer
or other programmable device. Alternately, the digital signal may be produced
by a
manually controllable switch.
A carrier signal generator 26 generates a carrier signal at a selected
frequency. While it is fully contemplated that the disclosure is suitable for
use with
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carrier signals at various frequencies, a carrier signal having a frequency of
about
400 KHz has been found to be suitable for the uses contemplated herein. In
alternate configurations, thereof, a ceramic resonator, for example, a model
EFO-
A400K048 ceramic resonator manufactured by Panasonic, or an RC oscillator may
be used to generate the carrier signal. After being buffered by a NAND gate
28, the
carrier signal produced by the signal generator 26 and the digital signal
generated
by the digital signal generator 24 are provided as first and second inputs to
a
NAND gate 29. The NAND gate 29 modulates the digital signal onto the carrier
signal by generating, as its output, an integrated signal.
The NAND gate 29 drives the integrated signal to a bandpass filter
consisting of a resistor 30, a capacitor 31, and an inductance of a primary
winding
of a transformer 32. This bandpass filter attenuates the higher harmonics of
the
carrier signal, changing it from a square wave to a sine wave, and also
attenuates
hash down in the audio frequencies. Having between 18 and 100 turns on the
primary winding and between 1 and 2 turns on the secondary winding, the
transformer 32 acts as a current mode transformer by stepping up the amplitude
level of the current for embedding the integrated signal. Since there are only
one or
two windings on the primary side, the voltage generated is negligible while
the
current changes are preserved.
The current mode transformer 32 can be very small and inexpensive, and
may include a toroidal core with 30 gauge magnet wire as the primary winding.
The secondary winding uses a relatively heavy wire passing through the center
of
the toroidal core, presenting a negligible impedance to the audio signal.
Further, a
small and inexpensive capacitor, typically a 0.1 ~F or less ceramic bead, is
used at
each end of the transmission line to bypass the current around the power
amplifier
output and speaker. However, the impedances of the power amplifier output and
the speaker are often low enough to make the bypass capacitor unnecessary in
some
applications.
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Referring now to Fig. 2, the wire pair carries the digital information to a
receiver end 11 of the mixed signal communication system. Provided along the
output line of the receiver end 11 is a current sense transformer 34 having
between
1 and 2 turns on the primary winding and between 18 and 100 turns on the
secondary winding. Because the primary winding has a very small inductance, a
small voltage is induced across the secondary winding because of the current
changes across the primary winding. The voltage induced in the secondary
winding of the current sense transformer 34 is applied to an input, e.g., pin
3 of a
tone detector 36 in this case, selected to detect and extract the digital
signal from
the combined signal. In this manner, the tone detector 36 extracts the digital
signal
from the received integrated signal.
The tone detector 36 drives the extracted signal to a digital data output
device 38 where the digital signal is decoded and any instructions contained
within
the signal are executed. In some cases, the digital signal may convey
information
related to the audio signal transmitted to the audio signal reproduction
system 40.
For example, the information may be text information associated with the audio
signal and the digital data output device 38 may be a display. Thus, in this
example, the audio signal reproduction device 40, typically, a low (e.g., 4
ohm)
impedance loudspeaker, would produce audible sound from the received analog
audio signal while the digital data output device 38 would display information
associated with the produced sound.
Of course, it is fully contemplated that the analog signal may contain
information other than audio information and that the digital signal need not
necessarily carry information related to the analog signal. In addition, if
desired,
the respective analog and digital signals may be used to drive unrelated
devices.
Referring next to Fig. 3, an alternate embodiment of the transmitter portion
10 of Fig. l, herein referenced as transmitter portion 10', may now be seen.
In
situations when a DC power is not available in a remote place, it is known in
the
art to add additional circuitry to the transmission wires so that a DC power
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extraction circuit 42, as one proposed by this disclosure, can serve as a
power
supply to the transmitter circuit 44 or any digital signal generator. For
example, a
plurality of switch circuits 46 and an oscillator circuit 48 will generate a
digital on
and off signal at different frequencies. The oscillator circuit 48, along with
a
selected switch circuit, will drive a signal 50 at a certain frequency to the
transmitter circuit 44. In another embodiment, a simple remote control IC
encoder
can be used to generate a digital pulse stream as the digital signal input.
In Fig. 3A, more details about the DC power extraction circuit are shown,
whereas an RC filter and a voltage regulating zener diode 52, or alternatively
a
series of regulators, can be arranged to output a DC voltage supply 54.
In Fig. 3B, one of the switch circuits 46 is illustrated. Each switch circuit
includes a switch 56, a CMOS device 58, and a resistor 60 with a resistance R.
A
debounce circuit is also provided between the switch 56 and the CMOS device 58
to
reduce transient noises. The resistance R for each switch is different,
thereby
providing a uniquely identifiable frequency for each switch. When the switch
56 is
closed, the CMOS device 58 turns on. As a result, the signal 50 connects to
the
oscillator 48 (described below) through the resister 60.
In Fig. 3C, an exemplary version of the oscillator circuit 48 is shown. In
essence, a frequency generated from the oscillator circuit 48 depends on the
resistance and capacitance connected to an oscillator 61. The frequency is
supplied
to the switch circuits 46 and the transmitter circuit 44, as discussed above.
In Fig. 3D, an embodiment of the transmitter circuit 44 is shown. The
transmitter circuit 44 includes a voltage-to-current transformer 62 and a CMOS
switch 63. When the CMOS switch 63 is switched on, the signal 50 from the
aforementioned oscillator circuit 48 and switch circuits 46 is transmitted
through
the voltage-to-current transformer 62 in accordance with the teaching of the
present disclosure.
Referring now to Fig. 4, an alternate embodiment of the receiver portion 11 of
Fig. 2, herein referenced as a receiver portion 11', may now be seen. The
receiver
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portion 11' includes a current sensing receiver circuit 64 for taking the
digital
signal off the integrated signal on the transmission wire 16. In the present
embodiment, the receiver circuit feeds the digital signal into one or more
tone
detector 66 for matching the particular frequency of each signal in the
digital
signal. The tone detector 66 provides the signals to an output device. Here,
as an
example, six light emitting diodes (LEDs) 68 turn on and off according to each
signal.
Although not shown, different variations can be made to the receiver portion
11'. In one embodiment, the receiver circuit can send the received digital
signal to a
computer processing unit and let the decoding be done by software. In another
embodiment, a matching remote control decoder can decode the digital signal
pulse
stream generated by a remote control encoder in the transmitter portion.
Referring now to Fig. 5, a system 100 represents yet another embodiment of
the present invention. The system 100 includes two nodes 102, 104 connected by
a
transmission medium 106. The transmission medium may be a pair of wires, such
as speaker or telephone wires, a computer bus, a power line, a trunk, or other
type
of link well known in the art. To clarify the description, reference will
continue to
be made to the example described above, it being understood that many
different
applications can also benefit from the present invention. In the present
example,
the transmission medium 106 will be a line consisting of a balanced paired
audio
cable typical of that used in recording studios.
The first node 102 includes an audio signal generation system 108 for
producing an audio signal 110a and a data processing system 112 for sending
and
receiving digital data signals 114a, 116a, respectively. The audio and data
signals
110x, 114a, 116a are also connected to a transceiver 120 which combines the
audio
signal 110a with the data signal 114a for transmission on the line 106 and
receives
the data signal 116a from the line.
The second node 104 includes an audio signal reproduction system 122, such
as a speaker, for receiving an audio signal 110b and a data input/output
device 126
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for sending and receiving digital data signals 116b, 114b, respectively. The
audio
signal 110a is essentially identical to the audio signal 110b, and the digital
data
signals 114a, 116a are essentially identical to the digital data signals 114b,
116b.
The audio and data signals 110b, 114b, 116b are also connected to a
transceiver I30
which separates the audio signal IlOb from the data signal 114b for reception
from
the line 106 and transmits the data signal 116b on the line.
Referring now to Fig. 6, the transceiver 120 receives the digital data signal
ll4a into a frequency shift keying (FSK) modulator 140a. It is understood, of
course, that different types of modulators, or no modulator at all, can be
used. To
continue with the above example, the digital data signal 114a is a non-return-
to-
zero (NRZ) serial data input at TTL logic levels (OV-5V) at a data rate of
19.2kbs.
The FSK modulator 140a transforms the digital data signal 114a into an FSK
signal 142 with carrier frequencies of 137 kHz (for a space) and 167 kHz (for
a
mark).
I S The FSK signal 142 is then provided to a filter 144a and the filtered
signal is
then provided to a line interface unit 146a. The line interface unit 146a also
receives the audio signal 110a and combines it with the FSK signal 142. These
combined signals are then driven onto the line 106.
In the present embodiment, the line interface unit 146a also receives a
modulated digital signal 148 from the line 106 and provides it to a second
filter
150a. In one embodiment, these functions of the line interface unit 146a may
be
performed by the current sense transformer 34 and the tone detector 36, all of
Fig.
2. The second filter provides the modulated digital signal 148 to an FSK
demodulator 152a. The FSK demodulator demodulates the signal, thereby
providing the data signal 114a. In continuance of the present example, the
data
signal 114a is a NRZ serial data' output at TTL logic levels (OV-5V) and at a
data
rate of 19.2kbs.
Referring now to Fig. 7, the transceiver 130 is in many respects similar to
the
transceiver 120 of Fig. 6. However, component values may be different to
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accommodate different carrier frequencies for each direction of transmission.
The
transceiver 130 receives the digital data signal 114b into an FSK modulator
140b.
The FSK modulator 140b transforms the digital data signal 114b into an FSK
signal 142b with carrier frequencies of 91 kHz (for a space) and 100 kHz (for
a
mark). The FSK signal 142b is then filtered by a filter 144b and filtered
signal is
then provided to a line interface unit 146b which drives the signal onto the
line 106.
The line interface unit 146 also receives the audio signal 110a and a
modulated digital signal 148b from the line 106. The line interface unit 146
provides the modulated digital signal 148b to a second filter 150b, which
filters and
provides the modulated digital signal to an FSK demodulator I52b. The FSK
demodulator 152b demodulates the signal, thereby providing the data signal
116b.
Referring now to Fig. 8, one embodiment for the two line interface units
146a, 146b is shown in extended detail. In continuance with the above example,
the line interface unit 146a serves an amplifier or a mixer, and will
hereinafter be
referred to as the hub node. The line interface unit 146b serves an output
device
such as a powered speaker, and will hereinafter be referred to as the speaker
node.
Each of the line interfaces units 146a, 146b are identical, and therefore use
identical reference numerals. The suffix of "a" or "b" are appended to the
components of the line interfaces units 146a, 146b, respectively. An example
of
operation from the line interface unit 146a to the line interface unit 146b
will
illustrate the functionality of all the components of both units.
The line interface unit 146a includes a pair of series resonant circuits 160a,
161a across a set of terminals 162a, 164a. The terminals 162a, 164a
selectively
connect with the audio signal 110a of the hub node. In the present embodiment,
the series resonant circuit 160a includes a capacitor in series with an
inductor for
providing a resonant frequency that matches the mean between the 91 kHz and
110
kHz carrier frequencies and the series resonant circuit 160b includes a
capacitor in
series with an inductor for providing a resonant frequency that is the mean
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between the 137 kHz and 167 kHz carrier frequencies. It is understood,
however,
that the carrier frequencies may be different for different applications, and
that one
of ordinary skill in the art can correctly choose and compensate for a desired
frequency. As a result, the terminals 162a, 164a will have a consistent
impedance
(a short circuit} at the carrier frequencies, regardless of whether or not any
other
components are connected to the terminals.
A current mode transformer 166x, similar to the transformer 32 of Fig. 1, is
placed serially in-line with the audio signal 110a and across the input
digital signal
114a. The number of turns on the primary of the transformer 166a (the side
connected to the input digital signal 114x) is significantly greater than the
number
of turns on the secondary. For the sake of example, the primary-to-secondary
ratio
of the transformer 166a may be 18:4. As a result, the transformer 166a
converts
the voltage variations on the input digital signal 114a with only a negligible
voltage
drop on the audio signal 110a. However, the transformer I66a modifies the
current
on the audio signal 110a, at the specified frequency (from the FSK modulator),
in
response to the input digital signal 114x. The audio signal 110a, as modified
by the
transformer 166a, is then connected across the line 106 to the terminals 162b,
164b
of the line interface unit 146b.
The terminals 162b, 164b can connect the audio signal 110b to a speaker, or
may be simply left open. The series resonant circuits 160b, 161b ensure that a
consistent impedance (a relative short) appears across the terminals at the
desired
frequencies. The input digital signal 114a can be received by the line
interface
unit 146b through a transformer 168b. The transformer 168b is part of a
resistor-
capacitor-inductor peaked high pass circuit 170b across the terminals 162b,
164b.
The transformer 168b senses the FSK carrier frequency voltage across the
secondaries of the transformer 166b for the output FSK data signal 114b.
Although illustrative embodiments, or examples, of the invention have been
shown and described, other modifications, changes, and substitutions are
intended
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in the foregoing disclosure. Accordingly, it is appropriate that the appended
claims
be construed broadly and in a manner consistent with the scope of the
invention.
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