System and method for multiplexing control signals over data signal conductors

A system and method for multiplexing control signals over data signal conductors comprises an audio source for generating audio signals, a speaker device for monitoring the audio signals, a first high-pass filter for filtering the audio signals, a signal cable over which the audio signals are transmitted, a second high-pass filter for again filtering the audio signals, and headphones for monitoring the audio signals. The system and method further comprises a current source for generating a control signal whenever the headphones are connected and for transmitting the control signals over the signal cable, and a detector device for receiving the control signals and responsively using them to mute the speaker device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to computer systems and more particularly 
to a system and method for multiplexing control signals over data signal 
conductors. 
2. Description of the Background Art 
The efficient and accurate transmission of electronically-encoded 
information is an important consideration of manufacturers, designers and 
users of electronic information systems. The goal of designing and 
building an effective and reliable electronic device can frequently result 
in improved performance and reduced manufacturing costs, and can thus 
provide significant economic benefits for both users and manufacturers. 
Referring to FIG. 1, a block diagram of a prior art system 110 for 
transmitting control signals and data signals is shown. Conventionally, a 
data signal source 112 generates data signals and then transmits the 
generated data signals over line 114 to data destination 116. Data 
destination 116 may then process and utilize the generated data according 
to the general design and purpose of system 110. System 110 may also 
conventionally include a control signal source 118 which generates control 
signals and then transmits the generated control signals over line 120 to 
govern the operation of a particular controlled device 122. The prior art 
system 110 of FIG. 1 thus requires two separate and discrete signal paths 
(lines 114 and 120) to effectively transmit the data signals and the 
control signals to respective data destination 116 and controlled device 
122. 
Multiplexing is a special transmission method which allows simultaneous 
communications to occur between multiple source and destination devices. 
Multiplexing techniques typically utilize a single signal path over which 
the various communications simultaneously occur. The use of a single 
signal path to perform several different communication functions 
advantageously results in a simpler and less costly system design. The 
increased efficiency and reduced cost of such a design may thus allow 
system manufacturers to provide a more economic overall system for the 
benefit of system users. Therefore, an improved system and method is 
needed for multiplexing control signals over data signal conductors. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a system and method are disclosed 
for multiplexing control signals over data signal conductors. In the 
preferred embodiment of the present invention, an audio source generates 
an audio signal which a first power amplifier amplifies and provides to 
drive an audio speaker. 
The generated audio signal is also provided to an audio driver which 
transmits the audio signal to an input high-pass filter that passes only 
those audio signal frequencies which are greater than a selected system 
cutoff frequency. The high-pass filter then provides the audio signal to a 
signal cable. The audio signal is thus transmitted from the input 
high-pass filter to an output high-pass filter which also typically passes 
only those audio signal frequencies which are greater than the system 
cutoff frequency. The output high-pass filter then provides the audio 
signal to a second power amplifier which amplifies the audio signal and 
then provides the amplified audio signal to a headphone jack. Headphones 
may then receive the amplified audio signal by inserting a headphone plug 
into the headphone jack. A computer system user wearing the headphones may 
thus advantageously receive and hear the amplified audio signal. 
In the preferred embodiment, when the headphones are connected to the 
headphone jack, a switch is closed to activate a current source which is a 
switched, rate-limited current source that generates a low-passed control 
signal. The generated control signal is then provided to a level detector 
via the aforementioned signal cable. The level detector is a low-passed 
direct-current (DC) level detector which detects and low-passes the 
received control signal to produce a detected control signal which is then 
provided to the first power amplifier to mute the audio speaker. The audio 
speaker is thus muted whenever the headphones are connected. 
The present invention thus efficiently multiplexes control signals and 
audio signals over a single signal cable. Furthermore, the control signals 
do not interfere with the audio signals provided to headphones and, 
similarly, the audio signals do not interfere with the control signals 
provided by the level detector to mute the audio speaker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the present invention, a system and method are disclosed 
for multiplexing control signals over data signal conductors. The 
preferred embodiment of the present invention comprises an audio source 
for generating audio signals, a speaker device for monitoring the audio 
signals, a first high-pass filter for filtering the audio signals, a 
signal cable over which the audio signals are transmitted, a second 
high-pass filter for again filtering the audio signals, and headphones for 
monitoring the audio signals. The system and method further comprises a 
rate-of-change limited current source for generating a control signal 
whenever the headphones are connected and for transmitting the control 
signals over the signal cable, and a detector device for receiving the 
control signals and responsively using them to mute the speaker device. 
Referring now to FIG. 2, a computer system 210 is shown for multiplexing 
control signals over data signal conductors, according to the present 
invention. Computer system 210 preferably comprises a central processing 
unit (CPU) 212, a video monitor 214, a keyboard 216, an input device 218 
and a memory 224. Memory 224 typically contains an operating system 220 
and at least one application program 222. Each element of computer system 
210 preferably has an input and an output coupled to a common system bus 
226. Memory 224 may alternatively comprise various storage-device 
configurations, including Random-Access-Memory (RAM), Read-Only-Memory 
(ROM), and non-volatile storage devices such as floppy-disks and hard 
disk-drives. System bus 226 may alternatively be connected to a 
communications interface to permit computer system 210 to output 
information to a computer network. 
Referring now to FIG. 3, a block diagram for the preferred embodiment of 
monitor 214 and CPU 212 is shown, according to the present invention. In 
the preferred embodiment, an audio source 310 generates an audio signal 
which power amplifier 314 receives via line 312. Power amplifier 314 
amplifies the generated audio signal and provides the amplified audio 
signal, via line 316, to drive speaker 318. The generated audio signal 
thus advantageously becomes audible to a user of computer system 210. 
The generated audio signal is also provided, via line 320, to audio driver 
322 which transmits the audio signal to a high-pass filter 326 via line 
324. High-pass filter 326 passes only those audio signal frequencies which 
are greater than a selected system cutoff frequency (for example, 50 
Hertz). High-pass filter 326 then provides the audio signal, via line 328, 
to a CPU connector 330. 
A signal cable 332 is connected between CPU connector 330 and monitor 
connector 334. The audio signal is thus provided from high-pass filter 326 
to a high-pass filter 338 via line 328, CPU connector 330, signal cable 
332, monitor connector 334 and line 336. High-pass filter 338 typically 
passes only those audio signal frequencies which are greater than the 
system cutoff frequency selected above for high-pass filter 326. High-pass 
filter 338 then provides the audio signal, via line 340, to power 
amplifier 342 which amplifies the audio signal and then provides the 
amplified audio signal to headphone jack 346 via line 344. 
In the preferred embodiment, headphones 350 may then receive the amplified 
audio signal, via line 348, by inserting a headphone plug, electrically 
connected to line 348, into headphone jack 346. A computer system 210 user 
wearing headphones 350 may thus advantageously receive and hear the 
amplified audio signal. 
During periods of headphone 350 use, the audio provided by speaker 318 may 
no longer be required and further, may create an unwelcome distraction in 
the work environment. The present invention provides a methodology for 
controlling the operation of a device such as speaker 318, in response to 
a control state, such as headphones 350 use. 
In the preferred embodiment, when headphones 350 are connected to headphone 
jack 346, a switch 352 is closed to activate current source 356 via line 
354. Current source 356 includes a low-pass filter (LPF) as described 
below in conjunction with FIG. 4. Switch 352 may be conventionally 
operated by insertion of the headphone plug into headphone jack 346. In 
alternate embodiments, current source 356 may also be activated by a 
variety of other devices which sense the connection of headphones 350 to 
headphone jack 346, including microprocessors and other logic devices. 
Further, in alternate embodiments, current source 356 may be activated in 
response to various control states other than the connection of headphones 
350 to headphone jack 346. 
Current source 356 is a switched, rate-limited current source which 
generates a low-passed control signal onto line 358. In the preferred 
embodiment, current source 356 only generates control signals having 
frequencies which are lower than the system cutoff frequency discussed 
above in connection with high-pass filters 326 and 338. The operation of 
current source 356 is further discussed below in conjunction with FIG. 4. 
The generated control signal is then provided to level detector 362 via 
line 358, line 336, monitor connector 334, signal cable 332, CPU connector 
330, line 328 and line 360. 
Level detector 362 is a low-passed direct-current (DC) level detector which 
detects and low-passes the received control signal to produce a detected 
control signal which is then provided to power amplifier 314, via line 
364, to mute speaker 318. Speaker 318 is thus muted whenever headphones 
350 are connected to headphone jack 346. Level detector 362 is further 
discussed below in conjunction with FIG. 4. In alternate embodiments, the 
detected control signal may control devices other than speaker 318, and 
may also alternately control a device directly or can be sensed by a logic 
device such as a microprocessor. 
The present invention thus efficiently multiplexes control signals and 
audio signals over signal cable 332. Furthermore, the control signals do 
not interfere with the audio signals provided to headphones 350 and, 
similarly, the audio signals do not interfere with the control signals 
provided by level detector 362. 
Referring now to FIG. 4, a schematic diagram of the preferred embodiment of 
the present invention is shown. As discussed above in conjunction with 
FIG. 3, an audio signal is provided to an input high-pass filter 326 via 
line 324. High-pass filter 326 is formed of capacitor 410 and resistor 
412. The high-pass filtering process is performed by capacitor 410 and 
resistor 412, in parallel with termination resistor 414, which are 
collectively selected to pass only those audio signal frequencies which 
are greater than the selected system cutoff frequency. 
High-pass filter 326 then provides the audio signal, via signal cable 332, 
to high-pass filter 338, which is comprised of capacitor 415 and the load 
characteristic of the input of power amplifier 342 on line 340. High-pass 
filter 338 preferably passes only those audio signal frequencies which are 
greater than the system cutoff frequency selected for high-pass filter 
326, as discussed above. High-pass filter 338 then provides the audio 
signal, via line 340, to power amplifier 342 which is connected to 
headphone jack 346. 
As discussed above in conjunction with FIG. 4, switch 352 is closed when 
headphones 350 are connected to headphone jack 346. The closure of switch 
352 causes current to flow through current source 356 to thereby 
responsively generate a control signal onto line 358. In the preferred 
embodiment, current source 356 is implemented to generate only control 
signals having frequencies which are lower than the system cutoff 
frequency discussed above in connection with high-pass filters 326 and 
338. 
Current source 356 is a switched, rate-limited current source which 
generates a low-passed control signal onto line 358. In the preferred 
embodiment, the control signal on line 358 has a rise time greater than 
thirty milliseconds. The generated control signal is triggered by switch 
352 and is injected onto line 358 after passing through a low-pass filter 
having a cutoff frequency set by capacitor 418 and the parallel 
combination of resistors 416 and 420. This low-pass filter in current 
source 356 thus sets the slew rate (amps/second) for the current flowing 
from the collector of transistor 426. The slew rate of current source 356 
determines the spectral content of the control signal and therefore the 
slew rate is selected to minimize the high-frequencies which may cause an 
audible artifact to be introduced into the audio signal. This 
high-frequency audible artifact is further prevented by passing the audio 
signal through high-pass filter 338, as discussed above. 
The generated control signal is further low-passed by the combined effects 
of capacitors 415 and 410, resistors 414 and 412, the input 
characteristics of power amplifier 342 on line 340 and the output 
characteristics of audio driver 322 on line 324. This low-passing of the 
control signal combines with high-passing of the audio signals to 
effectively prevent the control signals from being heard on speaker 318. 
The generated control signal is then provided to level detector 362 via 
line 358, signal cable 332 and line 360. 
Current source 356 operates to supply control signal current, however, 
introduction of the control signal by current source 356 does not affect 
the impedance of the audio path along signal cable 332. The output 
impedance of transistor 426 is substantially larger than the value of 
resistor 414. Regardless of the voltage on the collector of transistor 
426, approximately the same amount of current flows on line 358. Current 
source 356 thus generates the control signal through signal cable 332 to 
level detector 362 without affecting the impedance of the audio signal 
path along signal cable 332. Current source 356 therefore advantageously 
transmits the control signal through signal cable 332 without affecting 
the level of the audio signal which is simultaneously present on signal 
cable 332. 
Level detector 362 is a low-passed direct-current (DC) level detector which 
detects and low-passes the control signals received on line 360. Level 
detector 362 then responsively generates a detected control signal which 
is provided to power amplifier on line 364 to mute speaker 318. Level 
detector 362 includes a low-pass filter, comprised of resistor 428 and 
capacitor 430, which preferably passes only control signals having 
frequencies which are lower than the system cutoff frequency discussed 
above in connection with high-pass filters 326 and 338. This low-pass 
filter in level detector 362 effectively prevents audio signals from 
activating the control circuitry in CPU 212. 
The present invention thus utilizes different segments of the frequency 
spectrum to separate the audio signals from the control signals. In 
operation, when switch 352 is closed, the low-frequency control signal 
voltage present on signal cable 332 changes to a logical high. Transistor 
440 is then turned on by the received control signal voltage and 
responsively generates a logical low on line 364 to signal that switch 352 
is closed. 
In the preferred embodiment, termination resistor 414 and the current level 
from current source 356 are selected so that when signal cable 332 is not 
connected between monitor 214 and CPU 212, the open-circuit voltage across 
current source 356 and termination resistor 414 is low enough to prevent 
damage to electronic circuitry which is not designed to be compatible with 
computer system 210. Furthermore, termination resistor 414 and the current 
level from current source 356 are also preferably selected to prevent 
potential circuit damage caused by the short-circuit current output of the 
present invention's audio circuitry. 
The present invention may be effectively implemented using a number of 
alternate embodiments. Using slow, time-dependent signaling protocols, 
slow transmission of more complex data may be performed over a single 
channel (for example, 0.2 Baud serial data using MFM or NRZ encoding, or a 
slow version of Apple Desktop Bus by Apple Computer, Inc. could be used 
for slow asynchronous bi-directional signaling). 
Further, in stereo audio systems, use of separate signals on the left and 
right channels allows four different states to be signaled. Using two 
channels, slow synchronous communications may also be accomplished by 
using one line for the clock signal and the other line for data. A 
protocol such as I 2C by Philips, Inc. could be accommodated at slow 
rates, thereby allowing bi-directional communications. 
Multi-level signaling may also be accomplished on each line by sourcing or 
sinking current. A third state is the absence of a DC bias. Additional 
levels may be obtained by setting differing amounts of current. In some 
cases, it may be desirable for the current source and signaling circuitry 
to be powered by CPU 212 rather than by monitor 214, thereby allowing 
detection of headphones 350 when power to monitor 214 is turned off. In 
such a case, power may be provided on one of the audio lines, and the 
second audio line may be used to transmit the control signal. 
Referring now to FIG. 5, a graph of signal frequency ranges affected by the 
present invention is shown. A frequency spectrum 510 is depicted which 
includes low frequencies 512 located on the extreme left of spectrum 510. 
In the preferred embodiment, low frequencies 512 may approach a DC level 
at their lower limits. 
The low frequencies 512 increase linearly, moving from left to right, until 
relatively high frequencies 514, located on the extreme right of spectrum 
510, are depicted. A cutoff frequency 516 is also depicted on spectrum 
510. Cutoff frequency 510 is further discussed above in conjunction with 
FIGS. 3 and 4 and current source 356, high-pass filters 338 and 326 and 
level detector 362. 
The present invention utilizes different segments of frequency spectrum 510 
to effectively separate the audio signals from the control signals. FIG. 5 
illustrates this aspect of the present invention by depicting a control 
signal range 518 which is located below cutoff frequency 516 and an audio 
signal range 520 which is located above cutoff frequency 516. As FIG. 5 
illustrates, control signal range 518 and audio signal range 520 do not 
overlap and thus do not share any frequencies on spectrum 510. 
Referring now to FIG. 6A, an initial portion of a flowchart of preferred 
method steps for multiplexing control signals over data signal conductors 
is shown. Initially, audio source 310 generates 610 an audio signal and 
then provides 612 the audio signal to speaker 318 after amplifying the 
audio signal by using power amplifier 314. Next, audio source 310 
transmits the unamplified audio signal through audio driver 322 to 
high-pass filter 326 which filters 614 the audio signal and transmits 616 
the audio signal over signal cable 332. High-pass filter 338 then receives 
and responsively filters 618 the audio signal before providing 620 the 
audio signal, through power amplifier 342, to headphone jack 346. 
Referring now to FIG. 6B, a final portion of a flowchart of preferred 
method steps for multiplexing control signals over data signal conductors 
is shown. In the preferred embodiment, switch 352 indicates whether 
headphones 350 are connected 622 to headphone jack 346. If headphones 350 
are not connected, then the FIG. 6B process ends. 
If, however, headphones 350 are connected, then current source 356 
responsively generates 624 a low-frequency control signal and transmits 
626 the generated control signal over signal cable 332. Next, level 
detector 362 detects 628 and low-passes the transmitted control signal and 
then provides the detected control signal to power amplifier 314 to mute 
630 speaker 318. 
The invention has been explained above with reference to a preferred 
embodiment. Other embodiments will be apparent to those skilled in the art 
in light of this disclosure. For example, the present invention may 
readily be implemented to control devices other than the speaker 318 
described above in conjunction with the preferred embodiment. Furthermore, 
the control signal may multiplexed in combination with signals other than 
the audio signal described in the foregoing discussion of the preferred 
embodiment. Therefore, these and other variations upon the preferred 
embodiments are intended to be covered by the present invention, which is 
limited only by the appended claims.