Analog video fiber optic link

Described is an electro/optical system for transmitting video information. The system includes a transmitter assembly interconnected via a fiber optic assembly to a receiver assembly. The transmitter assembly includes a circuit arrangement which generates a data stream of analog signals interspersed with control pulses. The data stream is transmitted through the fiber optic assembly. Changes in the control pulses are detected at the receiver and are used to adjust the gain of a video amplifier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to communications systems in general and more 
particularly to electro/optical communications systems wherein the 
receiver restores transmitted signals to the condition they were in at 
transmission. 
2. Prior Art 
Optical communications systems that transmit digital and analog signals are 
well known in the prior art. A typical communications system includes a 
transmitter with an optical source such as, for example, a light-emitting 
diode for converting an electrical input signal to an optical signal. An 
optical fiber transmits the optical signal to a receiver. An optical 
photodetector converts the optical signal back to an electrical signal. 
The electrical signal is amplified by a circuit arrangement and is given 
to a utilization device. The systems are attractive because their 
radiation is in acceptable range even when information (data) with large 
bandwidth are transmitted. 
One of the problems that is associated with optical communications systems 
is that the transmitted signal is subjected to much more distortion than a 
similar transmitted via a conventional copper wire communications channel. 
It is believed that the distortion is caused by the characteristics of the 
optical components. The components usually include transmitting diodes, 
light receiving photodetectors, optical connectors, optical fibers, etc. 
As these components age, their characteristics change and contribute to 
the non-linearity problem. 
Another problem that is common in optical channels is that the gain is 
non-uniform. Non-uniform channel gain is particularly harmful to video 
channels which transmit information that is subsequently displayed on a 
monitor or cathode ray tube (CRT). In the case of monochromatic systems, 
only one channel is used to transmit the video data. Usually, for each 
transmission burst or interval a line of data is transmitted. The quality 
of the displayed data is a function of the channel gain. Therefore, for 
good quality display it is desirable to maintain a constant channel gain. 
The problem is even more severe with color transmission. Unlike 
monochromatic systems that require a single optical channel, color systems 
require three channels to transmit the red, green and blue colors required 
for color display. Therefore, with color transmission it is desirable that 
the gain be maintained at a uniform level across the channels. 
The prior art has used two approaches to address the signal distortion and 
non-uniform gain problems. In one approach the prior art seeks to improve 
the operating characteristics of individual components used in the optical 
channel. It is believed that if these characteristics are improved, the 
proclivity of the optical channel to distort the optical signal will also 
be reduced. More particularly, the light-emitting diodes and the 
light-receiving photodetectors are perceived as potential problem 
components and circuits are provided to compensate accordingly. 
One noted problem with these diodes and/or photodetectors is that these 
devices change their operating characteristics as the junction temperature 
changes. Usually, the junction temperature is a function of the modulating 
signal. Stated another way, the junction temperature differs according to 
whether the modulating signal is at low or high frequency. U.S. Pat. No. 
4,443,890 describes a direct-light modulation information transmission 
system wherein filtering means are provided at the emission side and the 
reception side for compensating for non-linear thermal effects of the 
emission means and the linear thermal effects of the reception means. In 
the patent the emission means is a light-emitting diode (LED) and the 
reception means is a photodiode. 
U.S. Pat. No. 4,654,891 is another prior art patent which addresses 
variations, due to temperature, in the characteristics of the optical 
emitter and receiver. In particular, the subject patent describes a video 
transmission system in which a pre-distortion circuitry is provided at the 
transmitter. The circuit dynamically modifies the signal level with 
respect to which the video signal is clamped. An automatic gain control 
(AGC) circuitry, at the receiver, detects a DC level in the received video 
signal. A shift in the DC reference level of the received video signal 
causes the AGC circuitry to correct the receiver gain. 
Even though the above prior art devices work well for their intended 
purposes, it should be noted that the problems which are associated with 
the optical channel are caused by the different components which are used 
in the channel. Thus, by correcting problems associated with only one of 
the problem sources would not necessarily correct the problem associated 
with the entire optical channel. To correct the problem which affects the 
entire optical channel requires a more comprehensive approach. 
U.S. Pat. No. 4,742,575 to Arita et al is an example of prior art patents 
in which a comprehensive approach is used to correct problems in the 
optical channel. The patent describes a light signal 
transmission/reception system with a means which superimposes 
predetermined different levels reference pulse signals on the 
informational signals. The receiver is provided with a means for deriving 
the amount of change in the reference pulse signals. The derived change is 
used to compensate the magnitude of the received signal. The reference 
pulse signals are then filtered from the informational signals. 
U.S. Pat. No. 4,249,264, Crochet et al, is another patent in which a low 
frequency reference signal is superimposed on an informational signal at 
the transmitter. At the receiver, the received signal is passed through a 
high pass filter which delivers the useful signal to a utilization device 
and to a low-pass filter which extracts the reference signal. The 
reference signal is compared with a predetermined voltage and an error 
signal is generated which is used for polarizing or biasing the reception 
diode. 
There are certain applications in which the Arita et al and Crochet et al 
patents are not suitable for use. In particular, these references raise 
several additional problems if used in video transmission systems. As 
video transmission systems exist today, there are no practical low 
frequency reference signals which can be superimposed without disturbing 
the video signal which is displayed. With regard to the Arita et al 
patent, another problem is that the range (high and low) of the reference 
signals must be sufficiently different from the information signal so that 
the reference signals can be detected at the receiver. This suggests an 
unnecessary wide bandwidth whose upper limit can only be used to transmit 
reference signals as opposed to informational signals. 
SUMMARY OF THE INVENTION 
It is, therefore, the main object of the present invention to provide an 
optical communications channel which is free from the above-described 
problems. 
It is another object of the present invention to provide an optical channel 
which is ideal for transmitting video signals. 
These and other objects of the present invention are achieved by generating 
and inserting "Reference White Level" pulses in a data stream during time 
periods when the informational contents of the data stream are absent. 
This creates a hybrid data stream in which the signals representative of 
useful data are displaced laterally from the "Reference White Level" 
pulses. The hybrid data stream is transmitted through the optical channel 
to the receiver. A control signal which indicates the absence of data in 
the hybrid data stream is also transmitted to the receiver. At the 
receiver, the presence of the control signal indicates a time period when 
there is no informational data in the received signal. The changes which 
the reference pulses undergo as they are transmitted through the optical 
channel are measured, during this time period, and generate error signals 
which are used to adjust the gain of the receiving amplifier. During the 
adjustment period the output of the amplifier is not available for use. 
Because of the nature of video signals, our invention is ideal for use in a 
video channel. Video signals have regularly recurring blanking intervals 
with associated synchronizing (sync) pulses. The "Reference White Level" 
pulses are inserted at predetermined times relative to the sync pulses. 
Since the "Reference White Level" pulses are occurring at known times, 
they are extracted at the receiver without adversely affecting the video 
signal that is displayed on the monitor. Changes in the pulses are 
determined and are used to adjust the gain of the receiving video 
amplifier. During the adjustment period the input of the display monitor 
is tied to the blanking level which is already available in video signals. 
Following the adjustment period, the input of the display monitor is 
switched to display incoming video information. In addition, the present 
invention provides appropriate circuit arrangements which generate the 
above described hybrid and control signals. 
The following and other features and advantages of the present invention 
will be more fully described in the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention may be used in any communications channel in which 
analog signals are to be transmitted. It works well in a communications 
channel in which video information is being transmitted. As such, it will 
be described in that environment. However, this should not be construed as 
a limitation on the scope of the present invention since it is well within 
the skill of one skilled in the art to adapt the teachings of the present 
invention to cover other types of analog channels. 
FIG. 1 shows a system block diagram of the video communications channel 
according to the teaching of the present invention. The video 
communications channel includes a video generator 8, electro/optical 
transmitter 10, fiber optic cable 12 and electro/optical receiver 14. The 
electrical video generator can be an off-shelf conventional generator such 
as the one called "A Triple 4-Bit D/A Converter" (P/N AD9702) by Analog 
Devices, Inc. A description of the generator or module is set forth at 
page 2-391 of the Data Conversion Products Data Book 1989/90. In addition, 
a subsystem which generates video for a display apparatus is set forth in 
"Computer Data Display" by Samuel Davis, Prentice-Hall, Englewood Cliffs, 
N.J., 1969 (pages 170-171). Another subsystem which generates video for a 
display is set forth in the section entitled "Video Subsystem", of the 
"IBM Personal System/2 Hardware Interface Technical Reference (68X2330)". 
The INMOS module P. N. IMS-G171 is used in the IBM subsystem. The named 
prior art references are herein incorporated. Such generators are well 
known in the prior art; therefore, further description will not be given. 
Suffice it to say that the video generator generates an electrical video 
input signal which is fed into the electro/optical transmitter 10. The 
electro/optical transmitter 10 includes control electronics 16 and 
infrared (IR) light emitting diode (LED) 18. Likewise, the electro/optical 
receiver 14 includes photodiode 20 and receiver control electronics 22. As 
will be explained subsequently, the transmitter control electronics 16 
accepts the electrical video input signal and generates a hybrid signal 
(to be described hereinafter) which is used to modulate the IR-LED to 
output a light signal representative of the hybrid signal. The modulated 
light signal is conveyed by optical fiber 12 to the electro/optical 
receiver 14. The photodiode 20 converts the light signals into electrical 
signals which are processed by receiver control electronic 22 to output an 
electrical video signal which is used to display information on a CRT or 
similar display tube (not shown). 
FIG. 2 shows a circuit arrangement for transmitter 10. The circuit 
arrangement is for a voltage mode video input signal. A circuit 
arrangement for a current mode video source input signal will be described 
hereinafter. The transmitter includes IR-LED 18, transconductance 
amplifier 24, white reference pulse generator 26 and IR-LED bias circuit 
28. The cathode or IR-LED 18 is connected to reference ground potential by 
resistor R3. The transconductance amplifier 24 accepts a voltage signal at 
its input and generates at its output a current signal representative of 
the input voltage signal. The current signal which is outputted from the 
transconductance amplifier 24 modulates IR-LED 18. The transconductance 
amplifier 24 further includes transistor T1, T2 and T3. The emitters of 
transistors T1 and T2 are connected through current source 32 to ground 
potential. The base of transistor T2 is connected to the cathode of IR-LED 
18. The emitter of transistor T3 is connected to the anode of IR-LED 18. 
The base of transistor T3 is connected to the collector of transistor T2. 
The collector of transistor T2 is connected through R2 to the collector of 
transistor T3 and to +V supply. Similarly, the collector of T1 is coupled 
to +V voltage supply and the base of T1 is connected to the Reference 
White Pulse circuit generator 26. 
The function of the Reference White Pulse circuit generator 26 is to 
generate a Reference White Pulse (details to be given hereinafter) which 
is inserted into the video data stream during the blanking period when no 
video information is displayed on the video screen. As will be explained 
subsequently, this blanking period is usually used as a flyback period 
when the beam retraces on the video display. Also, its presence is usually 
marked by the advent of the conventional horizontal sync pulse. The 
Reference White Pulse circuit generator 26 includes FET device 34 and 
pulse generator 36. Pulse generator 36 is a conventional off-the-shelf 
device which is activated by the presence of the horizontal sync pulse on 
its input. The output of pulse generator 36 is connected to the control 
electrode of FET 34. One of the terminals of FET 34 is connected to the 
base of T1 and another terminal is connected through resistor R1 to +V 
voltage supply. 
The biasing current for the IR-LED 18 is provided by resistor R0. Resistor 
R interconnects the electrical video input voltage signal to the voltage 
transmitter circuit. As stated earlier, the video generator which 
generates the electrical video input voltage signal is conventional and 
the details will not be given. 
FIG. 3 shows a circuit schematic for a current mode video generator. The 
circuit includes a conventional video generator including a video 
digital-to-analog converter (DAC). As stated previously, such a video 
generator is an off-the-shelf item, and details will not be given. The 
output of the video generator is coupled through switching assembly 38 to 
IR-LED 18. The switching assembly 38 includes transistor T4 and a 
transistor T5. The emitter electrodes of transistors T4 and T5 are 
connected through current source 42 to +V (voltage supply). As will be 
explained subsequently, current source 42 generates a pulse called 
"reference white pulse" which is inserted into the video signal during the 
blanking period. This pulse is used at the receiver to adjust the gain of 
the video channel. The base electrode of transistor T5 is tied to a 
voltage level which is approximately 2V. The collector of transistor T5 is 
connected to the anode of the IR-LED 18. Likewise, the base of T4 is 
connected to pulse generator 40. Pulse generator 40 is activated by 
horizontal sync pulse. When it is activated, T4 and T5 conduct and the 
current pulse generated by current source 42 is inserted in the video data 
stream and is used to modulate the IR-LED 18. When the pulse generator 40 
is not activated, the video signal is used to modulate the IR-LED 18. 
The biasing current which is provided by current source 41 (FIG. 3) and 
voltage means 28 (FIG. 2) is always present. It was determined that the 
IR-LED 18 has a very linear current to optical intensity transfer 
characteristics above a minimum current level. Therefore, current source 
41 and voltage means 28 are designed to bias IR-LED 18 to operate at or 
above the minimum level. 
FIG. 4 shows a graphical representation of the hybrid signal which is 
generated and transmitted from transmitter 10 (FIG. 1). FIG. 4 also shows 
the control signal which indicates the time when the Reference White Pulse 
is to be inserted in the data signal. In case of the video channel, the 
control signal is the horizontal sync pulse. Still referring to FIG. 4, 
the horizontal axis represents time and the vertical axis represents 
IR-LED current. It should be noted that the hybrid video signal which is 
transmitted to the fiber optical cable 12 (FIG. 1) is repetitive; 
therefore, only one cycle of the hybrid signal is represented in FIG. 4. 
As is shown in the figure, consecutive video signal bursts (such as 49 and 
51) are separated by the Reference White Pulse which is inserted during 
the blanking interval. Preferably, the blanking level is set to be equal 
to the minimum IR-LED current. For purposes of discussion, the maximum 
blanking interval is comprised of three sections identified as horizontal 
front porch 46, sync period 50 and horizontal back porch 48. As can be 
seen from the figure, the horizontal back porch 48 is longer than 
horizontal front porch 46, and therefore the Reference White Pulse is best 
positioned within the horizontal back porch section of the blanking 
interval. 
To better understand the structure of the hybrid video signal which is 
generated and transmitted according to the teachings of the present 
invention, a description of conventional video transmission will be given. 
In conventional electrical transmission of video signals on wire 
conductors, there is a blanking period such as the one shown in FIG. 4 
during each scan line when the video level is defined to be the blanking 
level and black is displayed on the screen. It is during this time that 
the CRT's flyback occurs. This convention is retained here with a blanking 
level corresponding to the minimum IR-LED current level being transmitted 
during most of the blanking period. The Reference White Pulse, instead of 
the blanking level, is transmitted via the IR-LED 18 into the fiber cable 
12 (FIG. 1) for a period during the blanking interval. This reference 
level is provided by a switched current source or an input to the 
controlled current source. The source of this reference pulse is keyed by 
the horizontal sync pulse from the video input. The reference level of the 
Reference White Pulse has a defined relationship to the brightest level 
signal available from the electrical video input. Typically, the reference 
level of the Reference White Pulse is a few percent brighter than the 
brightest video level normally available. Both the blanking level and the 
Reference White Level signal are used in the automatic gain control (AGC) 
scheme, details of which will be given hereinafter. 
Referring again to FIGS. 1, 2 and 3, IR-LED 18 outputs a light signal which 
is conveyed by fiber optical cable 12 to the receiver 14. A low cost 
surface emitting IR-LED is used in this application. As stated previously, 
IR-LEDs have a very linear current to optical intensity transfer 
characteristics above a minimum current level. Therefore, the IR-LED is 
biased with a current source such that it is always at or above this 
minimum level. The video signal to be transmitted is a current source or 
it feeds a current controlled current source or voltage controlled current 
source depending on whether the electrical video signal is considered a 
current or voltage. As shown in FIG. 3, in some cases the video current 
input can be directly connected to the IR-LED along with the other current 
sources described here. This current source then forces a current which is 
proportional to the video signal amplitude into the IRD/LED. As a result, 
light signals representative of the video signal are conveyed by the fiber 
optic cable into the receiver. 
FIG. 5 shows a block diagram of the receiver according to the teachings of 
the present invention. The primary function of the receiver is to receive 
optical signals from cable 12 (FIG. 1), reproduce an electrical signal 
that is similar to the original electrical signal that was transmitted 
into the optical fiber and use the reproduced electrical signal for 
driving a standard monitor or CRT tube (not shown). The reproduced 
electrical signal is identical to that which would have been carried by 
copper wire in a conventional video system. To this end, the receiver of 
FIG. 5 includes photodiode 20, trans-impedance amplifier 52, coupling 
capacitor 54, DC restore circuit 56, automatic gain control (AGC) circuit 
arrangement 58, switching assembly 60 and pulse generator 62. The AGC 
circuit arrangement 58 further includes video amplifier 62, peak detector 
64 and comparator 66. 
Still referring to FIG. 5, photodiode 20 receives the light from fiber 
optic cable 12 (FIG. 1) and produces an electrical current, which is 
amplified by trans-impedance amplifier 52 to a voltage at a useful level. 
At this point the signal which is referred to as "Raw Received Video", 
includes a linear replica of the original video signal with a DC offset 
and a "Reference White Pulse" during the blanking interval. The raw 
received video signal is AC coupled to the DC restore circuit 56 which 
brings the blanking level to the desired blanking level which is typically 
0 volts. The DC restore circuit is activated by the horizontal sync pulse 
which may be transmitted via a separate electrical wire. 
Next, the signal is passed through the AGC circuit arrangement 58 which 
corrects the amplitude of the signal so that the "Reference White Level" 
pulse is at the desired value. The peak detector 64 detects the amplitude 
of the Reference White Pulse, compares it with the desired reference level 
(on the conductor labeled White Reference Voltage) and generates an error 
signal (on conductor 65) which is used to adjust the gain of the video 
amplifier 62, thus forcing the "Reference White Level" pulse to be equal 
to the desired level. The signal which is now present at the output of AGC 
circuit arrangement 58 is identical to the original transmitted video 
signal except for the Reference White Pulse. This pulse is removed by 
generating a pulse from pulse generator 62 and activating switching 
assembly 60 so that the line marked electrical video output is connected 
to the blanking reference level. The DC restore circuit and the pulse 
generator are activated by the horizontal sync pulse. 
FIGS. 6 and 7 show circuit diagrams for the receiver according to the 
teachings of the present invention. The circuit components of the figures 
are identical except that in FIG. 6 devices C2, Q5, Q6 and R6 form a 
positive peak detector whereas in FIG. 7 devices C2', Q6', R6', Q5' and 
R9' form a sample and hold circuit. Also, in order to correlate FIGS. 5, 6 
and 7 and simplify the descriptions, common numerals are used to describe 
like components. 
Still referring to FIGS. 5, 6 and 7, the cathode of photodiode 20 is 
connected to +V and the anode is connected to the input of transimpedance 
amplifier 52. Transimpedance amplifier 52 is a conventional off-the-shelf 
device and the details of the amplifier will not be given herein. Suffice 
it to say that the amplifier accepts a current signal and converts it to a 
voltage signal. The output of transimpedance amplifier 52 is connected to 
coupling capacitor 54. The output plate of coupling capacitor 54 is 
connected through FET devices Q7 and R1 to ground potential. A controlled 
pulse from DC restore pulse generator (to be described hereinafter) is 
supplied to the control electrode of device Q7. The output plate of 
coupling capacitor 54 is also connected to video amplifier 62. 
The video amplifier is formed from transistors Q1, Q2 and Q3. The emitter 
of transistor Q3 is coupled through resistor R5 to ground and through R4 
to the base of Q2. Likewise, the emitters of Q1 and Q2 are connected via 
R2 to -V supply. The collector of Q2 is connected through R3 to +V supply 
and the base of Q2 is connected through R12 to FET device Q4. FET device 
Q4 is an off-the-shelf device. In the preferred embodiment of this 
invention, Q4 is identified with the number 2N5019. This device is 
manufactured by Siliconix. Of course, there are other types of devices 
which could be used without departing from the scope of the present 
invention. The control electrode of device Q4 is coupled to comparator 66. 
Comparator 66 includes a differential amplifier A2 with its positive input 
connected to ground by R8 and its negative input connected to a voltage 
divider formed by resistors R10 and R11, respectively. The input to R10 is 
the reference voltage level which is set to be equal to the magnitude of 
the White Reference Pulse which is inserted at the transmitter of the 
present invention and was previously described. 
Still referring to FIGS. 5, 6 and 7, sample and hold circuit 64' (FIG. 7) 
is coupled to the positive input of the differential amplifier. 
Alternately, peak detector 64 (FIG. 6) is coupled to the positive terminal 
of differential amplifier A2. As stated above, peak detector 64 is a 
positive peak detector which is always monitoring the output of video 
amplifier 62 whereas sample and hold circuit 64' samples the output of the 
video amplifier only when the blanking pulse from the blanking generator 
is present on the line labeled "From Reference Pulse Blanking Generator." 
With reference to FIG. 7 for the moment, the sample and hold circuit 64' 
is comprised of components R6' and R9', Q5', C2' and FET device Q6'. 
Likewise, with reference to FIG. 6, the peak detector 64 is comprised of 
transistors Q5, Q6, resistor R6 and capacitor C2. The switching assembly 
60 comprises of resistor R7 and FET device Q8. When the blanking pulse is 
active on the line labeled "From Reference Pulse Blanking Generator," the 
line labeled Video Out is connected through FET device Q8 to the blanking 
reference level. In the preferred embodiment the blanking reference level 
is ground potential. 
With reference to FIGS. 6 and 7, with no optical signal present at 
photodiode 20, input current of transimpedance amplifier 52 is 0. The 
output of amplifier 52 is at the quiescent, DC level, with no current 
flowing in coupling capacitor 54. The base terminals of both transistors 
Q1 and Q2 are at approximately ground potential. The emitter of transistor 
Q3 is likewise very close to ground potential. Transistors Q5 and Q6 (FIG. 
6 only) form a positive peak detector circuit where the voltage across 
capacitor C2 is equal to the positive peak value of the signal present at 
base of transistor Q6 or the amplifier output. 
For the no-signal condition of the amplifier previously described, the 
voltage across C2 is essentially 0 volts. A precision reference voltage 
(REF VOLTAGE) is connected through R10 and R11 to the negative terminal of 
differential amplifier A2. The voltage present across capacitor C2 is 
compared to this reference by the differential amplifier. The output of A2 
is proportional to the difference between the reference voltage and the 
voltage across C2. The A2 output level is connected to the gate terminal 
of field effect transistor Q4. The drain-to-source or channel resistance 
of Q4 is determined by the voltage magnitude applied between the gate and 
source terminal. For this particular device identified by numeral 2N5019, 
which is fabricated by Siliconix. The channel resistance varies from a low 
value of 150 ohms or less with no voltage across the gate of source 
terminals to a value of several megohms when a voltage of 5 V is applied 
across the gate to source terminals. In the no-signal condition, the Q4 
gate to source voltage (V.sub.GS) is 0 or even negative, and the Q4 
channel resistance is minimal. In this condition the amplifier 62 voltage 
gain is determined by the expression A.sub.V =R4+R12+RQ4/R12+RQ4 where R4 
is a resistance interconnecting the base of Q2 to the emitter of Q3, R12 
is a resistor interconnecting the base of Q2, the source of Q4 and RQ4 is 
a channel resistor of FET Q4. Since the value of RQ4 is minimum, the 
amplifier voltage gain is maximum. 
Still referring to FIGS. 6 and 7, when a signal is applied to photodiode 
20, current is generated and flows in the input of transimpedance 
amplifier 52. A positive going signal will be coupled via coupling 
capacitor 54 to the base of Q1, and an amplified response will appear at 
the amplifier output. This signal is also applied to the input of peak 
detector circuit Q6 and Q5 (FIG. 6) or sample and hold circuit 64' (FIG. 
7) where it causes C2 (FIG. 6) or C2' (FIG. 7) to charged to the most 
positive peak level of the input signal. This voltage level is compared to 
the reference voltage by differential amplifier A2, which controls the 
gate voltage, therefore, the channel resistance of Q4. The gain of 
differential amplifier A2 can be chosen such that small differences 
between the reference voltage and the peak detector output will 
significantly alter the channel resistance of Q4 and therefore the voltage 
gain of the video amplifier (Q1, Q2, Q3). 
As stated previously, a "Reference White Pulse" is inserted at the 
transmitter and is transmitted during the "back porch" portion of the 
horizontal sync interval. This pulse is chosen to be more positive than 
any of the normal video signals and is identical in magnitude to that of 
the reference voltage. If the detected Reference White Pulse (RWP) level 
is less than the reference voltage, the amplifier gain is increased. A 
greater detected RWP level has the effect, of reducing the amplifier gain. 
The net result is that the detected RWP level is made to be equal to the 
reference voltage. 
FIG. 8 shows a circuit arrangement that generates the control pulses which 
activate DC restore circuit 56, sample and hold circuit 64' and switching 
assembly 60 (FIGS. 5, 6 and 7). The circuit arrangement includes two pulse 
generators 64 and 66 connected in tandem. Any conventional off-the-shelf 
pulse generator can be used for generating the respective pulses. In the 
preferred embodiment of this invention, identical conventional pulse 
generators, identified by numeral 74HCT221, were used. This pulse 
generator is fabricated by Signetics TM. Pulse generator 64 is connected 
through C1 and R1 to a +V supply. R1 and C1 are chosen to set the width of 
the pulse which is desired from the pulse generator. Likewise, R2 and C2 
interconnect pulse generator 66 to a +V supply. Similarly, the values of 
R2 and C2 are selected to set the width of the pulse which is outputted 
from pulse generator 66. As stated above, for proper operation of the 
subject invention, the DC restoration and output video blanking must occur 
during the presence of the horizontal sync pulse. To this end, when the 
leading edge of a negative going horizontal sync pulse is applied to the 
"A" input of pulse generator 64, a positive going pulse begins at "Q" 
output, and a negative going pulse begins at the Q not output. The time 
duration of these pulses is solely dependent upon the values selected for 
R1 and C1. 
The positive pulse from the Q output of pulse generator 64 is connected to 
the gate of field effect transistor Q8 (FIGS. 6 and 7). This pulse causes 
Q8 to saturate and effectively short the video output line to ground for 
the pulse duration. As said before, this is referred to as the blanking 
interval or period. During this time interval, the gain of the AGC loop is 
adjusted and the "Reference White Pulse" is removed from the restored 
video signal. 
In those cases where a sample and hold circuit is used (FIG. 7), the 
negative going pulse from the Q not output is connected to the input side 
of resistor R9' (FIG. 7). This pulse causes Q5' to turn off which will 
cause Q6' to saturate and allow capacitor C2' to charge to the value of 
the output video which, at this time, should be the reference white pulse. 
The input transimpedance amplifier 52 (FIGS. 6 and 7) is coupled to the 
video amplifier via coupling capacitor 54. The leading edge transition of 
the horizontal sync is used to generate a positive going pulse with a 
duration determined by R2 and C2 (FIG. 8). This pulse is applied to the 
gate of transistor switch Q7 which saturates and grounds the coupling 
capacitor 54. This action forces a voltage equal to the average value of 
the video signal across capacitor 54. Therefore, the video will begin all 
scan lines from ground potential, regardless of the average value of the 
signal. 
The above description addresses a single video channel which can be used 
for monochromatic systems. For color systems, three identical channels 
transmitting Red, Green and Blue (RGB) are required. In such a 
configuration the transmitter side of the color system is such that for 
each video channel there is a separate infrared light emitting diode 
IR-LED which is coupled to an optical channel as shown in FIG. 1. Each LED 
is connected to a current source which causes a bias current to flow in 
the LED. This bias current is chosen such that the LED is operating in its 
linear region without any additional current flowing. The video current 
source is also attached to the LED such that the current from it also 
flows through the LED in an additive fashion. 
The receiver side for such a color system is shown in FIG. 9. The Red, 
Green and Blue video signals are handled by video channels 68, 70 and 80, 
respectively. Since each of the channels 68, 70 and 80 are identical, only 
one will be described. Each of the channels includes a video amplifier 82, 
AGC circuit 84, sample and hold circuit 86, and switching arrangement 88. 
The control input of each sample and hold circuit 86 is connected to 
Reference pulse and blanking pulse generator 90. As described above, when 
the horizontal sync pulse is present on the line labeled horizontal sync, 
a pulse is generated on the output of pulse blanking pulse generator 90 
which causes sample and hold circuit 86 to sample the pulse outputted from 
video amplifier 82. Error signals which result from a mismatch between the 
Reference White Voltage and the sample and hold signal create error 
signals which are used to adjust the gain of the video amplifier. During 
the time interval when the gain of the amplifier is being adjusted, the 
output of the line labeled Red video is connected to ground. The structure 
and function of video channels 70 and 80 are identical to video channel 
68; therefore, a further description of those channels will not be given. 
The present invention ensures that the relative amplitude of signals are 
maintained constant across the respective video channels. This is of 
maximum importance for color transmission wherein variations in the 
amplitude of received signals causes improper reproduction of the colors. 
Although the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention.