Low-pass filter with remotely controllable rise time

A remotely controlled chroma key soft edger is described for use with an analog switcher. The keying signal is a low passed filtered and differentially summed with substantially an unfiltered keying signal and the resultant high frequency components are summed with the low pass filtered signal. The gain of the peaking signal is remotely controlled to adjust the summed rise time of the keying signal.

BACKGROUND OF THE INVENTION 
This invention relates to a variable low pass filter which is remotely 
controllable and more particularly to a chroma key soft edger. 
In a chroma keying system, the red, green and blue video signals from a 
camera are received by a chroma keyer which in response to a particular 
color signal provides a relatively high amplitude keying signal which 
exceeds a given threshold level. When a special effects device receives 
this keying signal, the device causes an electronic switch to switch from 
one camera to a different camera for the duration of that keying signal. 
It has been found that if this keying signal switches the electronic 
switch too rapidly, the rapid change in video causes bearing and noise 
generating effects which are undesirable. Broadcasters and users of such 
equipment are requesting some form of more gradual switching from one 
camera to the other so as to prevent these beating and noise effects. 
Since this switch which is switching the cameras may be located at a point 
remote from the controller, it is also desirable to have the control 
signal processing means be remotely controlled in order for the operator 
in the studio control room to vary the switching time for special effects 
and for eliminating the beating noise effects. It is therefore desirable 
that the chroma control signal which is going to control the switcher have 
a variable rise time which is selectable from a remote location. 
SUMMARY OF THE INVENTION 
Briefly, a remotely controllable low pass filter system is provided for 
selectively and remotely varying the rise time of a control signal such as 
used to control an analog switcher. The system includes a low pass filter 
and adjustable means for differentially summing the low pass filtered 
signal and an unfiltered signal for providing a peaking signal that 
contains only the high frequency components. The peaking signal and the 
low pass filtered signals are summed to provide the desired filtered 
control signal. The gain of the differential summing means is adjusted 
remotely to vary the peaking signal level to thereby control the rise time 
of the control signal.

DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, there is illustrated a chroma keying system. The 
signals from camera 10 for example are encoded in encoder 11 and applied 
to terminal 15a of a switcher 15 of special effects device 18. The camera 
signals from another source camera 12 are applied via encoder 14 to 
another terminal 15b of switcher 15 of device 18. The signal at the 
switcher 15 is applied via switch arm 15c and terminal 15d to the output 
device 20. A chroma keyer 22 responsive to signals for example from camera 
10 provides a keying signal at a selected color from camera 10. The keyer 
22 is responsive to control voltages for example from remote unit 24 for 
selecting that particular chroma signal at which an output exceeds 
threshold and provides a signal to the special effects device 18. For a 
description of a particular remote controlled chroma keyer see applicants' 
application (RCA 72,885), entitled "Chroma Keying Selector System" filed 
on even date herewith. The switcher 15 in the special effects device 18 is 
responsive to signals via a low pass filter 16 according to the present 
invention for switching the signals from camera 10 to camera 12. The scene 
viewed by camera 10 for example may include a background which is of the 
particular chroma selected by the remote unit 24 in the chroma keyer 22 to 
cause the switch 15 to couple the signals from camera 12 when the scene 
scanned by camera 10 reaches the color of that background. 
According to the present invention, a low pass filter 16 is responsive to 
the chroma key signal for providing a soft edger so as to cause the switch 
15 to move to the position to couple the signals from camera 12 over a 
given time period determined by the low pass filter 16. By softening the 
edge of transitions from the signals from camera 10 to camera 12 and back, 
the beating and noise effects are eliminated. A variable D.C. control 
voltage from the remote control unit 24 to the low pass filter 16 provides 
an adjustable voltage to the low pass filter 16 to change the rise time 
characteristic of the keying signal applied to the analog switch 15. The 
switch 15 is not a simple toggle switch as represented in FIG. 1. The 
switch is an analog switching device in which the time period for 
switching from camera 10 to camera 12 is delayed in proportion to the rise 
time of the output of filter 16. The analog switch 15 is preferably 
operated such that the video from camera 10 is faded as the input of 
camera 12 is added in proportion to the rise time of the signal from 
filter 16. The analog switch 15 may be like that illustrated in FIG. 2 
using 4-quadrant multipliers 17 and 18 and summer 19. The video (video 1) 
from camera 10 is applied to x input of multiplier 17 and the video (video 
2) from camera 12 is applied to the x input of multiplier 18. The keying 
signal is applied to the Y input terminal of both multipliers 17 and 18. 
As the keying voltage rises for example the video from camera 10 falls and 
the video from camera 12 increases. The video is summed at the summer 19. 
Between switching positions video from both cameras is applied in 
proportion to the rise or fall time of the keying signal. At the midpoint 
of the rise time equal signal levels are provided from both cameras. 
Referring to adjustable low pass filter 16 in FIG. 3, the chroma keying 
signals exceeding the threshold in the chroma keyer 22 are coupled via 
isolating emitter follower 31 to a low pass filter 33 and to a delay line 
35. The output of the delay line 35 is such that the signal has full band 
width (not band width limited in the delay) and is delayed an amount equal 
to the delay from the low pass filter 33, which may be, for example, a 
2-stage low pass active filter. The filter 33 provides a rise time of the 
signal that is selected to be the maximum desired rise time. A resistor 
divider network of equal valued resistors 37 and 39 is coupled between the 
outputs of low pass filter 33 and delay line 35. The junction of resistors 
37 and 39 is coupled to the + input terminal of operational amplifier 41. 
The output of low pass filter 33 is also coupled to the -x input of a 
4-quadrant multiplier 45. The output of delay line 35 is coupled to the +x 
terminal of multiplier 45. The -Y input of multiplier 34 is coupled to 
ground or a reference potential. The +Y input terminal of multiplier 45 is 
coupled to the remote unit 24. At the remote unit 24 is located a+ and -DC 
source of potential and the D.C. voltage to the +Y terminal is varied from 
+ to -. The output from the multiplier 45 contains just the high frequency 
information only or the high frequency difference signal from the low pass 
filter 33 and the delay line 35. By varying the D.C. voltage from + to - 
to the +Y input of multiplier 45 the output level or the gain and the 
phase of the high frequency signal is varied. The signal from the output 
of multiplier 45 is inverted via inverter 47 and applied to the minus (-) 
input of operational amplifier 41. The high frequency adjustably varying 
peaking signal is added to the summed voltage (E1+E2/2) at the plus input 
terminal of operational amplifier 41 to form a keying control signal in 
which the rise and fall times vary symmetrically as a function of a remote 
D.C. control signal while the low frequency amplitude remains constant.