Video signal processing system architecture

An edit suite combines a vision mixer and digital video effects device. A routing matrix (100) is arranged on a back board and interconnected with a number of signal processing devices including two mixers, two effects devices and a key store. Video inputs are supplied from time base controllers (102-108). The matrix comprises a plurality of dedicated buses and each processing device selects an input from one of a number of buses and returns an output to a different bus where it can be selected for further processing by a different device.

CLAIM OF PRIORITY 
Applicant hereby claims priority under 35 USC .sctn. 119 to UK Patent 
Application Ser. No. GB9020539.4 filed in the United Kingdom on Sep. 20, 
1990. 
FIELD OF THE INVENTION 
This invention relates to video signal processing and in particular to 
television production and post-production systems. 
BACKGROUND TO THE INVENTION 
An edit suite used in television production and post-production systems 
will typically contain a vision mixer, digital video effects unit (DVE) 
and edit controller. Additionally a routing switcher, color corrector and 
an assortment of other processing devices may well be made available. 
Often each of these products will have been produced by different 
manufacturers. Although every attempt will have been made by these 
manufacturers to ensure conformity to industry standard protocols and 
interfaces, some non-conformity is inevitable in order to extract optimum 
performance and functionality from a particular device. 
This non-conformity often causes problems for operators and engineers 
working with the system. An operator will have to learn the operating 
style of each individual device and the best way to communicate between 
devices. An engineer may have to address timing problems and interface 
protocols requiring a great deal of communication with individual 
manufacturers. 
Several manufacturers have recognised the problem and have begun to produce 
products which are designed to work together. 
FIG. 1 shows the traditional layout of a mixer and digital effects unit 
which is used in some recent units. In this figure a mixer 10 comprises a 
mix-effects device (M/E) 12 and a down stream key (DSK) 14. Primary inputs 
are taken into the mixer where they may be mixed, wiped or keyed together. 
A dedicated loop (usually only one) provides an exit and re-entry system 
allowing a primary source to be manipulated by the separate digital 
effects device (DVE) 16 and recombined within the mix-effects without any 
appreciable timing or communication difficulties. The output of the mix 
effects and/or pgm/pst bus is then put through the downstream key 14 which 
is the last section of processing in the chain and is generally used for 
the addition of captions or credits. This approach of FIG. 1 works well, 
however, it does introduce some constraints which have been accepted in 
the industry but which apply limitations. For example, signal processing 
through the system takes place in a very structured way. 
The complexity of the system obviously depends on the particular choice of 
mixer and digital effects device combination. Some very powerful systems 
can deal with up to 10 layers of video in a single pass and may well 
command a price tag of around 250,000. However, very often even these 
very powerful systems will only be required to produce a composite of 4 or 
5 layers. The extra functionality being required due to the inflexible 
architecture and timing problems inherent in such a system. 
Such systems can be wasteful of circuitry, as a good deal of built in 
redundancy is required to overcome the inflexibility of this very 
structured architecture. Some installations have made these systems work 
extremely well with the aid of external matrixes and processing devices. 
However, those are in the minority. 
The inflexibility of the prior art can be appreciated by considering a 
situation where a primary input was fed to M/E 12 and external key and 
external fill applied followed by manipulation in DVE 16. 
SUMMARY OF THE INVENTION 
The present invention aims to overcome the limitations and disadvantages of 
the prior art as discussed above. 
The invention provides a system which integrates the mixer and digital 
effects device sections of the edit suite. This solution avoids the need 
for complex interfaces between these two pieces of equipment. Furthermore, 
a system is provided which has an integral routing matrix which enables 
traditional processing problems experienced with the prior art to be 
overcome and ensures a high degree of flexibility in routing. 
This invention is defined more specifically by the claims to which 
reference should now be made. 
The advantages of flexibility is achieved as a result of the central 
processing system. This preferably conforms to the CCIR601 4:2:2:4 format 
which yields a high level of transparency for the processing of video and 
layer signals. 
A preferred embodiment of the invention provides a system in which the bus 
structure is adopted to enable each processing block to communicate its 
output to the input of the other blocks within the system. Preferably each 
of the blocks is built on an individual PC board and shares a common 
interconnected backplane with the other PC boards containing other system 
blocks. 
An embodiment of the invention has the further advantage that each device, 
on its own PC board, can select which digital video inputs it processes 
(for example mixes), and then send the resultant signal back to a 
different bus on the backplane. This processed signal can then be selected 
for further processing by different device. 
Although the actual processing performed by any one device is similar to 
that performed in the prior art, a system embodying the invention can have 
significant cost savings resulting from using common circuitry where 
possible and the overall operation of the system is much more simple. 
Picture quality is preserved by keeping the signal in digital format 
rather than processing through A to D and D to A converters required with 
many of the existing devices. 
The invention also provides a method of color correction in which a 
transfer curve is calculated by a system processor from a user controlled 
input and the curve is written into a memory in the luminance path to 
create a color balance correction.

DESCRIPTION OF PREFERRED EMBODIMENT 
The system of FIG. 2 comprises five separate elements, a mix-effects unit 
(M/E) 20, first and second digital video effects units DVEs 22 and 24 and 
first and second flex key 26, 28. The mix-effects and DVE units are 
similar in principal to those used in the prior art. The flex keys are 
analogous to the DSK 14 of the prior art, however their position within 
the system is not rigid. The flex keys take a background video signal to 
which a key and fill signal is applied. These key and fill signals may be 
generated by external character generators such as manuscript or floating 
point. This operation is illustrated in FIG. 4. The flex keys may be 
introduced into the system at any point, for example before or after the 
DVE's, up-stream, midstream or downstream. Thus, as will become clear from 
a detailed examination of FIG. 6 in due course, the architecture is 
inherently flexible and does not suffer from the limitations of the prior 
art. 
The processing performed by any one of the M/E, DVE's and FLEX's of FIG. 2 
is similar to that of the prior art. For that reason, no further 
description of their composition is necessary. However, it should be 
appreciated that M/E unit 20 is a one and a half level device as can be 
seen from FIG. 3. The unit has three inputs, MEA, MEB and MEC and a single 
output. The first two inputs MEA and MEB are mixed or processed as 
required in a first mixer effect unit 30 and the output mixed and 
processed with the third input MEC in a second mix-effects units 32 to 
produce the output signal ME ABC. 
Referring back to FIG. 5, the system shown achieves flexibility of routing 
by the inclusion of a digital matrix within the unit. This matrix is 
illustrated in FIG. 6 and has the effect of turning the edit suite inside 
out. Thus, the timing problems that are present in the prior art that 
result from sources being re-routed in an n external matrix and then 
re-entered are obviated. 
Consider now the problem posed earlier with respect to the prior art, in 
which a primary input, to which an external key and external fill are 
applied followed by manipulation in a digital effects device. The 
realisation is shown in FIG. 5. The primary input (VTR 1) is fed into the 
mix effects bank while a secondary input (VTR 2) is fed into flex key 1 
(26). The output of flex key 1 is then fed into a digital effects channel 
(DVE 1) 22 with the subsequent output being fed into M/E 20. The output of 
M/E 20 is VTR 1 as the background with a flying VTR 2 with captions laid 
over VTR 1. 
Typically, a pair of vision mixers, dual video effects generators, A to D 
convertors (ADC) and color correctors are each provided on an individual 
PC board and share a common backplane. A microprocessor communicates 
instructions to each of the boards on a standard VME bus. 
Two additional connectors on each board communicate several simultaneous 
digital video signals to each other at the standard rate of 27 MHz. This 
bus structure communicates the outputs of each board to the inputs of 
every other board that uses digital video inputs. The ADC boards are 
treated differently and receive several analog inputs from a TBC I/O board 
that also puts analog signals on a limited part of the backplane. 
The microprocessor instructs each board as to which signals to select from 
the backplane and on each step of the processing by addressing a board 
slot and registers or memories on that board. These registers and memories 
are then loaded from the processor with digital words which perform 
specific tasks on that particular board. Several registers may be loaded 
on several different boards to perform several tasks simultaneously for a 
special video effect. In this way the processor controls the vision mixer, 
digital video effects, ADC's color correctors, and routing selectors 
simultaneously for a single complexed effect. 
Each board has pipeline delay devices on each its inputs to compensate for 
the accumulative delay or lack of same dependent upon how many boards the 
selected signal has been through and the delay introduced by each board. 
The processor keeps track of the routing of each signal, calculates the 
accumulated delay of every signal and then instructs each board on how to 
compensate for each particular signal routing. This is done by addressing 
and loading the delay control registers on every input of every board with 
data words so that every signal on every board is properly timed. This is 
necessary in this system because there are a large number of different 
digital video signals on the backplane. Some of these signals may pass 
through up to six different delays while others may pass through 0 to 5 
delays. The number of combinations is very high but the processor is 
programmed to calculate all of the delay compensation requirements during 
every video field so that the system routing can be programmed to change 
drastically every video field, if required. 
The system configures several different video processing boards on a 
backplane in such a way that each board can select inputs from several 
different buses, process these signals and put them back on the bus for 
further processing. The system gives more flexibility and more powerful 
utilization of each individual element of the system than previously 
achieved with any other technique. Also, this flexibility gives benefits 
that can only be attained by much larger accumulation of much more 
expensive equipment. Furthermore, the nature of the system gives it the 
capability to store the instructions of routing, mixing, wiping of a 
pattern or keying signal, digital effects instructions of size, rotation 
in 3D space, direction of movement, rate of movement, digital trails or 
sparkles, mosaic patterns, posterization and solarization, ADC input 
selection and format selection, digital color correction, and video 
processing values simultaneously for each and every effect in an editing 
sequence. This makes it possible to preview, run, edit and achieve edit 
sequences with more detail and greater ease than present equipment. 
In FIG. 6, the backplane is indicated generally by reference 100. The 
backplane has 15 buses each of which are dedicated as follows: 
TBC 1-4: are dedicated buses each of which receive an input signal from a 
correspondingly numbered time base controller TBC 102, 104, 106, 108. The 
time base controllers receive inputs via input buffers 110 and provide 
digital outputs to the buses TBC 1-4. As an alternative, TBCs 102-108 may 
be replaced with analog-to-digital convertors. 
DVE 1-2: are the two buses which communicate, respectively, with DVE1 112 
and DVE2 114. Both DVE's 112 and 114 can take fill inputs from any one of 
the four TBC buses 1-4, and key signals from any one of buses MAT 1-4. 
STOR: is a bus which corresponds to keystore 116. The keystore 116 is a 
framestore device in which pictures may be manipulated by external keys or 
fill signals as-shown. 
DSK 1,2: are the two flexkey buses referred to previously. The DSK key may 
be applied at any point in the signal processing path. 
ME 1,2: are the two mixer/edit buses and correspond to the mix/edit units 
120, 122. Only one mix/edit unit is necessary and each comprises a pair of 
mixers 120 A,B, 122 A,B, as described with reference to FIG. 3. In each 
case the mixing is controlled by the system computer 150 which applies 
most of the wiper and other edit functions. However, some controls may be 
supplied directly from a mix control and pattern generator 124. One 
example would be a circular wipe which is a complex pattern to apply. 
MAT 1-4: The final four buses are external mat buses on which external keys 
are applied. As can be seen, the key inputs of the keystone, DVEs, and 
mix/edit units are taken from a selected one or ones of these buses. 
In addition to receiving analog input signals through TBCs 102-108, 
provision is made for 3 digital inputs which are placed onto TBC 1, 2, or 
3, respectively via digital interface 130. A fourth digital input may be 
used and placed on any selected bus, with the exception of the four MAT 
buses. 
Keys may be put onto the flexkey buses DSK 1, 2 and two of the MAT buses 
MAT 1, 2 via a mask generator and assignable key device 140. This device 
takes four key signals and two sets of luminance and color difference 
signals from external inputs via an interface 142 provided with input 
buffers. The final output is processed into a number of video formats by 
processors 160 and then passed to an output buffer 162 at which outputs 
are available in component (RGB), composite, or Y/C sub carrier form. 
The overall system is controlled by system computer 150. The computer is 
connected by VME bus 152 to all the hardware units described and also to a 
hard disk memory 154. The user controls the system from control panel 156. 
The control panel interfaces with four video tape inputs VTR1-4 and 
enables the user to select a variety of keys and also to configure the 
hardware as required. For example, the key input to DVE1 is selected from 
any one of MAT 1-4 and the fill input from any one of TBC 1-4, STOR, DSK 
1, 2 or ME1. The output from DVE1 is put onto the DVE1 bus and can be 
picked up as an input to, for example, either of the M/E devices, DVE 2 or 
the key store. 
The control panel 156 uses, for example, an electroluminescent display 
which is surrounded by soft keys. The software is structured such that all 
commonly used menus are no more than one layer or one push of a button 
away. All menus give a current status indication and can include relevant 
information about other menu screens. As users get to know the device, 
they will be able to program frequently used sequences of button presses 
for use as macro commands, which can be recorded on floppy disk. 
Thus, it will be appreciated that the embodiment described provides a very 
flexible post production system in which the operator can decide the 
processing path without being constrained by an inflexible system 
architecture. 
The system can store a number of keyframes. A keyframe is a snap-shot of 
how the control panel is set up at any given time. A sequence of 
keyframes, for example 25, can be stored as an event. Any number of events 
can be stored on disk or elsewhere for future use. 
The system also provides a digital color correction technique which uses a 
processor to calculate a transfer curve by reading an operator's input 
from a shaft encoder, pot, fader, or joystick and writing that transfer 
curve to a memory in the luma path to create a color balance correction. 
The color corrector may be included in the TBCs 102-108 of FIG. 6. This 
technique is very simple and low cost in hardware but gives very powerful 
color correction by using an available processor. This circuit and 
software allows the user to set color black balance, white balance, and 
gamma curves from a menu and continues to add new correction values to the 
old ones until the operator is satisfied with the result. The processor 
can recalculate the correction curves based on a simple set of 
coefficients and load them to the color correction memory during each 
vertical interval, if required. The chroma correction value derived from 
this method is then added to the multiplexed chroma path. In this system 
two sets of correction values are used or the R-Y and B-Y chroma vector 
sets are time multiplexed out of memory by using a clock equal to the 
R-Y/B-Y rate as an address bit. The video keyer and key shape generator 
are unique to the system described. A key shape generator (key generator) 
is used to control a pair of digital multipliers. The key signal causes 
one of the multipliers to reduce the level of the background video signal, 
while the other multiplier increases the level of the foreground signal. 
This is a linear process so the edges of the keying signal are not 
quantized to the sample rate of the video. The process must obey the 
nyquist criterion of a minimum of two samples to define a rising edge on a 
signal. It is sometimes necessary to use a small portion of a video signal 
to create a usable keying signal. The portion of the signal near video 
black or white level may be noisy, or the scene may have uneven lighting 
to create a tilt on the video waveform. Therefore, the signal may have to 
be amplified and clipped at both ends of the amplitude scale. In the 
keyer, a fixed clip level is created using two diodes wired in parallel 
with emitter to collector and collector to emitter. A variable gain 
amplifier precedes the diode clipper so that the percentage of clipping 
can be controlled. DC control of the amplifier is used to set the video 
signal that is within the linear portion of the diode clipper. A filter 
follows the clipper to limit the risetime of the signal to 5 MHz. an 
amplifier follows the filter to amplify the signal back up to slightly 
exceed the linear range of the analog to digital converter. In this way, a 
digital keying signal is created that does not produce unwanted aliasing 
artifacts when used to create digital video key effect.