IE64133B1 - Video mixer unit - Google Patents

Abstract

The unit is capable of selectively combining a current input video signal with an earlier video signal, both signals being decomposed 20-30 into Y, U, and V components and digitised 46, 50, 52, the earlier signal being stored 80. The stored signal components, and the current components present at latches 56, may be combined in their digital form at mixer 102, eg in complementary amounts as determined by the setting of gain units 92,94,110,112, passed through a D/A converter 68 for reformation at 76 as a composite signal for a gradual or stepped fade for example. One set of components could be selectively inverted before combination. Alternatively one set of signals could be wiped to the other, with superimposition of cross-hairs at the border between the two images; or the live and stored images could be displayed in frame alternation, so that differences will appear as flickering areas. The unit is arranged 20-30 to act on either PAL or NTSC inputs.

Description

The present invention relates to a video mixer unit, more specifically to a unit which is capable of selectively mixing an input video signal, and a stored video signal, to produce a composite output video signal which can then be further processed or displayed on a monitor in the form of a composite image.

One of the requirements of the film industry, particularly when special effects are to be achieved, is the ability selectively to be able to mix images emanating from two different sources, for example a video camera and a VTR. Mixers of this general type TO are known, but tend to be somewhat limited in applicability primarily because they will operate only in black and white and not in colour. A need for colour mixing has been evident in the industry for some time, but this need has never previously been met because of the technical difficulties that arise in moving from black and white to multi colour operation.

The present applicant has overcome these difficulties by making use of very rapidly clocked digital circuitry.

It is an object of the present invention at least to alleviate the difficulties of the prior art.

It is a further object of the invention to provide a video mixer unit which can operate on full colour video images.

According to the present invention there is provided a video mixer unit comprising an input for a M colour video signal, decoder means for splitting the input signal into its Y, U and V components, A/D 4 conversion means for digitising the said Y,U and V components, memory means for selectively storing the digitised values, means for later reading the stored values and selectively combining them with digitised Y, u and V components of the current input video signal to produce a composite digital output signal, and means for producing a composite analogue video output signal representative of the composite digital output signal.

In the description and claims, the digitised Y, u and V components which are stored in the. memory means will be said to be representative of a stored image, while the Y, U and V components of the current input video signal will be referred to as being representative of a live image. It will be appreciated, of course, that the live image need not in fact actually be live since the input video signal might just as easily be derived from a video tape recorder as from a video camera which is actually filming a live scene.

There may be automatic video standard detection means arranged to detect the type of video signal applied to the input, and to process the signal accordingly. Conveniently, the standard detection means may comprise a PAL/NTSC decoder arranged to detect which of the two standards has been used for the input video signal, and selector means arranged to process either an output from a PAL notch filter or an output from an NTSC filter accordingly.

Two or more inputs for colour video signals may be provided, with corresponding user-operable switches for choosing which of the inputs is to be used. The mixer may be arranged to accept inputs either from video cameras and/or video tape recorders.

To ensure proper synchronisation of the digital part of the circuitry, clock means may be provided % which are desirably locked to the line frequency of the input video signal. If no input video signal is # provided, the clock means may be arranged to default to a predetermined frequency, consistent with producing a final analogue video output signal which is suitable to operate a conventional monitor. This could be done by means of a sync pulse generator, locked to the clock means, and arranged to produce a sync pulse at for example 50 MHz; if a sync pulse is however detected from an input video signal, then the sync pulse generator may instead lock itself to the input sync.

Conveniently, either the PAL or the NTSC standard may be chosen depending upon which was last applied to the video input.

So that the unit may operate in an entirely self contained way, a battery back up for the memory means may be provided. Thus, if mains power is lost it is still possible for a user of the unit to view an image which has been stored in the memory means.

There are a number of possibilities for the means for later reading the stored values and selectively combining them with the corresponding digitised Y,U and V components of the current input video signal.

Firstly, the respective stored and live values could be summed to produce a composite digital output signal representative of the summation of a live and a stored image. A suitable inverter for one of the sets of signals could also be provided so that the user could have the option of a composite digital output * signal representative of the difference, rather than the addition, of the two sets of values. A two- ·♦ position user-operable switch would be a possibility.

A second.way of selectively combining the two sets of values is to form a composite digital output signal representing alternately the live and stored images, for example on a frame by frame basis. When such a composite digital output signal is converted into analogue video form, and is supplied to a suitable monitor, one would see the stored image for one frame, then the live image for one frame, repeatedly. Any differences between the live and stored images will manifest themselves in an apparent flickering of the display on the screen.

It will be appreciated that a digital multiplexer can be used to provide a composite digital output signal which alternates between live and stored values.

Another way in which the stored and live Y, U and V components could be combined is on a line by line basis so that, as seen on the output monitor, one has the stored image on part of the screen and the live image on another part. By suitable timing of the multiplexer, the live and stored images may be positioned on the screen either side by side or one above the other. By means of timing control or wiper means, the horizontal and/or vertical positions on the screen at which the live image changes to the stored image or vice versa may be moved, so giving the impression either that the live image is being wiped over the stored image or that the stored image is being wiped over the live image.

Conveniently, switch means allow the user to view either the live image alone, or the stored image alone. As will be appreciated, this is achieved either simply by producing an analogue video output signal which corresponds exactly to the digitised Y, U and V f components of the current input video signal, or by producing an analogue video output signal « representative only of the stored Y, U and V components.

There may be a user operable gain control, for use when the unit is set up to produce a mixed (additive or subtractive) composite output signal. Operation of the control enables the user to fade gradually or in discrete steps from the stored image to the live image, or vice versa. The gain control, of which may be by means of an analogue potentiometer, preferably operates on the digital part of the circuit, in particular on the Y, U and V components of the live and/or stored image. This can easily be done by means of one or more look up tables, wherein a change in the position of the gain control changes the address for entry to the look up table, and consequently the multiplier or additive constant to be applied to the Y, U and V values. If the gain control is analogue, the output thereof may first be digitised before being applied to address the look up table.

A single gain control may concurrently control in a complementary manner the gain to be applied to the live and stored images, so that for example by turning a single knob the user can fade in one while fading out the other.

Horizontal and vertical adjustment means may be provided for adjusting the position on a monitor output screen of the stored image. This is conveniently · achieved by applying additive constants to the addresses at which the Y, U and V values for a > particular point are stored in memory.

The invention may be carried into practice in a number of ways and one specific video mixer will now be described, by way-of example, with reference to the accompanying drawing which is a schematic block diagram of an exemplary mixer.

The purpose of the mixer illustrated is to enable a film maker selectively to mix two separate video signals: a live signal from for example a video camera, and a stored signal, in other words a single video frame which has previously been stored within the unit. In the following description the live and the stored signals will be referred to accordingly, but it should be appreciated that in fact the live signal may in fact be instead derived from the output of a running video tape recorder (VTR). * The user of the unit has a number of options available to him by means of a series of switches and knobs on a front control panel (not shown). The first control available is a five-position selector switch labelled live, store, mix, matched, wipe. In the live position, the live image can be seen on a display monitor, and in the store position the stored image can be seen. In the mix position, the display is a composite mixture of the two, while in the matched position the live and stored images are alternately displayed frame by frame. Finally, in the wipe position, the monitor screen is split with part of the display showing the live image and part showing the stored image.

For use when the main selector switch is in the mix position, there is a gain control knob, rotation of which changes the relative amplitudes of the live and stored pictures in the composite display. By making use of this, a film maker can easily fade” from the live to the stored image or vice versa. There is also an additional two-position add/substract switch enabling him to control whether the live and stored images are additively or subtractively mixed.

For use when the main selector switch is in the wipe position, there is a four-position switch enabling the user to choose which of the four quadrants of the screen will contain the stored image, with the other quadrants containing the live image. Slider controls are provided so that the horizontal and vertical limits of the stored image quadrant can be moved respectively up and down and across the screen, so giving the impression, as the quadrant is gradually *made larger, that the stored image is being wiped over the live image.

Another switch enables the user to reverse the positions of the live and stored images, so that the live image instead of the stored image is shown within the chosen quadrant. In this way, the live image can then be wiped over the stored image.

If the live and stored images are very similar, it is often difficult to see exactly where the horizontal and vertical limits of the quadrant are, and to overcome this a push button is provided to display a cross hair line on the screen, the arms of which delimit the quadrant.

The stored image, when it is being displayed, can be moved horizontally and vertically on the screen, by suitable horizontal and vertical alignment controls.

When the main selector switch is in the matched position, alternate frames of live and stored images are displayed. Naturally, if both the live and the stored images are identical, then the resultant display is no different from either of them on their own. However, if one small part of the images differ (where both show the same scene except for a single object which has been moved) then the difference will clearly appear as a rapid flickering on the screen. In this way, a moved object can be easily identified and can if necessary be placed back in exactly its original position by alternating a live action shot of where the object is at present with a stored shot of where it was originally. The object can then be moved by hand until, in the live image, it aligns exactly with its position in the stored image at which point it will cease to appear to flicker on the monitor screen.

The live input to the mixer illustrated in the diagram is via one or the other of two input video connectors 10,12 which are connected to a two-position video select switch 14. The user operates the switch to select one of the two sources: these may be, for example, a VTR and a camera, or two cameras, or two VTR's.

The selected video input is supplied to a sync pulse generator and clock, which detects the line and frame sync pulses and uses a standard phase locked loop to produce its own sync pulses and clocking signal.

The sync pulse generator is an SAA1043, having an oscillator that runs at 5 MHz. The line-locked clock generator is based on a 48 MHz tunable Colpitts voltage-controlled oscillator, the output of which is locked to that of the sync pulse generator.

The clocking signal from the sync pulse generator and clock 16 is used, after suitable divisions, to control multiplexing within the mixer and in particular to synchronise the timing of the various latches, to be described below. For simplicity, the lines carrying the clocking information to the various latches are not shown in the diagram.

The selected video input signal is supplied first to a buffer amplifier 18 and then in parallel to a PAL notch detector 20, an NTSC filter 22, a pulse generator 24, and a croma filter 26. The pulse generator 24 (for example a TDA2579) generates a set of timing pulses to drive a PAL/NTSC decoder 30 ( for example a TDA4555). Once the decoder 30 has determined, from the input supplied from the pulse generator 24, whether the input video signal is in PAL or NTSC format, it sends an appropriate signal along the line 32 to a selection circuit 28 which accordingly chooses for further processing either the output of the PAL notch filter 20 or the NTSC filter 22. In either case, the resultant signal, from which the colour information has now been removed, is sent along a Y channel line generally indicated at 34.

Since the PAL/NTSC decoder 30, in combination with the selector 28 automatically selects processing appropriate either for a PAL or for an NTSC input signal, there is no need for the user of the unit to know whether the camera or VTR that he is plugging in to the connectors 10,12 operates on the PAL or on the NTSC standard.

The standard outputs from the decoder 30 are two colour signals in quadrature, corresponding to the U and V channels. These are respectively indicated by numerals 36 and 38. The sub carrier signal 40 is also separated out for future use.

The Y channel (luminance) signal passes first through a delay unit 42, then through a further buffer amplifier 44 before being digitised in an A/D converter 46. A further delay 48 follows, the purpose of which is to ensure that the Y,U and V channels are properly timed before being multiplexed. The delays in the Y channel are necessary to compensate for the time lag introduced by the decoder 30 in the U and V channels.

The U and V channels are simultaneously digitised by respective A/D converters 50,52.

Further processing of the signal at this point depends upon the status of two series of latches indicated generally at 54 and 56. When the latches 54 are enabled the input signal is passed to a frame store, and when the latches 56 are enabled the live video signal continues to be processed for viewing on an external monitor.

If the user merely wishes to see the live image (ie a simple reproduction of the input video signal on the connector 10 or 12) he operates the mainselector switch on the front panel of the mixer to select live; this actuates a latch 58 by means of which the Y, U and V channels are multiplexed onto a digital bus 60 and are latched off at the far end by respective Y, U and V latches 62, 64 and 66.

The Y, U and V channels are then converted back into analogue form by a D/A converter 68. Output multipliers 70, 72 on the V and U channels add back into the signal the subcarrier which had previously been extracted along the line 40 by the decoder 30 and, following a delay 74 in the Y channel to bring all the signals back into line, the three channels are re11 combined in a summer 76 to give a resultant output video signal 78. It is at this stage that the sync pulse, generated by the sync pulse generator 16, is added back. The final video output signal would typically be supplied to an external monitor (not shown) and/or to a VTR (also not shown).

When the save push button on the front panel is pushed, the latches 54 are enabled and the latches 56 disabled, so that the digitised input video signal becomes stored in an addressed RAM generally indicated at 80. Data for the RAM is presented in three separate streams: the Y-channel at 12 MHz, and the U and V channels each at 6 MHz. The Y-channel information is de-multiplexed by the latches 54a, 54b and stored alternately in RAM locations 80a, 80b; the U-channel information is stored in RAM location 80c; and the Vchannel is stored in location 80d. Address generation is common to all of the RAMs, with each read/write cycle taking 160 ns.

A battery back up is provided for the RAMs 80, enabling the stored image to be retained even if the mains power fails.

When the user wishes to view the stored image, he places the mainselector switch on the front panel of the mixer in the store” position. This causes the stored information to be latched out of memory by a series of latches indicated generally at 82, following which it is multiplexed onto a databus 86 in a 24 MHz stream by a multiplexing latch 84. The information is then latched off the bus 86 by the latches 62, 64, 66 and are converted into an analogue video output signal 78 in the same way as has already been described for the live signal.

If the mainselector switch on the front panel of the mixer is set to match, the device is designed to produce an output signal 78 which alternates, frame by « frame, between the image stored in the RAM 80 and the 5 live image. How this is achieved will now be described.

Operation of the switch first enables the latches 82 to read the stored values out of memory, and then pass the information to subsequent parallel latches 88, 90. The first of these multiplexes the U and V channels and passes the resultant signal through a U/V gain PROM 92, while the second picks up the Y-signals and passes them through a Y-gain PROM 94. The PROMs 92,94 act to increase or decrease the strength of the' respective signals, and operate under control of the user-operable rotary gain-control knob 96 on the front panel of the mixer. By suitable timing of a multiplexing latch 100 the U/V and Y-signals are multiplexed at 24 MHz onto a bus 98. They are then supplied through a summing mixer 102, and a further latch 104 from where they can be demultiplexed again by the Y, U and V latches 62, 64, 66.

The gain control knob on the front panel operates a potentiometer, the output of which is converted by a suitable A/D converter (not shown) to a four-bit level signal. This is used to select one of 16 pre set gains programmed into the PROMs 92,94 and 110, 112.

When a single frame of the stored image has been passed to the output, the latches 56 operate to enable a single frame of the live image to be passed. The latches 54 and 56 are operated in conjunction with each other, so that when one set is enabled the other is disabled, and vice versa. The parallel latches 106, 108 operate in a manner similar to the latches 88, 90 to pass the U/V and Y signals respectively to a U/V gain PROM 110 and a Y gain PROM 112. These PROMs are operated in conjunction with the PROMs 92,94, previously described, under control of the front panel knob 96: the two pairs are complementary, so that as the gain is increased on one pair it is correspondingly reduced on the other.

The U/V and Y-signals are then passed respectively to a difference latch 114 and to a sum latch 116 (not used here), following which the signal is passed to the mixer 102 and via the latch 104 to the de-multiplexing latches 62,64,66.

In the '’wipe’’ position of the main selector switch, the latch timing is effected such that the live and stored signals are alternatively passed to the output on a line-by-line basis. The front panel sliding wiper controls alter the relative timings to effect an apparent movement of the live or of the stored image across the screen.

In the mix position of the front panel main selector switch, both the live image and the stored image are displayed at the same time. In this position, the latches 82 and the latches 56 operate more or less together, with the two resultant live and stored streams of data being mixed in the mixer 102. Depending on the position of the add/subtract switch on the front panel the difference and some latches 114, 116 are actuated to cause the mixer 102 to effect either additive or subtractive mixing.

Finally, there is a save push-button, by operating which the user can read the current live signal into the memory for future reference.

The front panel horizontal and vertical alignment controls act by adding horizontal and vertical offsets to the respective values before they are stored in RAM.

So that the video mixer can be operated with no input either from video connector 10 or 12 and still provide a usable output picture from the store, the sync pulse generator 16 can be allowed to free run. Since the sync pulse generator oscillator is a voltage controlled oscillator of the L-C variety, with no input the phase locked loop would naturally settle at one end of its lock range. This might be as much as five per cent away from the standard line frequency, and not necessarily acceptable to all monitors. To avoid this problem, the absence of syncs is detected by the sync pulse generator 16; as a result of this the clock generator defaults to a predetermined frequency ( 24 MHz) with generated syncs being locked to this.

As a result, the mixer of this embodiment is capable of displaying a stored picture even if there is no mains voltage supply, and no video syncs going into the system.

Claims (13)

CLAIMS:

1. A video mixer unit comprising an input for a colour video signal, decoder means for splitting the input signal into its Y, U and V components, A/D 5 conversion means for digitising the said Y, U and V components, memory means for selectively storing the digitised values, means for later reading the stored values, means for selectively combining the read values with digitised Y, U and V components of a current input 10 video signal to produce a composite digital output signal, and means for producing a composite analogue video output signal representative of the composite digital output signal.

2. A video mixer as claimed in Claim 1 including 15 automatic video standard detection means arranged to detect the type of video signal applied to input, and to process the signal accordingly.

3. A video mixer as claimed in Claim 1 or Claim 2 synchronised by clock means which are locked to the 20 line frequency of the input video signal.

4. A video mixer as claimed in Claim 3 in which the clock means are arranged to default to a predetermined frequency when no input video signal is provided.

5. A video mixer as claimed in any one of the 25 preceding claims in which the means for combining comprises a summer, the composite analogue output signal being representative of an image which corresponds to the summation of the images represented by the input video signal and the stored values.

6. A video mixer as claimed in Claim 5 including an inverter arranged to invert either the digitised Y, U 5 and/or V values or the read values before summation.

7. A video mixer as claimed in any one of the preceding claims in which the means for combining is arranged to produce a composite analogue output signal which alternates between a signal representative of the 10 video input signal and a signal representative of the stored values.

8. A video mixer as claimed in Claim 7 in which the composite analogue output signal alternates between a signal representative of one frame of an image coded 15 for by the input video signal and one frame of an image coded for by the stored values.

9. A video mixer as claimed in Claim 7 in which the composite analogue output signal alternates between a signal representative of at least one line of a frame 20 of an image coded for by the input video signal and at least one line of a frame of an image coded for by the stored values.

10. A video mixer as claimed in Claim 9 including means for adjusting the number of lines in the 25 composite output signal taken from the respective images.

11. A video mixer as claimed in any one of the preceding claims including switch means in a first position of which the composite analogue output signal is representative solely of the video input signal and 5 in a second position of which it is representative solely of the stored values.

12. A video mixer as claimed in any one of the preceding claims including user-operable gain controls for selectively increasing or decreasing the digitised θ Y, U and V components and/or the read values prior to combination.

13. A video mixer substantially as specifically described with reference to the drawing.