1. Field of the Invention
The present invention relates to a digital video processor, and more particularly to a secondary digital color processor (DCP) which is usable with modern standard-definition digital telecines and is also compatible with high-definition television systems, for correcting video color and for other purposes.
2. Background Art
Telecines and external secondary color correctors (ESCC's) are used for the production of high-quality television pictures in a post-production television studio environment. The highest quality pictures normally originate from motion picture film, normally 35 mm film. These pictures are transcribed to video on a telecine, such as the Rank Cintel MkIII, the Rank Cintel URSA, or the BTS FDL90. Telecine machines convert the picture to a digital format and contain processing elements to enable simple manipulation of the color reproduction of the resulting images.
This sort of color manipulation may also be performed by an external analog system such as the RCA Chromacomp or its derivatives. This simple RCA-type color processing has the limited capability of, for example, intensifying the red component, over the entire picture, in all colors that have a red component. However, it is incapable of, for example, altering only the dark reds and leaving the light reds unchanged.
Over the years, improved ESCC's such as the PRISM from Encore Video Industries, Inc., of California, have emerged. The PRISM is an analog device which is capable of more selective color adjustment than the RCA Chromacomp. Other analog ESCC's are the Da Vinci, and the "Sunburst" made by Corporate Communications Consultants.
Color correctors have also been used for many years for tape-to-tape correction. The output of one videotape recorder (VTR) is processed by the ESCC, whose output is then recorded by another VTR.
ESCC's were traditionally analog, but now some color correctors are available that are partly or fully digital. The output of most modern telecines is digital.
Analog color correctors have several problems. Since a logical way to interconnect the telecine to the color corrector is to connect the output of the telecine to the input of the color corrector, it has normally been necessary to convert the digital signal to analog, apply the color correction, and then convert the corrected signal back to digital for storage or further processing. The conversion from digital to analog and back again causes degradation, and is therefore undesirable.
This phenomenon is most easily visible when one feeds a color signal through a color corrector with all of the controls set to zero (which in theory, should not change the colors). When one compares the output of the color corrector with a signal which bypasses the color corrector, quite often, there will be a difference between these two signals, which will become apparent when the two images are viewed "split-screen".
One reason for this discrepancy is that the signals in the electronic signal path (particularly in analog) are unintentionally modified due to the imperfections of real electronic circuits.
Another reason is that since the color corrector is an analog device, it will suffer from drift, which is inevitable in analog devices. Drift originates from many sources, one of which is temperature.
A third reason for the visible differences is noise. This noise may be visible as a change in level (and therefore color) or a difference in texture of a given flat color bar.
Even if, as has occasionally been done, the signals are processed in a totally digital domain, there have been degradations.
One reason may be digital "rounding", such that a color passing through a color corrector unintentionally becomes slightly modified, compared with the input.
Further, in known color correction systems (particularly digital systems), discontinuities occur. This happens where colors below a given limit are to remain unmodified, but colors above that limit are to be modified. If the picture to be modified contains an increasing ramp of that color, then the portion of the picture around the limit will show an unnatural discontinuity when it is color-corrected.
The known color-correction devices will be discussed further below, in the context of the following discussion of their use with telecine devices.
Referring first to FIG. 1, there is seen a schematic block diagram of the most basic type of stand-alone telecine 20, which is usable to broadcast a film directly over the air in real time. A film transport 21 runs a film 22 past a CRT scanner 24, which incorporates photomultipliers for generating red, green, and blue component signals designated R.sub.v, G.sub.v, and B.sub.v. Gain, lift and gamma of the three signals are adjusted by a processor 26, and color is adjusted by a processor 28. Simple overall picture adjustments such as lightening and darkening are provided by a local control panel 30. Adjustments are performed live, as the film is broadcast. The local control panel 30 also contains controls such as a START control for the film transport and an ON control for the CRT scanner. A disadvantage of this basic system having only the foregoing components is that, in order to provide interlace for producing a conventional broadcast signal, it is necessary for the lines in each film frame to be scanned non-sequentially, that is, lines 1, 3, 5, . . . , followed by lines 2, 4, 6 . . .
An improvement upon the foregoing basic system, developed in the late 1970's, is the addition of a store 32, also shown in FIG. 1. The store improves the scanning process, by permitting the lines of each frame to be scanned sequentially, and then read out of the store with interlace. This feature was patented by Rank Cintel in British patents 1,542,213 and 1,535,563, and equivalent U.S. Pat. Nos. 4,184,177 and 4,127,869, respectively.
An improvement shown in FIG. 2 is a controller/programmer 30', which as shown is an accessory for the telecine 20', although it could conceivably be built into the telecine 20'. The controller/programmer 30' provides a control panel for the processors 26, 28, and the film transport 21. It further includes a programming function, whereby an operator can rehearse, slowly, the optimum grading and picture adjustments for each scene in the film, which the programmer stores. A key feature is that the programmer is driven by time code, or by film footage, so it can tell one scene from another by the time code or by the position of the film.
One example of a controller/programmer is the POGLE telecine controller/programmer manufactured by Pandora International Ltd., which is designed for use with a range of telecine machines. It is capable of providing a large number of control signals simultaneously, for example 96 control channels, either analog (a voltage within a predeterminable range) or digital. The channels can be used to control a telecine and/or other peripheral devices, such as noise reducers, VTR's, still stores, mixers, and color correctors (such as the DCP disclosed herein).
Another example of a controller/programmer is the "Rainbow" system of Corporate Communications Consultants.
A further improvement, shown in FIG. 3, is the external secondary color corrector (ESCC) 34. Examples are the RCA Chromacomp, the PRISM, and the Da Vinci. ESCC's can be either digital or analog. Instead of simple RGB control, which only gives the ability to add or subtract red, green, and blue, everywhere in the picture, this advanced generation of ESCC's provides 6-or-more-channel color control. That is, for example, 6 separate colors can be selected and then modified, without modifying any other colors. The ESCC 34 is controlled by the controller/programmer 30".
The ESCC 34 takes its inputs from the color processor 28 in the telecine 20", and provides its outputs to the store 32. The reason for this signal path is that the store is usually capable of handling lower bandwidth (i.e., fewer bits or lower resolution) than the signals from the processor 28, so the number of bits is reduced, for example, by rounding, before the signals are stored. For example, the URSA's output is medium-bandwidth "4:2:2" color (D-1 format). Thus, taking the input to the ESCC 34 from the output of the processor 28 allows the ESCC to operate on a higher-bandwidth signal than if it operated on the output from the store.
The trend is for telecines to be made digital and to incorporate improved color correction facilities. For example, one advanced digital telecine, the Rank Cintel URSA telecine, has a built-in digital implementation of a 6-vector RCA-type processor, in place of the analog processor 28 shown in FIGS. 1-3. It would be desirable to provide an ESCC which is capable of processing the digital output from the internal digital color processor 28 in the URSA (or the like), having greater capacity and flexibility than that RCA-type color corrector.
A further trend is to increase the bandwidth of the store 32, which will make it practical for the digital color processor to perform its functions on the digital output of the store, making it unnecessary to connect the ESCC to any internal circuits of the telecine.
In either case, an ESCC is needed which will not noticeably reduce the bandwidth of the digital color signal or degrade the picture.
The disclosures of all prior art publications mentioned herein are expressly incorporated by reference.