1. Field of the Invention
This invention addresses the synchronization of alternative sources of video to a video displays particularly a television or computer monitor. Display devices must be capable of displaying a variety of video formats, and of displaying multiple video images simultaneously.
The use of a Phase Lock Loop having a slowly changeable frequency and phase is shown to be effective for the synchronization of differing and simultaneous video images to a display device.
2. Discussion of the Prior Art
Sources of video information include conventional television signals, such as NTSC and PAL; advanced digital televisions such as ATV and HDTV; computer graphics and animations; and others. Each of these sources have different characteristics, such as the number of frames per second to be displayed. A display device designed to accommodate each of these formats would typically contain some means, such as a phase locked loop, to synchronize the display to the source signal. To be effective, this synchronization must address two aspects of the source signal: its frequency and its phase. The frequency must be synchronized to maintain the proper display rate. The phase must be synchronized to assure the proper correspondence between the source image and displayed image; for example, to assure that the top of each source scene is displayed at the top of the display device, or at the top of its associated display area within the overall display device area.
Although two signals from sources which conform to a particular standard can be expected to have the same nominal frequency, they may well be asynchronous to each other with regard to phase. Thus if the display is synchronous with a first source, it will not necessarily be sychronous to a second source. The synchronization of the second source to the display is typically accomplished by delaying or buffering the second source to compensate for any phase differences. The effects of the asynchronous phase among alternative sources becomes apparent whenever the source of the signal is changed, and the display circuitry attempts to synchronize the display to the new source. This resynchronizing is, for example, the cause of the momentary loss of signal, or "jump" of the image, when a viewer changes channels on a conventional television receiver. Although most viewers may have become acclimated to this disruption during channel changing, the disruption is particularly troublesome for less conspicuous channel changes. For example, consider a viewer of a television with Picture-In-Picture (PIP) who chooses to display the PIP as the main image. To the viewer, this is a mere enlargement; but, in reality, it is similar to changing the channel, because, in a PIP television, the display is synchronized to the source of the main image. When a viewer chooses the PIP image as the main image, the display must be resynchronized to this new source signal, with the corresponding momentary loss of signal, or image jump, caused by an abrupt change of phase.
Two signals from sources conforming to different standards can be expected to differ in both frequency and phase. A display device must accommodate for each, and, must allow for both to be viewed simultaneously. In the above PIP scenario, the formed sub-image must be both frequency and phase synchronized to the main image. Typically, this is accomplished by providing a memory, or buffer for adjusting the rate and phase of the received sub-image relative to the display. The buffer is written to as it is received, and the information in the buffer is extracted in sync with the display. If the display frequency (read clock) is greater than the source frequency (write clock), the buffer will be read faster than it is written to. At some point there will be no new data available to be displayed. Typically, the system would be designed to redisplay the prior image, resulting in periodic pauses in the displayed motion. If the display frequency is less than the source frequency, the buffer will be written to faster than it is displayed. Assuming a finite buffer size, eventually the buffer will become full. Typically, the system would be designed to periodically skip an image for display, resulting in periodic jumps in the displayed motion. For example, if the source frequency is 70 Hz and the display frequency is 60 Hz, the system would be required to skip the display of every seventh frame, to equalize the source and display rates. This repetitive skipping or repeating of a frame is commonly termed "judder".
As noted above, most systems are designed to adjust the display frequency and phase to match the frequency and phase of the source of the primary video image. In this way, the primary image is free of the judder caused by a difference between the display rate and the primary image frame rate. Thus, when the source of the primary signal is changed, the display must be resynchronized to the new source.
One would expect a display device to be able to accommodate a change of signal source quickly. It would be unacceptable, for example, for a PIP television to lose the signal for a noticeable period of time, while the circuitry resynchronizes to a changed frequency or phase. Thus, most televisions and other monitors employ means for rapidly resynchronizing to a changed source, typically by the use of Phase Locked Loops (PLLs) which can rapidly lock to a changed signal, as discussed in U.S. Pat. No. 5,473,385. Traditionally, without the use of a fast PLL, a change of channel on a television would result in visual disturbances in the new image, such as "rolling" and "tearing" until phase and frequency synchronization is established. Conversely, using a fast PLL causes a momentary, but highly noticable, loss, or jump, of the video image, as a natural consequence to the fast reaction of the circuitry to this changed video source. As stated in the aforementioned '385 patent. "the interference is not objectionable because the viewer expects the brief disturbance during turn on and channel changes."
The change of source of video images with become increasingly common, even if the viewer does not explicitly change the channel or expand a PIP. Under the newly adopted ATV (Advanced Television, including conventional TV and HDTV) standards, providers of services will have the ability to "splice", for example, commercials specifically targeted for a particular viewing area. Thus, the "source" of a video image on a single television channel may, in fact, be a composite of multiple sources, each at their own frequency and phase. Although the aforementioned standards include provisions for "seemless splicing", a poor splice will result in the jumps typically associated with a change of channel.
Further compounding the problem, some "displays" have inertial elements which preclude their rapid resynchronization. Some projection display systems incorporate color wheels which spin in synchronization with the color decoding process, as presented in EP 0710016-A2. White light is projected to and through a portion of this wheel, synchronized to the color which is to be projected at that particular time. A change of the source of the video necessitates a resynchronization to the new source. During this resynchronization period, improper colors will be displayed unless means are provided to synchronize the new source to the spinning wheel quickly. Traditionally, this is accomplished by maintaining a constant spin rate, and buffering the source, as discussed above with regard to a constant display rate. This fixed display rate solution, however, has the deficiencies discussed above when the source rates may differ, or when the aforementioned ATV composite signal is improperly spliced. As discussed in the aforementioned EP '016 patent, this constant display rate may be selectable by the user, or may be derived from knowledge of the format of the primary source. The selection of a new display rate will cause the visual artifacts discussed above with regard to a selection of a new image source.
Computer displays also require synchronization to multiple signal sources. As typified by U.S. Pat. No. 5,155,595, it is common to place one video image within another on a computer screen, similar to PIP television. In U.S. Pat. No. 5,155,595, a phase locked loop is used to synthesize a clock which synchronizes the horizontal sync frequency of the smaller image to that of the larger image. A separate phase locked loop circuit is used to synchronize the vertical sync frequencies. If the source of the larger image changes frequency, display activity is paused until the system reestablishes synchronization. If the vertical signals are out of phase, a reset signal is asserted to reinitialize both the larger and smaller images' vertical line controls, thereby forcing a vertical resynchronization. The horizontal sync signal, being significantly faster than the vertical sync signal, is utilized to establish synchronization so that such synchronization can be accomplished quickly, minimizing the time the screen is frozen. Once horizontal synchronization is established, a reset signal is utilized in order to minimize the time required to reestablish vertical synchronization. That is, a change of the source of an image on a typical computer display results in a momentary `freeze` of the image until synchronization is reestablished, followed by a jump of the image to its resynchronized state Although this phenomenon may be acceptable for text and graphics, it is likely to be found unacceptable for full motion video and multimedia applications.
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