1. Field of the Invention
The subject invention applies to the provision and display of video information and has particular application to apparatus for displaying recorded video programs, although not being limited thereto.
2. Description of the Prior Art
The detrimental effect of flutter on the display of video signals is well known, as is the fact that video information recording and playback equipment is a typical, although by no means exclusive, source of flutter.
The thrust of prior-art endeavor has been to improve the equipment responsible for the flutter. For instance, design and precision of video recording and playback system, including video tape recorders, have been continuously improved toward a reduction of flutter to tolerable proportions. This has neither been consistently successful nor has it helped in realistically bringing the equipment in question within the reach of the public or of educational and other institutions on a large scale. Rather, the requisite improvements for an implementation of prior-art proposals in this area have greatly increased the complexity and expense of manufacture and design of the apparatus in question.
An aggrevating factor in this area has been the tendency of conventional television display sets to amplify the effect of flutter in received signals. The source of this problem has been traced to a phase lock loop or electronic flywheel circuit present in most sets for stabilizing synchronizing signals against noise above a certain frequency. In general, prior-art stabilization circuits of this type have the advantage of permitting the synchronization drive to adjust to synchronization frequency differences when switching stations or to drift occurring in any station, while at the same time rendering the synchronization drive relatively immune to frequently occurring noise disturbances.
However, it will now be shown that these otherwise beneficial stabilization circuits amplify the effect of flutter in certain instances. Ideally, the horizontal position of a picture element in a video display is related to the phasing of the corresponding horizontal sync pulse. In practice, this is no longer the case if the horizontal sync has been stabilized by the stabilizing circuit of the set to a relatively steady time averaged value.
For instance, the line rate as delivered by a video tape recorder in the playback mode will have a flutter component. If .nu..sub.o is the nominal horizontal sync frequency and A is the peak amplitude of a flutter component of frequency .apprch., then the delivered line rate .nu. will be EQU .nu.=.nu..sub.o [1+A sin 2.pi..psi.t] (1)
The horizontal position of a picture element ideally will be at some fixed phase with respect to the beginning of the particular line. If .phi..sub.h is the absolute phasing of the line beginning, we have ##EQU1## so that .phi..sub.h is EQU .phi..sub.h =2.pi..intg..nu..sub.o [1+A sin 2.pi..psi.t]dt (3)
Since we are interested in the excursion of the variational component due to flutter and the peak amplitude of the excursion, the integral is taken between EQU t =0 and t =1/4.psi. (4)
variational component is ##EQU2## or EQU .phi..sub.h =.nu..sub.o A/.psi. (6)
as the amplitude excursion of the screen element when the sync is stabilized in the television display set.
Since the horizontal sweep is linear with time, this excursion expressed as a fraction of screen width is ##EQU3##
wherein .GAMMA. is the peak amplitude of the excursion of a picture element and A is the peak amplitude of the flutter component whose frequency is .psi..
As to the effect of flutter on the image display, we are concerned with the ratio ##EQU4##
For a nominal horizontal sync frequency of .nu..sub.o =15, 750 Hz and a flutter component frequency of .psi.=100 Hz, we obtain a ratio .GAMMA./A of ##EQU5##
Video recording and playback machines in which all flutter components remain below 0.1% or A A.ltoreq. 0.001 are expensive and complex. But even at that low value, the effects of flutter due to the above mentioned stabilizing action is still very objectionable.
For instance, if we take the above example of equation (9), we find that the picture element would execute excursions with an amplitude of EQU 25.sup.. 10.sup..sup.-3 =0.025 (10)
as a fraction of screen width.
For a screen width of about 20 inches (508 millimeters), for instance, the excursion amplitude of picture elements would be about 0.5 inches (12.7 millimeters) or 1 inch (25.4 millimeters) peak-to-peak.
Such a deviation is intolerable on its face. To reduce such excursions to, say, 0.04 inches or about one millimeter under the permises of our above example, it would be necessary to reduce the flutter of the video recording and playback machine to about 4.10.sup..sup.-5 or 0.004%!
The general equation applying to these cases is ##EQU6## wherein: A is the peak amplitude of flutter;
.OMEGA. is the peak-to-peak excursion of the picture element on the display screen; PA1 .nu..sub.o is the nominal horizontal sync frequency; and PA1 .lambda. is the screen width.
The resulting precision requirements are far too stringent in most practical applications.
In their quest for a workable solution, proposals have been made which necessitate an invasion and modification of the existing circuitry of the television display set. With respect to such proposals, most experts in the field agree that systems which are inherently incapable of operation without invasion and modification of the existing receiver circuitry have practically no chance of widespread acceptance among television receiver owners.