This invention is related to a method of and means for identifying a precise location on a video tape so as to facilitate editing of the tape.
In order to edit video tape it is necessary both to identify the location of desired program material and to preserve the synchronization of the edited material so that the program material is continuous and free of interruptions caused by momentary loss of synchronization. Several systems are used in professional recorders made for video tape that is 3/4-inch or more in width. However, most tape used in home video systems is 1/2-inch in width. Such tape may be either in the form of a cassette or it may be designed for reel-to-reel use. In either case, this video tape is normally sold with synchronizing bits magnetically encoded in a synchronization track. Two changes of magnetic flux, comprising together a single rectangular or square pulse, are recorded to serve as a synchronizing pulse for each frame of the picture that is being recorded. Each such frame represents a two-to-one interlace of the video raster, so that the frequency of the synchronizing pulses is 30 Hz for the NTSC system used in the United States and other countries on a 60-Hz power system and 25 Hz for systems designed for use with the PAL and SECAM systems that are used in Europe and other areas where the power frequency is 50 Hz.
The foregoing features are a part of the Video Home System (VHS) that is in common use for home video recording. In this system, synchronization of recording is achieved by deriving a pulse from differentiating the leading edge of the synchronizing pulse. The portion of the pulse after the leading edge is not normally involved in synchronizing the operation.
Editing of video tapes requires that the synchronization be maintained in order to avoid distracting jumps or other interruptions in the picture. The process of editing also requires that the editor have some measure of the location of areas on the tape at which to add material to the tape or delete it from the tape. The systems in common use are typified by the broad teachings of U.S. Pat. No. 4,167,028, entitled "Method And An Apparatus For Time Signal Encoding/Decoding," assigned to the assignee of the present invention. The '028 patent teaches, inter alia, pulse-width modulation of the synchronization pulses over an interval of one second, defined by thirty synchronizing pulses in a 60-Hz system to identify the particular interval in terms of hours, minutes and seconds from an arbitrary starting time Individual frames can than be identified within that interval by counting bits within the interval.
A system similar in some ways to that of the '028 patent was disclosed in my U.S. Pat. No. 4,703,311, entitled "Method And Apparatus For Transferring An Information Code Onto The Synchronization Track Of A Video Tape And A Video Tape Produced According To Said Method." The '311 patent encodes over the existing sync pulses. The system of the '311 patent is a single-frame retrieval and access program in digital code that has been sold under the registered trademark RAPID. This system encoded an eight-bit header or starting section, a data field of twenty bits to encode five BCD numbers, a user field of sixteen bits to contain four BCD numerals, and an end section of six bits. The total number of bits that were entered by extending the portion of the synchronizing pulse that followed the leading edge to effect pulse-width modulation of the synchronizing pulses was thus fifty bits, occupying two seconds in a 50-Hz system.
A system that is in use for editing VHS tapes is VASS. This is an acronym for VHS Address and Search System, which was described in IEEE Transactions on Consumer Electronics, Volume CE-33, Number 3, August 1987, at page 220. VASS uses pulse-width modulation of the synchronizing pulse by extending the trailing edge of the pulse to produce a wider pulse representing a binary "0" and shortening it to produce narrower pulse representing a binary "1." VASS places a specified eleven-bit header, four four-bit BCD numbers in the next sixteen bits, and a four-bit closing header. This pattern is repeated three times over an interval of about 3.1 seconds. Individual frames can then be determined by counting synchronizing pulses within the interval.
The VASS system has several disadvantages. One disadvantage is that the time interval that is identified by VASS coding is approximately 3.1 seconds in length. Locating a desired interval and then locating a particular desired frame within that interval can take an appreciable amount of time. It would be useful to have a shorter identifying interval so as to speed the location of a desired frame within the interval Another disadvantage of the VASS system is its encoding of only four BCD numbers, thus spanning the range from 0000 to 9999. This is a time span of about 81/2 hours. It would be easier for an editor to select frames by using an identifier that is related to seconds, minutes and hours, preferably over a range of twenty-four hours.
The header of the VASS system is a binary "0" followed by nine binary "1s", then a binary "0." It would be useful to have an improved system that uses the same header so as to be compatible with the VASS system.