Ampex Corporation

Maurice G Lemoine

Leonard A Pasdera

In a digital tape recording system for video and sound signals, in which information in each track is stored in plural sectors having synchronizing preambles that aid in synchronizing the playback of the information, special preambles containing a frequency component that is lower than normal are used for the first sector of the first track in each video frame or field. The normal and special preambles are produced by passing the binary code'. . . 01010101 . . . ' (in the case of all but the first sector of each frame or field) or '. . . 011011011011 . . . ' (in the case of the first sector) through a Miller Channel Code encoder to produce, in the first case, a synchronizing square wave fluctuating at one-fourth the bit presentation frequency, and in the second case, a synchronizing square wave fluctuating at one-sixth the bit presentation frequency. Special resonant circuits associated with the playback circuitry detect the special lower frequency component and signal the precise start of a new frame or field. This signal is used for capstan synchronization, head positioning during playback, and head positioning during editing when the record heads are used as playback heads so that they may be properly positioned to overwrite individual sectors on a track without overwriting other sectors.

Synchronization of record media transports and tracking adjustment

Errors in data words recovered following the transmission of digital data having internal correlation are concealed by substituting recovered correlated words for recovered words suspected of being in error. Concealment may follow the correction of errors according to an error correcting code. Error flag signals are used to identify corresponding respective recovered words that are suspected of being in error. These error flag signals are counted over a predetermined totaling interval, and a concealment interval signal is generated for a concealment interval when the count exceeds a predetermined threshold count. Recovered correlated words are substituted for recovered words corresponding to respective error flags and for all other recovered words corresponding to a respective concealment interval. In accordance with one aspect of the invention, the concealment interval is centered upon the corresponding totaling interval. According to another aspect of the invention, the concealment interval signal is generated upon the occurrence of an error flag signal while the count exceeds the predetermined count. Also, the concealment interval signal is preferably restarted upon the occurrence of a subsequent error flag signal while the count exceeds the predetermined count.

Digital signal error concealment

A deskew processor includes first and second memory units each having independent data write control circuits and data read control circuits. The write control loads successive data into respective memory units as it is received, and the read control reads the data out of the memory units into a data selector circuit as determined by a data selector control circuit. The latter determines the presence of any data overlap and delays read out of data via the data selector circuit until the previous data is completely read, thereby deskewing the successive data.

Apparatus for deskewing successively occurring blocks of data

A method and apparatus is disclosed for recording and reproducing color television signals on a magnetic media such as magnetic tape, wherein the signals recorded on and reproduced from the magnetic media are in a digital domain. The analog color television signal is converted to digital signals in a manner whereby the horizontal blanking interval is compressed and digital synchronizing information is inserted in the compressed horizontal blanking interval for every line. Upon reproduction, the digital synchronizing information is extracted from the reproduced information and is provided to servo circuits for controlling the relative transport of the magnetic tape and operatively associated magnetic heads to effect synchronous reproduction of the recorded information relative to a reference signal.

Method of controlling apparatus for recording and/or reproducing on a

A method and apparatus is disclosed for recording and reproducing color television signals on a magnetic media such as magnetic tape, wherein the signals recorded on and reproduced from the magnetic media are in a digital domain. The apparatus utilizes a rotating head wheel having a plurality of transducing heads spaced around its circumference which operate to record and reproduce the digital data transmitted through two separate channels at a very high data rate. The analog color television signal is converted to digital signals in a manner whereby the horizontal blanking interval is compressed and digital synchronizing information is inserted in the compressed horizontal blanking interval for every line. The portion of the real time horizontal blanking interval for which recording is not performed enables time base correction to be accomplished on an individual line by line basis. The digital television signal is processed through memory during recording to enable recording of the data on two channels at a slower than real time rate. During reproducing, the data is again processed through memory and is converted back to real time rate and then converted to an analog signal to which typical synchronizing signals are added.

Method and apparatus for digital video signal processing

Apparatus is disclosed for acquiring and maintaining a generally continuous clock signal from a very high frequency encoded data stream of the type which is self-clocking and has logic state transitions occurring at two locations within each bit cell of the data stream. The apparatus is particularly useful in decoding apparatus, such as may be used in a magnetic recording and reproducing system. The apparatus effectively generates the clock signal from the encoded data stream and reclocks the data stream utilizing the generated clock signal to accurately position the logic signal transitions within the bit cells and thereby assure accurate decoding of the data. The apparatus processes two simultaneously transmitted synchronous data streams, and has the capability that permits the generated clock signal from one data stream processed in one channel to be used to maintain synchronization of the other processing channel notwithstanding a loss therefrom of the data stream for short periods of time.

High frequency digital PCM decoding apparatus

A method and apparatus for detecting errors in the accuracy of multi-bit data words, i.e., a parity method and apparatus, is disclosed. The invention is intended for use in determining the accuracy of multi-bit data words that are being transmitted through a communication channel or are being recorded and reproduced using magnetic recording or other technique. One embodiment of the invention involves examining at least three significant bits in at least three successive words and generating parity bits with predetermined logical states determined by the content of the examined significant bits and combining one of the parity bits with each of the data words for transmission or recording and subsequently examining the data words and parity bits upon receipt and generating an error signal for the data words when one of the parity bits combined with the examined data words has a logical state other than that that would have been transmitted or recorded if determined by the original content of the received data words.

Parity checking circuitry for use in multi-bit cell PCM recording and

Apparatus is disclosed for providing compensation for missing, incorrect or otherwise defective data in a data stream preferably comprised of video information. The apparatus is particularly useful in a magnetic recording and reproducing system for digital video data signals. The apparatus effectively stores the digital video data that occurred 262 video lines prior to the current data at the input and continuously updates, (i.e., stores in place of) the stored data with current data unless defects occur in the current data, in which case the defective current data is not stored. The data from the store is inserted in the video data streams when the drop-outs or parity information associated with the data stream indicates that the current data is defective. By inserting digital data which represents data that occurred 262 horizontal lines previously, the content of the substituted video information has a high probability of being visually equivalent to that of the original current data, since in most cases, it represents data from an adjacent horizontal line in a prior television field. The apparatus inhibits writing of data during substantial portions of the horizontal and vertical blanking intervals and thereby reduces the capacity of the storing means which is required to provide compensation for essentially a full field of active video information.

Apparatus for providing drop-out compensation in recording and

A method and apparatus is disclosed for controlling the sampling of a composite analog color video signal so that the analog signal is sampled at precise locations relative to the phase of the color subcarrier of the color video signal in the absence of modulation. The invention thereby enables a digitized signal having a constant phase relationship relative to the unmodulated subcarrier phase to be provided which can be advantageously used in magnetic recording and reproducing apparatus. Since the constancy of the location of the samples relative to the phase of the color burst is maintained prior to recording, there is no phase error that would pose a problem during reproducing of the signal. The apparatus stores a set of reference samples of a selected number of cycles of the color burst interval of the color video signal and generates an output clock signal that is phase synchronized with the reference samples, subsequently stores sets of comparison samples of color burst cycles from subsequent horizontal video lines using the generated clock signal and detects any error in the phase location thereof. The apparatus adjusts the phase of the generated output clock signal in response to any detected error signal and stores a new set of reference samples in the event that the error detected is beyond predetermined limits.

Fast acting phase shifting apparatus for use in digital sampling systems

Timing errors in a color television signal equal to a fraction of the nominal period of one cycle of color burst are corrected by clocking an analog-to-digital converter during each horizontal line interval of the television signal with two clock signals having the same nominal frequency equal to a multiple of that of the color burst. During the color burst interval of each horizontal line, the analog-to-digital converter is clocked by a stable time base reference clock signal and the obtained digitized color burst is stored in a recyclable digital memory. Following the color burst interval, the stored digitized color burst is regenerated for the remainder of the horizontal line and a clock signal derived for use in clocking the analog-to-digital converter. The digitized television signal provided by the analog-to-digital converter is written into a clock isolator at times determined by the clock signal derived from the digitized color burst and, thereafter, read from the clock isolator at times determined by the reference clock signal. Timing errors exceeding the nominal period of one color burst cycle are corrected by writing the digitized television signal from the clock isolator into a following random access memory and incrementally adjusting the memory' s read time in accordance with such errors measured in steps equal to the period of one color burst cycle. The stable time base reference clock signal is coupled to control the signal processing following passage through the clock isolator.

Time base compensator

A time displacement adjustment device compensates time base errors that progressively cumulate in a time varying signal having a synchronizing information component from which the signal time base can be determined. Periodically, the synchronizing component is sampled and a representation thereof continuously generated with the component's timing relationship to the signal retained. Each continuously generated representation and the succeeding sample of the synchronizing component are compared to determine the time base error that progressively occurs in the signal between the successive samples of the synchronizing component. That signal is expanded or compressed accordingly to correct the erroneous timing.

Timing error compensator

In last line velocity compensation of color television signals reproduced by video recorders, the velocity error signal for the next to last line of information before the head switch that occurs during the vertical blanking interval is stored for a time extending beyond the period of the last line. The switching between transducers that occurs during each vertical blanking interval is delayed so that a color burst signal following the last line is reproduced and a velocity error for the last line generated. This last line velocity error signal is differentially compared with the stored next to last line velocity error to provide a velocity error difference signal. The error difference signal is stored for the period between the foregoing and succeeding vertical blanking intervals. The stored velocity error difference signal is added to the velocity error of each of the next to last lines of video information reproduced before switching between transducers at times between such vertical blanking intervals to provide last line velocity error compensation for those intervening last line.

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Redwood City, CA, United States of America

Maurice G Lemoine