Synchronization and automatic resynchronization of multiple incremental recorders
The United States Government has rights in this invention pursuant to U.S. Contract No. F19628-85-COO53 awarded by the Defense Logistics Agency of the Department of Defense. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of dynamic information storage and retrieval, and in particular to a method and apparatus for synchronizing and resynchronizing multiple incremental digital cassette recorder (DCRSi) units. 
     2. Description of Related Art 
     Incremental recorders are information storage devices which buffer the variable user data rate from the internal fixed high-speed-read-write rate of the recorder. An example of an incremental recorder is the digital magnetic recording tape cassette (DCRSi) unit sold by Ampex Corporation. 
     Synchronization of multiple DCRSi units, or other incremental recording units, permits the user to achieve a high bit rate recording device that appears to the system as a single device. Data received from the host system is disassembled by the synchronization controller and buffered by the individual recorder units for simultanous recording. Retrieved data is reassembled and sent to the host system in its original format. Such synchronization results in recording capabilities beyond what is available for a single machine. 
     A disadvantage with using multiple DCRSi and other recording units is that, when one or more of the units loses synchronization during playback, operator intervention is required to attain resynchronization of the recorders. At present, no system is known for achieving synchronization and automatic resynchronization of multiple DCRSi or other incremental recording units. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the invention to provide a system capable of synchronization and automatic resynchronization of multiple DCRSi units. 
     This objective is achieved by injecting a frame sync word at periodic intervals simultaneously into each unit during recording, and monitoring retrieved data for the frame sync words during playback. If synchronization is lost, each unit is automatically adjusted to achieve resynchronization. 
     A frame sync decoder is provided for each recording unit in order to monitor the data stream for the occurrence of the frame sync word in each of the DCRSi channels, after which a frame sync detect sequencer inputs the decoded frame sync words and outputs individual frame sync gates for each of the DCRSi channels to a synchronization monitor. 
     The synchronization monitor monitors the occurrence of the frame sync gates on each operating channel and adjusts the synchronization of the data if out of alignment. The adjustment is made by controlling the DCRSi units to add additional reads to the channels that slip behind. Resynchronization of the input data is therefore automatic upon the loss of frame sync. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a multiple incremental digital cassette recorder synchronization and automatic resynchronization system constructed in accordance with the principles of a preferred embodiment of the invention. 
     FIG. 2 is a schematic circuit diagram showing in detail a portion of the sync monitor/resynchronizer illustrated in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The synchronization and automatic resynchronization system of the preferred embodiment is suitable for use with an incremental digital cassette recorder such as the AMPEX DCRSi recorder. The system shown in FIG. 1 includes three DCRSi unit input/output buffer control circuits DCRSi X, DCRSi Y, and DCRSi Z, a record multiplexer 1, a playback multiplexer 2, and synchronization monitor/resynchronizer 3. The input/output circuits represent those typically found on AMPEX type DCRSi units. However, it will be appreciated by those skilled in the art that the synchronization and resynchronization techniques of the preferred embodiment are applicable to recording systems other than the illustrated system. 
     For example, incremental recording devices other than digital magnetic tape cassette recorders may easily be adapted for use with the preferred system. Also, the preferred synchronization/resynchronization system is adaptable for use with systems containing more or less than three recorder units. For those systems which are not incremental, a large enough input data rate buffer must be designed to isolate the output data from the recorder electronics during start/stop operation, but such systems are nevertheless intended to be included within the scope of the invention because the design of such a buffer is well within the capability of one skilled in the art. 
     In the illustrated embodiment, data to be recorded is received in 32-bit format from a host system computer by record multiplexer 1, which multiplexes 32-bit data received from the computer to 24-bits. The 24-bit data, in the form of eight bit signals XIN, YIN, and ZIN, is output from multiplexer chip 4 via registers 5-7 to respective DATA IN terminals of the three eight bit input/output circuits DCRSi X, DCRSi Y, and DCRSi Z. The data is output in the exemplary embodiment at 13 MHz with a 96.2% duty cycle (100 Mbps), although the data format and rate may be varied within the scope of the invention, depending on both the host system&#39;s and the recording system&#39;s input/output configurations. 
     Input of the data from registers 5-7 is synchronized by clock signals generated by sync monitor/resynchronizer 3, described in detail below, and input to the DATA IN terminals of the DCRSi input/output circuits. The frame sync words are inserted into the output of multiplexer chip 4 at periodic intervals and recorded by the DCRSi units. In the exemplary system, the frame sync word contains three bytes, although any suitable frame sync word format may be used. However, the interval between frame sync word insertions should be at least half the depth of the playback multiplexer&#39;s FIFO memories, as will become apparent from the following description. 
     Playback multiplexer 2 includes three First-In-First-Out memories (FIFOs) 8-10, one for each recorder. FIFOs 8-10 correct for skew between the reproduce clocks (REPRO) of the respective recorder input/output circuits DCRSI X, Y, and Z. Data XOUT, YOUT, and ZOUT is written into each FIFO in response to signals XREPRO, YREPRO, and ZREPRO from a reproduce clock of the associated recorder. When a FIFO is half full, a signal XHF, YHF, or ZHF is sent from an HF terminal of a respective FIFO to the sequencer 11, which generates signals RDX, RDY, and RDZ at fixed, predetermined intervals, thus ensuring that the reads are in sync so long as the data output by the DCRSi units does not get ahead or behind by more than one half of a respective FIFO. Data is therefore read from the FIFOs under the control of FIFO read sequencer 11 which outputs the data to a 24-bit to 32-bit multiplexer 15 at a rate of 300 Mbps. The 24-bit to 32-bit multiplexer 15 in turn outputs 32-bit data to the host system computer. 
     The data output by FIFOs 8-10 under the control of sequencer 11 is also read by a plurality of frame sync decoders 12-14, one for each recorder. Decoders 12-14 monitor the data output by the FIFOs for the occurrence of the frame sync word in each of the recorder channels. The frame sync detect sequencer 16 inputs decoded frame sync words FRX, FRY, and FRZ from decoders 12-14 and outputs individual frame sync gates FRAMEX, FRAMEY, and FRAMEZ for each of the channels to the synchronization monitor 3. In addition, sequencer 16 generates an in-synchronization signal (INSYNC) which is asserted once the frame syncs have occurred for two or more consecutive intervals, as counted by counter 17. Counter 17 outputs a terminal count signal, TERMCNT, each time the frame sync word insertion interval has been reached. 
     Synchronization monitor 3 monitors the occurrence of frame sync gates FRAMEX, FRAMEY, and FRAMEZ on each operating channel and adjusts the synchronization of the data if the frame sync in the channels is out of alignment. Adjustment is made by adding additional reads to the channels that slip behind. Resynchronization of the input data is automatic upon the loss of frame sync. 
     A block diagram of the synchronization monitor/resynchronizer circuit 19 which accomplishes the automatic adjustment is shown in FIG. 2. A READDCRSI gate output by playback multiplexer read sequencer 11 is input to read gate controller 18 (FIG. 1) to request data to be read simultaneously when sequencer 11 receives half full signals XHF, YHF, and ZHF, unasserted (not half full) from each of respective FIFOs 8-10. DCRSi read gate controller 18 inputs the READDCRSI gate, and in response outputs data ready gates GATEX, GATEY, and GATEZ and clock signal CLK to each recorder unit input/output control circuit. 
     The frame sync monitor and resynchronizer circuit 19 includes an input register which generates a signal INFRAMESYNC in response to receipt of the three frame sync gates FRAMEX, FRAMEY, and FRAMEZ and signal INSYNC. Offset sequencer 21 is activated by the loss of the in sync signal INSYNC, which causes the INFRAMESYNC signal to be disabled. Offset sequencer 21 includes three input terminals for frame sync gate received signals XRCVD, YRCVD and ZRCVD, only two of three are present at the terminal depending on the last frame to be received, as determined by the last frame programmable array logic (PAL) circuit 38, described in more detail below. Offset sequencer 21 also includes channel enabling terminals ENDCRSiX, ENDCRSiY, and ENDCRSiZ, and power reset terminal PWRRST. The output of offset sequencer 21 includes output enable signals OEXY, OEXZ, and OEYZ, which enable the output of 8-bit difference counters 22-24, and counter load signals LDX, LDY, and LDZ, which cause offset counters 25-27 to load differences between the respective frame sync words for each channel via offset bus 39. 
     Difference counters 22-24 operate by incrementing a sum based on the presence of enable signals ENXY, ENXZ, and ENYZ, which are generated in response to differences between the respective XFRAME, YFRAME, and ZFRAME signals. Exclusive NOR gates 28-30 cause counters 22-24 to input and count a difference signal whenever only one of the two respective frame sync signals XFRAME, YFRAME, or ZFRAME are present at an input, indicating a lack of synchronization. Each time there is a difference, flip-flops 31-33 output a signal ENXY, ENXZ, and ENYZ to a respective difference counter 22-24 which measures the extent of difference between decoded sync words in the respective channels. Each of the difference counter values are individually transferred to offset counters 25-27 by enabling the difference counter output OEXY, OEXZ, OEYZ and the destination counter load LDX, LDY, LDZ for each of the values to be transferred. 
     Count gates COUNTGATEX, COUNTGATEY, and COUNTEGATEZ, which are proportional to the number of read gate clock signals which are to be inhibited, thereby causing additional reads to be added to the data output, are then output by the offset counters 25-27 and input by read gate controller 18 as difference signals SLIPX, SLIPY, and SLIPZ, in response to which read gate controller modifies the data output from input/output circuits DCRSiX, DCRSiY, and DCRSiZ by inserting reads until resynchronization for two consecutive frames is detected by counter 17 and sequencer 16. 
     Sync monitor 3 therefore operates as follows: Initially, last frame PAL circuit 38 determines the recorder unit or channel whose frame sync signal has arrived last. Offset sequencer 21 loads the offset counters 25-27 with values from the difference counters 22-24 based upon the last frame sync signal received. Only those channels requiring a correction factor are loaded as a result of X-NOR circuits 28-30. The offset counters then output count gates proportional to the number of clock periods that are to be inhibited. The count gates are output as ready gates to the recorder units to achieve resynchronization. 
     Last frame PAL circuit 38 includes a logic gate 34 with outputs a signal when either of three sync signals are present. A LAST signal is output by J-K flip-flop 36 to the offset sequencer, which reads which frame is present at inputs XRCVD, YRCVD, and ZRCVD, thus determining the last frame, whenever logic gate 35 indicates that two out of the three sync signals have arrived based on the output from 2/3 detector 37 and the last frame sync has arrived. Those skilled in the art will of course recognize that any of the specific logic gates shown may be replaced by combinations of other logic gates to accomplish the same results. 
     When the INFRAMESYNC signal is present at the input to the offset sequencer 21, the offset sequencer outputs a reset signal RST which resets all flip-flops and counters in the sync monitor. The XFRAME, YFRAME, and ZFRAME signals are only allowed to pass through to the j-input of flip-flops 44-46 by logic gates 41-43 until two of the three signals have been detected by 2/3 detector 37, thus enabling detection of the last frame by offset sequencer 21, and consequent control of difference counters 22-24 to measure the difference to the last frame, thereby causing a count gate representative of the difference to be sent to the read gate controller to automatically cause the recorder units to resynchronize their data output. 
     Having thus described a specific embodiment of an incremental recorder synchronization and resynchronization system, it is nevertheless intended that the invention encompasses all variations of the embodiment specifically disclosed above which will occur to those skilled in the art. Included within the scope of the invention are, for example, variations in which the recorders are not specifically DCRSI recorders, and variations in which a number of recorders is not three. Further, it will be appreciated that the specific circuitry shown in FIGS. 1 and 2 may be modified in innumerable ways while still performing the same essential functions. Consequently, it is intended that the invention not be limited in any way by the above description, but rather that it be limited solely by the appended claims.