Transmission recording and reproduction of digital data and time information in transport packets using a compression ratio

#The present invention relates to a transmitting apparatus and to a digital video tape recorder and, more particularly, to apparatus and method for recording and reproducing MPEG2 transport packets to and from a magnetic tape.

#As is known, digital video tape recorders compress a digital video signal using discrete cosine transformation (DCT) and variable length encoding prior to recording the digital video signal on a magnetic tape. NTSC video signals generally are recorded in a standard (SD) mode and high definition television (HDTV) signals generally are recorded in a high definition (HD) mode. Typical recording rates in the SD and HD modes are 25 Mbps (megabits per second) and 50 Mbps, respectively.

#Currently, a format known as MPEG2 (Moving Pictures Expert Group) for video signals has been developed. In MPEG2 systems, plural programs generally are time division multiplexed prior to being transmitted, and time information, which is included in those video signals, identify the real time transmissions of each of the programs. MPEG2 transmissions are time compressed and thus transmitted in a shorter time than the real "play" time of each of the programs contained therein.

#The following detailed description, given by way of example and not intended to limit the present invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, wherein like reference numerals denote like elements and parts, in which:

#FIGS. 1A-1D illustrate the data structure of a transport packet of an MPEG2 signal;

#FIG. 2 is a block diagram of an MPEG2 system which time divisionally multiplexes plural television programs and which transmits and records those television programs on a magnetic tape;

#Referring now to FIG. 5 of the drawings, apparatus for transmitting digital video data, and for recording and reproducing digital video data to and from a magnetic tape in accordance with the present invention is shown. Like the apparatus shown in FIG. 2, programs P-1, P-2 and P-3, which have data rates of 5 Mbps, 4 Mbps and 3 Mbps, respectively, are digitally compressed and supplied to input: terminals 1A, 1B and 1C, respectively. As is appreciated, these programs previously are divided into transport packets, each consisting of 188 bytes of data, are transmitted in a burst manner, and include PCR values which represent the time when each of the packets is formed. Programs P-1, P-2 and P-3 are supplied to a program selecting and time compressing circuit 2 which, in response to a control signal supplied from a controller 3, selects and time compresses one of the programs. For example, circuit 2 selects program P-1 which has a bit rate of 5 Mbps and which is illustrated as the shaded portion of the timeline shown in FIG. 6B, and wherein FIG. 6A represents the relative time of the 27 MHz system clock. Circuit 2 converts the bit rate of program P-1 from 5 Mbps to 20 Mbps; thus, since the bit rate of program P-1 is increased by a factor of 4, the amount of time required to transmit program P-1 is reduced from, for example, two hours to one-half of an hour (i.e., 30 minutes). Circuit 2 supplies the time compressed program to a PCR capture circuit 4 which detects the PCR value therein and supplies the detected PCR value to a PLL circuit 5.

#Since the selected program P-1 is time compressed by a compression ratio of, for example, 4, the rate at which the PCR value changes is 4 times (i.e., the compression ratio) greater than the rate change of the PCR value in the uncompressed signal. PLL circuit 5 compares in a subtracter circuit 9 the captured PCR value supplied from circuit 4 and a counter value which is multiplied by the compression ratio. That is, a counter 7 supplies a counter value to a multiplying circuit 8 and controller 3 supplies the amount of compression, that is, the compression ratio (e.g., 4) also to multiplying circuit 8 and circuit 8 supplies the product of the counter value and the compression ratio to subtracter circuit 9. Circuit 9 supplies the compared result to a PCR recovery circuit 6 which controls counter 7 so that the output of counter 7 is synchronized with the captured PCR value, thus producing a 27 MHz system clock.

#PCR capture circuit 4 supplies the time compressed selected program to a multiplexer 10 which adds to the time compressed signal stuffing bits that have been supplied to a terminal 11 so that the data rate of the signal equals 30 Mbps. For the example stated above, where the data rate of the time compressed signal is 20 Mbps, stuffing bits which have a data rate of 10 Mbps are added to the signal to produce the 30 Mbps video signal.