Pulse or digital communications – Spread spectrum – Direct sequence
Patent
1987-11-25
1990-06-26
Olms, Douglas W.
Pulse or digital communications
Spread spectrum
Direct sequence
371 36, H04L 710
Patent
active
049378430
DESCRIPTION:
BRIEF SUMMARY
The present invention pertains generally to the synchronization of a digital data stream, and is more particularly directed to the accurate recovery of information in a data stream received from a transmission channel.
In data transmissions systems, there is a need for synchronization of the data in order to successfully recover the information transmitted in the data stream. Generally, in the past, many systems have appended synchronization sequences to the beginning of respective information data blocks to facilitate this task. Recovery is achieved by recognizing a synchronization sequence and identifying the end of the sequence. Once the synchronization data indicating the end of a sequence have been identified, the information data block is identified as those data which follow the sequence over a predetermined interval.
It is difficult to detect the synchronizing sequence of a transmitted data stream reliably because the sequence itself may be partly affected by transmission errors. Of course the problem of detecting the synchronizing sequence becomes more difficult with higher data rates. While longer sequences increase the probability of being able to detect the transmitted sequence, an increase in the length of the sequence also increases the probability that one or more of its constituent parts may be received differently from the manner in which it was transmitted.
In general, each of these synchronizing sequences is comprised of a set of predefined characters or symbols which occur in a time ordered sequence. With such a technique, if a particular character is received which is not in the character set, then it can be immediately discarded as not part of the sequence. However, there is an inherent ambiguity in the method when a received symbol is one of the characters in the set, but out of the correct order. It is ambiguous as to whether the preceding symbols are in error or the presently received symbol is in error. Therefore, to detect a synchronization sequence of this type reliably, it is necessary to provide means for resolving this ambiguity.
While the invention described herein is compatible with any digital data stream transmission system, it has specific utility in recording and reproducing data with magnetic tape record/reproduce devices. In recently developed helical scan magnetic tape record/reproduce devices for digitized television signals, the same tracks on the magnetic tape include video and audio data separately recorded at spaced locations along each track. The video data account for as much as 90% of the data recorded on a track, and the audio data represent only a small part of the data on each track. The synchronization sequence included in the audio data may be different and without the benefit of the repetitiveness that is the case in synchronization sequences included in the video data.
Thus, a reliable sequence detection system different from that used in video recovery must be used to recover audio data successfully. An audio recovery system should be able to recover a synchronization sequence in one pass because there are no, or only a few, repetitions of the sequence for the audio data, and it should be reliable as it must detect a relatively short sequence of data.
Further, such system should be able to discriminate between false synchronizing sequences which contain a partial string of synchronizing sequence symbols in the correct order, and actual synchronizing sequences which are in the correct order but have some symbol errors included therein. Because the magnetic recording and reproducing medium is likely to have been recorded previously, there is a possibility that partial synchronization sequences (previously recorded sequences which have been overwritten but not completely erased) will be detected, and this possibility should be excluded by conclusively identifying a complete correct sequence for the presently recovered information, in spite of some of the sequence symbols being in error.
SUMMARY OF THE INVENTION
Therefore, it is an aspect of the presen
REFERENCES:
patent: 4112498 (1978-09-01), Reiner et al.
patent: 4214124 (1980-07-01), Jarus
patent: 4598413 (1986-07-01), Szechenyi
patent: 4703479 (1987-10-01), Ikeda
Ampex Corporation
Gilbert Douglas M.
Huseman Marianne
Olms Douglas W.
Strnad Elizabeth E.
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