Coded data generation or conversion – Digital code to digital code converters – Substituting specified bit combinations for other prescribed...
Patent
1986-05-12
1989-07-25
Shoop, Jr., William M.
Coded data generation or conversion
Digital code to digital code converters
Substituting specified bit combinations for other prescribed...
341 50, 341106, 360 40, H03M 700
Patent
active
048518379
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method and apparatus for processing digital signals prior to recording. More particularly but not exclusively the digital signals represent television video signals.
A channel code which had a minimum separation of 11/2 bit cells and which also was capable of statistically controlling the low frequency components was described in our co-pending U.K. Application No. A-2,141,906. For each successive group of eight bits it is possible, theoretically, (at least on average) to identify just over 453 discrete combinations and this should make it possible to construct a blockcode with controlled low frequencies with a minimum transition separation of 11/2 bit cells.
An alternative proposal has been made which operates on a bit-by-bit basis. Consideration has been given to the realisation of this alternative proposal but there are a number of difficulties which stem from the combination of the necessary high speed, the look ahead and the modulo-three requirements of parts of the coder.
SUMMARY OF THE INVENTION
The present invention provides a method for processing digital information for recording comprising the steps of inputting a succession of input digital words each comprising a plurality of bit locations, generating a respective code word for each digital word with each code word being of the same duration as a digital word but having a number of time slots greater than the number of bit locations in an input word providing a plurality of groups of code words, and selecting for each digital word a code word from one of the plurality of groups on the basis of the immediately preceding code word used.
Preferably, in each code word there is a period of three time slots between adjacent transitions. It is also advantageous to use more than two groups of code words e.g. two main groups may be used with each main group having a transformed or "opposite" group.
An advantage of the method is that less read only memory is required whilst approaching close to the theoretical limit of just over 453 different combinations in each block of 8 cells.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the present invention be more readily understood an embodiment thereof described by way of example with reference to the accompanying drawings, in which :
FIG. 1 shows the basis of a first code book for 4-bit words;
FIG. 2 shows the basis of a second code book for 4-bit words;
FIG. 3 shows the time of transitions for certain combinations in the first and second code books;
FIG. 4 shows the basis of a first code book for 8-bit words;
FIG. 5 shows the basis of a second code book for 8-bit words;
FIG. 6 shows in block diagram form an encoder which utilizes the first and second code books for 8-bit words;
FIG. 7 shows truth tables to assist understanding of the encoder shown in FIG. 6;
FIG. 8 shows in more detail one way in which a part of the encoder of FIG. 6 may be implemented and is accompanied by its truth table;
FIG. 9 shows in block diagram form a decoder for decoding the signal encoded by the encoder of FIG. 6; and
FIGS. 10A-10J are a print out of the complete first and second code books.
DETAILED DESCRIPTION
Before describing the invention in detail, it is considered helpful to explain some basic principles of the coding system being used. It is to be remembered that the techniques disclosed in our co-pending application identified above are still being used and attention is directed to that case for further details. Suffice to say that the present invention comprises a method of processing digital information for recording purposes. The processing is organised on a word-based arrangement and generates a code word for each input digital word to be recorded, the code word occupies the same time as the input digital word but has at least double the number of time slots with respect to the number of bit locations in an input digital word. In a code word, transitions are not permitted at specified locations.
For a minimum transitio
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Independent Broadcasting Authority
Shoop Jr. William M.
Young Brian
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