Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-05-24
2002-11-19
Phu, Phuong (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S340000, C714S758000, C714S759000
Reexamination Certificate
active
06483882
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a circuit and method for EFM demodulation, and more specifically, to a circuit and method capable of correcting data errors caused by invalid EFM bit patterns during demodulation by using alconversion table, based on fuzzy logic.
2. Prior Art
Data is stored on a compact disc as a single spiral of pits and lands. The data is encoded as eight-to-fourteen modulation or EFM. EFM is an encoding technique where 8-bit digital values are converted into 14-channel bit symbols. The basic block diagram of all compact disc players includes an EFM demodulation stage. The EFM demodulation stage can be implemented using a ROM look-up table or a logic array.
The standard EFM conversion table defines the conversion of 8 bit digital to 14-channel bit symbols. The assignment of 8-bit digital to 14-channel bit symbols called out by the standard EFM conversion table was done arbitrarily and the 14-channel bit symbols do not follow a specific sequential binary progression. Bit patterns in the standard EFM conversion table were chosen carefully to minimize the number of transitions required to represent the original 8-bit digital data making the compact disc easier to manufacture and to establish a minimum and maximum length of pits and lands. The number of possible patterns for a 14-bit binary number is 16,384 and the number of patterns used to represent EFM is 258 (256 for data and 2 patterns for sub-code synchronization purposes) which leave 16,126 invalid patterns unused and undefined by the standard EFM conversion table.
The compact disc player uses a laser head assembly to read the reflective contrast of pit versus land and convert that contrast into a high frequency wave that is sliced at the zero crossing point into a digital series of ones and zeros. The format of the compact disc defines the data to be grouped into 588-channel bit frames. Each 588-channel bit frame includes a 24-channel bit synchronization pattern; one 14-channel bit sub-code symbol, twenty-four 14-channel bit symbols of data and eight 14-channel bit symbols of error correction code. Three merging bits separate the synchronization pattern and each 14-channel bit symbol. The merging bits are used to maintain the minimum and the maximum pit and land lengths and to minimize the DC content of the signal over time.
The pits on a compact disc are among the smallest structures ever manufactured. A compact disc may have as many as 3 billion pits. Pits of nine separate lengths are used on the compact disc. The pits range in size from the shortest that is 0.833 micrometers to the longest that is 3.05 micrometers in length. All of the pits are approximately 0.5 micrometers wide and approximately 0.11 micrometers deep. It may be easier to visualize just how small the pits on the compact disc are if you consider that a human hair is approximately 75 micrometers in diameter. Precise manufacturing control of the shape, length, width and depth of the pits is required to make a disc playable. Variations in the manufacturing process of compact discs can cause problems such as jitter or variation of pit and land lengths, which may cause data errors during playback. Data errors are common place and error correction circuits are required and can be found in all compact disc players. Traditional error correction on the compact disc player occurs after EFM demodulation (fourteen bit symbols back to 8-bit digital values). Existing compact disc systems do not prevent invalid EFM 14-channel bit patterns from entering the system and creating data errors during translation into 8-bit digital values and rely solely on an elaborate multiple stage error correction scheme for providing error-free data.
Error correction in a compact disc system:carries a format burden of 8 symbols of error correction code for every 24 symbols of user data. This format burden affects both data transfer rate and overall storage capacity of the compact disc. The error correction system is precise and effective, but at the same time is limited in the number of corrections it can make per frame of data before it fails and an entire frame of data is lost. An improvement to the system, which reduces errors, will also reduce the chance that the error correction circuitry will reach its limit and fail. The ideal improvement of the compact disc error correction system would be one that increases the error correction capability without increasing the burden or creating incompatibility.
Fuzzy logic is a form of logic where absolute or one-to-one correspondence is not required to determine the correct or most likely choice. The demodulation of EFM offers an ideal opportunity for the application of fuzzy logic. The patterns selected for EFM tend to be as different as possible from each other. The fact that EFM uses only 258 out of 16,384 patterns leaves many invalid patterns, which are very similar to valid EFM patterns.
The integrity of data in digital systems is of crucial importance. A single uncorrected data error in a software program can cause an entire system to fail. The manufacturers of compact discs and compact disc players go to great lengths to create a system which can deliver error-free data under less than ideal conditions. The consumer's demands for less expensive and faster methods to transfer data from the compact disc continues to drive the industry. Industry's response to this demand has tested the compact disc specification based on a 1× data rate. The original compact disc specification detailed how to build a disc and player for 1× data rate, where present day compact disc drives commonly used in computer systems are now approaching data rates of 50× and beyond.
SUMMARY OF THE INVENTION
Briefly, in accordance with the invention, a method for EFM demodulation is provided which includes a fuzzy logic-based ROM look-up conversion table capable of correcting data errors caused by invalid EFM bit patterns. The ROM look-up table contains the conversion data of valid EFM 14-channel bit patterns to 8-bit digital values. In addition to the valid pattern conversion, the ROM look-up table also includes fuzzy logic-based conversion of all invalid EFM 14-channel bit patterns to 8-bit digital values. The conversion of invalid EFM 14-channel bit patterns to 8-bit digital values allows error correction to begin during EFM demodulation using the inventive method.
Considering that there are 16,384 possible 14-channel bit patterns and EFM uses only 258 of those patterns, it is not surprising that a high percentage of data errors that occur in EFM tend to fall outside of the 258 valid 14-channel bit patterns. All data errors caused by invalid EFM 14-channel bit patterns have a chance of being corrected during conversion back to 8-bit digital values by the invention. The invention is not limited to how many symbols it can correct per frame as the traditional error correction circuitry. The invention improves the depth of the overall error correction system by correcting many of the data errors caused by invalid 14-channel bit patterns and most of the simple “off by one clock period” invalid patterns. The inventive method relies on traditional error correction circuits for the errors that do not show up as invalid patterns or those that it is unable to resolve. If the inventive method is unable to correct an error caused by an invalid EFM 14-channel bit pattern, the attempt to correct the error is transparent to the preceding error detection and correction circuits. The error detecting and correcting circuits detect and correct the error just as if the invalid EFM 14-channel bit pattern has been demodulated by a traditional EFM demodulation circuit.
The inventive method is not limited to compact disc applications, but can be used in other systems such as digital versatile disc (DVD), digital audio tape (DAT) and other systems which use EFM or similar block encoding.
An alternate method and circuit to correct errors caused by invalid EFM 14-channel bit patterns
Phu Phuong
Tachner Leonard
LandOfFree
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