An error detection and correction system in which at least the highest order bit of each data word is given a greater protection against errors in transmission than lower order bits. Useful for digitized TV signals where errors in received high order bits have much worse effect on picture quality than errors in low order bits. A parity is generated for each data word in accordance with a self-orthogonal convolutional code constructed by the use of the base elements of a difference triangle for each bit of the data word, the difference triangles being disjoint, the triangle associated with the highest order bit or bits having a large number or numbers of base elements, and the triangles associated with the lowest order protected bit or bits consisting of a single base element. Errors in received code words are detected and corrected in a decoder including majority logic threshold detectors.
An error correcting method for use with a continuous serialized stream of digital information, such as present with digitized stereo audio signals, in which data is formatted in frames containing a preselected number of data words, parity words which in combination with data words of other frames enable correction of data in yet other, spatially remote frames, error check words from which errors in any frame may be detected, and frame sync words, and wherein the parity words are encoded from three orthogonal data sets having in common a single data word so that that word can be reconstructed by operating on any of the three sets, thus enabling correction of that word even if one or two of the three sets also contains an error.
Coder-decoder for a transmission channel for digital signals, said signals being constituted by a sequence of bits between a more significant bit formed by a sign bit and a less significant bit. The coder-decoder comprises on the one hand a coder constituted by a circuit able to extract the more significant bit e.g. 1 from the indicated incident digital signal, a recurrent coding circuit of ratio 2 receiving bit EB1 and supplying two redundant bits EB1a and EB1b, means for forming a coded signal having the same number of bits as the incident signal with bit EB1a as the most significant bit, the least significant bit of the incident signal being eliminated and bit EB1b being inserted between two given bits of the incident signal and on the other hand a decoder which comprises means for extracting the redundant bits EB1a and EB1b from the coded signal, a recurrent decoding circuit receiving the two bits EB1a and EB1b and supplying a sign bit EB1b, a pseudo-random bit generator and means for restoring a digital signal having the signal bits received other than EB1a and EB1b between the sign bit taken as the most significant bit and the pseudo-random bit taken as the least significant bit.
Decoding performance of threshold-decodable forward-error-correcting codes is improved through the use of three methods. These methods employ measurements of received-digit reliability, and extended examination of syndrome digits. The first disclosed method allows correction of most partially-masked channel errors. These errors are not corrected in the prior art. The second and third disclosed methods correct almost all channel correctable with prior art, but furthermore disable action by the decoder in most cases where prior art would erroneously alter an information digit that had been received correctly.
A method and apparatus for digital radio communication employs separation of a frame of data to be transmitted into key bits, critical bits and unprotected bits. The key bits are processed to provide parity bits. The parity bits, and key bits are convolutionally encoded using a tail-biting scheme and merged with unprotected bits, and then transmitted. At the receiver, the decoder splits the received data into convolutionally encoded bits and unprotected bits, and trellis decodes the convolutionally encoded bits into a number of possible paths through a trellis using a generalized Viterbi algorithm. The tail-biting scheme reduces the number of bits that must be transmitted. Paths having errors in the key bits are rejected, and the path having the best metric without key bit errors is used in decoding the transmitted information. In the event that there is no such path, a previously selected path is substituted and decoded.
In a Viterbi Algorithm decoder (204) as used to decode convolutionally encoded information, reliability information is developed for various path discard decisions made within the Viterbi Algorithm. These decisions are made for discard opportunities that impact one or more error detection windows (601). Based upon these metrics, a reliability factor sequence can be provided and compared against a fixed (or varying) threshold. When unreliability appears, appropriate action can be taken. For example, all of the information can be discarded, or only certain portions of the information can be discarded, as appropriate to the particular application.
