High speed variable length code decoding apparatus

A variable-length code decoding apparatus for decoding, at a half clock rate, sequential variable-length codewords supplied from an input buffer which stores an input bit stream to be decoded in fixed-length segments having a length equal to twice a longest length of the variable-length codewords, said apparatus comprising:

• first and second bit storage means for storing consecutive fixed-length segments of the input bit stream in the input buffer;
• first shift means connected to the first and the second bit storage means and having a first output window for producing a first window output sequence from bits in the consecutive fixed-length segments from the first and the second bit storage means, the bit length of the first window output sequence being equal to the longest length of the variable-length codewords and the first output window being shifted across the bits in the first and the second bit storage means in direct response to a window control signal;
• second shift means connected to the first shift means and having a second output window for producing a second window output sequence from bits contained in the first window output sequence and a previous decoding output sequence applied thereto, and generating high-order M bits from the first bit in the second window output sequence as a code value, the bit length of the second window output sequence being equal to the longest length of the variable-length codewords, said second output window being shifted in direct response to a codeword length and M being an integer smaller than the longest length of the variable-length codewords;
• relay means for latching the second window output sequence for one-half the clock cycle and producing the latched second window output sequence as the decoding output sequence;
• first memory means coupled to the relay means for producing a codeword length in response to a pre.sub.-- fix code which consists of high-order P bits of the variable-length codeword that begins at the first bit position of the decoding output sequence, the maximum value of P being an integer smaller than the longest length of the variable-length codewords;
• second memory means connected to the first memory means and the second shift means for producing a fixed-length word in response to the codeword length and the code value;
• and
• accumulation means for adding the codeword length to a previously accumulated codeword length in order to produce the window control signal indicating the added and accumulated codeword length, said accumulation means generating a read signal to retrieve a next fixed-length segment stored in the input buffer when the accumulated codeword length is greater than twice the longest length of the variable-length codewords, the next fixed-length segment being stored in the first bit storage means and the fixed-length segment previously stored in the first bit storage means being transferred to the second bit storage means.