Matsushita Graphic Communication Systems, Inc.

Tsukasa Sakai

Nobuhiko Noma

Hiroyuki Nemoto

Keiichi Tomita

Mikio Mizutani

Osamu Noguchi

Kazuo Kurita

Genzou Takagi

When carrying out pre-processing of image signal transmission using the control channel of a full-duplex modem and switching image signal transmission to the primary channel of a half-duplex modem, the answer modem stops the transmission carrier upon confirmation that all &ldquo;1&rdquo; signals with 40 bit or more have been received from the calling modem, restarts the reception of all &ldquo;1&rdquo; signals after the elapse of a certain period of time, confirms the end of all &ldquo;1&rdquo; signals and switches to the primary channel.

Modem control method

An encoding (or decoding) method, by which raw data of 1 line is converted into, for example, modified Huffman (MH) code (or MH code is converted into raw data) by obtaining a cumulative run-length code every transition point, at which there occurs a transition or change between a run of white pixels to another contiguous run of black pixels, from the beginning of the line and then converting the cumulative run-length code to the MH code (or converting the MH code to the cumulative run-length code). The cumulative run-length code is defined by both of data, which indicates that a pixel of a run just prior to the transition point is black or white, and data representing a cumulative run-length of the run up to the pixel thereof just prior to the transition point. Thus, an encoding or decoding processing can be performed at a high speed.

Encoding and decoding methods for use in facsimile

A Viterbi decoding method, in which consecutive input data bits Q.sub.6n .about.Q.sub.1n are first converted into consecutive code bits Q.sub.6n .about.Q.sub.3n Y.sub.2n Y.sub.1n Y.sub.0n at a time instant n by use of a Trellis encoder. Then, a branch metric, which is a minimum Euclidean distance between a reception signal point and each of signal points (namely, Trellis codes Q.sub.6 .about.Q.sub.3 Y.sub.2 Y.sub.1 Y.sub.0) in a signal space diagram, corresponding to each of eight states 0 to 7 and a corresponding signal point are obtained. Further, a sum of the branch metric corresponding to each state and a corresponding path metric at a time instant (n-1), which is a sum of corresponding branch metrics since a time instant 1 to the time instant (n-1) is calculated. Then, the path metric corresponding to each state is updated by replacing it with the calculated sum of the corresponding branch metric and the corresponding path metric obtained at the time instant (n-1). Subsequently, a minimum one of the path metrics is determined as the latest part of a surviving path. Moreover, the surviving path is traced back to a signal point Q.sub.61 .about.Q.sub.31 Y.sub.21 Y.sub.11 Y.sub.01 at the time instant 1. Thus input data bits Q.sub.61 .about.Q.sub.31 Q.sub.21 Q.sub.11 at the time instant 1 is restored. In case of this method, the signal points are classified according to three bits Y.sub.21 Y.sub.11 Y.sub.01 into eight groups. Further, one of the groups is determined as a reference group. Furthermore, tables representing the signal points of the other groups are calculated from a table representing the signal points of the reference group.

Viterbi decoding method

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Tokyo, Japan

Tsukasa Sakai