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Browse Prior Art Database

Three Quarter Speed Method for Four Levels Modem

IP.com Disclosure Number: IPCOM000090766D
Original Publication Date: 1969-Jul-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Related People

Pierret, JM: AUTHOR

Abstract

The method decreases the signalling rate in order to improve the transmission without modifying the analog circuits of the modem. This method, using a three-level encoder, allows a gain of 3 to 4 db in the signal-to-noise ratio, the line signal spectrum being similar to the one of the four-level modem.

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Three Quarter Speed Method for Four Levels Modem

The method decreases the signalling rate in order to improve the transmission without modifying the analog circuits of the modem. This method, using a three-level encoder, allows a gain of 3 to 4 db in the signal-to-noise ratio, the line signal spectrum being similar to the one of the four-level modem.

In many modems, the four-level modulation is achieved by adding the outputs of two encoders with weighting PA = 1 and PB = 2. The resulting amplitudes arc +3, +1, -1, -3. The data at the input are entered alternatively on each channel through channel selector device CSD.

This method consists in processing three bits at the same time to generate trits according to the table. Each group of three bits corresponds finally to a trit which is composed of two consecutive, three possible level digital elements. The result is a three-level signal with the restriction that the middle level does not occur two times in a trit. For obtaining such a signal, it is possible to use the above-mentioned encoders, their outputs being added with the weighting PA = PB = 1.

In the drawing, the table of the various combinations of three bits is represented under the form of a message entering the transmission system. Each three-bit group corresponds to two groups of two bits, one group on each channel. Signals on channel A and channel B being continuously composed, each composition of two corresponding groups of two bits corresponds to a gr...