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Code Efficient Carrier Detect for a Bell 103 Equivalent Modem

IP.com Disclosure Number: IPCOM000062708D
Original Publication Date: 1986-Sep-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 41K

Publishing Venue

IBM

Related People

Mueller, JR: AUTHOR

Abstract

A Bell 103 equivalent modem is implemented by a general-purpose signal processor card designed for the IBM Personal Computer family. Carrier detection is the function of a modem that determines if the audible energy present on a telephone line is of the proper frequency and amplitude characteristics of a Bell 103 modem on the opposite end of the telephone line. Figs. 1 and 2 depict a classical approach to the Bell 103 equivalent modem and the new design. The Bell 103 specifications define two modes of operation: Originate and Answer. These modes further define the frequencies used to denote binary 1 and 0 values. The table below gives these frequencies: (Image Omitted) The Carrier Detect function must detect both the 1's and 0's frequency for a given mode and strongly reject the 1's and 0's frequency for the opposite mode.

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Code Efficient Carrier Detect for a Bell 103 Equivalent Modem

A Bell 103 equivalent modem is implemented by a general-purpose signal processor card designed for the IBM Personal Computer family. Carrier detection is the function of a modem that determines if the audible energy present on a telephone line is of the proper frequency and amplitude characteristics of a Bell 103 modem on the opposite end of the telephone line. Figs. 1 and 2 depict a classical approach to the Bell 103 equivalent modem and the new design. The Bell 103 specifications define two modes of operation: Originate and Answer. These modes further define the frequencies used to denote binary 1 and 0 values. The table below gives these frequencies:

(Image Omitted)

The Carrier Detect function must detect both the 1's and 0's frequency for a given mode and strongly reject the 1's and 0's frequency for the opposite mode. It must be noted that while receiving in one mode the modem is also transmitting on the other, hence the need for strong opposite band rejection. The classical approach to this problem is given in Fig. 1, where a Carrier Detect Filter is implemented following the Bandpass Filter. However, this approach yields redundant code, since the HI Band Filter can be used to perform this function. By placing the Carrier Detect Integrator after the HI Band tuned filter, as in Fig. 2, several benefits are realized: -Less code is required to accomplish the same function. -False Carrier detection i...