Modem with low part count and improved demodulator

• US 4,431,867 A
• Filed: 12/13/1982
• Issued: 02/14/1984
• Est. Priority Date: 06/15/1981
• Status: Expired due to Term

• Current Assignee: Telogy Networks, Inc. (Texas Instruments, Inc.)
• Original Assignee: Hayes Microcomputer Products, Inc.
• Inventors: Heatherington, Dale A.
• Primary Examiner(s): Rubinson, G. Z.
• Assistant Examiner(s): Brady, W. J.
• Application Number:
  US06/449,464
• Time in Patent Office: 428 Days
• Field of Search: 179/2 C, 179/2 DP, 179/5 R, 179/5 P, 179/6.16, 179/84 L, 179/84 C, 179/99 LS, 179/99 P, 375/8, 375/9
• US Class Current: 379/93.28
• CPC Class Codes: H04L 27/10 Frequency-modulated carrier systems, i.e. using frequency-shift keying H04L27/32 takes precedence
• CPC Class Codes: H04L 27/30 wherein each code element is represented by a combination of frequencies

Abstract

An FSK modem for coupling a standard data port (50) to a telephone line (45) in which the signal path between the phone line and the data port passes through a processor (55). The same frequency detection apparatus (83, 192, 180) is used to both demodulate FSK encoded signals and detect the presence of a telephone ringing signal on the line. A pair of impedances (161, 162) provide alternate paths either through an input filtering network (167) or bypassing the network (135, 69) in a manner which allows the ring signals to pass directly to the input (69) of the frequency detector while assuring that only filtered FSK signals reach the input. Also shown is the use of the same frequency synthesizing apparatus (86, 253, 259) for generating FSK output signals and high group DTMF signals with appropriate switchable attenuation (88) provided in this signal path.

Claims

1. In a data communications adapter including means for coupling a telephone line to a first point in a signal path, said first point being an input point to a filter network, said filter network including an output, and a decoding means for decoding signals from said telephone line, the improvement comprising in combination:

a first impedance connecting said first point to a second point;

means for connecting said output of said filter network to a third point and for providing a low output impedance at said third point;

a second impedance connecting said second point and said third point;

wherein said first impedance, said second impedance and said output impedance provide a low pass function between said first point and said second point; and

said output impedance and said second impedance provide a high pass function between said output of said filter network and said second point; and

means connecting said second point and said decoding means.

2. In a data communications adapter for accepting angle modulated signals from a telephone line, the improvement comprising:

means for coupling a ringing signal from said telephone line to a junction point;

a filter including an input and an output;

means for coupling said angle modulated signals from said telephone line to said input of said filter;

means for coupling said output of said filter to said junction point;

frequency responsive means for detecting the frequency of an alternating signal present at said junction point, said frequency responsive means being responsive to said alternating signal being of a frequency characteristic of said ringing signal for providing a ring detection signal when said data communications adapter is on-hook and alternately being responsive to said alternating signal being of a frequency characteristic of said angle modulated signal for decoding said angle modulated signals.