Carbon microphone interface for a modem

A traditional modem includes a data access arrangement circuit located between the computer and the RJ11 connector. An alternate connection is made from this data access arrangement circuit to an input of a carbon microphone interface circuit. This carbon microphone interface circuit includes a galvanic isolation circuit and an impedance matching circuit. The input to the carbon microphone interface circuit is thus transformed to a format which is compatible with a traditional carbon microphone found in the handset of practically all telephones. Therefore, the output of the carbon microphone interface circuit can be directly connected to the handset of a telephone (or other carbon microphone audio terminal device), independent of the location or type of subscriber interface at the wall jack.

FIELD OF THE INVENTION 
The present invention relates generally to computers and specifically to a 
modem which is connected to a computer so that the computer can 
communicate with a remote device located far away from the computer. More 
specifically, the invention relates to an interface connection for 
connecting a traditional modem to a carbon microphone interface. 
BACKGROUND OF THE INVENTION 
A traditional modem is connected to a computer so that the computer can 
communicate with an external device located remotely from the computer, 
over, for example, telephone lines. The modem is connected to a telephone 
line using a well-known interface, known as a POTS interface, usually 
located on a wall of a room. In North America, the POTS interface most 
commonly employs an "RJ11" style connector. 
Problems have occurred with this conventional arrangement. Sometimes, it 
may be difficult for a person wishing to connect his modem to the wall 
outlet to properly access the wall outlet. For example, perhaps there is a 
heavy object such as a piece of furniture in front of the wall outlet. 
There are many other reasons why a wall outlet may be physically 
inaccessible to a person. For example, perhaps the wall outlet is located 
quite low and close to the floor. An elderly person would have a difficult 
time bending down to plug in the modem into the wall connector. 
Another problem which exists is that perhaps the user would like to send 
and receive computer data over a different type of line besides the 
traditional telephone line which is associated with the POTS interface 
RJ11 wall outlet connector. For example, perhaps a particular type of 
digital subscriber loop is to be used. The traditional modem connection to 
a POTS interface would not allow such versatility. 
It has been known (see German Patent No. DE 3,718,742 December 1988) to 
provide for a dual output arrangement for a modem so that a modem can be 
simultaneously connected to both a POTS interface and to an ISDN 
connection. This allows a modem to connect to either of the two lines. 
However, this system is dedicated to providing the ISDN connection as the 
alternative to the POTS interface. That is, the device is designed in a 
particular way, with the proper circuitry, so that only the ISDN system 
can be accommodated through the alternate connection. Other types of 
lines, such as one of many digital subscriber loops used by PBX's cannot 
be accommodated through this arrangement. Further, this prior art system 
does not allow a traditional telephone line to be easily accessible to a 
user in situations when an RJ11 wall outlet is not readily available, as 
discussed above. 
SUMMARY OF THE INVENTION 
An object of the present invention is to make it very easy to connect a 
modem to a traditional telephone line, even when an RJ11 wall outlet is 
not readily accessible to a user. 
A further object of the invention is to allow a modem to be easily 
connectable directly to an audio terminal device which may be connected to 
any type of line, such as one of many types of digital subscriber loops, 
in public or private networks. 
These objects, among others, have been attained as follows. 
A traditional modem includes a data access arrangement circuit located 
between the computer and the RJ11 connector. An alternate connection is 
made from this data access arrangement circuit to an input of a carbon 
microphone interface circuit. This carbon microphone interface circuit 
includes a galvanic isolation circuit and an impedance matching circuit. 
The input to the carbon microphone interface circuit is thus transformed 
to a format which is compatible with a traditional carbon microphone 
interface found in the handset of practically all telephones. Therefore, 
the output of the carbon microphone interface circuit can be directly 
connected to the handset of a telephone (or other carbon microphone audio 
terminal device).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A traditional modem 1 is well known in the art and contains a 
digital-to-analog encoder 100 and an analog-to-digital decoder 101 for 
interfacing between a computer 9 connected to the modem 1 at line 4 and a 
data access arrangement circuit 102 which transfers information to and 
from a line 5 connecting the modem 1 to the RJ11 connector (wall outlet). 
See FIG. 1. 
The preferred embodiment of the invention modifies this traditional modem 
structure by taking an electrical wire 6 and connecting it at one end to 
the input of the data access arrangement circuit 102 and at the other end 
to a carbon microphone interface circuit 2, as shown in FIG. 1. The 
circuit 2 includes a Galvanic isolation circuit 200 which prevents DC 
current from flowing, in a well-known manner. The circuit 2 also includes 
a well-known impedance matching circuit 201 for enabling the output of the 
circuit 2 on line 7 to be of the proper impedance for direct connection to 
an existing carbon microphone interface located in an audio terminal 
device 3 (which is preferably a telephone). The impedance matching circuit 
can either be with or without automatic gain control (AGC). 
This embodiment operates as follows when data is to be sent out from the 
computer 9. Digital data coming from the computer 9 along line 4 is 
transformed to analog form by encoder 100. The analog data is then passed 
to the data access arrangement circuit 102 for sending out along line 5 to 
the wall outlet RJ11 connector (POTS interface) in a well known manner. 
However, according to the invention, the analog data output from encoder 
100 is also passed along a parallel route along line 6 to the carbon 
microphone interface circuit 2 which transforms the analog signal to a 
format compatible with a carbon microphone interface located in the audio 
terminal device 3. Power for powering-up the impedance matching circuit 
and AGC is passed through the Galvanic isolation circuitry 200 from the 
modem 1. 
In this way, if it is impractical to connect the modem along line 5 to the 
RJ11 wall outlet, the modem can be alternatively connected through the 
interface 2 along line 7 to the audio terminal device 3. The audio 
terminal device 3, which is, for example, a regular telephone which 
includes a handset, is usually placed in an easily accessible location. 
Thus, it becomes very simple to connect the modem to the telephone line. 
Further, if the audio terminal device 3 is not connected to a regular 
telephone line at output 8 but is instead connected to a digital 
subscriber loop, a PBX or an ISDN network, the modem 1 will simply be 
connected to whatever the device 3 is connected to. There is no need for 
special dedicated circuitry to be included before the device 3, such 
circuitry being dedicated to a particular type of system existing at 
output 8. Instead, by simply converting the modem output 6 to the carbon 
microphone format at line 7, the modem can be instantly connected to 
whatever the device 3 is connected to, be it a digital subscriber loop, a 
PBX or an ISDN network. 
The interface circuit 2 does not necessarily have to be connected to the 
input of the data access arrangement circuit 102. For example, it could be 
connected to the output of circuit 102. 
The scope of the invention is not to be limited by the above-described 
preferred embodiment, but only by the below-appended claims.