GB2172773A - Data transmission using a telephone handset - Google Patents

Abstract

Apparatus for the transmission of data using a telephone handset (3) having an earpiece (4) and a mouthpiece (5) between data transmitting and receiving equipment (11), in which a box housing a pair of oppositely wound induction coils (L1, L2) and an amplifier circuit is positioned in a holder adjacent the earpiece, and a loudspeaker (6) is positioned adjacent the mouthpiece (5). The box 7 is shielded with an aluminum foil which is earthed. The induction coils can alternatively be positioned within the earpiece. <IMAGE>

Description

SPECIFICATION Data transmission using a telephone handset The present invention relates to the transmission of data using a telephone handset.

It is often desired to transmit data, rather than speech, along a telephone line, e.g. when connecting a remote terminal to a computer or when using facsimile transmission apparatus.

Generally speaking, when this is done the data signal (normally digital) is fed first to a modulator/demodulator (commonly known as a modem), which modulates the signal for transmission and then puts it on the telephone line. At the receiving end, another modem takes the incoming signal from the telephone line, demodulates it, and passes it to data receiving apparatus, which may be a computer, facsimile machine, or other equipment capable of receiving and handling data.

Frequently the term "modem" is used only for such devices when they connect directly to the telephone line, bypassing the handset, and the term "acoustic coupler" is used for a device which expresses acoustically the outgoing signal to be transmitted, for reception by the telephone handset mouthpiece, and receives the incoming signal acoustically from the telephone handset earpiece. However, even with an acoustic coupler a modulating/demodulating operation may be necessary. It is generally considered that acoustic couplers are inferior in performance to directly connected modems, but they are often more convenient as they need no special installation before use and thus are portable.

The present invention is concerned with the way in which a data signal reception and emission apparatus receives a data signal from a telephone line. Such an apparatus will normally perform a modulation/demodulation function, and so for convenience will be referred to hereinafter as a modem, but this is not intended to imply that the apparatus is for direct connection to the telephone line bypassing the handset, nor is it intended to imply that the apparatus necessarily performs a modulation/demodulation function.

In one aspect, the invention provides an input means for a modem which takes the incoming signal from the telephone handset earpiece, but instead of receiving the signal acoustically responds to the electromagnetic flux of the earpiece.

The input means may have two "back to back" induction coils with windings wound in the opposite sense, so as to filter the signal and reduce sensitivity of the input means to noise. Preferably the input means also includes an amplifier, the coils providing input to the amplifier. By providing an amplifier in this input or pick-up means, the overall signal to noise ratio may be improved.

The input means circuit may be contained in a box which is shielded, e.g. by means of an aluminium foil which is earthed against external electrostatic or capacitive influence, the box being held adjacent the telephone handset earpiece in use.

The input means may be associated with output means for the modem, the output means comprising a loudspeaker. The input and output means may be located in the earpiece receiving section and the mouthpiece receiving section of a telephone handset holder.

The handset holder may also include the modem.

The present invention also provides a data signal reception and emission apparatus comprising the said input means, and also preferably said associated output means.

An embodiment of the present invention, given by way of example, will now be described with reference to the accompanying drawings, in which: Figure 1 shows diagramatically a modem input/output means embodying the present invention, with an input circuit; Figure 2a shows a box containing the input circuit of Figure 1 open; Figure 2b shows the box of Figure 2a closed; and Figure 3 shows a handset holder equipped with the input circuit of Figure 1 in a box and a loudspeaker output.

Referring first to Figure 1, the upper dashed box 1 shows a circuit diagram for a modem input means embodying the present invention, and the lower dashed box 2 shows a diagram for a loudspeaker output means to be associated with the input means of box 1. A telephone handset 3 is shown with its earpiece 4 positioned opposite the coils L1 and L2 of the input means and its mouthpiece 5 positioned opposite to the loudspeaker 6 of the output means.

The input means includes an amplifier for the output of the coils L1 and L2. The amplifier includes transistors TR1 and TR2, resistors R1 to R4, and capacitors C1 to C4. the amplifier has a gain of approximately 60dB. In order to reduce the number of interconnections, the power supply and signal line of the amplifier share the same line. The input circuit is powered by the 5V supply applied to the output line via a resistor R5. Resistors R1 and R4 control the base bias of the respective transistors TR1 and TR2 and have a negative feedback effect. It should be noted that these resistors have a much greater resistance than the resistors R2, R3 and R5. Resistors R2 and R3 are load resistors for the transistors TR1 and TR2 respectively.The capacitor C1 couples the induced E.M.F. in the coils L1 and L2 to the transistor TR1. The capacitor C2 couples the amplified signal to the transistor TR2. the output of the transistor tR2 is directly connected to the demodulation circuit on the computer side. The capacitors C1 and C2 also serve to block low frequency hum.

the capacitor C3 and resistor R3 form a lowpass filter to derive the D.C. component of the output for operation of the transistor TR1 and the bias voltage for the transistor TR2. It should also be noted that capacitor C4 has a much smaller capacitance than the capacitors C1, C2 and C3 and is used to filter out high frequency noise picked up by the coils L1 and L2. the cut off frequency of the filter, constituted by the capacitor C3 and the resistor R3, is above the carrier frequency of the modem in question. the coils L1 and L2 have the same number of turns, but are wound in the opposite sense, so as to filter the signal and reduce the sensitivity of the input means to noise.

In a preferred construction, the coils L1 and L2 each have 6400 turns and are connected in parallel with their windings wound in the opposite sense. The transistors TR1 and TR2 are both NPN transistors of the bipolar type (1402D).

The capacitors and resistors in the circuit have the following values in the preferred values: C1 = 0.iiiF C2 = 1 O,u C3 = i0,uF C4 = 1000pF R1 = 330 Kohm R2 = 6.8 Kohm R3 = 6.8 Kohm R4 = 330 Kohm R5 = 4.7 Kohm.

Connections to the computer interface indicated at 11 are via a capacitor C5 from the amplifier circuit shown in the block 1 and via a transistor TR3 from the block 2 containing the loudspeaker 6.

Figures 2a and 2b show a box 7 for the input circuit of the upper dashed box 1 of Figure 1. In Figure 2a the box 7 is shown with its lid 7a open, and the coils L1, L2 and other circuit components, which are designated as X for transistors, Y for resistors and Z for capacitors, are mounted on a circuit board 8. the box 7 is sitting on aluminium foil 9 for electrostatic and capacitive shielding.

When the lid 7a of the box 7 is in the closed position as shown in Figure 2b, the aluminium foil 9 is earthed by means of a bare wire (not shown).

It will be appreciated that instead of the components X, Y and Z being mounted on the board 8, they can be incorporated into a printed circuit board, which will form the board 8, by any known printed circuit technique.

Figure 3 shows a telephone handset holder 10 in section, with a handset 3 received in it.

The box 7 wrapped in the aluminium foil 9 and containing the modem input circuit is located in the part of the holder 10 which receives the handset earpiece 4. The modem output loudspeaker 6 is located opposed to the position occupied by the handset mouthpiece 5. Modulation/demodulation circuitry may also be located in the handset holder 10.

It can be readily shown that when two identical coils are positioned as shown in Figure 1, viz. with their longitudinal axes lying on the same line, but having their coils wound in opposite sense, and are connected in either series or parallel, then the induced energy will be inversely proportional to the eighth power of the distance from the electro-magnetic wave source. In a practical embodiment, it was found that using a small double coil located at a distance of 25.4 cm from a noise source, a noise energy of 100 X 106 times smaller could be picked when the coil was located at a distance of 2.54 cm. Compare this with the acoustic wave employed by a known modem which obeys an inverse square law.

It is preferable that the pick-up coils should be placed near to the source, for example the coils could be positioned inside the ear-piece 4. In this case, distant noises will have negligible effect on the pick-up circuit.

The above described modem input/output means in which electro-magnetic means replace the traditional acoustic means has the following advantages: (a) it has substantially less distortion than known modems; and (b) relatively free from noise interference as compared with known modems.

In connection with distortion, it should be noted that phase distortion in particular will impair communication accuracy. Frequency responses of acoustic transducers, such as microphones and ear-phones are generally not flat, which will result in both amplitude and phase distortion. The energy conversion process involved in an ear-piece consists of the following stages: (a) electric current-magnetic field, (b) magnetic field-mechanical force, (c) mechanical force-vibration of a thin membrane, and (d) vibration of membrane--vibration of air particles.

It should be noted that the same stages apply to the case of a moving coil microphone except that the order is reverse.

At each stage distortion is introduced. At stages (a) and (b), distortion is due to hysteresis of ferro-magnetic substances present in the transducer inside the ear-piece. At stages (c) and (d) distortion arises from the fact that the mechanical systems involved are of second or even higher order which implies a nonuniform frequency response.

On the other hand the frequency response of electro-magnetic transducers are comparatively flat because the only energy conversion involved is in stage (a) namely electric current magnetic field or vice versa.

In the case of noise acoustic couplers are susceptable to unwanted sounds since only a vacuum can isolate sound. Although most acoustic couplers employ a pair of close-fitted rubber rings to enclose the mouthpiece and earpiece of the handset, a considerable portion of noise can get into the transducers since the handset itself can convey sound. In the present modem input/output means, because of the design of the electro-magnetic transducer, unwanted electro-magnetic noise is largely reduced by the double coil arrangement and unwanted electro-static noise are shielded by the aluminium foil.

Claims (10)

1. Apparatus for the transmission of data using a telephone handset between data transmitting and receiving equipment, said apparatus including: means for responding to the electro-magnetic flux of the earpiece of the telephone handset induced therein by said data transmitting equipment; and means for transmitting the received data to the data receiving equipment.

2. Apparatus according to claim 1, wherein said means for responding to the electro-magnetic flux of the earpiece comprises two induction coils connected in series or parallel and wound in the opposite sense.

3. Apparatus according to claim 1 or 2, wherein the means for transmitting the received data comprises an amplifier, which is connected to receive an input from the electro-magnetic flux responding means.

4. Apparatus according to claims 2 and 3, wherein said induction coils and amplifier are contained in a box, provided with electro-static shielding means, the box being positioned against the earpiece of the telephone handset.

5. Apparatus according to claim 4, wherein said box is shielded by means of an aluminium foil which is earthed.

6. Apparatus according to any one of the preceding claims, wherein it additionally includes a loudspeaker positioned adjacent the mouthpiece of the telephone handset and connected to data transmitting equipment.

7. Apparatus according to claims 5 and 6, wherein a holder is provided for receiving the telephone handset, the box shielded by the foil which is wrapped therearound being located in that part of the holder which receives the earpiece of the handset, and wherein the loudspeaker is located in that part of the holder which receives the mouthpiece of the handset.

8. Apparatus according to claim 2, wherein the energy induced in the two coils is inversely proportional to the eighth power of the distance from the earpiece of the handset.

9. Apparatus according to claim 2 or 8, wherein the two induction coils are positioned within the earpiece of the telephone handset.

10. Apparatus for the transmission of data using a telephone handset between data transmitting and receiving equipment, said apparatus being constructed substantially as herein described with reference to and as illustrated in the accompanying drawings.