Circuit arrangement for providing isolation between the transmission paths of a hands-free subset.

The present invention relates to a circuit arrangement as set forth in the preamble of claim 1.

A hands-free subset with such a circuit arrangement is described, for example, in an article by M.Slawik and H.Wiedmann published in "Elektrisches Nachrichtenwesen", Vol. 53, No. 4, 1978, pages 288 to 293.

In such a hands-free facility, background noise is particularly disturbing, because such noise, like the voice signals, may cause the send path to be de-attenuat­ed, and the receive path to be attenuated. As a result, the send path is constantly de-attenuated, and the sub­scriber at the opposite station can no longer get through because the receive path is blocked. Use of the hands-free facility is thus rendered very diffi­cult. In the above article, a circuit for reducing the effect of continuous interfering noise is given on page 291, right column, penultimate paragraph, but this circuit is effective only in the presence of per­sistent, continuous noise, not in the presence of short-­time noise with a high dynamic range.

It is the object of the present invention to provide a circuit arrangement of the above kind which also re­duces the effect of short-time background noise on hands-­free telephone operation.

This object is attained by the features set forth in the characterizing part of claim 1.

The invention makes use of the fact that nearly every background noise occurring in an enclosed space contains a frequency component lying below the lower limit of the voice-frequency range, while this is not true for voice signals; this was found by a great number of measurements. The circuit arrangement in accordance with the invention interprets a low-frequency-containing sig­nal picked up by a microphone as noise, and inhibits or reduces the de-attenuation of the send path and the attenuation of the receive path by the voice control cir­cuit. Thus, background noise, such as the noise of running machines, ventilating fans, cooling installations, but also traffic noise or short-time noise, such as the slamming of doors, lose their effect on the voice control. Noise produced in the immediate vicinity of the microphone mostly contains frequencies above the voice-­frequency range. Such noise (e.g., paper rustling, typewriter), which may also be very disturbing, can be rendered ineffective in analogous fashion by deriving a noise criterion from the level of a frequency range lying above the voice-frequency range.

Further advantageous features of the circuit arrange­ment in accordance with the invention are claimed in the subclaims. Claims 2 to 4 describe different circuits for influencing the voice control by means of a previously derived noise criterion.Claim 5 specifies an advan­tageous frequency range for deriving a noise criterion. Claim 6 gives a circuit in which noise criteria are derived from frequency ranges lying above and below the voice-frequency range.

An embodiment of the circuit arrangement in accordance with the invention will now be described with reference to the accompanying drawing.

The single figure of the drawing is a block diagram of the embodiment.

The figure shows schematically the send path of a hands-­free subset, comprising a voice control circuit 1 which acts on the send path via a line 15, and on the receive path via a line 16.

Voice signals from the subscriber and ambient noise are picked up by a microphone 2, which converts them into electric signals. The frequency response of the micro­phone is chosen so that sound signals lying below the voice-frequency range are not completely suppressed. From the output of the microphone, the converted voice signals are applied through a first preamplifier 3, a first band-pass filter 4, and a variable-gain amplifier 5 to the subscriber line 6. The band-pass filter 4 re­moves the signal components lying outside the voice-­frequency range, and the variable-gain amplifier amplifies the signal from the band-pass filter 4 under control of signals from the voice control circuit 1, thus producing the send signal. The send-path levels for the voice control circuit are tapped at the output of the first preamplifier 3 and at the output of the variable-gain amplifier 5. The two tap signals are summed in a first adder 7 and applied to a second band-pass filter 8 which limits the sum signal to the voice-fre­quency range or to a frequency band lying within the voice-frequency range. The output of the second band-­pass filter is amplified in a first control-signal amplifier 9 and rectified in a following first rectifier stage 10. Before being applied as a control signal to the voice control unit, the rectified signal is com­bined in a second adder 11 with a second control sig­nal which represents the noise level in the send path and is opposite in polarity to the first control signal. The noise-level signal is tapped from the send path directly at the output of the microphone and is fed to a low-pass filter 12 which rejects frequencies above 90 Hz. The output of the low-pass filter, like the first control signal derived via the band-pass filter 8, is amplified in a second control-signal amplifier 13 and rectified in a second rectifier stage 14. The diodes of this rectifier stage are so connected that the control signal appears as a negative potential at the ouput of this stage and, thus, at the input of the second adder 11. The latter delivers a sum signal in which the in­fluence of low-frequency noise signal components in the send path is eliminated or at least greatly reduced. This sum signal and a control signal derived in the re­ceive path (not shown) of the hands-free facility and applied over a line 17 are used to control the voice control circuit 1.

In Fig. 1, an additional circuit is indicated by broken lines. It contains a high-pass filter 18, an additional control-signal amplifier 19, and an additional rec­tifier stage 20. This circuit derives a noise-level signal from the frequency range located above the voice-­frequency range and is connected in parallel with the circuit consisting of the low-pass filter 12, the control-­signal amplifier 13, and the rectifier stage 14. It counteracts the control by the voice level, too, and serves to reduce the effect of interfering noise pro­duced near the microphone and containing high frequencies on the voice control.