GB2297463A

GB2297463A - Improving reception in a mobile phone by controlling the bandwidth of a filter in the signal path in dependence on some signal parameter

Info

Publication number: GB2297463A
Application number: GB9602258A
Authority: GB
Inventors: Pertti Vaekiparta
Original assignee: Nokia Mobile Phones Ltd
Current assignee: Nokia Oyj
Priority date: 1995-01-27
Filing date: 1996-01-24
Publication date: 1996-07-31
Also published as: GB9602258D0; FI950380A; FI99068B; FI99068C; FI950380A0

Description

A METHOD FOR IMPROVING THE HANDS FREE OPERATION IN A MOBILE STATION AND A MOBILE STATION The invention relates to a method for improving the hands free operation in a mobile station or radio telephone.

When using a mobile station in a moving vehicle the received signal level may occasionally be very low due to a tunnel or other shadow region. Such a situation may result in a channel handover or a disconnection because of too weak a signal or the connection may be maintained as the signal quality stays barely acceptable for a relatively long time. In any case, there may occur even 10second periods with a very low quality of the received signal, which in an analog mobile station means that the received speech is very noisy. The problem is emphasized when using the hands free operation, as is often done when using a mobile station in a moving vehicle. The received audio signal is then considerably amplified, and the noisy speech is not only difficult to understand but also very unpleasant and annoying for the listener.In a moving vehicle, the ambient noise also increases the noise level, especially in the hands free operation, and makes it more difficult to understand the received audio signal.

In accordance with the invention there is provided a method for improving the hands free operation in an analog mobile station, where a signal is received from the radio path containing an audio signal modulated to a radio frequency, the received audio signal is demodulated, the quality of the received audio signal is measured, the received audio signal is band pass filtered, and the band pass filtered audio signal is taken to a speaker device, characterized in that in the band pass filtering of the received audio signal the pass band is narrowed in response to the quality of said audio signal dropping below a predetermined limit.

Embodiments in accordance with the invention may be based on the realization that, in the hands free operation, noisy speech can be made more easily distinguishable and, at the same time, more pleasant for the listener by narrowing, in the filtering of the received audio signal, the pass band, especially in the upper range, thus cutting out the most disturbing high frequency noise and yet maintaining the frequency band that in most cases is sufficient for the recognizable reproduction of a speech signal. The band may be narrowed in response to the deterioration of the audio signal quality below a predetermined limit.

In a situation where the received signal level is low and, therefore, the reproduced audio signal noisy the operation of the phone is improved using adaptive filtering of the received audio signal.

In the preferred embodiment in accordance with the invention, the pass band is narrowed from the 300 to 3200 Hz band typically used in analog mobile telephone systems to 300 to 2400 Hz. Embodiments in accordance with the invention can also be implemented by narrowing the pass band stepwise according to the degree of deterioration of the signal quality.

Deterioration of signal quality primarily means increased noise in the signal.

Since the increase of noise is generally directly proportional to the weakening of the received signal, the audio signal pass band is narrowed in the preferred embodiment in accordance with the invention in response to the deterioration of the received signal strength below a predetermined limit. In this implementation, it is possible to use the so-called RSSI signal (RSSI = Received Signal Strength Indicator) already used in mobile stations which indicates the strength of a signal received from the radio path.

A special advantage in accordance with the invention is that in the preferred embodiment it is possible to use the RSSI signal and that in addition to that there is need for only minor and easily implementable changes in the band pass filter of the receive branch audio frequency part and its control. Tests show that this simple method results in a considerable improvement in the convenience of use of the hands free operation of a mobile station.

Embodiments in accordance with the invention can also be applied in the normal operation of a mobile station in which speech is listened to through the earpiece of the hand set.

Below, there is a more detailed description of embodiments of the invention with reference to the enclosed drawings, in which: Figure 1 shows frequency responses in the band pass filtering of a received audio signal in an embodiment of the method in accordance with the invention, Figure 2 shows a simplified block diagram of a mobile station in accordance with the invention, and Figure 3 shows diagrammatically an embodiment of a band pass filter for a received audio signal in a mobile station in accordance with the invention.

Figure 1 shows the frequency responses for the band pass filtering of a received audio frequency signal in an embodiment of the method according to the invention. In normal operation, the response is represented by curve A and the pass band is from 300 to 3200 Hz. This is the pass band generally used in analog mobile telephone systems such as the NMT, TACS, AMPS and NAMPS.

In the transmit end, the audio signal is filtered with a band pass filter with the same pass band. The gradient of the line in Figure 1, -6 dB/octave, is due to the fact that the filter carries out in the same known manner a so-called deemphasis, which is an operation opposite to the pre-emphasis performed in the transmit end. In the pre-emphasis, the higher frequencies of the pass band of the modulating audio signal are more emphatically amplified and, correspondingly, in the receive end, the higher end of the band is attenuated more. When the quality of the audio signal drops below a predetermined value the pass band is limited in the upper range according to curve B so that the lower limit is still 300 Hz but the upper limit becomes 2400 Hz.

The method can also be implemented in the following manner: the audio signal pass band is narrowed by setting an upper limit of, say, 2900 Hz, 2600 Hz, or 2400 Hz, selectable according to the signal quality. In other words, the idea is that the poorer the signal quality, the more the pass band is narrowed. That way, the operation in accordance with the invention becomes more inconspicuous but at the same time the implementation of the invention becomes more complex.

As stated before, the deterioration of the signal quality, primarily means increased noise in the signal, which in practice is almost directly proportional to a decrease in the received radio signal strength. Therefore, in the preferred embodiment of the invention, the RSSI signal indicating the strength of the received radio signal is directly used to measure the signal quality and a predetermined voltage value of the RSSI signal is used as the predetermined limit for the deterioration of quality.

It is possible that the quality, of a received audio signal can be measured in other ways, too; e.g. the noise is separately indicated in the receive branch and the signal-to-noise ratio is determined for the received signal and a certain value of the signal-to-noise ratio is used as a predetermined limit for the deterioration of the signal quality. It is possible to use, in principle, almost any measurement system for the signal quality that primarily measures the amount of noise in the signal.

Figure 2 is a simplified block diagram of the preferred embodiment of a mobile station in accordance with the invention where the RSSI signal is used to measure the quality of the received audio signal and to control the narrowing of the pass band. In addition to the internal earpiece 10 and internal microphone 11 an external speaker 14 and an external microphone 16 may also be connected to the mobile station 1 for hands free operation. In addition, the mobile station comprises an antenna 2, duplex filter 3, block 4 comprising the receive branch radio frequency and intermediate frequency parts and correspondingly block 5 comprising the transmit branch intermediate frequency and radio frequency parts, and blocks 6 and 7 representing the audio frequency parts in the receive and transmit branches.The receive branch block 4 includes means for generating the RSSI signal, and the receive branch block 6 includes a band pass filter for the received audio frequency signal, with some of its possible frequency responses in accordance with the invention shown in Figure 1. To narrow the frequency response of the band pass filter when the strength of the signal received from the radio path is below a predetermined value the RSSI signal obtained from block 4 of the receive branch is brought to the microprocessor 17 controlling the operation of the mobile station. The band pass filter included in the audio frequency block 6 includes means for narrowing the frequency band in response to a control signal LPC. The microprocessor 17 uses signal LPC to issue a command to narrow the band in response to the RSSI signal received by the microprocessor indicating that the strength of the signal received from the radio path has dropped below a predetermined limit.

From the audio frequency block 6 the received and filtered audio frequency signal is taken in the normal operation to the internal earpiece 10 of the telephone or in the hands free operation to the external speaker 14.

Correspondingly, in the transmit branch, the signal from the internal microphone 11 of the telephone, in the normal operation, or from the external microphone 16, in the hands free operation, is brought to the transmit branch audio frequency block 7. The mobile station 1 is set into the hands free state with signal HF by means of switches 8 and 9. For the hands free operation the telephone also includes a usual attenuator block 12 in which, mainly to prevent acoustic feedback, the signal coming from the microphone is attenuated according to the strength of the signal coming to the speaker. The operation of the attenuator block 1 2 can also be controlled with signal HF that can be obtained from the microprocessor 17 controlling the operation of the mobile station.

Below it is explained in more detail an example of the generation and use of the RSSI signal in the method and mobile station in accordance with the invention.

The implementation of the receive branch may include an FM/IF detector circuit which may be, for example, of type Toshiba 31136FN EL which includes the generation of the RSSI signal. This circuit includes a mixer and receives the first intermediate frequency (45 MHz) in the receive branch and generates the second intermediate frequency (450 kHz) for the mixer output. After the filtering, the second intermediate frequency is taken again to this circuit for amplification and the amplifier generates at the same time a d.c. voltage proportional to the second intermediate frequency level for the RSSI output of the circuit. The higher the first intermediate frequency level coming to the circuit, the higher the d.c. voltage at the RSSI output. The dynamic range of the circuit is 70 dB and the level range for the first intermediate frequency is 1 5 to 90 dBu EMF.Corresponding to this range, the RSSI output produces a d.c.

voltage in the range 0.2 to 2.4 V. The microprocessor 17 controlling the operation of the mobile station receives the RSSI signal and reads it with its A/D converter. When the microprocessor 1 7 notices that the RSSI signal level has dropped below a predetermined limit it sends control signal LPC to the band pass filter in the receive branch audio frequency part instructing the band pass filter to narrow the pass band in the manner described above. It is a general specification of analog mobile telephone systems that a telephone should operate and meet a certain signal-to-noise ratio requirement even when the strength of the signal coming from the radio path is -116 dBm. The noise becomes particularly annoying only when the signal strength drops very near to this level.In the implementation in accordance with the invention it is therefore advantageous to set the limit for narrowing the pass band in close vicinity to this level in order to avoid unpleasant jumping between the normal band and the narrowed band in circumstances where the signal quality is poor but the audio is still satisfactory. The predetermined limit for narrowing the pass band can be, for example, the value of the RSSI signal corresponding to the signal strength of -115 dBm. The jumping between normal and narrowed band can be avoided by using a hysteresis where, for example, when the RSSI signal level drops to -115 dBm or below the control signal LPC will get the logical value 1 whereby the band is narrowed. This will happen e.g. for a value of -120 dBm.

However, the RSSI signal level has to rise to -115 dBm or higher e.g. -105 dBm before the control signal LPC is changed to a logical 0, whereby the band is changed back to normal. However, should the RSSI signal level only rise back to -110 dBm, the band will not be changed. A second possibility is to use time hysteresis where, for example, if the RSSI signal level drops to -115 dBm or less and has a value < -115 dBm for, at least, one second, then the band is narrowed. Again, if the RSSI signal level rises back above -115 dBm and keeps a value > -115 dBm for, at least, one second, then the band is changed back to normal.

In the manufacturing process the mobile station is calibrated in such a way that it, or more specifically the microprocessor 17 controlling it, knows on the basis of the RSSI signal level the exact level of the signal coming from the radio path to the antenna. As the noise level coming from the radio channel and circuits of the mobile station to the pass band of the audio frequency filter remains approximately the same all the time, the signal quality, ie. mainly the noise level, can quite well be measured by measuring the strength of the signal coming from the radio path.

Figure 3 is a simplified and diagrammatic presentation of an embodiment of a band pass filter for the received audio signal in a mobile station in accordance with the invention. Such filters are usually SC (switched capacitor) filters implemented with CMOS technology. A filter circuit implemented in this way is usually part of an integrated circuit including other circuit connections as well.

The filter described here as an example inputs signal FIN and outputs band pass filtered audio signal FOUT and has four filtering stages the first of which is a low pass stage LP1, the next a band pass stage BP, the one after that a high pass stage HP, and the last a low pass stage LP2. The invention is implemented in such a filter by adding in the low pass stages LP1 and LP2 of the CMOS filter circuit capacitors and switches to selectively switch the capacitors and an input for the control signal LPC that controls the switching of the capacitors. In the preferred embodiment, the control signal LPC comes from the microprocessor 17 controlling the operation of the mobile station as a response to the value of the RSSI signal monitored by the microprocessor.In practice, an implemented filter needs in both low pass stages LP1 and LP2 two switches and, correspondingly, two control signals opposite to each other and both comprising one control bit.

Above there was mentioned an embodiment in accordance with the invention where there are several alternatives for the narrowing of the pass band of the band pass filter according to the degree of deterioration of the signal. In this embodiment a filter like the one described above has to be modified more: it has to have more switched capacitors and switches and it requires more complex control signals for the controlling. Accordingly, the implementation of the control, preferably the microprocessor controlling the telephone, is then more complex. However, the implementation is in principle similar to a series of implementations consisting of one alternative, where the narrowing of the pass band is done on the basis of one limit value.

If the operation in accordance with the invention is used in a mobile station in the hands free operation only, it can be arranged to activate automatically when the telephone turns on the hands free operation. The arrangement can also be such that the user can select the operation of an embodiment in accordance with the invention either by a switch or with any usual method specified for the mobile station. It is possible to specify that the selection can be done only in the hands free operation or both in the hands free operation and in the normal operation of the mobile station. It is also possible to implement in the mobile station several alternatives of the operation in accordance with the invention, e.g. the above-mentioned implementation of one limit and one narrowing alternative or the implementation of several limits and several narrowing alternatives. Especially in the one-alternative implementation it is easy to see the possibility that the limit may be set or selected by the user.

Above it was described preferred embodiments in accordance with the invention but the implementation possibilities of the invention are not in any way limited to those embodiments. The band pass filtering of the received audio signal and the narrowing of the pass band as a response to a control obtained by measuring the quality of the audio signal can be implemented within the scope of the invention in many other ways, too. For the control it is natural and advantageous to use the microprocessor controlling the operation of the mobile station, but, within the scope of the invention, controlling can also be implemented by other means and in other ways known to one skilled in the art.

The present invention includes any novel feature or combination of features disclosed herein either explicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed.

In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A method for improving the hands free operation in an analog mobile station, where: a signal is received from the radio path containing an audio signal modulated to a radio frequency, the received audio signal is demodulated, the quality of the received audio signal is measured, the received audio signal is band pass filtered, and the band pass filtered audio signal is taken to a speaker device (14), characterized in that in the band pass filtering of the received audio signal the pass band is narrowed in response to the quality of said audio signal dropping below a predetermined limit.

2. The method of claim 1, characterized in that the pass band is narrowed by changing the low pass limit to a lower frequency.

3. The method of claim 2, characterized in that in the filtering of the received audio signal the pass band is normally 300 to 3200 Hz and it is narrowed to 300 to 2400 Hz.

4. The method of claim 1 or 2, characterized in that there are several predetermined limits for the deterioration of the quality of the received audio signal and the pass band is narrowed selectively according to the limit below which the quality of the audio signal drops.

5. The method of claim 4, characterized in that the low pass limit for the pass band is normally 3200 Hz and when narrowing the pass band the limit is changed selectively into the frequencies of 2900 Hz, 2600 Hz, and 2400 Hz.

6. The method of any one of the preceding claims, characterized in that the quality of the received audio signal is measured by indicating the strength of the signal received from the radio path and in the band pass filtering of the audio signal the pass band is narrowed in response to the fact that the strength of the signal received from the radio path drops below a predetermined limit.

7. The method of any one of the preceding claims, characterized in that it is used in the normal operation of an analog mobile station where the band pass filtered audio signal is taken to the earpiece (10) of the hand phone.

8. An analog mobile station including at least: receiver means (4) to receive from the radio path a signal containing an audio signal modulated to a radio frequency and to demodulate the received audio signal, audio frequency means (6) to receive the demodulated audio signal which include a band pass filter to filter the audio signal, normally having a first pass band (A), speaker means (14, 10) to which the band pass filtered signal is taken, measurement means to measure the quality of the received audio signal and to issue the signal indicating the quality, and control means (17) to receive the signal indicating the quality of the audio signal, characterized in that the band pass filter includes the means for creating and selecting at least one pass band (B) narrower than the first pass band according to a received control signal (LPC) and the control means (17) have been adapted to send to the band pass filter a control signal (LPC) for the selection of a pass band narrower than the first pass band in response to that the signal indicating the quality of the audio signal indicates that said quality has dropped below a predetermined limit.

9. The analog mobile station of claim 8 in which the receiver means (4) include means for indicating the strength of the signal received from the radio path and for generating a signal (RSSI) indicating the strength, characterized in that the measurement means for measuring the quality of the received audio signal comprise said means for indicating the strength of the signal received from the radio path and the signal indicating the quality of the audio signal is said RSSI signal and the control means (17) have been adapted to send to the band pass filter a control signal for the selection of a pass band (B) narrower than the first pass band in response to that the strength of the signal received from the radio path has dropped below a predetermined limit.

10. The analog mobile station of claim 8 or 9, characterized in that the first pass band is 300 to 3200 Hz and the band pass filter includes means for creating and selecting a pass band of 300 to 2400 Hz in response to that the signal indicating the quality of the audio signal shows that said quality has dropped below a predetermined limit.

11. The analog mobile station of any one of claims 8 to 10, characterized in that the control means are included in a microprocessor (17) controlling the operation of the mobile station.

12. A radio telephone operable to receive signals in a radio telephone system, comprising an adjustable filter having a frequency response defining the frequency range of audio signals derived from the received signals, and processing means operable in dependence on an indication of the quality of the audio signal to adjust the frequency response of the adjustable filter

13. A method for improving the audio quality of a radio telephone operable in a radio telephone system in which the frequency range of audio signals derived from received signals is defined by an adjustable filter, the method comprising adjusting the frequency response of the adjustable filter in dependence on an indication of the quality of the audio signal.

14. A radio telephone substantially as herein described with reference to Figures 1 to 3.

1 5 A method for improving the audio quality of a radio telephone substantially as herein described with reference to Figures 1 to 3.