A METHOD FOR USE IN A TELELOOP SYSTEM, A TELELOOP SYSTEM AND AN AMPLIFIER FOR A TELELOOP SYSTEM

Abstract: The invention relates to a method for amplifying a signal in an amplifier for a teleloop system, the method comprising generating the output signal by means of pulse width modulation. The invention further relates to a teleloop system comprising a teleloop cable, an amplifier connected to the teleloop cable and means within the amplifier for generating an output signal by means of pulse width modulation.

Description: TITLE A method for use in a teleloop system, a teleloop system and an amplifier for a teleloop system FIELD OF THE INVENTION The invention relates to teleloop systems, particularly to amplifiers for amplification of signals, which are supplied to a teleloop and to the method of amplification. BACKGROUND OF THE INVENTION Teleloop systems are well known assistive listening devices used for generating a magnetic signal, which can be picked up by a telecoil in a hearing aid worn by a hearing impaired person or by a tele-receiver. Teleloops are commonly used in public facilities, such as churches, railway stations and the like. A typical teleloop system comprises a teleloop cable and an amplifier adapted to amplify an input signal from a microphone or the like. The purpose of the amplifier is to make a suitable amplification of the signal supplied to the amplifier in order to generate a magnetic field of a magnitude sufficient for the receivers, e.g. the coil in the hearing aid, to sense this signal and process this further for output to the user. The impedance in the teleloop cable is constituted by a self-induction connected in series with a resistance. When a linear amplifier is connected to such impedance an output effect corresponding to the output voltage multiplied with the output current is expected. However since only real effect is disposed in the resistance a blind effect is disposed within the amplifier. In the previously known amplifier of the linear type a significant part of the effect supplied to the amplifier is therefore converted to heat in the amplifier. This requires a significant cooling surface area and a power supply having a capacity significant larger than the necessary output. The result is a high power consumption and high production costs. One objective of the present invention is to provide a method for amplifying a signal in a teleloop system, which will make it possible to lower the production costs and the power consumption of the teleloop system. A further objective of the present invention is to provide a teleloop system, where the production costs and the power consumption of this is lowered. A still further objective of the present invention is to provide an amplifier for a teleloop system, where the production costs and the power consumption of this is lowered. SUMMARY OF THE INVENTION According to the invention first objective is achieved by means of a method as defined in claim 1 of this application. By utilizing a pulse width modulation in the amplification of the signal the amplification has been achieved in the time domain and the heat generation will remain limited since the blind effect disposed in the amplifier is reduced. By this it is possible to dimension the cooling surface area smaller. The capacity of the power supply can at the same time be reduced. Both these factors contribute to lower production costs. Furthermore the operation costs are reduced due to the lower power consumption. The invention further lead to a possibility of increasing the output from the teleloop system at a maintained power consumption and hereby allowing a hearing aid user to reduce gain in his or her hearing aid with possible audible benefits such as lowering of noise and distortion. According to the invention the second objective is achieved by means of a teleloop system as defined in claim 3. For the same reasons as mentioned above it is hereby possible to dimension the cooling surface area smaller and the capacity of the power supply can at the same time be reduced. This may reduce the production costs and the power consumption. According to the invention the third objective is achieved by means of an amplifier as defined in claim 5. For the same reasons as mentioned above it is hereby possible to dimension the cooling surface area smaller and the capacity of the power supply can at the same time be reduced. This may reduce the production costs and the power consumption as the efficiency has been improved. Advantageous embodiments are described in the dependent claims. The invention may further advantageous comprise the elements or part thereof as described in the detailed description of the preferred embodiment, with the advantageous functions as described in connection with these. The invention will be described more detailed in the following description of the preferred embodiment with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing showing a preferred embodiment of the amplifier according to the invention; FIG. 2 is a schematic drawing showing the frequency /voltage characteristics of the amplifier; FIG. 3 is a schematic drawing showing the principles of the PWM generator; FIG. 4 is a diagram showing a PWM signal. DESCRIPTION OF THE PREFERRED EMBODIMENT From FIG. 1 it appears that the amplifier comprises a highpass filter 1 followed by a lowpass filter 2. The highpass filter 1 is of a variable type controlled by a frequency compensation control 12, which preferably is automatic and which gets input from a current transducer 7 at the induction loop or the teleloop 6. The signal from the lowpass filter 2 is mixed with a filtered (filter 3) feedback signal 4. The mixed signal is lead to an amplifier 11 and afterwards to a PWM generator 10 and to a driver 9 before the output stage 8 is reached. After the output stage the signal is lowpass filtered (filter 5 and led to the induction loop 6. The feedback signal 4 is obtained from the output before the lowpass filtering 5. This feedback signal is likewise led to the pulse width modulation generator 10. The purpose of the amplifier is to generate current with constant amplitude and a defined frequency response at a given load, the teleloop or induction loop. The teleloop is a cable arranged as a single loop coil. The cable can be equalized by an impedance (Z) consisting of a self-induction (L) and a resistance (R) connected in series. If a current is applied to the impedance the characteristics as shown in FIG. 2 appears, f, and f u are the lower and the upper border frequency, respectively and f o is the border frequency of the teleloop. f 0 is determined by L and R, which are dependent on the area of the teleloop and the cross section area of the cable, and may therefore vary in the entire bandwidth of the amplifier. The amplifier comprises three main elements, which are a signal adaptation part, a regulation part and an output stage. The signal adaptation part: Tone control: The loop loads the output stage. The frequency characteristic of the output stage can therefore not be completely linear between f, and f u . The tone control is used for correcting this error. A linear characteristic is necessary for achieving optimal f 0 correction. The tone control is further used for tone regulation of the input signal. f 0 correction: The circuit forms a variable treble raising element. The knee point frequency f 0 is determined by a digitally controlled potentiometer. f u of the amplifier is likewise determined by this circuit. Attenuator: A voltage-controlled attenuator is used for protection purposes (temperature and excessive current) Clipper: Due to the load of the output filter primarily by a self-induction there will be significant resonance at frequencies 3-4 times the f u . If the amplifier is heavily loaded the output stage will clip against the power supply, which will lead to increased distortion, as the output filter will start oscillating. The clipper circuit will limit this phenomenon as the input signal is damped before the output stage is clipping. By using the supply voltage for the output stage as reference it is ensured that the limitation always is in relation to the actual supply voltage. Filter: A filter is applied serving the purpose of damping the input signal to a level where the output filter is not oscillating. Controller: A micro-controller, an AD converter and a tone generator constitute the controller. The controller sends a sine signal in the loop and the ADC measures the resulting current. Based on measurements at different frequencies the necessary correction is carried out. The regulation part: PWM generator: A crystal controlled oscillator generates a basic clock signal for the PWM generator. A logic circuit divides the frequency to the desired switching frequency. A schematic embodiment of the generator is shown in FIG 3. In FIG. 3 Al is a operation amplifier coupled as a integrator and A2 is a comparator , the output of which represents the output of the amplifier. The coupling reacts as a common inverting amplifier coupling, as Al will adjust its output voltage at OV between the + and - inputs. The output signal will shift at a frequency corresponding to the clock frequency. If a positive signal is applied to Rl (Uin) Uin is mixed with the clock signal. Al will try to maintain OV between the inputs. Hereby the triangle signal is displaced in negative direction. Most of the triangle signal will be below OV. The negative output pulses will therefore be broader than the positive output pulses. As R3 creates a coupling the mean value of the output signal is proportional with the input signal. The triangle signal appears from FIG. 4. Coupling: In order to achieve a low distortion the PWM generator is coupled back to the input side of the generator. The feedback signal comes directly from the output stage and is damped by a band stop filter. Current limitation: In order to avoid to high currents levels in the output FET's the current level is monitored and in case the current exceeds predetermined levels the system goes into a shut down mode and remains there until the current is again at an acceptable level. The output stage: Push-pull coupling: Two FET transistors are arranged in a +- supplied push- pull coupling. Output filter: the filter functions as a demodulator. The PWM signal is integrated and the amplified input signal is regenerated. The filter furthermore removes noise. Current measurement: The output current is used for determining the output voltage, which is used for f 0 correction. Temperature measurement: The temperature is measured. At to high temperature the signal is damped and the temperature is reduced.

Claims: CLAIMS 1. A method for amplifying a signal in an amplifier for a teleloop system, the method comprising: generating the output signal by means of pulse width modulation. 2. A method according to claim 1, where an input signal is pulse width modulated, is successively amplified and finally demodulated. 3. A teleloop system comprising: - a teleloop cable an amplifier connected to the teleloop cable - means within the amplifier for generating an amplified output signal by means of pulse width modulation. A teleloop system according to claim 3, where the amplifier comprises means for pulse width modulating an input signal, means for amplifying the pulse width modulated signal and means for demodulating the amplified pulse width modulated signal. An amplifier for a teleloop system, the amplifier comprising means for generating an output signal by means of pulse width modulation. 6. An amplifier according to claim 5, where the amplifier comprises means for pulse width modulating an input signal, means for amplifying the pulse width modulated signal and means for demodulating the amplified pulse width modulated signal.

Inventors: JOERGENSEN LEIF, JOERGENSEN, LEIF
Applicants: JOERGENSEN LEIF, LOGIA A S, JOERGENSEN, LEIF, LOGIA A/S
Application year: 1999
Main classification: H04B5
IPC codes: H03F3, H04B5, H04L25, H04R25, H04R27