Transmission Link

This invention relates to a transmission link having high interference immunity and, more particularly, to a transmission link for transmitting a succession of pulses separated by intervals to a receiving circuit of a receiver, in which each pulse is modulated with a frequency.

In known types of transmission links, such as infrared transmission links, each pulse signal is usually modulated with one frequency. In a transmission link of this type, pulse signals have a frequency of approximately 1 to 10 kHz, and the modulating frequency in a known system called "Infra-fern", lies at approximately 40 kHz. When disturbances appear in the pulse frequency or in the modulating frequency, the transmission link cannot always be kept free of disturbances despite the increased reliability through the use of a modulating frequency.

There is a need for a transmission link having a high interference immunity, low energy consumption, and simple design.

This and other needs are satisfied by the present invention in the first instance by using a transmission link having at least two frequencies for modulating the pulse signals. A separate evaluation circuit is provided in the receiver for each modulating frequency. The outputs of the evaluation circuits are each coupled to an AND gate. The signal appearing at the receiver circuit is only accepted when it is received without disturbance over each of the transmission links, each transmission link corresponding to a modulating frequency. The higher the number of transmission links, the more likely erroneous information will be excluded at the receiver.

The signal, which contains the message or information, can be a baseband or an encoded signal. The signal can be encoded by a variety of methods including dual phase, PPM, PAM etc.

In a further development of the present invention, the transmission link is constructed so that a first modulating frequency which modulates the pulses is modulated by a second, higher modulating frequency. This principle can be continued accordingly with a third or higher modulating frequency. In the case of two modulating frequencies, the first modulating frequency preferably lies in the 30 to 50 kHz range and the second modulating frequency lies in the 400 to 500 kHz range . A frequency of 455 kHz, which is held open in AM radio for intermediate frequencies, can be used in a technically favorable manner in order to avoid electrical shielding measures such as on an infrared input amplifier.

The transmission link can also be constructed so that after the pulse signals have been modulated with a first modulating frequency, these modulated pulse signals are then alternately modulated by a second modulating frequency and third modulating frequency. Fourth and higher frequencies can be used in this manner. The interference immunity can be increased when the additional modulating frequency has only a relatively small frequency differential compared to the previous frequency.

The present invention can be realized using many signalling methods, in particular, the transmission link can operate with infrared signals.

FIG. 1 is a block diagram of a transmission link.

FIG. 2 shows a waveform comprising a sequence of pulses and intervals that represent a signal or a sequence of signals.

FIG. 3 shows a waveform comprising pulses that are modulated with a first modulating frequency according to the present invention.

FIG. 4 shows a waveform comprising a sequence of pulses that has been modulated with a first modulating frequency and then modulated with a second, higher modulating frequency according to the present invention.

FIG. 4a is a block diagram of a transmitter using two consecutive modulating frequencies constructed according to the present invention.

FIG. 5 shows a waveform comprising a sequence of pulses that has been modulated with a first modulating frequency and then each modulated pulse is alternately modulated with a second and a third modulating frequency according to the present invention. FIGS. 3, 4, and 5 are not drawn to scale.

FIG. 5a is a block diagram of a transmitter using three modulating frequencies constructed according to the present invention.

FIG. 6 is a block diagram of a receiver comprising two evaluation circuits for two different modulating frequencies coupled to an AND gate.

Referring to FIG. 1, a transmission link 1 comprising a transmitter 2 and a receiver 3 is shown. A sequence of pulses 4 spaced by intervals 5 are transmitted in a medium such as infrared light to the receiver circuit of the receiver 3.

Referring to FIG. 2, a waveform comprising a sequence of pulses spaced by intervals is shown. The pulses and intervals signals can represent the high and low signals of a binary signal system. In this known system high and low signals represent the binary digits 1 and 0.

Referring to FIG. 3, a waveform of pulses generated using a known method of modulation is shown. In this method, each pulse signal of FIG. 2 is modulated with one frequency (e.g. f1). In the prior art, modulation stops here. With the present invention the waveform of FIG. 3 is further modulated, i.e., according to the present invention, several frequencies are used to modulate the pulses.

Referring to FIG. 4, a waveform of one full pulse and a part of a pulse generated using a method of the present invention is shown. In this embodiment, two frequencies, f1 and f2, are used to modulate the pulse signals. The first modulating frequency f1, corresponding to f1 in FIG. 3, can have a frequency magnitude in the 30 to 50 kHz range, and the second modulating frequency can have a frequency magnitude in the 400 to 500 kHz range.

Referring to FIG. 4a, the transmitter 2 for implementing the aforementioned method is shown. Pulses are modulated by a first modulator 41. The first modulator 41 is coupled to a second modulator 43. Pulses modulated by the first modulator 41 are then modulated by the second modulator 43. The resulting modulated waveform is then transmitted by the IR transmitter 45.

Referring to FIG. 5, a waveform of pulse signals generated using another method of the present invention is shown. In this method of transmission, the pulse signals are modulated using three modulating frequencies. In a first step, the pulse signals are modulated by a first modulating frequency to obtain a waveform such as that shown in FIG. 3. Then, pulse signals are alternately modulated with a second and third modulating frequencies.

Referring to FIG. 5a, a transmitter which alternately modulates pulses using second and third frequencies to obtain the waveform of FIG. 5 is shown. Pulses are modulated at a first frequency by a first modulator 5. The first modulator 51 is coupled to a multiplexor (Mux) 53. The input waveform is also coupled to a flip-flop 52. Each pulse signal appearing at the input of the first modulator 51 causes the flip-flop 52 to alternate its output between a high and low state. This alternating output controls the output of the Mux 53. Mux 53 alternately sends modulated pulses from the first modulator 51 to a second modulator 55 having a second fequency and a third modulator 57 having a third frequency. The second modulator 55 and the third modulator 57 are coupled to an IR transmitter 59. Every pulse modulated by the second modulator 55 and the third modulator 57 is transmitted by the IR transmitter 59.

The methods mentioned above for modulating pulse signals can be used in conjunction with known methods for encoding messages such as dual phase, PPM, PAM, etc.

Referring to FIG. 6, a block diagram of the receiver 3 is shown. The receiver 3 comprises at least two evaluation circuits 6 and 7 and an AND-gate circuit 8. The modulated pulses appear at the inputs of the evaluation circuits 6 and 7. The evaluation circuits 6 and 7 correspond to the two modulating frequencies f1 and f2. Therefore, only pulse signals having a modulating frequency equal to f1 or f2 will pass, respectively, through the evaluation circuits 6 and 7. The outputs of the evaluation circuits 6 and 7 are concatenated in an AND-gate circuit 8. Therefore, the evaluation circuits 6 and 7 block all noise components appearing in the transmission signal.

Infrared signals can be used as the medium for the transmission link when modulating frequencies have a magnitude of approximately 455 kHz.