GB2305580A - Data transmission over telephone systems - Google Patents

Abstract

In a method of transmission of digital signals over links intended for audio purposes, the method involves the generation of pulses with a recovery period between the pulses. The recovery period can be modulated with a compensating signal or no signal. A further example is where audio and digital data is transmitted simultaneously by modification of the audio signal before transmission and adding the pulse signals to the audio. The pulses once detected can be removed or filtered from the audio on reception.

Description

Methods of Modulation and Communication for Radio and Telephony Systems.

[Technical Field] The invention relates to the transmission of data over telephony circuits.

[Background] There are various forms of communication that are well known. Amplitude modulation is used for speech in which the amplitude of a carrier is varied with time. To transmit data over FM and AM telephony links various schemes have been used such as QPSK, FSK and Tone signalling systems. The invention relates to novel form of transmission of data over a telephony circuit that is fast, cheap to implement and relatively immune to problems of interference. It can also be used simultaneously to speech.

Title of Patent: Methods of modulation and communication for radio and telephony systems.

[Essential Technical Features] This describes: 1. A method of communication for data over telephony circuits resulting in improved reliability and data rates achieved with simple circuitry.

2. A variation on this method that allows for simultaneous voice and data communication.

3. Further variations on the above method that result in increased data rates.

The essential technical features are: Modulation of pulses, typically single sine waves or modifications of sine waves onto the transmitted signal with a recovery time between each one. The successive pulses to have similar characteristics in the frequency components so that any distortions in the transmission medium have no effect. The amplitude and/or phase, presence or non presence of these pulses can be detected. During recovery periods no signal or a compensating signal can be transmitted. The recovery periods can be constant or varied. These signals can be superimposed on an audio signal if the audio signal is modified during the time that the data is transmitted.It is not essential but the complete audio signal can be re-created by a summation process if the characteristics of superimposed signal are detected by correlation analysis, detection during a period of low audio activity or other method. Systematic nonrepetitive variation can be used as a method of encryption.

[Example] The method of Quadralure Phase Shift Modulation is well known. The drawback to this method is in some circumstances the where there is complex filtering of the audio tone distortion can occur making demodulation more difficult.

Tone modulation methods are well known. This have a limited data rate due to several cycles of tone being required to establish a bit of data.

Both these methods are not easily adaptable to the circumstance where it is required to transmit data and telephony simultaneously.

Various forms of phase shift and amplitude modulation have been used and adaptive systems have been used to overcome the variation in characteristics of voice telephony circuits. The disadvantage of using these methods is that an adaption period is required when the system 'learns' the characteristics of the transmission system.

In civilian and military environments messages must be short to make best use of available radio spectrum. An example of practical use would be 100-1000 vehicles reporting position every few minutes. Another example would be maintaining position information of personnel in a dangerous environment.

In many circumstances it is desirable to transmit data and voice simultaneously. Although this can be done on digital systems these may not be appropriate or economic in many circumstances.

A specific example is as follows: The invention can be produced in various forms: 1. Reference Phase Method.

A system similar or not similar to that in Fig. 1. used to generate the wave form in Fig. 2.

This is distinguished from QPSK in the following respects: The amplitude of the signal can be O or vary.

The phase shift is typically greater.

In Fig. 1. an independent oscillator is used to generate the required wave form. Two switches and a gain control block are used controlled by a microprocessor. These are synchronised by oscillator output so that a timer within the microprocessor system can be used to operate the switches at the required times.

In Fig. 2. another system is used. Here a d/a converter is used to generate the wave form.

Any combination of electronic components can be used to generate the wave forms in fig. 3.

Establishment of reference phase and amplitude.

At this stage the demodulator will establish the phase and amplitude of the signal. Although 3 sine waves are illustrated any number may be used.

At a time t after the end of the reference burst a further signal is transmitted in Stage 2.

Stage 2. provides for a signal that has 2 or more conditions at a time t after the reference: Typically these would be: Sine wave.

Sine wave with 180 phase shift.

No output.

Typically a 360 deg. segment of sine wave would be transmitted before a period of no output to allow the circuits to recover. This is recovery or unmodulated section. It is this feature that allows the signal to be easily detected although filtered through complex filter circuits.

The recovery period can be from .1 cycle to 5 cycles.

In a variation of this system the sine wave pulses are amplitude modulated.

A further variation is that the form of the sine wave or the frequency is changed to encode information.

A block diagram of a suitable demodulator is shown in Fig. 4.

In Fig. 4. the parts of the demodulator are: An input signal from the line/radio interface.

This will have the audio signal.

A limmiter or other circuit is used to convert the signal to a square wave. This is used to detect the zero crossing points and present signals to the microcomputer of a suitable amplitude.

An optional gain control block is used to adjust the signal amplitude input to an a/d converter. The a/d converter can be an integral part of the micro-computer. Other interconnecting items can be used. The a/d converter can be of any type and may incorporate a sample and hold mechanism.

The micro-computer program will typically: Find a reference time from the square waves.

Simultaneously measure the amplitude of the incoming signal using the a/d converter.

At a set time after the reference time TO make a series of a/d measurements.

From these measurements determine the sate 0,1,2. in the case of a system having the following states: Sine, 180 deg. shifted sign and no output.

At a later time after the recovery time a further series of measurements can be taken. From this can be calculaled the digital value of the signal.

If gain control is present than an adjustment of the gain can be made and the thresholds adjusted accordingly. The characterises of the signal are derived from the series of measurements. If the measurements are taken at the appropriate time a signal with a dv/dt of above a certain level can be coded into 0, below a certain level into a 0, negative and below a certain level to a 2. This is one example of a coding scheme.

According to the signal found at the detection point in the wave form the following automatic adjustments can be made but are not absolutely necessary: 1. Time for next detection.

2. Amplitudes for the various thresholds.

3. A coding algorithm be used to calculate the time for the next detection.

4. If the signal is super-imposed on another signal, for example audio, then the signal may be processed to remove the pulse and an emphasis circuit may be used to restore the attenuated frequency component. Appropriate components that need to be added the block diagram Fig. 4. are as follows Digital Signal Processor, or Delay line. summing amplifier, Digital to analogue block attached to the microcomputer.

Typically the signal may detected as follows: Detection 1. 0, 1, or 2.

Detection 2. 0, 1, or 2.

Detection 3. 0, 1, or 2.

For each 3 detection's there are 27 possible states and these are translated into letters of the alphabet. A shift can be used to encode numerals etc.

Methods of transmitting voice and data simultaneously are achieved as follows: The voice audio signal is selectively filtered so that when it is desired to transmit the single sine wave of with a high amplitude high frequency pulse containing the digital information the high frequency content of the audio signal can be atlenuated by a fixed percentage for example 50%. The digital pulse during this period can be summed with the audio wave form. On demodulation a correlation technique, or otherwise is used to determine the amplitude of the superimposed digital pulse. This is then subtracted from the audio wave form and through a process of further filtering the audio signal can be reconstituted without the digital information pulses. The digital information can be presented to the user in the form of words or pictures.

Claims (3)

[Claims]

1. A method of transmission and suitable apparatus for a form of pulse modulation in which the pulses vary in amplitude and phase but are separated by a recovery period.

2. A modification of the above method and apparatus in which a compensating signal is transmitted during the recovery period.

3. A further modification of the system in which the pulses are added to the audio signal that has been modified to reduce the amplitude of the frequency component at the required frequency, typically that characteristic of the pulse and on the receiving end the audio signal can be filtered to remove the pulse, or alternatively processed so that the original audio signal is completely restored.