Multitone Signalling System Employing Quenched Resonators for Use on Noisy Radio-telprinter circuits by H. K Robin, D Bayley, T. L. Murray and J. D. Ralphs. published in Proc IEE vol 110 No 9 September 1963 pp 1554-1568 paper presented at Convention on HF connunication 26 March 1963 authors work for the Diplomatic Wireless Service of the British Foreign Office 1. Introduction Talks about the requirements involving low power, simple antennas, low amount of traffic of high urgency. Prior systems mainly used morse code with skilled operators. The system is an audio frequency multitone signally system. Detection is performed with quenched resonator circuits. Other systems using quenched resonators are nominated as: Telemiprimeur Coquelet, predicted wave signalling (Doelz M L, Predicted-wave radio teleprinter in electronics 1954 no 27 p167) Kineplex, (Doelz M L, Held E T, Martin D L Binary data transmission techniques for linear systems, Proc IRE 1957 No 45, part 1 p 656) The existing equipment uses AF frequencies of 320 to 660 Hz. The signalling rate is 10 characters per second, which gives 100 words per minute or 75 baud equivalent in baudot. Each tone represents a character from the 32 character teleprinter alphabet. The name piccolo was derived from the distinctive sound. A Piccolo terminal takes up a cabinet 22 by 21 by 26 inches. The terminal can be switched from receive to send. Duplex operation requires two terminals. The performance is claimed to be 15 dB better than wide band binary fsk, 2db better than any other bit by bit encoding system, and within 8db of the Hartley-Shannon law limit. 2. Circuit Description 2.1 Fundamental principle talks about the response of a resonant circuit if its got losses, no losses, or the frequency differs. FOr the no loss resonant circuit the envelope of the response will increase linearly with time. The non tuned circuit will respond with a series of zero crossings and will not buid up. British Patent 19147: 1957 describes the basic system of a set of lossless tuned circuits. The intervals of the reesonant frequencies is arranged to be so that all except for one circuit will pass through a zero in its response to a tone. The frequency spacing is 10 Hz, the tone length is 0.1 second. 2.2 Sending system 32 LC oscillators spaced at 10 Hz generate the tones. Claimed to be A = 330 Hz, B = 340 Hz, C=350 Hz, letter shift = 630 Hz, blank = 640 Hz. The input is a punched 5 unit paper tape photoelectric reader. A master oscilator drives an hysterisis moter that rotates a shaft. This rotating shaft is the master time and phase reference. It generates tape transport, generates timeing waveforms, and creates the standby frequency. Five silicon photodiodes placed below the paper tape read the light shining through the holes from a linear filament incandescent light. A new tape head will use a gallium arsenide semiconductor lamp with the strobe pulse operating the 'lamp'. An amplitude discriminator determines mark or space and stores its result in a five bit binary store. Five wires take the data to a 10 by 32 diode matrix to damp all except one oscillator. The tone changes in frequency every 100 milliseconds as a new 5 unit combination is read by the photocells. The output is amplitude modulated by a 10 cycle per second square wave to give an extra synchronisation clue. 2.3 Receiving System 32 tuned circuits have positive feedback to balance any losses. the oscillators are damped at the beginning of the tone, the damping is released and the oscillations build up. The resonator outputs are detected separately. A comparator detects the highest level. It can detect between 0.3 and 10V and only needs a difference of 0.2 Volts. One out of 32 wires is pulsed to indicate the tone received. The oscillators are then all quenched ready for the next tone. The 32 wires go to a 32 by 5 diode matrix that converts to a 5 bit output which tranfers to a 6 bit shift register with start bit and 5 data bits. Timing is derived from the timeing system. 2.4 Synchronization the 10 Hz square wave modulation on the AF tone is used to derive a timing clock. The input is half wave detected and then synchronously detected by the clock square wave. This makes a DC offset that is applied to a reactance modulator on the master oscillator. ANother 10 Hz signal 90 degrees out of phase with this main signal is generated by the clock system. This is a phase locked loop. The DC signal also drives a small moter varying a variable capacitor in the master oscillator circuit to make the phase error zero. This is so that the circuit can hold synchronisation even when the signal has faded away. 2.5 AGC Automatic gain control is derived from the detected output of the best tone using a set of diodes. The DC drives a front panel meter and is fed back and added to a 10 kHz sawtooth wave wich is then put through a trigger detector. This makes a series of pules with mark to space ratio proportional to the output. This string of pulses is used to chop the input signal (wild!) This is in British patent application 6631:1960. The AGC has a time constant of 200 to 300 ms and adds 20 to 30 dB to the dynamic range of the receiver. 2.6 Combination of send and receive functions The send and receive oscillators are the same items. The character recognisers wirk in send mode, so that sent traffic will be printed too. The rotating shaft spins a disk with holes and shutters in it. A lamp and photocell then generate the periodic wave forms. 2.7 Standby facilities A 33rd tone is generated in standby. It sends mark signal to the printer. It is generated when the tape reading bulb is lifted, when there is no tape on the head or when the unit is swiched to receive. Standby tone is supposed to be the 65th harmonic of the character rate(650Hz) 3 Calculated performance 3.1 Error probability 3.1.1 2 tone system 2.1.2 2 tone 5 bit system 2.1.3 32 tone system 2.1.4 comparason of 2 tone and 32 tone system 3.2 Limitations on the application of the theory to the practical case Quenching time loses about 5% of the signal, This time also causes the comparason to occur when adjacent channel voltages have not reached zero. the 50/50 square wave at 10 % depth causes a residual voltage on the other channels leaving 7% unwanted signal. Detector limitations. Synchronisation error. With about 1 in 100 error rate synchronisation was still to with 5% correct. That is 0.005 seconds rms for the jitter error. Non linearity caused cross modulation. resonator frequency changing with amplitude changes. 4 Measured Performance 4.1 Random Noise and Fading 4.2 Impulse Noise and Interference 5 Comparason with the performance of other systems 5.1 Theoretical considerations 5.2 Comparason with the performance of various systems The systems Ideal systems 32 tone piccolo Bi-orthogonal code (less band width and errors thatn piccolo) Binary Phase modulation Pioneer V (BPSK coherent) Law's ideal telegraph receiver (2 tone) Law's double diversity 2 tone piccolo Hamming code Kineplex (differential phase modulation, 2 channels per carrier, 20 carriers, carrier spacing orthogonal, synchronisation off a separate carrier channel. Claimed to send 3000 bits per second in 2kHz bandwidth 0.66 cycles per bit). Predicted Wave signalling, (2 tones, tone spacing not orthogonal, synchronising information sent as start stop signalling on a third frequency, 23 cycles per bit) 7 frequency multitone system described by Jordan. Start-stop narrow deviation fsk (45 baud, +/- 50 Hz shift bandwidth 170Hz) Start-Stop wide deviation fsk (45 baud, +/- 425 Hz bw=1.6 kHz) morse (15 words per minute) 5.3 Results of Field Trials (a) Piccolo verses Hellschreiber on England to Europe radio link. Hellschreiber transmittted A1, Piccolo A3. Piccolo had 3 times less errors at 10dB less received signal. (b) FSK vs Piccolo DSB Frequency used was 2750 kHz (c)FSK vs Piccolo SSB power used was 50mW (d)FSK vs Piccolo ssb on 5890 kHz (e)FSK vs Piccolo ssb on 18.392 & 18.347 Mhz, England to Middle East. 6 Conclusions Performance is better than existing systems including morse. 7. Acknowledgement R D Neale and J W Nichols constructed and tested many circuits. R Muggeridge constructed many prototype designs and did field trials. 8 References 9 Appendixes 9.1 Transient Response of resonant circuits 9.1.1 General case 9.1.2 Lossless circuits 9.1.3 Lossy circuits - limits of linearity 9.2 Probability distribution of resonator outputs in the presence of noise 9.3.1 2 tone system 9.3.2 double diversity two tone system 9.4 Specification of current Piccolo model 9.4.1 Mechanical and environmental specifications dimensions (56*53*66 cm) weight 50kg with cabinet 77kg maximum temperature 50 degrees C 9.4.2 Electrical Uses transistors! Power supply 190-250V 50 Hz Consumption 60 W 9.4.3 Signalling router requirements Summary by Graeme Bartlett