,1969, No. 3 71 The use of digital circuits in data transmission P. J. van Gerwen Data transmission is the name given to the transmission of digital signals in such a form that they can be applied directly to a computer. Now that computers are being more and more widely used, it is becoming much more important to be able to transmit these signals reliably and at high speed. In this article it is shown that where data signals have to be transmitted over ordinary telephone lines, digital circuits have several advantages over conventional circuits containing inductors and capacitors. A survey is given of digital cir- cuits designedfor this purpose. The use of such circuits has made it possible to design a data transmitter in the form of an integrated circuit on a chip a few millimetres square. A significant development of the last few years is the transmission, but in order to use a telephone system growth of a new branch of computer technology: the for binary pulse trains an extra operation is required. combination of computer and telecommunications. To Telephone lines are, after all, designed for speech: only make the most economic use of the computer, it is signals at frequencies required for intelligibility are located at a central point and connected to the more or transmitted (ranging from 300 to 3400 Hz). Data sig- less distant places where the users are situated. The nals, however, have' a spectrum that also possesses a information to be processed then has to be supplied to d.c. component and a number of a.c. components out- the computer, often over a telephone line, and the side this band, including components at very low fre- resultant information is communicated to the user in quencies. Another difficulty is caused by the fact that the same way. The same situation is found where a carrier telephony systems with many channels are used computer memory store is used as an information pro- for long-distance communications. In this case there is cessing centre, e.g. for the book-keeping of bank trans- usually a small frequency difference between the carrier actions. Connection with the store allows any branch that is modulated in a channel at the transmitting end of the bank to have details of the state of customers' and the carrier that is .used for demodulation at the accounts without human intervention, and thus permits receiving end. Because ofthis there is a small frequency demands for payment to be accepted without delay. shift in the spectrum of the transmitted signals. This In this kind of procedure the information (the data) does not matter for speech, where only the spectral has to be transmitted in such a way that it can be distribution of the signals is of importance; for data directly processed by the computer; this is known as transmission, however, the shape of the signals is data transmission. Existing means of telecommunica- important, since it is determined by the time function. tion would seem to be the most convenient means for The shift in frequency upsets the harmonic relation this purpose, in particular telegraph and telephone between the components of the spectrum, and this may lines. Can these be used directly, without modification lead to more than the admissible level of distortion. for transmitting data signals, i.e. signals consisting of This distortion could also be aggravated by the fact series of binary pulses? Although telegraph systems are that the phase characteristic of telephone circuits is capable of handling binary signals, the bandwidth strongly curved, particularly at the edges of the trans- . available per telegraph channel is so small that the mitted spectrum. If one were to attempt to transmit speed at which the characters can be transmitted is far signals below 300 Hz in a telephone channel, the signals too low for modern data transmission. In telephone would be severely distorted. circuits the bandwidth is greater, giving a higher rate of However, it should be possible to avoid these difficul- ties by applying the extra operation mentioned above. P. J. van Genven is with Philips Research Laboratories, Eindhoven. In this operation the signals to be transmitted modulate 72 PHILIPS TECHNICAL REVIEW VOLUME 30 a separate carrier in such a way. that the principal com- phony, it is necessary to use filters in the transmitter. ponents of the modulated signallie within the frequen- As indicated in fig. 1, the data signal is usually put cy band available for telephony. The original signal is through a low-pass filter before being applied to the then obtained by demodulation with this "data car- modulator. After the filtering process, the higher- rier" in the receiver. Now if there is a frequency shift frequency components in the data signal are no longer of the spectrum because the signal has arrived via a present; the signal is then no longer binary. The output carrier telephony system, there will also be a shift in signal from the modulator is passed through a band- the data carrier. If the data carrier used for demodula- pass filter to suppress unwanted modulation products. tion in the receiver is given the same frequency shift, If both sidebands of the modulated signal are trans- which can be done by conventional methods, the mitted, the design of this filter presents no great prob- demodulated data signal again has the required shape. lems. In vestigial-sideband modulation however (see Amplitude, frequency or phase modulation may be below) this band-pass filter also has to nearly suppress used. Each of these methods has its advantages and one of the sidebands. The design of the filter is then disadvantages for data transmission, but we shall not much more difficult. go into this here. Fig. 2 shows the block diagram of a conventional receiver for data signals. A band-pass filter is used here The essentials of a data system at the input to suppress interference outside the re- Fig. 1 shows a block diagram of a conventional data quired frequency band. After demodulation the signal transmitter. The binary elements ofthe data signal, the again goes through a low-pass filter, and the original BP i/UV\r .----:::~::-:----,I____ - __ 1_ ~ Fig. 1. Block diagram of a conventional transmitter used for transmitting data via a tele- phone line. DATA data-signal source, Cl clock-signal source, Mod modulator, Carr carrier source, LP low-pass filter, BP band-pass filter, L telephone line. The waveforms ofthe signals are shown beside the appropriate connections. The modulation system used here is phase modulation. "bits", are usually supplied to the transmitter in time form of the data signals is then reconstituted with the intervals controlled by a clock signal, which consists of aid of the clock signal. a series of equally spaced pulses, The frequency of this signal, the clockfrequency, is thus equal to the number Digital circuits of bits per second, the bit frequency. Transmission using The conventional filters widely used in telephony, a clock signal is known as synchronous data transmis- made up from inductors and capacitors, can in prin- sion. The clock signal can be transmitted in one form ciple also be used for. data transmission. Their use, or another to the receiver, where it is used for reconsti- however, involves particular problems. Besides the tution of the data signal already demodulated with the attenuation characteristic (attenuation of the signal appropriate carrier frequency.' as a function of frequency) the phase characteristic The data signal modulates the carrier at the trans- (phase shift as a function of frequency) now has to be mitter in a modulator. The carrier signal can be pro- taken into account as well. To transmit the bits duced by an oscillator. Ifthis is tied to the clock signal, correctly it is desirable that the phase shift should be so that a. fixed relation exists between the clock fre- proportional to the frequency, in other words that the quency and the carrier frequency, the data system is phase characteristic should be linear. (In telephony the said to be fully synchronized. Instead of using a separate phase characteristic is not usually important.) This oscillator, the carrier signal can also be derived from makes it difficult to build conventional filters for data the clock signal. transmission, especially if a sharp cut-off is required at Since the output signal from the transmitter has to the edge of the frequency band; i.e. if the attenuation remain within the frequency band laid down for tele- characteristic has to have steep sides. There is then no 1 1969, No. 3 DIGITAL CIRCUITS IN DATA TRANSMISSION 73 alternative but to apply a phase correction. Another feature of digital circuits that makes their A new principle of filter design has now been devel- use for data transmission attractive is their great ver- oped which presents new and attractive possibilities. As satility. We mean by this that an existing system can will be shown, signals in certain frequency bands can often be combined with other units to allow operation be suppressed by means of delay elements. This makes at other transmission rates, and that other features such it possible to obtain an attenuation characteristic which as the attenuation characteristic of a filter can easily be . has steep sides yet still has a linear phase characteristic. modified. In conventional techniques this usually re- Such filters therefore require no phase correction. These quires an entirely new circuit design. filters are particularly attractive for digital signals, since they can be designed entirely with digital tech- Transmission rate and bandwidth niques, using bistable circuits (flip-flops) as delay ele- The binary elements of a data signal can be trans- ments.