1.7 The Electrical Interface

1.7 The Electrical Interface

Although digital interfaces are primarily concerned with the transfer of binary data, there are 'analog' problems to be considered , since the electrical characteristics of the interface such as the type of cable used, its frequency response and impedance will affect the ability of the interface to carry data signals over distances without distortion.

1.7.1 Balanced and Unbalanced Compared

In an unbalanced interface there is one signal wire and a ground, and the data signal alternates between a positive and negative voltage with respect to ground (see Figure 1.13). The shield of the cable is normally connected to the ground at the transmitter end, and may or may not be connected at the receiver end depending on whether there is a problem with earth loops (a situation in which the earths of the two devices are at different potentials, causing a current to circulate between them, sometimes resulting in hum induction into the signal wire). The unbalanced interface tends to be quite susceptible to interference, since any unwanted signal induced in the data wire will be inseparable from the wanted signal.

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Figure 1.13: Electrical configuration of an unbalanced interface.

In a balanced interface there are two signal wires and a ground (see Figure 1.14) and the interface is terminated at both ends either in a differential amplifier or a transformer. The driver drives the two legs of the line in opposite phase, and the advantage of the balanced interface is that any interfering signal is induced equally into the two legs, in phase . At the receiver any so-called 'common mode' signals are cancelled out either in the transformer or differential amplifier, since such devices are only interested in the difference between the two legs. The degree to which the receiver can reject common mode signals is called the common mode rejection ratio (CMRR). Although it is a generally held belief that transformers offer better CMR than electronically balanced lines, the performance of modern differential line drivers and receivers is often as good. The advantage of transformers is that they make the line truly 'floating', that is independent of the ground, and there is no DC coupling across them, thus isolating the two devices.

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Figure 1.14: Electrical configuration of a balanced interface. (a) Transformer balanced. (b) Electronically balanced.

The balanced interface therefore requires one more wire than the unbalanced interface, and will usually have lines labelled 'Ground', 'Data +' and 'Data -', whereas the unbalanced interface will simply have 'Ground' and 'Data'. For temporary test set-ups it is sometimes possible to interconnect between balanced and unbalanced electrical interfaces or vice versa, by connecting the unbalanced interface between the two legs of the balanced one, or between the ground and one leg, but often the voltages involved are different, and one must take care to ensure that the two data streams are compatible. Balancing transformers are available which will convert a signal from one form to the other.

1.7.2 Electrical Interface Standards

The different interface standards specify various peak-to-peak voltages for the data signal, and also specify a minimum acceptable voltage at the receiver to ensure correct decoding (this is necessary because the signal may have been attenuated after passing over a length of cable). Quite commonly serial interfaces conform to one of the international standard conventions which describe the electro-mechanical characteristics of data interfaces, such as RS-422 5 which is a standard for balanced communication over long lines devised by the EIA (Electronics Industries Association). For example, the AES3-1992 audio interface is designed to be able to use RS-422 drivers and receivers and specifies a peak-to-peak voltage between 2 and 7 volts at the transmitter (when measured across a 110ohm resistor with no interconnecting cable present), and also specifies a minimum 'eye-height' (see section 3.5) at the receiver of 200 mV.

(In passing it should be noted that standards such as RS-422 are mostly only electrical or electro-mechanical standards, and do not say anything about the format or protocol of the data to be carried over them.)

Unbalanced serial interfaces such as RS-232 are not used in audio and video systems, since they are not designed for the high data rates and long distances involved, and are more suited to telecommunications. The voltages involved in an RS-232 interface can be up to 25V, and thus it is not recommended that one should interconnect an RS-232 output with an RS-422 input (which may be damaged by anything above around 12volts). An RS-422 interface is designed to carry data at rates up to 100 kbaud over a distance of 1200 metres. Above this rate the distance which may be covered satisfactorily drops with increasing baud rate, as shown in Figure 1.15, depending on whether the line is terminated or not (see section 1.7.3).

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Figure 1.15: The RS-422 standard allows different cable lengths at different frequencies. Shown here is the guideline for data signalling rate versus cable length when using twisted-pair cable with a wire diameter of 0.51 mm. Longer distances may be achieved using thinner wire.

Other manufacturer-specific interfaces often use TTL levels of 05 volts over unbalanced lines, and these are only suitable for communications over relatively short distances.

1.7.3 Transmission Lines

At low data rates a piece of wire can be considered as a simple entity which conducts current and perhaps attenuates the signal resistively to some extent, and in which all components of the signal travel at the same speed as each other, but at higher rates it is necessary to consider the interconnect as a 'transmission line' in which reflections may be set up and where such factors as the characteristic impedance and terminating impedance of the line become important.

When considering a simple electrical circuit it is normal to assume that changes in voltage and current occur at the same time throughout the circuit, since the speed at which electricity travels down a wire is fast enough for this to be a reasonable assumption in many cases. When very long cables are involved, the time taken for an electrical signal to travel from one end to the other may approach a significant proportion of one cycle of the signal's waveform, and when the frequency of the electrical signal is high this is yet more likely since the cycle is short. Another way of considering this is to think in terms of the effective wavelength of the signal in its electrical form, which will be very long since the speed at which electricity travels in wire approaches the speed of light. When the wavelength of the signal in the wire becomes of the same order of magnitude as the length of the wire, transmission line issues may arise.

In a transmission line the ends of the line may be considered to be impedance discontinuities, that is points at which the impedance of the line changes from the line's characteristic impedance to the impedance of the termination . The characteristic impedance of a line is a difficult concept to grasp but it may be modelled as shown in Figure 1.16, being a combination of inductance and capacitance (and probably, in reality, also some resistance) which depends on the spacing between the conductors, their size and the type of insulation used. Characteristic impedance has been defined as the input impedance of a line of infinite length 6 . The situation at the ends of a transmission line may be likened to what happens when a sound wave hits a wall a portion of the power is reflected and a portion is absorbed. The wall represents an impedance discontinuity, and at such points sound energy is reflected. At certain frequencies standing wave modes will be set up in the room, at frequencies where multiples of half a wavelength equal one of the dimensions of the room, whereby the reflected wave combines constructively with the incident wave to produce points of maximum and minimum sound pressure within the room.

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Figure 1.16: Electrical model of a transmission line.

If an electrical transmission line is incorrectly terminated, reflections will be set up at the ends of the line, resulting in a secondary electrical wave travelling back down the line. This reflected wave may interfere with the transmitted wave, and in the case of a data signal may corrupt it if the reflected energy is high. The mismatch also results in a loss of power transferred to the next stage. If the line is correctly terminated the end of the line will not appear to be a discontinuity, the optimum power transfer will result at this point, and no reflections will be set up. A line is correctly terminated by ensuring that source and receiver impedances are the same as the characteristic impedance of the line (see Figure 1.17).

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Figure 1.17: Electrical characteristics of a matched transmission line.

The upshot of all this for the purposes of this book is that long interconnects which carry high frequency audio or video data signals, often having bandwidths of many megahertz , may be subject to transmission line phenomena, and thus lines should normally be correctly terminated. The penalty for not doing so may be reduced signal strengths and erroneous data reception after some distance, and such situations can sometimes arise, especially when audio data signals are sent down existing cable runs which may pass through jackfields and different types of wire, each of which presents a change in characteristic impedance. In video environments people tend to be used to the concepts of transmission lines and correct termination, since video signals have always been subject to transmission line phenomena. In audio environments it may be a new concept, since analog audio signals do not contain high enough frequencies for such matters to become a problem, yet digital audio signals may. It is not recommended, for example, to parallel a number of receivers across one source line when dealing with digital signals, since the line will not then be correctly terminated and the signal level may also be considerably attenuated.

1.7.4 Cables

The types of cables used in serial audio and video interfaces vary from balanced screened, twisted pair (e.g. AES/EBU) to unbalanced coaxial links (e.g. Sony SDIF-2) and the characteristic impedances of these cables are different. Typical twisted pair audio cable, such as is used in many analog installations (and also put to use in digital links), tends to have a characteristic impedance of around 90100 ohms, although this is not carefully controlled in manufacture since it normally does not matter for analog audio signals, whilst the more expensive 'star-quad' audio cable has a lower characteristic impedance of around 35 ohms. Typical coaxial cables used in video work have a characteristic impedance of 75 ohms, and other RF coaxial links use 50 ohm cable.

One problem with the twisted pair balanced line is that its electromagnetic radiation is poor compared with that of the coaxial link. A further advantage of the coaxial link is that its attenuation does not become severe until much higher frequencies than that of the twisted pair, and the speed of propagation along a coaxial link is significantly faster than along a twisted pair, resulting in smaller signal delays. But the twisted pair link is balanced whereas the coaxial link is not, and this makes it more immune to external interference, which is a great advantage.

Cable losses at high frequencies will clearly reduce the bandwidth of the interconnect, and the effect of this on the data signal is to slow the rise and fall times of the data edges, and to delay the point of transition between one state and the other by an amount which depends on the state of the data in the previous bit cell . The practical result of this is data link timing jitter, as proposed by Dunn 7 , which may affect signal quality in D/A conversion if the clock is recovered from a portion of the data frame which is subject to a large degree of jitter. Links which suffer HF loss can often be equalized at the receiver to prop up the high frequency end of the spectrum, and this can help to accommodate longer interconnects which would otherwise fail. This matter is examined further in section 6.4.1.

Thus a number of factors combine to suggest that the type of cabling used in a digital interconnect is an important matter. A cable is required whose characteristic impedance matches that of the driver and receiver as closely as possible and is consistent along the length of the cable. The cable should have low loss at high frequencies, and the precise specification requires a knowledge of the bandwidth of the signal to be transferred. Cable manufacturers can usually quote such figures in specification sheets or on request, and it pays to study such matters, especially when recabling a large installation. More detailed discussion of these matters is contained within the sections of this book covering individual interfaces.

1.7.5 Connectors

The connectors used in digital interfacing fall into a number of distinct categories (see Figure 1.18). First, there are the unbalanced coaxial connectors, normally BNC type, which differ slightly depending on the characteristic impedance of the line; 50ohm BNC connectors have a slightly larger central pin than 75ohm connectors and can damage 75ohm sockets if used inadvertently. RCA phono connectors, such as are often found in consumer hi-fi systems, are also used for unbalanced consumer digital audio interfaces using coaxial cable, although they are not proper coaxial connectors and do not have a controlled characteristic impedance. Both these connector types carry the data signal on the central pin and the shield on the outer ring.

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Figure 1.18: A number of connector types are commonly used in digital interfacing. (a) RCA phono connector. (b) BNC connector. (c) XLR connector. (d) D-type connector.

The XLR-3 connector is used for one balanced digital audio interface, and it has its roots as an analog professional audio connector. The convention is for pin 1 to be the shield, pin 2 to be 'Data +', and pin 3 to be 'Data -'.

The D-sub type of connector stems from computer systems and remote control applications, and it has a number of individual pins arranged in two or three rows. The 9 pin D connector is often used for RS-422 communications, since it allows for two balanced sends and returns plus a ground, whereas some custom digital interfaces (either parallel or multichannel serial) use 25, 36 or even 50 pin D-type connectors.

These are not the only connectors used in audio and video interfacing, but they are the most common. Miscellaneous manufacturer-specific formats use none of these, Yamaha preferring the 8 pin DIN connector, for example, as its digital 'cascade' connector in one format.



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

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