Fullscreen HTML5Med Res (??MB)HTML4

BOOKSHELF Understanding Telephone Electronics Understanding Telephone Electronics, by Stephen J. Bigelow. Third edition published 1997 by Butterworth-Heineman. Soft covers, 232 x 187mm, 367 pages. ISBN 0 7506 9994 2. Price $34.95. Among the many different fields of electronics, telephone electronics has to be one of the most inscrutable to anyone who has a conventional electronics background. Partly this is because telephone systems were developed long before electronics technol­ogy appeared on the scene but there also appears to be a particu­lar mindset or philosophy amongst tele­ phone engineers which makes it difficult for ordinary mortals to penetrate. Which is all the more reason why this book entitled “Under­ standing Telephone Electronics” is so welcome. Originally produced by the staff of the Texas Instruments Information Publishing Centre in 1983 & 1984, it has since been revised and updated in 1991 and is now in its third edition. Most text books produced by Texas Instruments are well-written and it was the same with this book. There are 10 chapters in all and the first one goes right back to basics, talking about the telephone system as it is in the USA, local loops, the public switched telephone network (PSTN), channel bandwidths and levels and so on. It then becomes more technical, and mentions multiplexing, DC signalling, tone sig­nalling, digital codes and PCM. It also covers older technology such as Strowger switching and uniselect­ ors, crossbar switching, reed relays and then brief­ly goes on with radio relay links, wave guides and optical fi­bres. Chapter 2 covers the conventional telephone set and gives simplified circuits of a rotary dial set and anti-tinkle and speech muting. Anti-tinkle prevents the bells in phone handsets on a line from tinkling when one of the phones is being used to dial out. Now that phones use tone dialling it is not much of a problem any more. Carbon microphones are covered, as well as dynamic and electret types. Ringer circuits and ring cadence are described and, most importantly, there is a good description of the “induction coil” and the “hybrid circuit” carryovers from ancient times. Briefly, the hybrid circuit is a multiple winding transformer which converts the incoming two-wire circuit to a four-wire circuit with separate transmit and receive signals. As part of this description, there is a discussion of line balancing and sidetone. Sidetone is often misunderstood but it refers to the amount of sound from the microphone which appears in the earpiece. Sidetone is necessary so that the caller can hear his/her voice in the earpiece, so that he/she knows how loudly to speak. With insufficient sidetone, the caller will tend to shout. Conversely, too much sidetone leads to acoustic feed­ back whistles and so the caller tends to speak too softly. Chapter Three is devoted to electronic speech circuits and it is preceded with a good rundown on conventional speech cir­ cuits. The electronic speech circuit discussed is the Motorola MC34014 and it is covered in considerable detail. Once you under­­stand this device, you will have a good knowledge of virtually any small single IC phone although this chip does not incorporate tone dialling. This function is covered in Chapter Four. Again, as part of the coverage, the text gives good back­ground on rotary pulse dialling. Electronic dialling chips are briefly covered and DTMF (dual tone multi-frequency) is dis­ cussed, along with well-known decoders such as the Texas Instru­ments TCM5087. Electronic ringers are also featured, both single tone and multi-tone types, with the latter type used almost universally today. The chapter wraps up with a complete electron­ic phone using Motorola chips: an MC­ 34014 for the speech side, an MC34017 as the ringer and an MC145412 as the dialler. Most of this technology is now old-hat and most phones are now completely integrated with one chip providing all the func­tions discussed above. Hence, Chapter Five covers the continued on page 92 May 1997  77 Electromagnetic wave meter I am writing to you to suggest a design for an electromag­ netic wave meter in a future issue. With the recent interest in the effect that transmissions from mobile tele­phones and towers might have on the human brain-box, I thought it would be useful if these radio transmissions could somehow be measured. I had in mind that the device would be very simple and one that could be connected to a multimeter to show the amount of electromagnetic radiation in the surrounding atmosphere. These measurements could then be done and seen by anyone who might be concerned about the transmissions. (B. F., Morphett Vale, SA). • As you suggest, such a device is fairly simple in concept but not easy to produce in practice. They are normally referred to as signal strength meters and are widely used in RF communica­ t ions, particularly by installers of TV antennas and microwave dishes. The catch is that they are not simple devices in practice because you need a tunable antenna, usually a dipole with adjust­able tele­ scopic elements, and the circuit itself needs to be tunable to the frequency of interest. Most critical of all, it needs to be calibrated and have a flat response over a very wide range of frequencies, if it is to be of use in measuring most communications services. In fact, if it was to measure mobile phone and microwave services, it would need a calibrated response up to 2GHz or more. Commercial units just covering TV services can easily cost $1000 or more. With these aspects in mind, we do not envisage publishing a suitable circuit. Second, when operated with a speed control, you can expect that the brushes may wear more than usual and one brush may wear more than the other, by dint of being operated with DC rather than AC. The specified Triac has a rating of 40A and so there should be no need to substitute a higher-rated unit. Fan timer wanted for 10A currents. Second, we would strongly suggest the substitution of a cartridge fuse instead of the glass-link 2AG fuse. You would need to wire the unit so that heavy currents were not carried by the PC board conductors. In other words, the Active wire would go direct to a chassis-mounted cartridge fuse­holder and then to the A2 terminal of the Triac. The Triac itself would not be wired into the PC board but would need to be mounted on a fairly substantial heatsink. The whole circuit would there­fore need to be mounted in a larger case. Finally, we have two warnings about using a speed control with a heavy duty router. These appliances generate large amounts of heat in their windings and they depend on the internal fan running at full speed to keep the whole motor cool. This means that you must not operate the unit at low speeds, otherwise you run a high risk of burning it out. I have a need for a circuit to automatically switch off a mains powered appliance after a preset time; specifically, bath­room exhaust fans which are left on by my family and sometimes run all day. I envisage a simple timer circuit using a Triac for switching and, if possible, being directly mains operated; ie, no need for a separate power supply. The circuitry could be housed in wall cavity behind the switchplate or alternatively, in a small jiffy box, plugging directly into the fan outlet. (N. W., Berowra Waters, NSW). • Such a timer is certainly feasible but it is unlikely to be much cheaper as a do-it-yourself project than a commercial timer from HPM or Clipsal. They are available in a mechanical form with a big button which you press in and then it takes 20 minutes (adjustable) or more to pop out and switch off. These pushbutton timers are widely used in home units for stairwell lighting. Alternatively, you can obtain them in electronic form and they can be part of a multiple switchplate. They are available from electrical wholesalers. Bookshelf – from p.77 Chapter Seven covers the circuit at the telephone exchange end of the loop and again the old methods are detailed before the SLIC is discussed. This is another of those inscrutable telephone terms. It stands for Subscriber Line Interface Circuit. By the way, ever wanted to know what PBX stands for? The answer is “private branch exchange”. PABX? Try “Private Automatic Branch Exchange”. Also covered are voice frequency filters, codecs, DTMF receivers and cross-point switching. Chapter Eight covers network transmission including channel banks, multiplexers and repeaters. Chapter Nine is on more famil­iar ground and discusses modems, fax machines and fax modems. Finally, Chapter 10 focuses on wireless telephones and these include cordless phones in homes and cellular phones. This last chapter is a little out of date, since this area of technology has been moving so fast. Overall, this is a very useful text for anyone wanting to familiarise themselves with telephone technology. It won’t make you an expert but will give you a good introduction to tele­phone electronics. The book is well-priced at $34.95 and is available from the SILICON CHIP SC office. (L.D.S). MC34010 and the use of microprocessors. Also covered are speaker phones (for hands-free use) and the featured chip is the Motorola MC34118. This is similar to the MC­34018 used in the Hands-Free Speaker­phone published in the September 1998 issue. Chapter Six explains digital transmission techniques and much of this will be familiar to anyone who knows about A/D and D/A converters although there are special twists such as u-law and A-law compand­ers, delta modulation and time-division multi­ plexing. 92  Silicon Chip