 A Fanfare Publication |
| "Rediscover FM-Stereo"; |
| by Marv Southcott |
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"A short musing
on the hows and whys of radio reception, and the means by which one might go about improving it." |
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Foreword When you think of it, reception of any kind of radio signal is a pretty amazing thing. Yet, it is something most of us take for granted as a particularly common occurrence. As a result, many would find it difficult to envisage the kind of excitement Mr. Marconi must have felt when he successfully transmitted the world's first 'wireless', transoceanic telegram in 1901, Even though it was just the single letter "S" in Morse code, it marked the beginning of an era in which radio would evolve into one of the most significant communication mediums known to man. At the end of 1985, there were close to 10,000 radio stations known to be operating in the U.S, Not surprisingly, more than 50% of these were FM stations, which continues to prove that most listeners have opted for the better fidelity and relative interference-free clarity of FM over AM. The American Heritage Dictionary defines the world "fidelity" as; "The degree to which an electronic system, such as a radio or phonograph, accurately reproduces at its output the essential characteristics of its input signal." And, nowhere is that a more critical prerequisite than in (a) the broadcasting of 'fine arts' programming, and (b) the reception and accurate reproduction of that program signal. At the broadcasting end, great progress has been made to improve on this standard. Most stations have installed 'cleaner' link-ups between studio and transmitter site. Low-noise, fibre optic telephone transmission lines and microwave air links have all but replaced the noise-ridden telephone wire. Add to that the compact disc, and the kind of high tech playback equipment for vinyl and tape now available and stereo has truly been given a whole new lease on life. Obviously, the segment of broadcasters most dependent on this new technology are those involved in the broadcasting of 'fine arts' programming. Here the term "fidelity" applies more as the station's mandate to its listeners rather than simply an endeavour. Similarly, manufacturers of FM tuners whose designs truly appreciate the needs of the fine arts listener share that responsibility. It is our hope that this booklet will provide some insight into this wonderfully inexpensive and highly entertaining medium called 'Stereo FM'. However, if past experiences with, the sound of FM have left you cold, we ask that you take the time and listen closely to the FM station of your choice. And, if you will allow us a commercial word, do so on a FANFARE FM tuner. There is an excellent chance that you will begin to 'Rediscover Stereo FM' all over again. Happy Listening AM and FM, THE FUNDAMENTAL DIFFERENCE The term 'FM' refers to a 'frequency modulated' method of signal modulation transmission and AM refers to 'amplitude modulated' signal modulation transmission. The term "modulation" refers to any information being carried by the signal, such as speech, music; or any kind of sound that is being brought to you via a 'carrier wave'. For a moment though, let us deal with the signal's 'amplitude' separately from its 'frequency' as a means of modulating the signal. In an AM transmission, the signal, or 'carrier', remains constantly at its allotted frequency, with the information being carried as variations of the signal's amplitude (look upon it as 'loudness'). Once received, the tuner 'skims' this information off the carrier and translates it into an audible signal for amplification by the tuner's audio amplifier stages. Unfortunately, most noise is amplitude modulated as well. FM, on the other hand, carries its information as a variation of the carrier's frequency around a central point. Even though the tuner is set to a specific frequency (FM station), the signal it is receiving will be changing in frequency very slightly with each bit of sound or modulation. In FM, the modulation is said to 'deviate' from the tuned or 'carrier' frequency. The deviating signal is translated instantly and continuously into an audio signal that varies in frequency and depth with the deviation of the signal from its central point. Also, noise does not deviate in frequency much therefore it is not as much affected by noise as with AM. The result of this process is usually a remarkably clear, crisp, FM-signal (in mono or stereo), the quality of which is determined by (a) the quality of the transmitted signal (noise level, dynamic range etc,), and (b) the sensitivity, selectivity and level of reproductive capability (fidelity) available in the particular FM tuner or receiver being used. AM, FM and DISTANCE It is here we can begin to really appreciate the problems inherent in getting a radio signal from transmitter site to your home receiver, Lower frequency AM signals travel much farther than do the very high frequency (VHF) FM signals and, in doing so, can easily 'bump' into other signals on the same frequency that are originating from AM stations hundreds of miles away. In contrast, FM broadcast band signals are not usually bothered by signals from distant stations on the same frequency. This is due to the relatively short distance VHF signals can be transmitted over the earth's surface (compared with the lower frequencies used on the AM connmercial broadcast band). With FM, an interfering signal from a distant station on the same frequency will be dealt with through the FM tuner's uncanny ability to 'capture' only the strongest FM signal available. Add to that FM's greater signal clarity, greater dynamic range and its lesser susceptibility to man-made noise and the advantages it offers become quite evident. IMPROVING YOUR FM RECEPTION Aside from a receiver to translate the signal, the most important part of any reception system is the antenna. If you recall the ancient crystal receivers used when radio was a pup, the efficiency of the antenna was what determined signal strength and reception distance. Many modern receivers and tuners now have such phenomenal sensitivity that relatively distant signals can be received clearly with only a small antenna. However, along with such sensitivity comes the need for shielding against interference. Hence the need for a metal cabinet. Curiously, this brings us full circle to the point where some type of antenna is now required for even the strongest signal. The most appropriate antenna is one that is tuned to receive the FM broadcast band of 88 tbrough 108 MHz. The best placement is the highest possible location. Care must be taken that it be kept away from obstructions such as walls, chimneys, etc. Also, it should not be placed near metal of any type, including furniture, eavestroughing, railings, beside TV masts or near their `guy' wiring. Such influences can reflect signals and create multipath problems. As well, the antenna could become detuned, thereby sapping its efficiency. Of the FM broadcast band antennas available, the following are most common; |
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Bidirectional Dipole (Fig.1) [Picture as a 60" piece of 1/4" metal tubing shaped into a round-cornered rectangle of about 3"(h) x 26"(w) with a space of about 3" between where the ends should meet . A conductor from each side of a 300 ohm twin lead-in wire is then attached to each of the ends.] It's receive pattern, as viewed from the antenna is very broad indeed, like that of a blast from an old blunderbus musket and the pattern of coverage that blast might have. We then view the 'pattern' as the area from which the signal can be received. The dipole is the most commonly used antenna for FM reception. It is also the basic element in any yagi antenna. The dipole receives equally well in a direction at right angles to its length (referred to as `front' and `back'). However, as the angle changes from the center towards the outside or ends, its sensitivity falls off markedly. n.b. The single, 1/2 wave, dipole design exists as an industry reference (0dB) point for most TV and FM broadcast band antennas. You have seen this design in its crudest form as the ubiquitous, `wire dipole' found packed with most FM tuners and receivers. Unidirectional Dipole(Fig.2) [Picture as a a short boom on which the above bidirectional dipole is attached perpendicular to, at its mid point, to one end of the mast. At the other end is attached, again at its mid point, and in perpendicular fashion, a single metal tube of about 25"". Its receive pattern is somewhat like a shotgun] With the addition of a second element, called a `reflector, it is able to concentrate its signal pickup mostly in one direction. This type of antenna mould be recommended where the desired station are in one general direction and moderately distant. Multi-element Array (Fig.3 ) [Picture the unidirectional antenna above with a longer central boom and a number of elements of predetermined length added at precise points, in perpendicular fashion, to the boom in front of a dipole. If one might liken the receive pattern of this antenna to the preceding two types, The multi-element array could be referred to as a rifle] This is a unidirectional antenna capable of pulling in very distant statons due to its high gain. However, this gain is directional and requires that the antenna be rotated each time you tune to a station that is `around the compass' from the one tuned previously. A good quality antenna of this type, mounted high on a mast, should provide you with the best, unaided, FM reception possible. `Yagis', `collinear arrays' and `log periodics' are the most common of these high efficiency, multi-element array types. Turnstile(crossed dipole) - (Fig.4) This design came to pass in an attempt to establish an omni-directional receive pattern, without the need for rotating the antenna. Unfortunately, tests conducted on this design have actually shown a reduction in efficiency (about -3dB) when compared with the results obtained with a single, bidirectional dipole. Vertical 1/2 Wave(Fig.5 ) This antenna is the newest and perhaps most revolutionary design on the market today. There are a number of distinct advantages offered by this design, over the dipole. First. it is fully omnidirectional in receptive capability. Second, it is receptive only to the vertically-polarized portion of the signal and is somewhat less sensitive to multipath interference. Another advantage is the gain provided, which is in the area of 2.5dB over that of the dipole. Finally. being a 1/2 wave design. no 'ground plane' is necessary for its function. It can be installed on a mast or on any surface (metal, wood, fiberglass etc.), including that of a car truck RV or boat. An excellent example of this antenna type is the Fanfare FM-2G. GAIN AND THE ANTENNA The perfect antenna receives equally well in any direction. However, because the desired concentration is the above ground level, a way has been discovered to transfer the antenna's receptive capability from one direction and 'add' it, electrically, to the antenna's receptive capability in another direction. Thus the antenna's directional response and its efficiency are improved dramatically. The best example of this is the "multi-element" antenna referred to earlier. Here, they have taken a yagi design, which has the dipole antenna as its basic element, and added a reflector behind the dipole to establish a concentration in another direction. The reflector's location is now called the "back". To the front have been added 'director' elements that establish a tuned attitude towards an established band of frequencies. In this case it would be the FM band of 88 - 108 MHz. The antenna's directivity is further 'shaped' by moving its concentration, again electrically, to a more forward attitude. The result is an antenna that has been 'desensitized' to signals arriving at its sides and back and has been made ultra-sensitive to signals arriving at its front. The amount by which it has become more sensitive, in relation to that of a single dipole, is called 'gain'. In terms of signal voltage, each 6 dB of gain represents a doubling of the signal voltage over that received using the reference dipole. (in this context, the dipole is referred to as having 0 dB gain.) ANTENNA DOWNLEAD CABLING There are two types of cable in general use for domestic, FM antennas. One is the flat, twin-lead cable that has been used since TV was first introduced. The second and most popular type is the 75 ohm coaxial cable; also referred to as RG59/U cable. Typical line loss for RG59/U is about 3 dB per 100ft at 100 MHz. More recently a 75ohm coaxial cable bearing the designation RG6/U has become popular. It is available in a single-shielded version and in a 'quad-shielded' (4 layers of shielding) version. This type of cable has a larger (18ga) center conductor and is capable of passing mpore signal. Therefore the line loss/100 ft. will be less than RG59/U. The RG6/U, 'quad-shielded' variety is right for the times as it offers maximum shielding against interference from extraneous interference from the likes of computers, CD players and other man-made interference-makers. However, while RG6/U has a similar spec. to RG59/U we do not recommend it be mixed with RG59/U in the same feed line. In fact, we offer this word of caution about coaxial cables in general. Do not mix different types of coaxial cable unless their technical specifications match exactly. ie. 'RG59/U-type' cable is not exactly the same as RG-59/U. Nor is cable labelled simply "75 ohm" to be regarded as the same as either of the previously-mentioned types. Speaking generally, the better grade of 300 ohm twin lead has about one half the losses that occur in coaxial cable. Unfortunately, its added efficiency is only of valuable in areas where interference is very low. Coaxial cable, on the other hand, provides (a) much better protection against interference pickup, and (b) a much easier and less expensive installation. The tradeoff of less signal for a quieter and less expensive installation is often well worth it. However, the rural listener who is not particularly bothered by local interference might want to take advantage of the higher signal yield offered by a good grade of 300 ohm twin lead when a long length run of lead-in cable is necessary. Typical line loss for a good grade of 300 ohm, twin lead is about 1.1 dB per 100 ft. at 100 MHz. Finally, would-be antenna installers should keep the following in mind as they plan their installation. The two major contributors to lasting installation efficiency are; (a) a neat, well-thought-out cable run and, clean, tight connections that have been protected against the elements. For coaxial cable, a little silicone grease on the connector shell threads will help guard against corrosive buildup. For 300 ohm twin lead, the avoidance of running the cable near metal is important. If the cable must go over eavestroughing, pipes etc., make sure it is kept a distance of at least 4 inches away to avoid picking up interference. The cable should also be twisted through 360 degrees for each foot of cable run to avoid becoming an antenna itself and detuning the system. N.B. The reference to 300 ohm cable excludes the shielded variety. This type has about the same loss factor as coaxial lead-in. HOW THE FM SIGNAL GETS TO YOU The FM broadcast band signal, being very high frequency (VHF) in nature, is available only within line-of-sight. With no hills, mountains or buildings to block its progress it will continue travelling into the atmosphere. However, a phenomenon known as diffraction causes the signal to 'hug' the earth to a distance (if unobstructed to approximately 30%, and more, beyond line of sight.(Fig. It is this phenomenon that determines fringe reception. The line-of-sight signal distance is more commonly referred to as the station's 'primary coverage radius'. How to determine primary coverage radius for a station signal is expressed in the formula 1.42 x square root of the height of the transmitting antenna (in ft.) x 1.30 (which is the usual distance-stretching effect of diffraction over reasonably flat terrain). Your line-of-sight reception distance is calculated similarly. Add the results of both transmit radius and your receive radius and compare the product with the actual straight line distance between your location and that of the transmitting antenna. If the actual distance is greater, you will need to employ extra measures to ensure reliable reception. Reception just outside the primary coverage radius area is referred to as "fringe" and, beyond that, "deep fringe". While reception in these areas must, officially, be classified as unreliable, satisfactory reception can be made possible through the use of a 'gain' antenna, such as the yagi mentioned previously. One could also add a 'booster amp'. However, few have a truly noiseless, universal capability. IMPORTANT.' It must be clearly understood at this point that height is the key to reliable, long distance reception, not antenna or booster amp gain. Finally, with the exception of a very few FM stations in North America, all transmit both a horizontally-polarized and a verticaIly-polarized signal. This means that both horizontal antennas, such as the yagi, and vertical antennas, such as the fender type in use on today's automobiles, will receive FM station signals with equivalent ease. FACTORS AFFECTING FM RECEPTION Interference problems generally increase as the distance between the transmitting and receiving antennas increase. Along with noise that can find itself 'mixed' inseparably with the signal, there can be a fluttering and fading of the signal caused by aircraft flying within that area. As well, there is 'tropospheric scatter' which causes part of a distant signal to be bent, or 'refracted' back down to earth some distance away. ( ) This is often evident when a temperature inversion is somewhere overhead in the signal's path. Such a signal reflection can create reception where there was none before. Or it can disrupt reception of a local station on that frequency. The obstructions to an FM signal range from small hills and buildings to office towers and mountains. Dependant on the signal's strength when meeting such an obstruction, the effect could be partial absorbtion or reflection. As a result you might end up with the main signal, plus any number of reflected signals; each of which will be out of step with the main signal ( ). We refer to these extraneous signals as "multipath interference". It is one or more of these multipath signals arriving at your antenna, out of time/phase synch with the main signal that usually disrupts the quality of the signal, rendering it 'fuzzy' and, very often, unlistenable. Aside from noise generated within the locale of the antenna, most other types of interference are easily dealt with by the characteristics inherent in a competent FM tuner design. The problem is that interference can make itself known at different points within the tuner's circuitry. The circuit's ability to deal with interference from other station signals lies mainly in its 'selectivity'. Some tuners may not have this quality in abundance as it could detract from the sensitivity of lesser capable 'front end' designs. Without the necessary selectivity, the rf stages in this design can 'saturate' with signal when an antenna with more gain than a dipole is used. The symptons of such a problem appear as a higher-than-usual noise level and strong, local stations presenting their unwelcome 'images' at more than one point on the dial. Finally, a good sensitivity figure allows the tuner to pull in a moderately distant station signal so that it can be heard in a mostly noiseless state, and in stereo. This is expressed in dBf (db per femtowatt) or microvolts (uv). While there are three different specifications used to express sensitivity; "IHF", "Usable" and "50 dB quieting", the latter, expressed in terms of 'mono' and 'stereo' is the most significant. It is at or above the 50dB quieting level that the signal is said to be quiet enough to provide full stereo separation with minimum distortion. This could vary from one tuner design to another. FM RECEPTION AIDS ACCESSORIES As you have already gathered, the factors affecting FM reception most, aside from the FM tuner are; (a) the type of antenna used, (b) its height above the ground, and (c) the location of the antenna in relation to surrounding obstructions. One must understand that there is nothing 'magic' about radio reception. It is therefore important to keep in mind that the most efficient antenna will, by design, present as much receptive surface as possible to the arriving signal. As a result, antennas with more tuned elements, or with the longest possible tuned, single element will most certainly do the best job. WHAT ATTRIBUTES SHOULD YOU LOOK FOR IN AN FM TUNER The more sensitive a tuner is, the less is the signal requirement to provide clear FM-stereo reception. A good FM tuner will have a sensitivity for 50dB quieting in stereo of 34 dBf, or less . In this case, the lower the figure, the greater the sensitivity. Most of this is accomplished at the most critical point in the tuner's circuitry, in its "front end", or first rf stages. However, we are already acutely aware that many FM stations can exist in every major listening area. Therefore, the ability to concentrate on the tuned station's signal and ignore signals from other stations on other nearby frequencies is most important. This is called 'Selectivity' and it is applied at different points in the tuner's circuitry in order to deal with specific interference problems. For example, 'Alternate Channel Rejection' refers to how well the tuner can effectively deal with interference from stations between 200 KHz. to 400KHz or more away from the tuned frequency. A good spec. here is 50 dB. Further attention is given to possible interference interruption by signals 200 KHz away through the tuner's 'Adjacent Channel Rejection'. A good spec. here is about 15 dB. Another significant spec. is the tuner's 'Spurious Response Rejection'. This deals with interference from sources that are beyond 400KHz from the tuned frequency. A good spec. here is 50dB. In all three cases above, the higher the number (in dB), the better. Of course, this selectivity must he accomplished without limiting the tuner's sensitivity or by introducing distortion to the incoming signal. A property balanced tuner design will provide equal billing to sensitivity, selectivity and sound (sonic accuracy). At Fanfare FM, they have been dubbed the '3 S's'. Also, a well-designed FM tuner will have different levels of selectivity that can be switched as necessary for best reception, i.e- switching to 'narrow bandpass' may deal with most urban receptions situations where stations are usually close together. However, this may or may not be be at the expense of fidelity, dependant on just how much design time has been spent on balancing the 3 S's. A lower level of selectivity ('wide' IF bandpass) would be used if the reception situation is less congested, and to maximize signal fidelity. As has already been mentioned, the trick here is to maintain balance in the tuner's design so the difference in signal quality is barely discernible, if at all, in either the 'wide' or 'narrow' IF band setting. Just as important, and as just as often missed in reading tuner specifications is dynamic range, or 'fidelity'. About this the antagonists once said 'The poor quality of FM broadcasting in North America did not warrant any special consideration of the tuner's fidelity specifications.' At the same there were just as many detractors for the records, tapes and compact discs. However, we all agree that, sonic accuracy being a highly subjective assessment and different with each individual, must be assessed within an audio system that can reproduce, faithfully, whatever is being played. This, of course, includes a playback units of equal resolution. Nowadays,as a result of the quality levels established by tuner designs such as the Marantz 10B and the like, most audiophile systems now include an audiophile grade FM tuner, such as the Fanfare FT-1. True, there are differences between the manner in which FM stations elect to process their audio signals, and this can be extremely aggravating. As an example, 'adult contemporary' and the more esoteric formats (heavy metal etc.) are often compressed, sometimes mercilessly. Fine arts broadcasts, on the other hand, are usually not processed beyond peak limiting. With the introduction of digital Compact Disc (CD) recordings, both the ambience and inherent signal-to-noise (SNR) quality of music reproduction has taken a quantum leap forward. And FM, rather than detracting from this has stimulated even more interest in the CD and vinyl formats as a result of sonically accurate broadcasts of 'First Air Plays'. And to all, including its one-time detractors, FM has never sounded so good. However, if your FM tuner is not leaving you with that impression, we strongly suggest you compare your FM tuner's sonic reproduction to that of a reference quality unit .... one that provides an accurate presentation of what is actually being broadcast. To accomplish this we suggest that you listen to the Fanfare FT-1, reference FM tuner and truly begin 'discovering stereo FM' all over again. Fanfare Electronics. Ltd. is pleased to bring you the FT-1A FM tuner ... designed for both the FM audiophile and the FM broadcast professional. Copyright © 1982-2003 Marvin C. Southcott. This publication 'Rediscover FM Stereo' is intended as a reference document only and may not be reproduced in any manner or used for any commercial purpose, except by permission of the author. |