Why "the higher, the better" is not good enough
by Edward J Foster
To MAKE SENSE of the specifications of a power amplifier is not generally a momentous task. You can look for the power you want, the lowest distortion you can find, frequency limits of 20 Hz and 20 kHz or better, and low noise, then audition the unit you've chosen with at least a reasonable expectation of hearing something good. But the job an amplifier does is not all that complicated.
A signal entering the antenna terminals of a stereo FM tuner, on the other hand, goes through a multitude of changes before appearing as audio output, and this complexity alone is enough to generate a bewildering array of specs. Worse yet, since the various specs interact in such a way that improving one often will degrade another, you cannot cover yourself by looking for the best you can find in each parameter. To evaluate a tuner sensibly, you need a firm grasp on what aspect of performance is described by each number and its relative importance for your particular situation.
In short, you need to know what you must have and what you can trade away to get it. And you will have to trade.
Consider selectivity, sensitivity, capture ratio, frequency response, and distortion. A tuner can be made more selective--that is, better able to reject stations near the frequency of a desired station by narrowing its IF bandwidth (that is, the "dial space" with which the intermediate-frequency section deals in tuning a particular station). More than likely, sensitivity-the tuner's ability to pro vide noise-free reception from weak signals--also will be enhanced. But the capture ratio will get worse, high-frequency audio response may be adversely affected, and the distortion content of the audio probably also will be increased. In a rural area far from transmitters, sensitivity may be paramount; in a metropolitan area with lots of local stations, it may be barely significant. But with many stations sharing the FM band in an urban area, it is more likely that stations will fall close together in frequency. For good reception, the tuner will need sufficient selectivity to reject unwanted stations while tuned to the desired one.
Unfortunately, improved selectivity is at odds with good capture ratio-also a desirable attribute in a "metropolitan tuner." Because of the many reflective objects around-buildings, aircraft, etc. FM reception in and near a city frequently is plagued by multipath problems. The reflected signals confuse the tuner and cause an increase in distortion-especially in the stereo mode. The first line of defense against multipath reception is a highly directive antenna, but a good capture ratio enables the tuner to suppress the late-arriving signals--which, from the tuner's point of view, resemble different transmissions on the same frequency. Capture ratio measures the tuner's ability to suppress all but the strongest of these.
So here's a case where two specs (selectivity and capture ratio), each important for good reception in the same locale, conflict. You will have to look at the frequencies of your favorite stations and decide whether you are more afflicted with multi-path or with alternate-channel interference.
Some tuners, we hasten to point out, manage through superior design to achieve a better compromise between conflicting specs than others do, but a compromise it remains. Faced with this basic tradeoff, some designers have introduced switchable-bandwidth tuners-those that provide a choice between high selectivity (narrow band) and high performance vis-à-vis distortion, capture ratio, stereo separation, etc. (wide band). Such tuners, in effect, allow you to make the tradeoff, at least in part, yourself-and to alter the chosen compromise at will depending on the specific reception problems of the moment.
The spec that seems to catch everyone's eye first, no doubt because it long has been shown so prominently in tuner advertising, is the so-called minimum usable sensitivity. This terminology is unfortunate, since an audio signal recovered by a tuner operating at this point is hardly usable. "Us able sensitivity" of a tuner is defined as the minimum input power (in dBf) required for the noise and distortion components in the output to be suppressed by 30 dB. (Note that this is a mono sensitivity figure.) Minimum usable sensitivity is measured at three different frequencies across the FM band (90, 98, and 106 MHz) for our test reports. The figures give an idea of the uniformity in sensitivity across the dial. Most measurements are made only at the midband frequency of 98 MHz. Good uniformity in sensitivity would suggest that the other lab data also are valid across the band. You can expect reasonable similarity--a spread of 1 dBf or less--in the three measurements; gone (fortunately) are the days of tuners whose performance deteriorates badly toward the ends of the dial.
Far more meaningful as a sensitivity figure is the power input (in dBf) required to achieve 50 dB of noise suppression. The lower this figure, the more sensitive the tuner. Average these days seems to be about 13 1/2 dBf in mono and 36 dBf in stereo. Note that the average tuner requires almost 23 dB more--roughly 200 achieve fairly quiet stereo operation than it does for equally quiet mono. That's why many stations on your dial will sound cleaner in mono than in stereo.
The minimum input power at which the multiplex circuitry in the tuner recognizes that the broadcast is in stereo and switches accordingly is called the stereo threshold. There is no particular reason to favor a low stereo threshold. If sufficient quieting (at least 40 dB) hasn't been achieved by the threshold point, you are unlikely to want to listen in stereo anyway. (Remember, below the threshold, the tuner will still receive the broad cast-but in mono rather than in stereo, and with less noise.) The stereo threshold, therefore, represents a point of information rather than a criterion of merit.
Other important aspects of tuner performance are how rapidly quieting improves with increasing signal strength, what the ultimate level of quieting is, and what happens when even stronger signals are received. In actual use, a tuner will seem more sensitive if it reaches its ultimate level of quieting at a relatively low level of input signal. Most tuners achieve ultimate quieting with 65 dBf or less of input signal, so by convention the quieting at that input level is defined as the signal-to-noise ratio of the tuner, both in mono and in stereo. The average stereo S/N ratio these days is about 65 dB; that for mono is about 70 dB or more and represents, for practical purposes, noise-free reception.
While the 50-dB quieting point is a convenient reference, it does not represent really enjoyable listening; noise suppression that low is not usually tolerated in other high fidelity components. It would be nice to know how much signal is needed at the antenna for 55 or 60 dB of quieting. This in formation is not usually contained in manufacturers' spec sheets, but it can easily be read from the graph of sensitivity and quieting characteristics included in our test reports.
At the other extreme is the ability of the tuner to cope with very strong signals-an important consideration if you live near a transmitter. A tuner whose quieting curves remain low above 65 dBf is coping well; if these curves rise at the high-input end, indicating overload and deteriorating performance, the model in question is less appropriate for urban use.
Frequency response of a tuner generally is restricted to the range between 20 Hz and 15 kHz.
Though FM-transmitter performance need not be maintained to beyond 15 kHz, in some new tuner designs that remove the 19-kHz stereo pilot by cancellation rather than by means of a filter the upper limit extends to around 18 kHz. The extra bandwidth will have little, if any, audible significance. Mono response is apt to be a trifle flatter than stereo response, but the difference is small: The average spread is under ± 1% dB in mono, ± 1 1/2 dB in stereo. It is not unusual to find the response trailing off below 30 Hz. Many designers do this to minimize low-frequency thumps while tuning and to make the low-frequency cue tones often used by broadcasters less audible.
Finding averages for channel separation is a bit more problematic. Tuners vary widely in just how much separation they achieve and in the frequency range over which they achieve it. Most manufacturers specify separation only at 1 kHz. In our test-report curves we show separation to 15 kHz and tabulate the best separation (40 dB, if the tuner can manage it-which most can these days) that it maintains throughout at least a reasonable portion of the critical midrange, together with the limits of the frequency band over which it is maintained. In addition, we show the frequency limits over which a figure 10 dB poorer (therefore, generally 30 dB of separation) is maintained. For practical purposes, 30 dB of separation seems adequate, even in the midband; FM stations' source material usually is records, and channel separation in the pickups used to play them generally is no better than this. So separation is more than adequate in the large majority of today's tuners.
Low total harmonic distortion is of course a paramount consideration. We tabulate the lab data for both the stereo and mono modes at three different modulating frequencies--80 Hz. 1 kHz, and 10 kHz. Again, mono performance is typically better than stereo, especially at 10 kHz. At 80 Hz and 1 kHz, the average tuner in the mono mode might exhibit under 0.15% THD, a bit more at 10 kHz. In stereo, you're more likely to find about 0.25% at 80 Hz,. 0.20% at 1 kHz, and 1% (sometimes considerably more) at 10 kHz. The II-IF/IEEE standard calls for measurement of high-frequency THD at 6 kHz, and this is the route taken by most manufacturers in their spec sheets. We believe that the 10-kHz measurement can be a more revealing test of the tuner and have therefore retained it, even though it is stretching a point to call the spurious frequency components revealed thereby "harmonic distortion." THD is, after all, really a "total garbage" measurement-distortion and noise.
You can glean further useful information about harmonic distortion (at 1 kHz) from the sensitivity/quieting graph in our test reports. The difference between the noise-only curves and the corresponding noise-plus-distortion curves represents distortion. Where the noise curve is low and the noise-plus-distortion curve relatively high, so is distortion; where the two curves are close together, distortion is swamped by noise, which therefore will be the factor limiting your enjoyment. So by examining the curves you can get some feeling for the way noise and distortion vary relative to one another with different input signal strengths.
For technical reasons, intermodulation in tuners is measured differently from the way it is done for other equipment; our interpretation, therefore, is not quite the same. We view it as an index of high-frequency distortion in the mono mode. Typically it runs about 0.1%.
The tuner must suppress the 19-kHz pilot and 38-kHz sub-carrier signals and all other by-products of stereo multiplexing that may contaminate the audio output, especially if the broadcast is to be taped. If these extraneous signals find their way into the tape recorder, they can "confuse" a Dolby circuit and result in a poor over-all frequency response or, if they're severe, result in intermodulation whistles on the tape. Fortunately, the average tuner knocks each of these pollutants down by about 65 dB, and that should be adequate.
Capture ratio, again, is a criterion that tends to conflict with selectivity, both of which have an effect on distortion and separation. For the record, the "typical" tuner has a capture ratio a bit under 1 1/2 dB (the lower, the better) and an alternate-channel selectivity of 70 dB (the higher, the better).
As with other equipment, lab data and specs alone do not tell the full story. How does the tuner act in practice? Is the tuning precise? Is the mute effective? How useful are the tuning meters? The answers to these questions and others of like importance appear in the text of our reviews-not in the tables and graphs.
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What to Look for (and What You'll Find) in the Tuner Ads
Advertising copywriters have a way of throwing out phrases ("epitaxial snark" or whatever) as though they represent unquestioned virtue-features to be conjured with however obscure in import. That's their business, just as it's ours to make plain as many rough places as we can.
With that in mind, we offer the following notes.
Phase-Locked Loop: This circuit for demodulating stereo information performs better than earlier demodulators and eliminates a number of costly parts. Both objectives are worthy--so worthy that you'll be hard put to find a current high fidelity tuner without PLL.
Tuning Meters: A variety of virtues and a few sins are covered by the term. Cheapie tuners and receivers have simple signal-strength meters, which--if they're any good-help you tune the station but not as quickly or as unequivocally as channel-center meters, which are standard from moderate price points up. Several types of metering address themselves to multipath distortion; their utility will depend on the behavior of the specific meter, on the severity of multipath problems in your area-and on whether you have an antenna rotator.
Scope: The next step up from meters (in price, anyway) is a small oscilloscope that gives you a visual display from which a number of signal parameters (including tuning, modulation, and multipath) can be judged simultaneously. It won't necessarily let you tune better than a good set of meters, but it may do so a bit faster. And it looks mighty sexy.
Interstation Muting: Virtually every FM device has for years had this feature, sometimes with a defeat switch or muting threshold control so you can "get at" extremely weak stations. A second rate design produces loud "thumps" every time you pass a station in rapid tuning; the better ones mute even this annoyance.
High Blend: For any particular signal strength, mono reception is almost always more noise-free than stereo reception, and the noise difference in creases as the received signal gets weaker. One way of suppressing stereo noise is to switch to mono; a useful in-between option preserves separation through the midrange (which largely deter mines stereo imaging) and cancels it in the highs (where the noise is most annoying) by blending the two channels. In a weak signal area this feature is virtually a must for stereo reception.
Automatic Frequency Control: Way back when, tubed tuners were subject to "drift" that would take them well off channel as they warmed up. AFC prevented drift, but at a price in distortion. So AFC virtually disappeared on component-grade solid-state equipment. Now it's back in forms that bypass the distortion, but it's a nicety rather than a necessity.
Bandwidth: Intermediate-frequency (IF) band width is what's referred to in those high-end units having a control so labeled. See the accompanying article for an explanation.
IF Filter: You won't see as much in today's ads as you once did about filter construction (crystal, Butterworth, for example), skirt characteristics, symmetry, and whatnot. Fine. The real story is in the specs, which of course reflect filter quality.
FETs and MOS FETs: The virtues of field-effect transistors, used in tuners partly for their wide dynamic range and hence relative freedom from overload with strong incoming signals, are de scribed better by the specs they achieve than by any in vacuo consideration of their nature.
Dolby: Here the obvious consideration is whether you have Dolby-encoded broadcasts in your area-or are likely to get any in the near future. If not, the feature has no value for you. If so, give the edge to a Dolby feature with the decoding circuitry built in. The alternative--Dolby switching to accommodate an outboard Dolby decoder--may be harder to set up and use (depending on the decoder, especially if it is built into a tape deck, even with a DOLBY-FM switch of its own), and the outboard decoders are getting more difficult to come by.
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(High Fidelity, Nov. 1977)
Also see:
HF's Music Critics Take On the Compact Disc (by Allan Kozinn) (Jan. 1983)
Hitachi SR-903 receiver (Equip. Report, Apr. 1977)
Sony STR-5800SD stereo FM/AM receiver (Equip. Report, Nov. 1977)