FM Specifications Revisited by Leonard Feldman (Apr. 1978)

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It is now just about three years since the IHF, the IEEE, and the EIA adopted new standards for the uniform measurement of FM tuner performance. The new standard (given IHF number IHF-T-200, 1975 and IEEE Standard Number 185-1975), issued on May 19, 1975, represented a much-needed updating of FM measurement techniques. (IHF-T-200, 1975 may be obtained from the Institute of High Fidelity, 489 Fifth Avenue, New York, N.Y. 10017 at a cost of $5.00.) The previous standard available from the IHF had been promulgated way back in 1958 (before the days of stereo FM), while the old IEEE standard was even older, having been last issued in 1947 when FM was still in its infancy as far as public acceptance was concerned.

Most manufacturers of FM tuners and receivers have adopted some of the measurement and reporting techniques spelled out in the new standard.

Very few manufacturers have actually published every specification called for in the new standard. A few manufacturers continue to ignore the new standard entirely, and what few specifications they do publish are still based upon earlier standards. Table 3 of the new standard is reproduced here as a partial explanation of why many manufacturers don't bother to detail all of the specifications of an FM tuner in their advertising and promotional brochures. No less than 22 separate published specifications are called for in the case of monophonic performance, while an additional 11 specifications are called for in the case of stereophonic performance. Several of these specifications require multiple listings (e.g., distortion at 65 dBf actually calls for readings at three modulating frequencies in mono and the same three frequencies must be used and listed for stereo distortion figures), making the list of required specifications even longer. In the case of the IHF (which is concerned primarily with manufacturers whose tuner products might properly be called "high fidelity" equipment), that trade association does not have any legal means to force even its members to embrace the new standard. The IHF can only suggest to and encourage its members to abide by the new standards. The choice of specifications which members of the IHF (and others) have made tells us much about the nature and usefulness of many of the new specifications and measurements.

Usable Sensitivity

Although much has been written about the relative unimportance of this specification, most manufacturers continue to "feature" it as the specification of primary importance at least as far as monophonic performance of the FM tuner is concerned. What's more, public awareness of "microvolt" notations of signal strength prompts many to continue to list usable sensitivity in that form, rather than to use the new and more meaningful "dBf" power notation for signal strength. Many continue to quote sensitivity in both forms. A simple formula for conversion from microvolts to dBf (assuming a 300-ohm antenna input impedance) is: dBf = 20log,oµV/0.55.

Not surprisingly, very few manufacturers tell us the value of "usable sensitivity" for stereo operation of their products (though these days, most FM listening is in the stereo mode). There are at least two reasons for this deliberate omission. Most FM tuners have automatic switching from mono to stereo. The switchover point is usually set by the manufacturer so that when a signal is strong enough to cause the transition from mono to stereo operation, the noise and distortion will be well under 3 percent (or more than 30 dB below the desired output signal level). If a manufacturer sets the switching threshold to, say, 5µV (19.2 dBf), that becomes, per torce, the "stereo usable sensitivity." On the printed spec sheet, such a number is not terribly impressive when compared with the 1.8, 1.7, or 1.6 microvolts figures (10.3, 9.8, or 9.3 dBf) normally associated with mono usable sensitivity of modern tuners.

Even where auto switching from mono to stereo is not featured, one can expect the stereo usable-sensitivity figure to be poorer than that of the mono usable sensitivity, and manufacturers are understandably reluctant to disclose this seeming disparity.

50-dB Quieting

The most important specification appearing in the new standard calls for a statement of signal strength required in mono and stereo for the noise to be suppressed 50 dB compared with the 100 percent modulated output signal at 1 kHz. Manufacturers have seemed more willing to publish these figures.

Readers of specification sheets should not be disappointed to find that at least 20 dB more of input signal is required to reach this "listenable" noise level in stereo than is required in mono. Thus, a tuner claiming a 50-dB quieting requirement of 12.0 dBf in mono (2.2 µV), will, at its best, require 32.0 dBf (22.0 µV) of input signal to achieve the same degree of quieting in stereo. Design limitations usually result in even greater discrepancies between these two published figures.


Above: Table 3--Receiver Performance Evaluation


Fig. 1-Mono and stereo quieting in the FM section.


Fig. 2-Mono and stereo distortion characteristics in the FM section.


Fig. 3-Ideal frequency response of an FM tuner follows the 75-µS de-emphasis to beyond 15 kHz with a sharp notch in response at 19 kHz.


Fig. 4-In a less-than-ideal response curve the kHz filter action interacts with the 75-µS de emphasis curve above 10 kHz.

Signal-to-Noise Ratio

In the case of this specification too, manufacturers have generally complied with the new standard and offered S/N values (in dB) for both mono and stereo performance. By the time a strong signal (65 dBf) is applied to the antenna terminals of most tuners, the difference in quieting between mono and stereo operation has usually reached a much lower figure. Typically, a tuner having a 70-dB S/N ratio in mono may exhibit quieting of 65 dB or even more in the stereo mode. Figure 1 shows the manner in which the stereo and mono quieting curves tend to converge as signal strength increases.

Distortion at 50-dB Quieting

One seldom sees this value published at all and, in my opinion at least, this is one specification which the standards committees responsible for the new standard might have been better off omitting and Fig. 2 shows why.

Note that in Fig. 1, 50 dB of quieting in mono was reached with an input signal level of 12.0dBf (2.2 µV). As we can see in Fig. 2, at that signal level, distortion is still quite high (1.0 percent in our example). The combination of incomplete limiting and minimal AGC levels restricts i.f. bandwidth and detector linearity so that 100 percent modulation at such low signal levels is "pushed" beyond the linear operating range of the i.f. detector circuitry of the tuner. Thus, the better the 50-dB quieting of the tuner in mono, the "poorer" the 50-dB quieting distortion is likely to be. Adding further to this contradiction is the fact that since a greater signal input is required to attain 50-dB quieting in stereo, the 50-dB quieting distortion in the stereo mode nearly always turns out to be lower than in the mono mode-a contradiction not easily understood by the average consumer.

Frequency Response

One would think that designers would have no difficulty in maintaining near-perfect frequency response in an FM tuner from 30 to 15,000 Hz (the legal limits of audio modulation in FM). Yet, as is easily seen from the many published tuner spec sheets, it is not unusual to find claimed response varying by ±1.0 dB, ±1.5 dB, or even worse. Again, two conflicting factors are at work here. Ideally, the response of the tuner to 19 kHz should be minimal in order to suppress any 19 kHz, stereo-pilot signal output from the tuner. Substantial output at this frequency can have an undesired effect when FM programs are recorded onto tape decks equipped with Dolby circuitry (the Dolby decoder senses this signal as a high frequency audio "program" signal causing false Dolby encoding) and can, in some cases, actually be injurious to tweeters. In still other instances, 19 kHz (and harmonic products of that pilot frequency) can cause audible "beats" when recorded on tape.

But rejection of this unwanted output requires a carefully designed 19 kHz, notch-filter circuit in series with the output signals. Unless that filter is of a multi-pole (and thus expensive) design, it is likely to start "rolling off" desired response in the range from 10 kHz and upwards. Figures 3 and 4 illustrate this problem. Response of an "ideal" filter is illustrated in Fig. 3, while the less-than-ideal situation (typically found in many tuners and receivers) is shown in Fig. 4.

Intermodulation Distortion

Most experts agree that the inter modulation distortion tests specified in the new standards tell more about the audible performance of a tuner than the harmonic distortion tests. Yet, virtually no tuner or receiver manufacturer bothers to specify this important number. Here, I think, the reasons arise more from practical considerations than from any attempt to withhold information. The IM tests require the use of two signals of equal amplitude (14 kHz and 15 kHz) with an instantaneous peak deviation of ±75 kHz. The resulting 1000-Hz IM product at the output must be measured using a 200 Hz-to-1500-Hz bandpass filter and expressed as a percentage of the output that would be obtained when 1000-Hz modulation at ±75 kHz is used. The test set-up is illustrated in Fig. 5. Unfortunately, none of the popular FM signal generators currently used by most manufacturers have this built-in test signal available, and the required bandpass filter is not standard either.

Thus, it would seem that manufacturers have simply been too lazy to build their own test fixtures for this important test. (That applies to this equipment reviewer as well, though I have, many times, vowed to correct that omission!)

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Table 1 -- FM Tuner Specification Priorities.

Order Of Priority From 0 to 15


Notes: These are the priorities author Len Feldman feels should be assigned to FM tuner specifications. Note that in many instances, a specification that is important for close-in dwellers is of lesser importance to fringe-area inhabitants.

*Figures must be provided for both mono and stereo reception.

**Of lesser importance in stereo, since tuner is well beyond full limiting and wide bandwidth reception at 50-dB quieting.

***Only of concern if you plan to record onto tape from FM.

****Only of concern if stations in your area transmit SCA signals.

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Unchanged Specifications

The following specifications and their methods of measurement remain substantially the same as they were in earlier tuner measurement standards Alternate channel selectivity, spurious response ratio, capture ratio, i.f, response ratio, image response ratio, and AM suppression ratio. Not surprisingly, most manufacturers find no difficulty in fully disclosing these specs.

On the other hand, adjacent channel selectivity, though clearly called for in the new standards, is almost never published by tuner or receiver manufacturers. Again, the manufacturer is faced with conflicting requirements here. If the tuner is designed with an extremely wide i.f. bandwidth in order to provide ultra-low distortion performance, the adjacent channel selectivity (and sometimes even the alternate channel selectivity) is going to be a relatively low number. After all, the tests for adjacent channel selectivity involve measuring the response of the tuner to a signal that is only 200 kHz removed from a desired signal. In an ultra-low distortion tuner, the adjacent channel selectivity figure may be as low as from 5 dB to 20 dB or so! Most manufacturers probably conclude that the audio consumer, having been conditioned to expect high "selectivity" figures, will be discouraged by such "low" selectivity values and will fail to understand the difference between adjacent and alternate channel selectivity. This is especially so since, for years, manufacturers have listed this specification as, simply, "selectivity"-without specifying the fact that it was measured for a signal that is 400 kHz (two channels away) from the desired signal.

As many readers are probably aware, a trend that has developed recently is to offer tuners with variable (or two degrees) of selectivity. "Narrow" settings are used when stations are close together on the dial (at the expense of ultra-low distortion), while "wide" settings are used when signal frequencies are far enough apart on the dial so that the lower selectivity afforded by such i.f. circuit configurations does not cause interference problems and yields the lowest distortion figures possible.

Subcarrier Product Ratio and SCA Rejection Ratio

These specifications and the methods used to measure them are two which relate to the stereo performance of tuners and receivers. We have already discussed subcarrier product rejection and how it is interrelated with a tuner's frequency response. Quite a few manufacturers are quoting this figure correctly and accurately.


Fig. 5-Setup required for measuring the IM distortion of an FM tuner.


Fig. 6-Suggested method of creating modulated 67kHz sub-carrier for proper measurement of SCA rejection.

As for SCA rejection ratio, I am certain that just about every dedicated FM listener has, at some time or other, been subjected to mysterious swishing and gurgling noises when listening to certain FM stations. As most readers know, many FM stations earn additional revenues by leasing their sub carrier facilities to such private communications firms as background music operators, private news services, and special services such as talking books for the blind. In the case of stereo FM stations, this service is transmitted via a 67-kHz subcarrier which is frequency modulated by the subscriber audio information. In theory, at least, regular listeners to FM should not hear any of this audio material, and special sets are leased to subscribers who are entitled to this SCA service. The gurgling and swishing sounds referred to are the result of cross modulation and intermodulation effects taking place either at the transmitter or (more likely) at the tuner.

I have often found, when listening to and bench testing certain FM tuners and receivers, that the published specification regarding SCA rejection does not provide reliable correlation with what I hear when tuning to a station which is known to be transmitting an SCA signal. I suspect that the reason why the audible effects of SCA interference are greater than might be expected from the published specs is very likely that manufacturers, in testing for SCA rejection, are merely applying a fixed 67-kHz modulating signal to their FM generators at an appropriate modulation level of 10 percent and measuring the resultant output at the audio output terminals of the tuner or receiver. That sort of static test will yield impressively high rejection figures for almost any decent tuner.

Unfortunately, that type of test does not correspond with what actually takes place when a transmitter sends out an SCA sub-carrier signal. A careful reading of the new standard discloses that the 67-kHz sub-carrier signal used must in turn be frequency modulated by a 2.5-kHz sine wave to an extent of ±6 kHz. In other words, the 67-kHz carrier is made to vary from 61 kHz to 73 kHz at a 2.5-kHz rate. It is this modulation of the 67-kHz sub carrier which causes the audible interference to a stereo listener, rather than the presence of the 67-kHz carrier itself as part of the recovered signal complex. While several FM signal generators (including the popular Sound Technology Model 1000A) do have a built-in 67-kHz signal capable of modulating the main carrier, these generators have no direct provision for frequency modulating that carrier, and so many manufacturers do not bother to construct or purchase the necessary equipment to make the test properly.

In my own lab, I have worked out a simple way in which to create this FM modulated 67-kHz sub-carrier. Since I own two FM signal generators anyway (two are required for such measurements as selectivity and capture ratio) as well as a "univerter" (which is normally used to create an i.f. signal at 10.7 MHz for measuring i.f. rejection), I found that I could create a "beat frequency" of 67 kHz between one of my signal generators and the univerter (in much the same way that a beat of 10.7 MHz is created using these two pieces of equipment). By then modulating the generator of this combination with a 2.5-kHz signal and with a deviation of ±6 kHz, out comes a modulated 67 kHz carrier which can then be used to modulate the main carrier of my second generator. The test setup is shown in the diagram of Fig. 6. Both the univerter and the generator associated with it must be fully stabilized and free of drift in order to maintain a constant 67-kHz beat frequency sub-carrier using this method, and I use a frequency counter to insure against possible drift of the critical 67-kHz frequency while making the SCA rejection measurement.

Summary

Much has been written lately about the lack of correlation between bench measurements made on modern amplifiers and their ultimate listenability. Indeed, I agree that there are probably many static measurements which we have not learned to make on an amplifier which would enable us to obtain a more meaningful set of test data if we but knew what those measurements might be.

In the case of FM tuner performance, however, I believe that the new standards do offer a comprehensive picture of the merits (or demerits) of a tuner or receiver insofar as its FM performance capability is concerned. A statement of all (or at least nearly all) of the specifications which the standard recommends be published can help the consumer who wants to purchase a superior FM tuner or receiver to make a proper judgment. Now it's up to more manufacturers to take the trouble to use the new standard for all it's worth.

(Source: Audio magazine, Apr. 1978; Leonard Feldman)

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