As the engineer for two stereo FM stations with SCA operations, I was intrigued by Len Feldman's comments in his Audio tuner reviews about SCA rejection and crosstalk measurement techniques. One of two audio-conscious stations, for which I act as Technical Consultant, has a classical format, with at least 40-dB dynamic range, and any SCA crosstalk must be kept to an absolute minimum.
"How can a stereo FM station be quality conscious and permit SCA operation?" you might ask. Before I began my career in broadcasting 13 years ago, then being an audiophile and avid radio listener, I was of the opinion that SCA transmission and high fidelity were mutually exclusive.
But now, thanks to modern solid-state transmitters and receivers, this is not so! And I am proud to have contributed to cleaning up the transmission end of some marginal SCA operations in the San Francisco Bay Area. The present day use of SCA results in only a very slight (and possibly inaudible) reduction in FM stereo quality, and need not destroy the listening pleasure of the most critical auditors.
Basically, the SCA (for Subsidiary Communications Authorization) is a frequency-modulated subcarrier, centered at 67 kHz, which is transmitted along with the station's "main channel" audio. For stereo stations, the SCA is sent, or "injected," at a maximum of 10 percent of the total modulation and a minimum of 8 percent.
This is only a fraction of a decibel loss and is not noticed even in a direct A-B listening test. Usually there are so many inaccuracies in FM monitor calibration, limiter performance, and differences in station policies on modulation levels that SCA and non-SCA stations cannot be told apart on the basis of loudness alone.
SCA crosstalk is usually, but not always, limited to the receiving end of the broadcasting chain. However, as James Tonne has pointed out in articles in Broadcast Engineering magazine, SCA "birdies" are also generated in the transmitter if it has greater than 0.3 percent distortion. Tonne, of Moseley Associates, manufacturers of SCA and stereo generators, characterizes the SCA crosstalk signals as 9and 10 kHz beat tones occurring from inter modulation between the second and third harmonics of the 19-kHz pilot (at 38 kHz and 57 kHz), the second harmonic of the stereophonic subcarrier (at 76 kHz), and the SCA subcarrier itself (at 67 kHz). Not only are these steady-state tones audible, but also "sizzling" sounds are heard from the sidebands of the 76-kHz harmonics of the stereo subcarrier mixing with the sidebands of the 67-kHz subcarrier.
Judging from the deplorable condition of many radio stations, we should expect dreadful interactions, but in reality the results are usually good to excellent, despite the crosstalk which occurs in two ways from the SCA into the main channel and from the main channel into the SCA! The people who pay the radio station for the use of the subcarrier for their programming are always the first to complain, speeding up a correction of a bad crosstalk situation.
In 1976, we performed an elaborate series of off-air measurements of the Bay Area's SCA operations, using a Marantz 10B tuner and a Tektronix SL4N spectrum analyzer, and made some significant discoveries. About two-thirds of the FM stations with SCAs were operating correctly, with complete separation of stereo subcarrier and SCA channel and proper injection levels; however, several stations betrayed severe spillage across guard bands between main channel, pilot, stereo subchannel, and SCA region, or improperly high pilot, SCA, or stereo subcarrier levels, or faulty 38-kHz and 76-kHz suppression. The stations with correct basebands had absolutely no audible SCA crosstalk on the Marantz, indicating that conscientious transmissions into a clean receiver produce outstanding stereo results.
Upon further investigation, we learned that the faulty stations had bad SCA and stereo performance due to these major factors: 1) Carelessness or ignorance on the part of the engineers who had not, for example, checked 38-kHz subcarrier null since the last station proof-of-performance measurements, as required by the FCC; 2) use of extremely old and obsolete early-60s stereo generators with no input low-pass filtering; 3) use of mis-calibrated modulation monitors with errors causing incorrect pilot and SCA injection levels or overmodulation, and 4) confusion about appropriate SCA deviation levels, resulting in excessive SCA modulation while also engaged in stereo transmission.
Table 1-- Comparison of mono and stereo noise, with SCA and without SCA, between
a Technics ST-9030 and a modulation monitor.
In the last-mentioned case, many SCA broadcasters were using 6-kHz carrier deviation or transmitting lots of high frequencies, permitting the 67 kHz subcarrier sidebands to intersect with the upper frequencies of the stereo sidebands out around 53 kHz and slightly beyond. This problem was exacerbated when old or cheap stereo generators without 19-kHz low-pass filters were employed, permitting the stereo sidebands to reach beyond 53 kHz. One mono-and-SCA station, with apparently no filtering of any kind, was transmitting as much "junk" out to 100 kHz and beyond as it was broadcasting usable audio. Crosstalk from main-to-sub and sub-to-main was devastating, and any time an announcer's voice sibilant or a cymbal crash was heard, the SCA audio was buried in annoying hash. We spoke to the SCA program people who leased air time on this station, and they said that the station's engineer has disavowed all blame for the problem.
Transmitter tuning and distortion was found to be of minor but noticeable importance. If low-level r.f. stages are not operated fully class C and if bandwidth is not broad, a perfectly clean baseband signal from the station's 10-watt FM exciter or carrier modulator-generator, will be improperly amplified and distorted. One notorious old transmitter, now retired, caused pilot and SCA levels to rise and fall several percent as its driver stage was tuned! The pilot signal recovered off the modulation monitor was no longer a perfect sine wave, but had become a virtual sawtooth. SCA birdies were dreadful unless the transmitter was tuned with a silent main channel while the engineer wore stereo headphones and nulled the crosstalk. Some old tube-type exciters were found to have leakage of their supersonic automatic frequency control signals into the audio, causing intermodulation effects. Stations which used wide band composite microwave studio transmitter links also sometimes picked up interference that beat with the pilot, stereo information, and SCA. And, as James Tonne pointed out, antennas with a large number of bays and high gain will encourage multi path and phase distortions which will, increase the whistles.
Nevertheless, the stations which looked good on our spectrum analyzer display sounded virtually noise free.
As I do not own a Marantz 10B tuner, I went looking for a modern equivalent which would give me equally quiet performance. Like Len Feldman, I was surprised that tuners with excellent published specs differed enormously in their SCA crosstalk rejection. Using one of my own stations as a reference, I listened to many receivers. The Marantz tuner would sound silky silent, while one tuner had a slight whistle, another had fuzz, and still an other reproduced ever-changing "birdies." For a while, I resorted to mating an old H. H. Scott rack- mount Model 335R matrix-type stereo demodulator, c. 1962, to an inexpensive Japanese tuner, and got clean reception with better than 30 dB separation.
Then recently I discovered the Technics ST-9030 FM tuner and use it now for all evaluation and measurements.
It is so clean that it even surpasses my station's modulation monitors. The table presents recent measurements comparing the station's performance on the $4000+ modulation monitors to the audio outputs of the Technics tuner, read on a Hewlett-Packard noise and distortion analyzer.
It is important to note that the FCC requires that FM stations only verify mono, noise performance (at-60 or better) during the yearly proof-of-performance, although stereo noise standards have also been established.
As you can see, the modulation monitor did not reveal the true station performance, as it did not have adequate filters to null out the pilot and SCA. We spoke to the modulation monitor manufacturer, who quietly acknowledged this flaw and promised that his new model would be much better! So, station engineers should learn not to trust broadcasting equipment merely because it is expensive.
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Fig. 1 Drawing of spectrum analyzer showing location of main and sub channels,
pilot, and SCA region.
For Figs. 2 through 5, a Tektronix 5L4N spectrum analyzer was connected to the detector output of a Marantz 10B tuner and operated in the 10-dB and 10-kHz per division modes. The top of the screen represents about 10 dB down from 100 percent modulation.
Because of cable capacitance between the 108 and 5L4N, 67 kHz appears to be down about 5 dB from its actual level. Original photos made May 1, 1976.
Fig. 2 Mono FM station with SCA. No filters are employed, and baseband is
filled with "junk" consisting of noise and harmonics. Because no
stereo subcarrier is used, 6-kHz SCA deviation is permissible.
Fig. 3 Stereo station with SCA. Due to cable capacitance in the test set-up, it was verified that correctly aligned stations showed their SCA carrier injection at-5 dB; however, this station's SCA carrier was high. Noticeable 76-kHz component indicates poor stereo generator performance. SCA crosstalk from this station was quite perceptible.
Fig. 4 Two views of a clean-sounding stereo and SCA station. Fig. 4A was made
with very loud, but legal, main channel stereo musical modulation, while 4B
shows very little stereo subcarrier modulation from an off-balance voice. The
4-kHz subcarrier deviation is used on SCA, and guardbands are maintained between
stereo signal and SCA.
Fig. 5 Another stereo and SCA station, though not quite as clean as the station in Fig. 4. A 57-kHz harmonic is apparent, which probably accounted for the slight whistle audible in stereo.
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Our tests with spectrum analyzer and a known clean receiver have shown that if the broadcaster meets the following operating requirements, his stereo quality will not be impaired by SCA: 1) The injection level must be no greater than 10 percent; 2) audio frequencies no greater than 5 kHz should be transmitted on the SCA (and James Tonne recommends a special audio filter with stepped rolloff); 3) no greater than 4-kHz frequency deviation of the SCA subcarrier should be attempted; 4) the transmitter itself should have well under 0.5 percent distortion, and 5) the stereo generator should have the proper filters, and its subcarrier null should be precisely adjusted and maintained at minimum (-60 or better is optimal). Under these conditions, we have found that with a good receiver, outdoor antenna properly aimed, and with good antenna feedline, it makes virtually no difference whether the SCA is modulated or not, and usually it may be turned on and off inaudibly.
I understand that the BBC is considering the use of SCA, and I would plead with them to phase-lock the SCA subcarrier to a harmonic of the pilot, out of the way of possible future quadraphonic subcarriers. For those of us in the United States who are stuck with conventional 67-kHz SCA, the situation is improving with current technology. And I wouldn't be surprised if my remarks encourage the gobbling up of any classic old Marantz 10B tuners which have fallen into disuse alongside their flashy solid-state brothers.
Finally, a last bit of advice for stereophiles: If you still have SCA crosstalk on your favorite station, even if your reception is multipath-free and their equipment is fine, try miniscule two or three-degree rotations of the cores of the i.f. cans and ratio detector in your tuner. You may be amazed and delighted, as I was, to hear the birdies and whistles fade away completely in many cases. More radical retuning will, of course, ruin the alignment and will necessitate expensive servicing, so be forewarned!
I would like to thank Robert Orban of Orban Associates for permission to include off-air spectrum analysis pictures of FM stations that he made for our SCA tests.
(Source: Audio magazine, Jan 1979; Stephen R. Waldee )
Also see: A View Through Different Windows by Richard C. Heyser (Feb. 1979)
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