SCA Primer for Broadcasters (2002 edition)


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There has been a huge drop in interest in leasing SCA space in the last couple of years. The prices that a station can get are therefore also down.

Foreign language broadcasters are still interested in SCA services from time to time, but those who previously used SCAs the most have been able to lease or buy AM or FM stations. In Los Angeles, Chinese, Vietnamese, Korean and Persian broadcasts are heard on stations. While Spanish broadcasts have never used SCA in Los Angeles, markets where there is not a large Hispanic presence may be candidates for leasing an SCA, as would other minority groups.

Several SCA data applications have been introduced, but most have failed. Seiko's Message Watch service is defunct, and Clariti's digital VOCA Wireless Messaging failed, among others.

Paging never worked that well on SCA because of the tiny antennas that had to be used in the pagers, the RF noise created by the digital electronics right next to the antenna, and the serious effects of multipath on digital SCA transmission causing packet loss as people moved about.

Some services, such as Muzak, have gone to satellite delivery. Command Audio (a digital service) also changed its delivery from SCA to satellite.

All that being said, here are my suggestions for optimal SCA channel usage:

1. Use the 92kHz subcarrier for analog services and 67kHz for digital services. 57kHz, used for RBDS, is close enough to 67kHz to be annoying in analog receivers. Digital signals on 57 and 67 coexist nicely.

2. Give SCA clients the injection they need to do the job. You'll just lose them if it doesn't work well. Ten percent is normal and generally necessary, particularly for analog. Remember that a station can increase its modulation 0.5 percent for every one percent used by SCA services. There's no point in limiting them to eight percent (a 20 percent drop for them) when it's only an undetectable one percent for the station. There is also no need to save injection for RBDS. Because it is phase-locked to the pilot, it does not really add to a station's modulation unless the injection is set to a level considerably higher than is needed. Typically, 0.5 percent to one percent should be fine.

3. Run analog SCAs at 6kHz deviation on 67kHz and 7kHz deviation on 92kHz. A higher deviation will reduce the chance that it will be heard as a birdie (whistle) in the station's main channel in areas of multipath. Most car radios blend to mono where there is multipath anyway, so this is not much of an issue nowadays and does not happen on mono stations. Even in stereo, it's almost a non-issue when an analog service is on 92kHz. Stories of SCA birdies mostly come from years ago, are related to 67kHz, and were sometimes caused or aggravated by transmitter mistuning.

4. Stations should own the analog SCA generators or ensure that the clients use high quality generators such as the CRL SCA-300B or the Modulation Sciences Sidekick. These units have peak limiters built in. The CRL needs an AGC in front of it, typically at the client's studio end. Other SCA generators can be used if a good peak limiter is placed in front of them. It should be set to 150uS pre-emphasis and the audio input should be band-limited to 5kHz. Modified Moseley TFL-280 peak limiters work well.

5. Be aware of the 'sound' of the digital stream that a digital SCA client will use if the main channel is in stereo, runs light audio processing and has multipath problems. The best digital data stream sounds like white noise, but there are some that are raucous. RBDS itself is noisy. When the data stream is essentially white noise, it is unnoticeable in multipath conditions.

6. Have an accurate SCA monitor at your transmitter and another one at your studio. A good directional antenna is required at the studio or inaccurate readings will be shown; typically higher than reality. The receive-only Scala log-periodic and the Radio Shack six-element antennas are good choices. The latest Belar monitors are digitally based and accurate. Do not depend on older SCA monitors to set digital injection. I have never seen one read correctly.

7. Use a decent SCA radio to listen to the quality of the signal, particularly for analog services. Have one of the client's radios, too. One of the most common client complaints is cross-talk from the main channel. The audio output from the SCA modulation monitor may not be the best source to listen to because it may not be band-limited. High frequency cross-talk from the main channel that is not audible on normal SCA radios will be heard.

8. Keep the transmitter's AM noise low, but even more important, keep the group delay through the transmitter to a minimum. This is mainly an issue with tube-type transmitters. The input and IPA tuning will have the most effect. Poor group delay can destroy a digital SCA and increase cross-talk on digital and analog SCAs. The effect of poor group delay on the main channel may be barely noticeable (if it's noticed at all), but it will affect stereo separation.

If a high-speed digital SCA tenant installs equipment at the transmitter that can show raw data errors, tune the transmitter for minimum errors and the group delay will be right.

You can also tune for minimum group delay by removing all modulation from the transmitter. Disconnect the stereo and SCA generators, too. Use a low-distortion audio oscillator and set it to exactly 9.5kHz. Connect it to the composite input and modulate the transmitter to about 100 percent. Use the station's stereo monitor to monitor pilot injection. Tune the transmitter for minimum indicated pilot, which is the second harmonic of your audio oscillator.

9. Be aware that the maximum total SCA injection is 30 percent for mono FM stations and 20 percent for stereo stations. The limit above 75kHz is 10 percent, so while a station can operate a 57kHz or 67kHz SCA at 20 percent on a stereo station, a 92kHz SCA can operate at no more than 10 percent. Incidentally, a 39kHz or 41kHz SCA at 30 percent will provide a phenomenal signal with excellent coverage on a mono station.

10. Note that some older composite STL receivers have an 85kHz low-pass filter and will not pass 92kHz. For 92kHz, be sure that a 110kHz low-pass filter is installed. Older units can be modified, of course. It is normally preferable to inject SCAs at the transmitter to keep cross-talk to a minimum. Many clients will not want to be at the mercy of a station's STL if it fails.


Henry, The Radio Doctor, is an SCA consultant and contract engineer for stations in Brazil, China, Taiwan and Southeast Asia. He is based in Los Angeles.




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