30Hz… Why is this freq the low end cut off point for FM?

[Sparky]

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Does anyone still use SCAs?

Here in the states SCA’s are still active, but not nearly as much as 25 years ago.

The largest users of audio type SCA programming today are ethnic broadcasters. This is a form of specialized broadcasting serving a small but highly targeted audience. Buying a radio station today is dreadfully expensive, but renting SCA subcarriers for this type of programming is an inexpensive alternative. Large population centers that are ethnically diverse are the most likely areas to have stations of this kind. New York City for example has a large assortment of stations on SCA. http://www.n2nov.net/NYCareaFM_SCA.html
Here in Denver a NPR affiliated public broadcaster has Korean programming on 92kHz.
I found this YouTube post on this very subject. http://www.youtube.com/watch?v=V0Qo7vKYQ-M (note the modified Physicians Radio Network table top radio)

The second largest users of SCA is "Radio reading services for the blind". As the name suggests this service targets blind people in providing news and other entertainment needs. Seeing that blind folks can’t use computers and internet connections very easily, it remains quite popular. This service is (or was) also carried on many analog TV station SAP channels. I would imagine it will carry over to HDTV secondary channels (or HD FM broadcasting too).

Most of the elevator music providers like Muzak have largely abandoned SCA and now use satellite distribution methods.
Other specialized audio services that were once active but now defunct are the Physicians Radio Network, and Farm Commodities Information Services.
On occasion, some FM stations use SCA channels for backhaul distribution to affiliated AM transmitters. I’ve even heard stations when on remote location (local ball game coverage, auto dealer grand openings ect…) the use the SCA channel for a studio to remote "cueing" channel. This always makes for interesting and sometimes colorful listening.

In the digital realm, a lot of SCA activity is still used for remote transmitter control and metering. Modulation methods are direct FSK of the subcarrier, or modem style audio tones.

Other digital services include things like Microsoft’s "directband" (which has been renamed MSNDirect http://www.msndirect.com), and mobile stock quote messaging (which may be defunct and has transitioned to internet).

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I believe it might be possible, at least from a technical standpoint, to generate a 67 kHz SCA in software, make it part of the regular Stereo MPX signal, and output it through a sound card

This might prove to be an interesting challange. You would probably need to have a audio processor for the SCA programming too. Breakaway SCA?

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