Vectronics VEC-422K - Manual (Page 20)

VEC-422K Owner's Manual SCA Decoder Kit
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OPERATING INSTRUCTIONS
Tuning in SCA signals: Unless you know which FM broadcasters are carrying
SCA programming, you will have to explore each station to find if it has hidden
SCA programming. Tune into the FM station; and then carefully tune R16 while
listening for SCA signals. If a SCA subcarrier is present, and is strong enough,
you should hear the SCA programming when the Phased Locked Loop (PLL)
locks onto the subcarrier signal. Volume control R17 is used when an external
speaker or headphones are attached to the VEC-422K to set the audio to a
comfortable listening level. Don’t ignore the Public Broadcasting and college
FM stations at the lower end of the FM dial, they are public service oriented and
often carry community oriented SCA programs.
What is an SCA signal? SCA (Subsidiary Communications Authorization)
programs are auxiliary revenue-producing services provided by many FM
stations. The use of the subcarriers is “sold”, just as commercial airtime is sold
for commercials. SCA subcarriers are located at 67kHz and 92kHz from the
main carrier, and transmit with a 7.5kHz deviation maximum. It is not a HI-FI
service, as its audio bandwidth is limited to about 5kHz. In theory, each FM
station can carry several subcarrier services. ARI, or Automobile Road Services,
normally operates on a 57kHz subcarrier. This is a limited bandwidth channel,
and requires special equipment for decoding the information. ARI provides
constant road advisory updates to motorists who subscribe to the service. Each
subcarrier reduces the main channel signal strength by about 1dB. Thus, few
stations are willing to lose 2dB of signal strength to carry two subcarrier (SCA)
services. Most SCA services will be found using the 67kHz subcarrier. Urban
areas with high population densities will generally offer more SCA services than
in a rural area. The original 41kHz SCA subcarrier is seldom used, due to its
incompatibility with the 38kHz stereo subcarrier.
The SCA signal represents about 10% of the total FM signal, thus it is unusually
weak and prone to fading or being noisy. The SCA subcarrier can also suffer
from “splatter” from the main audio channel, resulting in occasional noise bursts
on the SCA signal. This means even though the main FM signal sounds good,
the SCA subcarrier may still be too weak for the VEC-422K to decode properly.
Use a good outdoor antenna to capture the most signal level.
What you can hear: SCA programming is used for a variety of purposes.
Providing commercial-free background music for restaurants and waiting rooms
is just one of many uses. There are also SCA services for physicians giving the
latest medical news, reading and news services for the visually handicapped, and
second-language programming for ethnic populations.
Power requirements: Power is supplied via an internal 9-volt transistor battery.
Alkaline batteries are more expensive initially, but are more economical over the