1.0 FM "MAIN" MODULATION:
An FM broadcast signal can be a complex thing. There is the "main" carrier, for example 100.1 MHz which, by itself, contains no information. The main station information to be transmitted, for example a musical song, is then frequency modulated onto the main carrier. This can be monaural (mono) or stereo. If stereo is to be transmitted, then the stereo signal is modulated onto the main carrier using a subcarrier modulation scheme (see 4.0 below).
Other information can be modulated onto the main carrier using subcarriers. These include the RDS (57 KHz) (see 3.0 below) and Subsidiary Communications Authorization (SCA) channels (see 2.0 below).
Standard FM receivers detect the stereo channels. To receive the RDS or SCA information special receivers are required that have wide band IF's and audio responses, and special demodulators.
2.0 SUBSIDIARY COMMUNICATIONS AUTHORIZATION (SCA):
The SCA communications channels have been authorized since the 1950's. These channels are above-audio frequency subcarriers FM modulated onto the "main" FM carrier. The subcarriers are then also FM modulated with the desired information, i.e. music, voice, or data. The SCA subcarriers normally used are 67 KHz and 92 KHz although there are no restrictions on the subcarriers frequency other than technical limit or interference considerations.
To receive these SCA channels an FM receiver with a wideband IF and wideband audio output is required in order to pass these subcarriers to the subcarrier demodulator circuit. The wideband audio extends out to 100+ KHz and is called the "composite" output because it contains the main, stereo, SCA and all other modulation signals present.
There are several types of subcarrier demodulator circuits.
A phase locked loop (PLL) circuit "locks" onto the subcarrier and the information FM modulated onto the subcarrier is demodulated as the "error" signal from the PLL. An input filter is provided to pass only the subcarrier and modulation of interest.
Another type of demodulator accepts the subcarrier and mixes the subcarrier with a local oscillator generating an intermediate frequency (higher than the subcarrier) that is then amplified and applied to a discriminator circuit. The discriminator circuit output will be the information modulated onto the subcarrier. Dayton Industrial Corporation uses this type of demodulator. The I.F. used is 455 KHz. The SCA subcarriers are limited as to the allowed deviation of the main carrier. The limited is 10% (+/-7.5 KHz). This deviation limit results in lower effective sensitivity of the receiver to SCA subcarrier information than to the main carrier. Typical receiver sensitivity to the main can be 0.5 microvolt while the SCA subcarrier sensitivity may be some 5 microvolts. This then results in a reduction of the range from the transmitter that the SCA provides useful information.
Dayton Industrial Corporation designs and manufactures several models of receivers designed specifically for receiving the SCA information. These range from handheld models to professional rack mount versions. We also have designs for specific OEM applications for transmitting data, areawide proprietary applications such as ethnic radio, reading services, etc. Most of these are described within the web site.
3.0 RADIO DATA SYSTEMS (RDS):
A relatively new service, radio data service, is being used by the FM stations. This data service is a slow speed data that is used to identify the station, the artists name, song title, or promotional information. It uses a subcarrier at 57 KHz and is normally injected at +/-2.5 KHz, but could be the entire subcarrier allowed deviation. In addition to the data used by the station itself, there are numerous other data frames that can be used for such applications as paging, global positioning refinement data, and wide area local control. The data rate is about 1200 bits/second. More information is presented as "RDS Details" in this website. Dayton Industrial manufactures several professional models for receiving encoding and decoding the RDS signals.
4.0 STEREO:
Stereo audio signals are also transmitted by the FM station using a form of subcarrier. The stereo signal consists of Left and Right audio information. It is transmitted as "mono" (L plus R), and "stereo" (L minus R) signals. The "mono" signal is modulated onto the main carrier as the frequencies up to 19 KHz. A 19 KHz unmodulated signal is the "stereo pilot" signal. It is used to indicate that "stereo" is present and also to regenerate the stereo subcarrier in the stereo demodulator portion of the receiver. The "stereo" signal is sent as a suppressed-carrier (38 KHz), double-sideband AM signal. It will occupy the composite bandwidth from about 23 KHz to 53 KHz when fully modulated.
The standard FM receiver will demodulate the stereo signal by regenerating the 38 KHz subcarrier and recovering the (L minus R) portion of the signal. The two signals (L plus R) and (L minus R) are then added and subtracted to generate the Left and Right audio.
5.0 SUBCARRIER CONSIDERATIONS:
When working with subcarriers in FM receivers and transmitters there are several important items to consider:
5.1 RECEIVERS:
The receivers must be capable of receiving a composite signal bandwidth in excess of that normally associated with an FM receiver. For example, the stereo signal occupies the composite bandwidth out to some 53 KHz as in the standard FM receiver. If the station has RDS (57 KHz), SCA (67 KHz) and SCA (92 KHz), the composite signal will extend out to 100 KHz. The receiver must then have IF bandwidths wide enough to pass these signals with a minimum of amplitude and phase distortion. The first demodulator (composite) must be wide enough to recover the entire signal and be linear over the signal range.
Any distortions in the receiver operations will cause "crosstalk" as a result of unintentional mixing. The most often result is the main audio signal being heard in the SCA channel.
5.2 TRANSMITTER:
The transmitter has similar restrictions, in that, adding a subcarrier will require the flat response portion of the transmitted signal to extend out to that necessary to transmit the SCA or other subcarrier signals. Any distortions or non-linearities can cause high VSWR or narrow bandwidth characteristics in the transmitter-antenna system and serious main to SCA (or SCA to main) crosstalk.
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Last Updated October 16, 1998 p.m.