What's in the Signal Pt 1

There is more to radio than what you hear. I dont mean all the goings on behind the mike. I'm referring to the signals in the broadcast that you can't hear. The audio you hear is data on a carrier wave. There is more to it than audible sound.

There are 100 possible FM channels between 88.1 and 107.9 MHz. These are allocated in 200 kHz increments. Because of limited bandwidth a radio station can only transmit audio which is up to 100 kHz. But hey, that's still much wider than the range of human hearing. In practice, FM audio is usually restricted to 15 kHz. This leaves a lot of spectrum for other uses. FM Stations are permitted to place subcarriers in this unused portion of the spectrum.

Here's what's on that carrier wave that you can hear:
Normal baseband audio: consists of the right and left audio mixed together (R+L). This is so that when listening on a mono receiver, you hear both channels of sound.

Difference signal: This is harder to imagine, it's the difference between the right and left channels. When a songs in stereo both ears are getting very similar but not exactly the same audio. This is just the parts that are different or (L-R). This is transmitted on a 38 kHz subcarrier using FM modulation.

Mind blowing isnt' it? FM radio does not broadcast the Left and Right Channels. It's not a damn think like the channels on a cassete tape. Here is how how stereo audio is decoded. Of course, as much of a genius as Edwin Armstrong was, it was nto his system that gave us stereo audio. The Armstrong system was rejected by the FCC because it did not allow sub-carrier services. The the Zenith system has gone on to become the standard method in most countries.
The Armstrong system was more noise resistant I understand. Instead today's stereo FM signals are far more susceptible to noise and multipath distortion than mono FM signals. This is due to several factors, including the following:

1. the addition of the two sidebands of the difference subcarrier to the baseband signal increases the noise bandwidth of the signal by a factor of three (9.5 dB) as compared with a mono signal.
2. The pre-emphasis is applied to the audio signals results in the pre-emphasis acting in the wrong direction on the lower sideband of the difference subcarrier, i.e. decreasing the level as the frequency rises, which will have a further deleterious effect on the S/N of the difference signal. yadda yadda , I know thats too technical. Btu I cant think of a way to simplify it.

So heres what we got and how it works:
If you fed the baseband audio into a receiver's antenna input it will demodulate the signal. This demodulated signal would be the R+ L audio. If you combine this with the Difference (R-L) signal you get a mess, but that's kind of how it works. What it does is add the R+L and R-L signals, leaving just the right (R) channel. the remainder being the LEft channel left (L) channel. ta-dah!

The upside of this complexity and reduced reception qualityis that mono and stereo stations can all operate with this system as can all FM tuners. Also side bands can exist and aren't those nifty. More tomorrow!

Reading Services for the Blind

Braille was invented in 1816 by Charles Barbier. Several improvements were made to the language by Louis Braille including the addition of Musical notaitons in 1821. Braille allowed the blind to read books, magazines and newspapers. But radio made innovations to serve them as well. but with the growing popularity of books-on-tape how long will this novel public service last?

*Currently the FCC actually requires hat one SCA [Subsidiary Communications Authorization] channel in a community (not necessarily on an NCE station) had to be kept for a radio reading service for the visually impaired. http://reader.ku.edu/oldsite/scatech.htm#sca

The first two on this list offer Radio Reading Services as a primary service. The rest as a secondary sideband service that requires a special radio. Other services as noted.

WYPL 89.3 Memphis, TN
http://www.memphislibrary.org/wypl/index.html
The West Memphis Public Library started this station as a closed circuit service reading for the blind. In the beginning, the Memphis Lions Clubs bought specially made radios for visually impaired individuals. By the mid-1980s, there were about 11,000 listeners and 4,000 on the waiting list for the special radio that only carried the service. These days they reach about 2.4 million homes.

WRBH 88.3 New Orleans, LA
http://www.gnofn.org/~wrbh/
On September 12, 1982, WRBH signed on the air. since then they have been the recipient of numerous awards and accolades including being named the 257th Point of Light by President George Bush in 1990.

WWNO 89.9 New Orleans, LA
http://www.wwno.org/
Began offering a reading service on their side band in the 1970s with help from Robert T. McLean. The same Dr. Mclean went on to found WRBH.

WLRN 91.3 Miami, FL
The only reading service for the blind in South Florida and airs 24 hours a day, 7 days a week.
http://www.wlrn.org/rrs/

KDPS 88.1 Des Moines, IA
http://www.kdpsradio.com/
Iowa Commission for the Blind established a reading service in the 70s.
The service may now be defunct.

WVIA 100.3 Scranton, PA
The estemed Pell reading service launched in 1981. It is supported by a grant foundation of the same name. http://www.wvia.org/resources/prrs.html

WDET 101.9 Detroit, MI
http://www.wdetfm.org/dris/index.php
The Detroit Radio Information Service, a radio-reading service for people with disabilities. Provided by Wayne State University.

WTEB 89.3 New Bern, NC
This reading service is receivable by either sub-channel radios which are loaned free to qualifying individuals, or by televisions which have second audio capability.
http://www.wteb.org/blind.asp

KNOW 91.1 Minneapolis, MN
http://minnesota.publicradio.org/radio/stations/knowksjn/
This may be well the first one adding the service possibly as early as 1969. Reports indicate it may currently be suffering from interference likely caused by a translator on 90.7. The repeater runs christian K-love branded programming via satillite....

WIGH 88.7 Lexington, Tennessee
(no info)