WMBR's Transmitting Facilities|
WMBR's Transmitter is located at the top of the Eastgate Building (Bldg. E55) in Kendall Square, Cambridge.
WMBR's "Transmitter" is made up of three major components: a stereo generator, an exciter, and an RF power amplifier (PA). The exciter and power amplifier came together in a package from Broadcast Electronics, along with a rack and some equipment for controlling and monitoring the system. That whole thing is known as a BE FM-250B transmitter.
Aphex 2020 Mk III Stereo Generator
The stereo generator takes two audio inputs (left and right), and mixes them together into a mono signal (L+R). Then it mixes the left and right channels together 180 degrees out of phase (L-R). It takes the L-R signal and AM modulates it at a frequency of 38 kHz. This is called a "subcarrier". Then it takes the mono signal (L+R), the 38 kHz subcarrier, and a 19 kHz tone, and mixes them all together into one signal. This whole thing is called the "stereo composite" signal.
Brodcast Electronics FX-50 Exciter
The exciter is the piece of equipment that takes the composite signal from the stereo generator, and modulates it at a center frequency of 88.1 MHz. This means that it generates a signal with a frequency of exactly 88.1 MHz, then it varies that frequency back and forth, depending upon the input that it is fed. A stronger input signal will deviate the center frequency by a greater amount. The maximum deviation is plus or minus 75 kHz and this is known as 100% modulation. The exciter puts out about 20 watts of power.
Broadcast Electronics 250-Watt Solid State Power Amplifier
The output of the exciter is fed into the PA, which boosts the signal up to 240 watts and then sends the signal first through a low pass filter (to sqlelch any harmonics) and then through a 70-foot semi-rigid coax cable, up to the antenna on the roof.
Shively 8610-4D Antenna
Our antenna has a gain in the horizontal plane of a little more than three. When you take into account the loss in the transmission cable, that ends up giving us exactly 720 watts "effective radiated power".
How Stereo Works?
If you have a mono radio, it just filters out everything above 15 kHz. This means that all it gets is the mono signal which is an equal mix of left and right. If you have a stereo radio, it takes the L+R signal and adds it to the L-R signal to get the left channel, then it subtracts the L-R signal from the L+R signal to get the right channel. The 19 kHz tone is called the "pilot" and while some radios use it to demodulate the 38 kHz subcarrier, many radios just use it to light up the little red light on your radio that says "stereo".