Ramsey Electronics STC1 Instruction Manual - page 4
STC1
• 4
INTRODUCTION TO THE STEREO TRANSMITTER COMPANION
Knowing how troublesome different audio sources can be when trying to
transmit them over the air, we have come up with a product that will eliminate
many of the problems. This kit allows the FM10A and the FM25 stereo
transmitters to have the same quality audio and transmission that the
professional radio stations have. The STC1 has many of the same features the
professional sound processors have, and will be sure to please your listening
audience when used with your home brew transmitter.
If you have experienced a steady whine while trying to transmit a television
signal or audio from an older CD player, this kit will help to reduce the problem.
The cause of this problem is the horizontal sweep frequency in a television, or
the 44kHz sampling frequency of the CD player mixing with the 38kHz stereo
carrier. What you are hearing is the sum and the difference frequencies that are
the result of the mixing process. The same occurs with high frequency audio
such as cymbals and when someone says the letter ‘s’.
To help control this problem, this kit incorporates a 16kHz 8th order Butterworth
low-pass filter. This helps to eliminate interference with high frequencies, such
as cheap CD players and other high frequency interference. Some of you may
ask what 8th order means? Well, put simply it means a steep cutoff curve
above 16kHz. The more orders in a filter, the greater the slope of the cutoff
curve. Butterworth is the name given to the description of the curve’s shape. A
Butterworth curve means there are no dips or peaks in the audio response
before the cutoff point’s frequency. This means there is no “coloration” added to
the audio being processed in the circuit.
To prevent overmodulation in your transmitter, this kit incorporates a soft
limiter. This limiter prevents the audio from going over a certain set level. Since
the audio is limited, so will be your modulation. There are indicator LEDs
included so that when the limiter is activated, you will know. When an audio
signal is limited, it introduces distortion into the audio causing the sound to
become rather distorted. The idea is to run your audio just under the limiting
level so that the LEDs blink very rarely. It also is a good way to keep all of your
audio levels close to the same for sound volume consistency.
To add a little more functionality to the STC1, and make it difficult for the
engineer to stuff all the parts on a small, single-sided board with a very few
jumpers, a set of tone controls were added in. This allows you to compensate
for a lack or surplus of bass, treble or midrange before the sound is transmitted.
Some people don’t like a nice flat response in their sound, they like the bass
turned up to give a nice full sound to their listeners. Needless to say these tone
controls allow you to custom tailor your sound to your own tastes.