AUDIO BASICS
©1997
by
Eddie Ciletti

Before diving in to the electronic pool, here is a summary of technical terms followed by a block diagram approach to recording equipment.

P.S. This is a work in progress. Feel free to e-mail questions and become part of the program.

1.) What Is An Electronic Circuit?

Circuitry consists of passive and active components. Passive components, such as resistors and capacitors, create a working "environment" for active components such as vacuum tubes and transistors. An audio circuit can "amplify" a weak electronic signal and make it powerful enough to move a loudspeaker — such is the case when a microphone is used in conjunction with a public address system. An amplifier always has an input and an output.

2.) What Is Hybrid, Discrete And Integrated Circuitry?

If vacuum tubes and transistors are utilized within one product, that circuitry is called "Hybrid." Many transistors can be combined into a single off-the-shelf package called an Integrated Circuit (IC). A vacuum tube is considered "vintage" technology and is about seventy years old. The transistor is fifty years old and the integrated circuit is roughly thirty-five years old. Most modern equipment, such as computers, utilize integrated circuits along with minimal support from passive components and a few discrete transistors.

3.) What Is A Compressor/Limiter And Its Key Sub-Circuits?

A "compressor/limiter" is a device which acts as an automatic volume control (AVC). An AVC can optimize audio volume so that a weak signal is made louder and a loud signal is made softer. In addition to input and output amplifiers, the key sub-circuits of a compressor/limiter are the Gain Cell and the Detector. The Detector converts the audio signal — which is an Alternating Current, or "AC" — into a corresponding Direct Current or "DC." This signal, or control voltage, is sent to the Gain Cell which is responsible for processing the main audio signal level. There are many varieties of Gain Cell circuitry.

4.) What Is Meant By "Off-The-Shelf?"

Manufacturers wishing to "protect" circuit designs from piracy will often construct and then "pot" the finished assembly — via epoxy or silicone resins — into what is referred to as a "Black Box." A Black Box device can neither be serviced nor modified but merely used as intended by the manufacturer. An alternative to this more expensive approach is to design an Application Specific Integrated Circuit (ASIC). For this reason, many compressor/limiters are built around existing, off-the-shelf ASICs.

5.) Compression vs Limiting

The input amplifier serves as a buffer which then feeds both the detector circuitry and Gain Cell which for now will be treated as a "black box." Gain Cells typically only attenuate and so are followed by a "make-up" Gain stage. The output amplifiers serves to isolate what’s inside the box from the outside world.


About The Author

Eddie Ciletti is an engineering graduate of The Pennsylvania State University and a Technical Writer for EQ Magazine.



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