Series, 160s, 160sl – dbx 160SL User Manual

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Why You Need A Compressor

A remarkable feature of the human ear is that it can detect an extremely wide range of amplitude changes -
from the slightest whisper to a deafening clap of thunder. If one tries to record or reproduce this wide spectrum
of sound with the help of amplifiers, cassette recorders, records, or even digital recorders, one is immediately
restricted by the physical limitations of electronic and acoustic sound reproduction technology.

The useable dynamic range of electronic audio equipment is limited as much at low levels as at high levels. The
thermal noise of electrons in the components results in an audible noise floor and thus represents the bottom limit
of the transmission range. In such equipment, this is referred to as “hysteresis”. Referring to any magnetic media,
hysteresis is the amount of electrical impulse it takes for the tape recorder to begin to rearrange the magnetic
particles of the tape (record signal onto the tape). The more energy it takes, the more noise is introduced onto
the tape, making the noise floor more audible. The amount of hysteresis varies depending on the brand of tape
used, and depends on the materials used in the manufacture of the tape.

When magnetic tape was first used to record audio, hysteresis was a real concern, because it made the useable
dynamic range of magnetic tape very narrow. In the early 1960s, it was discovered that if a tape recorder supplied
an extremely high frequency tone to its record head (much higher than the magnetic tape could possibly record),
the tape’s hysteresis was greatly reduced, or, it took less electrical activity to start to rearrange the magnetic
particles of the tape. The result: a much lower noise floor level and a wider dynamic range. This tone is called a
“bias tone” or “bias frequency.” Today tape manufacturers produce tape optimized for specific bias frequencies,
and studio technicians align tape recorders’ heads according to these specifications, in order to take full advantage
of the tape recorders’ electronics, and the magnetic tape’s ability to record sound on its magnetic particles.

The upper limit of useable dynamic range is determined by the levels of the internal operating voltages; if they
are exceeded, audible signal distortion is the result. Although in theory the useable dynamic range sits between
these two limits, it is considerably smaller in practice, since a certain reserve must be maintained to avoid
distortion of the audio signal if sudden noise peaks occur. Technically speaking, we refer to this reserve as
headroom--usually about 10-20dB. A reduction of the operating level would allow for greater headroom, i.e. the
risk of signal distortion due to high level peaks would be reduced. However at the same time, the basic signal to
noise ratio of the program material would be increased significantly. It is therefore useful to keep the operating
level as high as possible without risking signal distortion in order to achieve optimum transmission quality. It
is possible to further improve the transmission quality by constantly monitoring the program material with the
aid of a volume fader, which manually changes the level of the program material. During low passages the gain
is increased, and during loud passages the volume is decreased. Of course it is fairly obvious that this kind of
manual control is rather restrictive; it is difficult to detect signal peaks and almost impossible to level them out.
Manual control is simply not fast enough to be satisfactory.

The need therefore arises for a fast acting automatic gain control system which will constantly monitor the signals
and which will always adjust the gain to maximize the signal-to-noise ratio without incurring signal distortion.
This device is called a compressor or limiter.

160SL

lue

series

160SL

controls

160SL

notes

160SL

Operations

160SL

Operations

Inspection

Operating

Connection

Installation

Technical

Introduction

to your system

considerations

contents

concept

diagram

applications

support and factory service

160SL

Specifications

160SL

Manual

Design

Block

Controls

Advanced

Warranty

lue

series

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