Operating instructions

The Technical Stuff
__________________________________________________________
- 20 -
Ratio
Another important term is compression ratio, which describes the amount of increase required in the
incoming signal in order to cause a 1 dB increase in output. A ratio of 1:1 therefore means that for
every 1 dB of increase in input level, there is a corresponding 1 dB increase in output level; in other
words, there is no compression being applied. A ratio of 2:1, however, means that any time there is
an increase of 2 decibels in the loudness of the input signal, there will only be a 1 dB increase in
output signal. A ratio of 4:1 means that even when there is a full 4 decibels of increase in loudness,
there will still only be a 1 decibel increase in output signal. (Bear in mind that decibel is a
logarithmic form of measurement, so a 2 dB signal is not twice as loud as a 1 dB signal; in fact, it
requires approximately 10 dB of increased gain for a signal to sound twice as loud.)
As you can see from this illustration, at a low ratio, a compressor has relatively less effect on the
incoming signal; at higher ratios, it has more effect. When operating in compression mode (that is,
when the front-panel Mode switch is set to COMP), the LA-610 uses a compression ratio of
approximately 3:1; however, as we will see shortly, this is program dependent, so that the actual ratio
changes according to the frequency content of the incoming signal.
While the terms “compression” and “limiting” are often used interchangeably, the general definition
of compression is gain reduction at ratios below 10:1; when higher ratios (of 10:1 or greater) are
used, the process is instead called limiting. Limiters abruptly prevent signals above the threshold
level from exceeding a certain maximum value. At very high ratios of 20:1 or greater (some limiters
even offer a theoretical infinite ratio of Infinity:1), “brick wall” limiting kicks in—that is, any change
in input, no matter how great, results in virtually no increase in output level. Infinity:1 is the ratio
used by the LA-610 when it is operating in limiting mode (that is, when the front-panel Mode switch
is set to LIMIT); however, it is designed so that the same warm sonic characteristics are retained,
even though more severe gain reduction is applied.
As an aside, an expander is the opposite of a compressor: a device which increases the dynamic
range of a signal. For example, a 10 dB change in the input signal might result in a 20 dB change in
the output signal, thus “expanding” the dynamic range.