Chris Melfi's PowerPoint on compression

1. By Chris

2.  Dynamic Range Compression (also called DRC, or simply Compression)
reduces the louder sounds, and amplifies the quieter sounds, narrowing the
dynamics, hence the name compression.
Clip

3.  Hardware compressors came first, in the late
1960’s, with compressors such as the optical
compressor, made by the artist Joe Meek.
 The first commercial studio compressors (not
custom-built) were the Teletronix LA-2A (1965) and
the Universal Audio 1176 (1968).

5.  Software Compressors are usually included in most
DAW’s, usually created to emulate real studio
compressors. Many of these software compressor
emulators do a nice job. The only limitation to
software compression is that its really only meant for
post-recording... Whereas studio compressors can be
used while recording with a much more successful
outcome.

6.  A compressor reduces the level of an
audio signal if its amplitude exceeds a
certain threshold. It is commonly set in
dB, where a lower threshold (e.g. -60
dB) means a larger portion of the signal
will be treated (compared to a higher
threshold of −5 dB).

7.  The amount of gain reduction is determined
by ratio: a ratio of 4:1 means that if input
level is 4 dB over the threshold, the output
signal level will be 1 dB over the threshold.
The gain (level) has been reduced by 3 dB:
 Threshold = −10 dB... Input = −6 dB (4 dB
above the threshold)... Output = −9 dB (1 dB
above the threshold)
 The highest ratio of ∞:1 is often known as
'limiting'. It is commonly achieved using a
ratio of 60:1, and effectively denotes that
any signal above the threshold will be
brought down to the threshold level (except
briefly after a sudden increase in input
loudness, known as an "attack").

8.  A compressor might provide a degree of control
over how quickly it acts. The 'attack phase' is the
period when the compressor is decreasing gain to
reach the level that is determined by the ratio. The
'release phase' is the period when the compressor is
increasing gain to the level determined by the
ratio, or, to zero dB, once the level has fallen below
the threshold. The length of each period is
determined by the rate of change and the required
change in gain. For more intuitive operation, a
compressor's attack and release controls are
labelled as a unit of time (often milliseconds). This is
the amount of time it will take for the gain to
change a set amount of dB, decided by the
manufacturer, very often 10 dB. For example, if the
compressor's time constants are referenced to 10
dB, and the attack time is set to 1 ms, it will take 1
ms for the gain to decrease by 10 dB, and 2 ms to
decrease by 20 dB.

9.  Another control a compressor might
offer is hard/soft knee. This controls
whether the bend in the response
curve is a sharp angle or has a rounded
edge. A soft knee slowly increases the
compression ratio as the level increases
and eventually reaches the
compression ratio set by the user. A soft
knee reduces the audible change from
uncompressed to
compressed, especially for higher ratios
where the changeover is more
noticeable.

10.  Because the compressor is reducing
the gain (or level) of the signal, the
ability to add a fixed amount of make-
up gain at the output is usually
provided so that an optimum level can
be used.

11.  When side-chaining, the compressor uses
the volume level of an input signal to
determine how strongly the compressor will
reduce the gain on its output signal. This is
used by disc jockeys for ducking – lowering
the music volume automatically when
speaking. The DJ's microphone signal is
routed to the compressor's side-chain so
that whenever the DJ speaks the
compressor reduces the volume of the
music. Many electro-house, dn’b and
dubstep tracks use side-chaining.
http://en.wikipedia.org/wiki/File:Upli
fting_Trance_Sidechain.ogg

12.  http://www.youtube.com/watch?v=3TlQo9
k827c&feature=related
 Here is a fairly biased view on the loudness
war... I personally don’t see a massive
problem with it, as now-a-days quite a lot of
the music we listen to actually sounds quite
good when compressed a lot.

13.  http://bit.ly/I4f8Aj
(http://www.sae.edu/reference_material/a
udio/pages/Compression.htm)