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AutoVelocity™ Dynamics and the dbx 160SL

Many audio compressors employ an "Auto" mode to set the attack and release

of the VCA so that the compressor will be well behaved when used in a variety

of situations such as compressing vocals, piano, or program material. "Well

behaved" means that pumping and breathing are minimized and that dynamic

distortion, caused by low frequencies modulating the control line feeding the

VCA, is not audible. The "Auto" mode works beautifully for most compression

needs, however, we often need to compress signals that require a quicker

attack time to catch leading edge transients such as a snare drum hit.

Many compressors on the market have manually adjustable attack and release

controls to give the engineer greater creative control over how the

compressor reacts to a signal. dbx compressors having manual controls

include the 266/266XL, 1066, 165/165A, the MC6, and new dbx 160S/160SL. If

we look at the markings around the dbx 1066 attack and release controls, we

find the attack range is 3 to .04 dB/msec and the release range is 250 to 5

dB/sec. These markings may seem cryptic at first but, upon further investi-

gation, we find that an attack setting of 3 dB/msec means that the VCA will

reduce its gain at a rate of 3 dB for every millisecond (0.001 second) of time,

or 3000 dB for every second of time. Whereas a release rate of 250 dB/sec

means that the VCA will release its gain reduction at a rate of 250 dB for every

second of time. These attack and release times may seem incredibly fast, but

believe it or not, they are not fast enough for all compression needs. So, why

not just speed up the range of the attack and release controls?

Well, let's examine the dbx 165/165A. Her attack and release times ranges are

400 to 1 dB/msec and 4000 to 10 dB/sec respectively. Anyone who has used

the 165/165A in the manual attack/release mode knows that it has an

extremely wide range of control and how effective it is for compressing fast

transients by dialing in a fast attack time. However, they also know how easy

it is to get a "bad" sound out of the 165A if the attack and release controls are

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AutoVelocityª Dynamics and the dbx 160SL White Paper - November 1998