"Nathan Higgins" -spam wrote in message
...
After learning that Scott Spock used a Distressor with Avril Lavigne, I
did
a google search for some info but without much luck, I have seen reference
to distortion but I need a layman's explanation...
http://store4.yimg.com/I/mercenary-audio_1749_2687275

It is a compressor but please don't judge its value from one recording, good
or bad.

I believe the following article is Mike Rivers' material. He didn't sue or
send me a cease and desist order last time I posted it, so...hopefully he
doesn't mind again.



A compressor is one of the most common outboard tools in a studio. All
ompressors perform the basic same function but, like microphones, various
models perform it differently, giving each different model a characteristic
sound or personality. Compression is a hot item among project studio
engineers these days, probably due to the variety of units on the market and
the perception that everything needs to be
compressed. This month, we'll look at what a compressor does, what
haracteristics separate one model from another, and look at situations where
compression is used, and abused.

Dynamic Range

The basic function of a compressor is to reduce dynamic range. Dynamic Range
is the difference between the loudest and quietest signal levels that pass
through the recording chain. The difference between a sound being barely
audible and being physically painful is about 130 dB, so this is what we
consider to be the dynamic range of human hearing. Anything below the
threshold of hearing will be lost, as
will anything above the threshold of pain. But how much dynamic range do we
need in our recordings, or can we really use?

Few of us listen in a totally soundproofed room. A well isolated control
room has an ambient noise level 10 to 15 dB higher than the threshold of
hearing. Since we want to keep ourselves safe from pain and hearing damage,
100 dB or so of dynamic range is about the practical limit. But where do we
typically listen to recorded music? A very quiet living room has an ambient
noise level about 25 dB higher than the threshold of hearing. The inside of
an automobile is 60 dB higher. Since most audio systems aren't capable of
producing painful sound pressure levels (I'm rethinking that as a car drives
by my house with the bass pumping loud enough to rattle my windows), a
typical listening environment can only support a dynamic range of 65 to 75
dB.

Any 16 bit digital system worth its dither can provide a dynamic range of
better than 90 dB. The theoretical limit is 96 dB (it's not really that
simple but this is an accepted working value) but necessities of life like
mic preamps, mixers, and power amplifiers add noise which eats into the low
end of the theoretical range. So, at a minimum, we have to squeeze our 115
dB of theoretical dynamic range into a 90 dB box. Practically, we have to do
more than that so that soft passages in our music don't get lost when your
next door neighbor starts up his lawnmower, ruining your nice quiet 25 dB
noise level living room, or when you're hearing the car radio through the
highway noise. So we can't use all the dynamic range that's available to us
if we expect people to hear all the music we record.



Compression To The Rescue

A compressor reduces the dynamic range of a signal. The range of amplitudes
coming out is less than what goes in. When used conservatively, the action
of a good compressor is hard to detect. But sometimes we want to shape a
sound, and a compressor is one of the tools we can use to do that. A
compressor may be inserted into a single channel in the recording chain, for
instance when recording or mixing a vocal track, or compression may be
applied to an entire mix or sub-mix.

In speech or singing, there are often periods of silence. Hard consonants
such as the letter `T' create high initial sound pressure levels, whereas
most vowels tend to be more even. The average volume level of a word may be
fairly low, but because of an initial loud consonant, we can only boost up
that word so far before we run out of
headroom. If there's music playing under the voice, even with the vocal
level boosted as high as possible without distortion on the initial attack,
part of the word may end up being far enough below the level of the music to
become inaudible or misunderstood.

By processing the voice with a compressor and adjusting it so that the loud
attack causes the onset of gain reduction, the compressed word can be
boosted to a level high enough to be understood over the music. Of course
you can't adjust the compressor for every word in the song (well . . . you
could on a digital workstation if you had the patience), but a combination
of a good average setting and a singer with some control will yield
effective results.

Another use for a compressor, and one that seems to be particularly popular
today, is to make a recording sound louder since, to most listeners, louder
sounds better. Often there will be a single sound (a snare drum is common in
pop music) that will be somewhat louder than anything else in the mix. If
the drummer hits the snare louder on some beats than others, the loudest hit
determines the maximum level that can be recorded, whether on disk, tape, or
CD. Increasing the level will
cause those peaks to distort. But by compressing the overall mix, those loud
transients can be controlled, allowing the average level to be raised.


Basic Theory and Buzzwords

We need to be able to adjust the compressor so that it will reduce the level
of signals above a certain volume and not affect lower level signals. This
is called the Threshold, and nearly all compressors have a control for it.
Those which don't have a fixed internal threshold and you set the point at
it starts compressing by adjusting the input level.

Except for the compressors built into multi-function microphone processors
or mixers, compressors are line-level input devices. The Threshold control
is generally calibrated in dB below (and above) the nominal line level of
the unit (typically +4 dBu or -10 dBV), though it's rarely a precise
calibration even at the 0 dB mark. But to keep the control in a good working
range, the compressor you choose should be
at least nominally matched to the line level of your console and other parts
of your system.

Below threshold, a compressor (ideally) has a linear gain characteristic,
just like a piece of wire or a linear amplifier. Whatever goes in comes out
unchanged except perhaps for a shift in overall level. When the input
increases by 6 dB, the output also increases by 6 dB. But we want the
compressor to reduce its gain when presented with an input level above the
threshold. If the compressor's output changes by 3
dB when an above-threshold input changes by 6 dB, we say that the
Compression Ratio is 2:1. If we want a peak coming in at 10 dB above
threshold to come out at a level that's 2 dB above threshold, we need to
compress with a ratio of 5:1.

We can also say that this represents 8 dB (10 minus 2) of gain reduction.
This expression of the amount of compression is an "eyeball average" since
the actual amount of gain reduction at any instant varies with the input
level at that instant. When someone says "I compressed vocals 2 to 3 dB",
they mean that they applied light compression, where most of the peaks don't
get more than 2-3 dB of
gain reduction. This is typical of the compression applied to a singer with
good dynamic control when tracking. It evens out the sustained notes just a
bit and provides a small safety net against surprise overloads.

If we never want the output level to exceed the threshold level, the
compression ratio approaches infinity (10:1 is usually practically close),
as a large change in input level over threshold will result in a very small
change in output level. In this case, our compressor becomes a limiter, as
the output level is limited to the threshold level.

Figure 1 illustrates the basic action of a compressor graphically for
several ratios. The slope of the line represents the gain. Notice that the
line changes its slope at the point where we've set the threshold, in this
case, at 0 dB. What makes a compressor a compressor is the fact that its
gain changes from unity to some lower gain when the input level exceeds the
threshold. Below the threshold level, the compressor has a gain of 1. For
every dB change in input, there's a corresponding change of 1 dB in output.
Above the threshold, the gain is lower. Look at the 2:1
line. Note that with 10 dB change in input, we get only 5 dB change in
output, a ratio of 2 to 1. Check out some points on the other compression
ratio lines to convince yourself.

The point at which the slope of the line changes is called the "knee". A
compressor is said to have a "hard knee" characteristic when the slope
changes abruptly as in this graph. Some compressors have a "soft knee"
characteristic (sometimes switchable) in which the break isn't a sharp
change of angle, but rather, it's rounded off so that the gain change is
gradual over some range of input. (dbx holds the trademark on the term "over
easy" to describe their brand of soft knee
compression) The gain change of a soft knee compressor actually begins
somewhere below the threshold level and gain continues to decrease to its
final value at some input level beyond the threshold. A hard knee tends to
do better at catching transients while a soft knee characteristic tends to
be less obtrusive on vocals. But these are only typical applications. Your
voice or kick drum may vary.

A compressor doesn't know what's coming at it until it happens, so it needs
a little time to figure out how much gain reduction is needed. This is the
response time and it's a function of the way the input level is detected.
It's fixed as part of the compressor design and is one of the things that
contributes to the compressor's personality. Another important time
parameter is Attack Time. Its definition is a bit loose. Sometimes it means
the amount of time it takes for the compressor to
reach full gain reduction when triggered by an over-threshold input,
sometimes it's defined as the time required to get most of the way (I've
seen 67% in print) there.

Adjusting the attack time can make a big difference in how a compressor
affects the signal. If the signal we're compressing has a loud initial
attack (such as almost any drum), we may want to assure that the attack gets
through unaffected even though it's louder than our desired average output
level. In this case, an attack time that's longer than the instrument's
attack time is desired. On the other hand, if
it's the transient that we want to sit on, we want a fast attack (short
time) so that gain reduction will begin as soon as possible after the input
crosses the threshold. The attack time is related to, but is not the same as
the shape of the compressor's "knee".

Once a high level signal falls back below the threshold level, the
compressor stops compressing and starts working like a piece of wire again,
but this doesn't happen instantly. Instead, the compressor's gain slews
gracefully (we hope) back to unity over some period of time. This time
period is called Release Time, and like attack time, it too isn't always
specified the same way.

Since the purpose of a compressor is to reduce gain, it is usually necessary
to amplify its output after the gain reduction to get back to your system's
nominal operating level. Most compressors have an Output or Gain control
which allows you to adjust its output to match up with the next point in the
signal chain.

Most compressors are equipped with a meter which looks like a VU meter but
that appears to work backwards. When using a compressor, it's useful to know
how much gain reduction it's doing. A typical compressor meter will read 0
dB when the input is below threshold and moves down scale as the input level
goes beyond threshold and the compressor starts doing its thing. When the
meter reads -6 this
represents a gain reduction of 6 dB.

Often there's a separate meter or a switch to allow the gain reduction meter
to read input level as a guide to setting the threshold. And some
compressors also allow you to monitor the output level so you'll know when
you're in danger of running out of headroom.



The Guts

On the surface, a compressor appears to be a fairly simple device. All the
action takes place in the gain control element with an amplifier on the
front and back end to match up signal levels to the outside world. There are
several different devices that are used as a variable gain element, and to a
large extent it's the characteristics of the different devices that give
each different compressor its "personality". Today's garden variety (and
some not-so-garden variety) compressors use a modular, usually IC voltage
controlled amplifier or attenuator (VCA) for
gain control.

There are two paths in a compressor, the main audio path and the side chain.
The audio path is what you put in and what you expect to get out. But in
order to derive a voltage used to adjust gain, the input signal must be
detected and applied to the gain control input of whatever element is used
for gain reduction. This path is called the side chain.

We aren't interested in following the input waveform cycle by cycle, but
rather, want our control voltage to follow the overall level, or envelope,
of the waveform. The method in which the side chain control voltage is
derived is another contributor to the compressor's personality.
Manufacturers have used simple averaging detectors, peak level detectors,
and true RMS average detectors. Each will give control voltage that follows
the input signal a little differently, so each type of
sidechain detector will impart a different control action on the main audio
signal. In addition, designers have applied their own "corrections" to the
side chain signal to compensate for non-linearity of the gain control
element.

A couple of terms associated with compressor design and description are
"feed forward" and "feed back". In a feed forward design, the input signal
goes to the side chain detector. In a feed back design, the output of the
gain control element is fed back through the detector to control the gain.
Again, each has its personality in terms of response time.

Several of the "classic" compressors use a photocell (light dependent
resistor or LDR) as a variable resistor in a voltage divider. An
incandescent light bulb, LED, or electroluminsecent (EL) panel glued to the
photocell is driven by a voltage that's derived from the level of the input
signal (not the input signal itself). The actual signal goes through the
LDR. The light gets brighter as the signal gets louder, causing the
photocell to change resistance and, in essence turn a pot to reduce the
gain. A Vactrol (R) is a sealed module consisting of a light source and LDR
that was used in several "classic" compressors, and today you'll sometimes
see the term "Vactrol" associated with a certain flavor of compression.

Another gain control element is a vacuum tube used as a variable resistor,
lending the name "Variable-mu" (u is the abbreviation for the tube's gain)
to yet another style of compressor. The classic variable-mu compressor is
the Fairchild 670, now selling on the vintage market for over $10,000! Altec
also made one, and today, Manley Labs builds one using the same principle of
gain control, but with a different tube and making use of a full
differential signal path which cancels out second
harmonic distortion caused by the tube's action.

VCA compressors are most common today due to the availability of expensive
and reasonably good VCAs in IC form. Since this style of VCA is all on one
chip, the designer's challenge is to keep the signal that's controlling the
gain out of the actual audio signal path. New designs are pretty successful,
and offer the most positive control of gain vs. control voltage. The VCA is
the fastest responding and most linear (or rather, most predictable) gain
control element, so it lends itself well to the new breed of digitally contr
olled analog compressors. Here, the side chain
control voltage can be shaped digitally to produce any imaginable response
curve, allowing the compressor's gain control element to emulate the sound
of any "classic" which can be measured.

Just about everything inside the box affects the sound of a compressor - the
gain control element, the way the side chain signal is derived and
processed, the sound of the amplifiers in the input and output, and even the
power supply. Today's much lusted-after "tube compressor sound" is really a
new development. Of the much emulated classic compressors, only the
Teletronix LA-2 had tube amplifiers. All the newer models in the series were
solid state although they used the same basic (with seasonal variations)
gain control elements. Much of what we think of as the warm tube sound of a
compressor is a result of a tube input and/or output stage, not the
compressing action itself. Certain compressors have a reputation for
creating irregularities in frequency response, both favorable and
unfavorable, but this in general isn't a result of gain reduction, but
rather, input and output stages.



Compressor Warts

Two terms often used to describe a compressor, unfortunately in an
uncomplimentary sense, are pumping and breathing. Breathing is most
noticeable on a solo voice and is, in fact, the sound of the vocalist
breathing. If the release time is short, the gain will come up quickly
during the pauses between words which is precisely when the singer breathes,
making the breath audible. Hearing a singer take a breath may not always be
desirable or dignified, but at least it's organic. But
few recordings are made in an absolutely silent environment. If there isn't
an honest breath to fill up the space, the ambient noise in the room will be
boosted with the gain increase, perhaps carrying leakage from the singer's
headphones or a not very well isolated instrument with it. All compressors
will exhibit some breathing", but careful adjustment (which includes
controlling room acoustics and mic positioning) can minimize it.

Pumping is another anomaly associated with compressors. It's most apparent
when compressing an overall mix rather than a single track. If one
instrument in the mix is louder than the others, this is what will trigger
the compressor into action. If that instrument stops playing, even for an
instant, the level of the mix will increase noticeably. Each time the
dominant instrument starts or stops, it "pumps" the level of the mix up and
down. Compressors that work best on full program material
generally have very smooth release curves and slow release times to minimize
the pumping effect.



Setting the Knobs

If you see that the input is peaking at +15 dB and you want to reduce those
peaks to a more manageable +5 dB, you might want to set the threshold at -5
dB and use a gentle 2:1 ratio. Or if you want to use a stiffer ratio, say
6:1, you'd set the threshold at +3 dB. As an exercise, try plotting out a
few combinations yourself. By adjusting the threshold downward while keeping
the compression fixed, you can reduce the
maximum output level by compressing over a larger portion of the input
signal's dynamic range. By keeping the threshold fixed but increasing the
compression ratio, you'll reduce the output level by only affecting the
loudest signals. There are no rules for this, let your ears be your guide,
with your meters as a sanity check.

Adjustment of attack and release times can change the timbre by rounding off
an attack or stretching out the sustain portion of the envelope beyond that
produced by the instrument being compressed. A drum note can be "stretched
out" be applying a long release time, a healthy gain boost, and fairly high
compression ratio. If an instrument or singer produces a soft note following
a loud note, release time should be short to let the gain come back up and
let that soft note through.

Material with significant low frequency content requires special care when
compressing. The attack and decay portions of a kick drum run 60 to 80
milliseconds, but a low pitched kick can have a fundamental frequency of
about 40 Hz. This means that only three of four cycles of the kick's
fundamental are heard on each hit, much of that in the decay phase.
Stretching this out with a fast attack and high compression ratio can make
more cycles of the fundamental audible. The beater attack is a higher
frequency (1 to 3 kHz ballpark) so a moderately fast attack will still let a
few cycles of beater through while grabbing the head
fundamental frequency. Slowing down the attack will allow more of the beater
to get through and letting you bring down the overall level of the kick in
the mix. Release time must be short enough so that the gain gets back to
unity between hits, however.

Compressors with a fast attack time may work well on vocals but don't work
well on kick drum or bass because the compressor actually tries to follow
the individual cycles of the waveform rather than the envelope of the note.
This characteristic can be used as a special effect, but usually it just
takes all the life out of a low frequency source.



Squeezing Out

Compression isn't a by-formula thing. No article will tell you how to set a
compressor for a particular source, because there are so many things that
make each recording different that rules don't apply. Hopefully, you know
better understand how a compressor works and what the knobs do, so you can
better understand what you're hearing when you twiddle with the knobs.


--
___
Rick Knepper
MicroComputer Support Services
Knepper Audio
Ft. Worth, TX
817-239-9632
413-215-1267 Fax
PC Tech Support & Equipment Sales
CDR Duplication & Audio Mastering
Recording
http://www.rknepper.com

My girlfriend.


--
David Morgan (MAMS)
http://www.m-a-m-s.com
http://www.artisan-recordingstudio.com

Rick Knepper wrote:
"Nathan Higgins" -spam wrote in message
...
After learning that Scott Spock used a Distressor with Avril Lavigne, I
did
a google search for some info but without much luck, I have seen reference
to distortion but I need a layman's explanation...
http://store4.yimg.com/I/mercenary-audio_1749_2687275

It is a compressor but please don't judge its value from one recording, good
or bad.

It's actually a compressor whose real forte is that it can deliver a huge
number of totally different compression effects out of one box. So judging
the sound from one recording is even less valid here.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Nathan Higgins -spam wrote:

After learning that Scott Spock used a Distressor with Avril Lavigne, I did
a google search for some info but without much luck, I have seen reference
to distortion but I need a layman's explanation...
http://store4.yimg.com/I/mercenary-audio_1749_2687275

I couldn't really guess from the control names, attack ? release? no idea!

Thanks for any advice,

As Tonebarge said recently in another thread:

"Each tool has its advantages and disadvantages but you need to learn
"what each tool does and a good place to start is with this group's FAQ
"which can be found he http://www.recaudiopro.net/faq/index.htm"

What he said.

--
hank alrich * secret mountain
audio recording * music production * sound reinforcement
"If laughter is the best medicine let's take a double dose"

"David Morgan \(MAMS\)" wrote in message . ..
My girlfriend.

My ex-girlfriend...and a cool comp/limiter. One of which I just
couldnt afford to have around my studio anymore. although they both
had nice racks

"David Morgan \(MAMS\)" wrote in message . ..
My girlfriend.

I was gonna say "my kids". However, I used to distress furniture when
I was in college as part of a refinishing process to make things look
antique. It was a blast...you took someone's old china cabinet and
beat it with chains, scratched it with screw drivers and generally
beat the **** out of it to make it look, well, distressed. And we
charged extra for the service! I guess we were adding distortion.

Tom Paul
www.tompaul.org