Questions about compressors

J

JonnyBoy12

New member
Hey. Lifelong musician, but new to the technical and mixing side.
I'm trying to learn all about compressors, but is there a big difference between FET, Vari-mu, VCA and optical compressors?

Also, this is probably a stupid question, but it's my understanding compressor can take a wide frequency range and "compress" it. But why when I add compression does it seem to make the audio louder, bigger and more "detailed". Shouldn't it almost have the opposite effect?

And lastly, when you have several instruments and some are starting to do
drown out, is it a good idea to add compression to get it back into clarity or is it wiser to EQ it differently? Sometimes EQ doesn't seem to do much when I EQ down the low end of a guitar and EQ up the low end of a bass, for instance.
 
Record some audio intro your DAW without compression, then do the same with compression added, or just duplicate the track and add it. compare the waveform. The valleys and peaks have peaks at around tame level, but you are raising the height of the valleys - this makes it sound louder as there are no quiet bits. Detail in the valleys, normally can't be heard - compression gives you much more detail from the quiet bits. The question always is if that stuff is good or bad?
 
I'm learning about compression, just like you JonnyBoy12.
I have a hardware compressor (Behringer 2600), a pre-amp with an optical compressor, two different compressor types on my SSL Big Six mixer, and then the plug-in compressors that come with Cubase. They're all going to have different characeristics.
No such thing as a stupid question.
Compressing may cause the overall signal to be louder, but you then adjust the output level to bring it back to be comparable with the uncompressed overall level.
There is a technique of placing similar frequency bands into a compressor just for those frequencies. So you might use several compressors to cover the whole
frequency spectrum.
Someone posted a link to a very good youtube explanation of compression here, a while ago.
Just do a search on youtube for compressors tutorial, and you shoud find some useful stuff.
 
Hi JonnyBoy12,
Compression's not all that complicated but, like anything, there's a few things to watch out for.

A basic compressor won't do anything to your frequency range - It deals with volume.
Say you take a click track peaking at -12 and a copy of it peaking at 0, then combine the two, offset, so you have a bounced track that goes
loud-quiet-loud-quiet

If you put a compressor on that track with the threshold set to -12 then the quieter clicks are going to be left alone because they fall below the threshold,
and the louder clicks are going to be compressed because they exceed it...essentially turned down.

There's more to it than that, of course. You have your ratio setting which dictates how aggressive the compressor is,
so a gentle ratio might make a subtle difference whereas an aggressive one could effectively be a limiter
and you have attack/release settings which control how quickly the compressor 'kicks in' and how long it takes it to relax back out when the threshold is exceeded.

For example a slow attack on a compressor on a snare track would ignore the crack of a snare completely then act on the body/ring sound,
resulting in a more aggressive, snappy, percussive snare,
whereas very fast attack and release could do the opposite,
catching the initial crack of the snare but disengaging quickly enough that the ring/body of the snare is untouched.
That last example is pretty common - Taming the crack of a snare and leaving the ring alone, allowing you to raise the volume of the whole snare track,
resulting in a fuller more resonant snare sound.

Some compressors have automatic make-up gain which automatically increases the level of the output after compression. Maybe yours has?
That would explain why your tracks sound louder/bigger to you when using compression.

Hope that's helpful.
If you can find someone doing a video tutorial with a compressor with visual representation of what it's doing, like Avid's Dyn3 compressor,
it might 'click' better for you. (y)
 
Mr. Maroo's post above is very helpful. Read it twice...or thrice.... maybe even quice......
JonnyBoy12 said:
FET, Vari-mu, VCA and optical compressors?
Click to expand...
Unfortunately, I can't help you with that one. Other more knowledgeable people will hopefully come in with a NON-CONVOLUTED, SIMPLE explanation. :)
JonnyBoy12 said:
my understanding compressor can take a wide frequency range and "compress" it.
Click to expand...
Not really. As mentioned, a compressor has nothing to do with "Frequency range". That's an EQ's job. A compressor is all about "Dynamic range", or simply put...volume. (I'm simplifying here, before someone comes in with their "Magic Alex" method of using a compressor to affect EQ)
JonnyBoy12 said:
But why when I add compression does it seem to make the audio louder, bigger and more "detailed". Shouldn't it almost have the opposite effect?
Click to expand...
I've never heard a compressor make anything louder unless:

1) "Automatic make up gain" is engaged.

or

B) You're using a look ahead limiter, which is usually only used in mastering (I said "Usually". There are always exceptions)

But a compressor generally turns DOWN the loudest signal so that the rest of the signal "seems" to get louder since they're now closer in volume.
JonnyBoy12 said:
And lastly, when you have several instruments and some are starting to do
drown out, is it a good idea to add compression to get it back into clarity or is it wiser to EQ it differently? Sometimes EQ doesn't seem to do much when I EQ down the low end of a guitar and EQ up the low end of a bass, for instance.
Click to expand...
That probably has as much to do with the choices you make as far as the guitar sound(s) and the bass sound you're using, as well as the parts they're playing. Not saying tools like compression and EQ can't help, but a big part of getting instruments to not step on each other is an arrangement choice. You should try to get that sorted out before depending on ouside tools, like compressors and EQ's. You then use those tools to enhance your recording.
 
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The optical old ones were interesting - using a small bulb to shine into a photosensor, made the changes gradual - the bulb took a short time to get to full brightness, or go dimmer, and the fillament kept a bit of heat in it so it had a nice gentle increase in compression and a equally nice release. Then electronics could do attack and release more adjustably. I had a friend who said he could 100% hear the difference, but I couldn't. It was an old Symatrix - and had a light on the front panel too that glowed in time with the music. Never figured out if it was a gimmic?
 
Random dude said:
probably has as much to do with the choices you make as far as the guitar sound(s) and the bass sound you're using, as well as the parts they're playing. Not saying tools like compression and EQ can't help, but a big part of getting instruments to not step on each other is an arrangement choice. You should try to get that sorted out before depending on ouside tools, like compressors and EQ's. You then use those tools to enhance your recording.
Click to expand...
This is really where I'm at. I'm still in the dark fully about compression. But I use it for sound shaping. For example, I might put the bass through the compressor for a little oomph {although most of the time I won't} or I might put some on the snare for a little difference in its sound....but then again I might not.
I have a kind of ignoramus's approach to compression. Most of the time I don't want to change the signal but out of curiosity, I might add a little compression {or a lot, it depends.....} just to see what it sounds like. However great it may sound on an individual element, it's how it sounds in the context of the whole that will determine whether I'll use it. Sometimes it sounds awful on something, but surprisingly, will sound great when all the other elements are added. There isn't a song of mine that I would say needs compression by default on any of its elements. The arrangement is where most of the heavy lifting is done.
 
I like the same idea - use compression as an effect - when it's the right thing, and maybe to avoid it as normal step without thinking.
 
Very interesting. How is it that sometimes when I have a drum part that begins to drown out under all the other instruments, increasing volume doesn't help BUT adding some "input/output" on a compressor starts bringing it out again? Is there some kind of indirect EQing going on in that instance?
 
It's not an eq thing that's happening. It's the compressor adjusting the shape of each hit.
What happens depends on your settings but most likely the easily audible transient, the crack of the snare is about all you're hearing in the mix.
A compressor can pull that crack down and leave the rest alone, making the ring of the drum louder relative to the crack.

Now when you turn the whole snare track up a bit so the crack sounds the same as it did before (or your compressor does this automatically), the drum now sounds much fuller.
 
JonnyBoy12 said:
Very interesting. How is it that sometimes when I have a drum part that begins to drown out under all the other instruments, increasing volume doesn't help BUT adding some "input/output" on a compressor starts bringing it out again? Is there some kind of indirect EQing going on in that instance?
Click to expand...
Compressors effectively make the sound of the drum hit last longer. That added length makes it more apparent in the mix.
 
A compressor essentially acts as a leveling amplifier. Not all amplifiers have positive audio output gain which is counter intuitive but the leveling part means that the quiet parts and loud parts become more evenly loud at output. Every compressor I have ever used makes the midrange shift, different types shape the midrange emphasis, so yes, a tonal change does happen. A frequency analyzer shows it.

A compressor can make a drum hit longer or shorter, depending on the attack and release settings.
 
The thing with compression and talking about what you actually hear is that the technical side is very straightforward. Sticki8ng them through frequency analysis displays exactly what should happen - but the way these display is open to some confusion. The usual logarthmic display always puts emphasis on midrange. It's where the most complex waveforms exist - bass, by comparison has almost individual notes visible in a horizontal display. HF is a sort of harmonic mess, but at lower levels, so reveals very litte bar odd whistles. In the mids - the display is more details because there are more fundamentals and harmonics coming and going. As a result - compressors change this portion significantly at certain settings, while in the bass - it's only the kick, bass and piano left hand messing things up. Limiters that split the band and process separately hive more versatility - but changing one from 1:1 to 1:4 can be totally impossible to hear on some music, but mega obvious on other material. Visually - you can see something happened, but that doesn't mean it can be heard.

I'm not afraid to say that for nearly half my career I failed to hear gently compression in an AB. I think I could detect something - but my ears and my knob turning hand often failed to find a link. Then - suddenly I heard it and it was totally NOT what I thought it was. I don't think to this day I can explain in words what I now hear?
 
Besides just taming peaks and raising levels , compression will glue tracks together better. An example would be 3 or 4 acoustic guitars each with compressors taming just their high peaks (without any gain increase). They will sound better , but still a little jumbled up and scattered. Send them all to a bus and use a bus or mastering compressor on it. They can sound more like they're all working together instead of separately. For me the is an important focal point. ms
 
I liked this explanation of leveling, compression.....riding the gain

(clip>
Jim Lawrence was born in 1924 and served in the Navy during WWII as a radar operator, studied electrical engineering and worked at JPL in his younger years, but he was also able to apply his skills to his true passion — radio and broadcasting. During a stint at KMGM in Los Angeles, he became frustrated with having to ride the gain to maintain a constant signal level on the air. From that frustration was born the idea of a “leveling amplifier.”

His revolutionary idea was to create the world’s first leveling amplifier utilizing optical sensors. Lawrence drew from his background with military optical sensors to design a circuit which would “level” the incoming audio signal. He combined a luminescent panel with photo resistors (whose impedance changes relative to light intensity) and sealed them in a vacuum-tube-sized metal canister. This optical attenuator, known as the T4, is what gives the LA-2A its gentle, program dependent optical compression, revered to this day by audio professionals worldwide.

www.uaudio.com

A History of the Teletronix LA-2A Leveling Amplifier | Universal Audio

In the early 1950s, Jim Lawrence, a young University of Southern California-degreed electrical engineer, was quietly asked to join Cal Tech’s
www.uaudio.com www.uaudio.com

as for types, I always thought of the Optical as like using a dimmer switch to a light bulb, soft, subtle response and a VCA more like a light switch, faster, on/off, more precision.....
LA2A, JoeMeek Optical types, Voice Channel units are more pleasing to voice compression, imo, maybe more noticeable to the ear and softer sounding...can be almost an effect.
I think of VCA as maybe better at limiting snare drums as its faster, more controlled, less coloration.
 
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