The Compression Thread

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The Compression Thread

Postby Mathieu Benoit » Thu Mar 03, 2011 4:06 pm

...Yes, I think it's about that time. I'll start it out with some basic information about compression to get everyone caught up and then I'd like to get in to some advanced concepts with Andrew and Malcolm.

For starters we are talking about dynamic range compression and not data compression which is another can of worms for some other time.

The text book definition for dynamic range compression is "a process that reduces the dynamic range of an audio signal, that is, narrows the difference between high and low audio levels or volumes." That's the most basic definition of what compression is.

On most compressor you'll find many common functions such as "Threshold", "Attack", "Release", "Ratio", and in some cases "Input" and "Output (or make-up gain.)"

The threshold simply defines at what level the compressor will start acting on the signal. In some compressor designs (like the 1176) There is no threshold parameter simply because the threshold is predefined and the input determines hom much signal gets pushed over the threshold. The threshold is usually expressed in dB.

The ratio is the amount of compression that will occur once the signal goes above the threshold. The higher the ratio the more compression is applied. Basically the ratio is direct representation of the amount of input you have to produce above the threshold to get a certain amount of output. Here's a graphical representation:

Image


The Attack and Release functions affect how quick the compressor acts when the signal goes above the threshold and how how quick the compressor let's go once the signal goes below the threshold respectively. So the slower the attack for example, the longer it takes for the compressor to grab the transient of the sound. Here's a graphical representation:

Image


The Input funtion simply determines the amount of gain you are pushing in to the compressor. It's found mostly on compressors with a fixed threshold, because it determines how much the signal is going to get compressed.

The Output (or make-up gain.) is simply a funcion that determines the gain of the signal coming out of the compressor.

I think that covers the basics, am I missing anything?
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Re: The Compression Thread

Postby macrae11 » Thu Mar 03, 2011 4:42 pm

That's probably good for now. Now what?
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Re: The Compression Thread

Postby Mathieu Benoit » Thu Mar 03, 2011 4:47 pm

macrae11 wrote:That's probably good for now. Now what?

What types of compressors exist and how do they differ?
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Re: The Compression Thread

Postby gamblor » Thu Mar 03, 2011 4:57 pm

Great topic Matt!

I’ve always known what compression physically does to a sound signal, but I find I have limited understanding of when, why, and how to use it. I find I’m able to grasp applications where I’m trying to clean up the dynamic range of a signal (usually on a vocal track) but I have a hard time relating it back to affecting the tone of an instrument (see the bass thread).

So my question for you experts is: Where compression applies only to the dynamic range of a signal, how does this relate back to trying to achieve a perceived tone?
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Re: The Compression Thread

Postby Mathieu Benoit » Thu Mar 03, 2011 5:18 pm

gamblor wrote:So my question for you experts is: Where compression applies only to the dynamic range of a signal, how does this relate back to trying to achieve a perceived tone?

I'm not an expert but I'll try my best. It has to do with this part of my post:

Drumwaiter wrote:The Attack and Release functions affect how quick the compressor acts when the signal goes above the threshold and how how quick the compressor let's go once the signal goes below the threshold respectively. So the slower the attack for example, the longer it takes for the compressor to grab the transient of the sound.

Image


Those functions can seriously affect the general envelope of the transient. For clarification, by transient I mean the initial part of the sound. If the attack is really fast it will grab the transient quickly and clamp it down, thus changing the attack and the perceived tone of the transient. If the attack is really slow the it will let the transient go by without compressing it and it will only start compressing the sound after the initial attack of the transient has passed. These kinds of things can really change the overall tone of your sound. Similarly the release set really fast means it will let go of the signal as soon as it drops below the threshold, but a slow release means that the compression will continue to affect the sound even below the threshold thus changing how the tail end of the transient will sound. It's all about shaping the overall envelope and that changes the sound on different levels than just dynamics control.

Andrew or Malcolm might be able to explain it better though.
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Re: The Compression Thread

Postby macrae11 » Thu Mar 03, 2011 5:42 pm

Wow well ok, I'll put down a few, although I don't think I'm really the person to dispense technical wisdom on the topic.

FET
Optical
VCA
Vari-Mu

FET stands for Field Effect Transistor. Typically a fast comp/limiter with a musical sound. Originally designed to mimic the sound of tubes. Most famous- 1176

Optical, uses a light bulb or LED and a photo sensitive transistor to control the signal level. Very smooth sounding, slower response, can be tube(LA2A) or transistor(LA3A) based.

VCA, or Voltage Controlled Amplifier, is typically the fastest type of compressor, and also has more flexibility, and is generally cheaper to manufacture. DBX 160, and SSL compressors.

Vari-Mu, I don't really know how it works, something to do with tube bias/voltage/something or other. I don't have any experience with any, the only one I know of is the Manley Vari-Mu, although I think Fairchild compressors and their clones are Vari-Mu
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Re: The Compression Thread

Postby Alain Benoit » Fri Mar 04, 2011 12:20 am

macrae11 wrote:Wow well ok, I'll put down a few, although I don't think I'm really the person to dispense technical wisdom on the topic.

FET
Optical
VCA
Vari-Mu

FET stands for Field Effect Transistor. Typically a fast comp/limiter with a musical sound. Originally designed to mimic the sound of tubes. Most famous- 1176

Optical, uses a light bulb or LED and a photo sensitive transistor to control the signal level. Very smooth sounding, slower response, can be tube(LA2A) or transistor(LA3A) based.

VCA, or Voltage Controlled Amplifier, is typically the fastest type of compressor, and also has more flexibility, and is generally cheaper to manufacture. DBX 160, and SSL compressors.

Vari-Mu, I don't really know how it works, something to do with tube bias/voltage/something or other. I don't have any experience with any, the only one I know of is the Manley Vari-Mu, although I think Fairchild compressors and their clones are Vari-Mu


Correct the Fairchild 670 is a Vari-Mu based compressor originally designed to physically limit the cutting depth and width of the stylus during printing of the master. The Urei 176 is also a Vari-Mu type, there are others. Vari-Mu refers to a tube circuit design that varies the Mu or gain factor of a tube thus the gain reduction portion of the device.

Another popular topology is PMW (Pulse Width Modulation) based gain reduction scenario. Examples of this are the famous Cranesong STC-8 and the PYE compressor which I believe was the earliest commercial example of this.
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Re: The Compression Thread

Postby Christian LeBlanc » Fri Mar 04, 2011 10:14 am

Before we start talking about noise floors and creative uses of, and differences between, compression/limiting/gating, how about we start with common uses of compression? What instruments typically get more compression, and which get less? And for what reasons?

My understanding is that bass guitar, especially when locked in with the rhythm, will get more than most instruments.
Lead guitar may get a touch, while compression will vary from singer to singer, and from piece to piece. If one singer's volume levels are unintentionally all over the place, then a compressor is the best solution (other than, you know, vocal training and practice), right?

What about singers who appear to sing at almost a whisper, yet manage to get amplified quite loudly without horrible feedback from their microphones? What's going on "behind the scenes" to keep the noise floor at bay? Or is there something else at play? (as an example, I'm thinking of Julie Doiron, but anyone with 'quiet' vocals will do)
EDIT: I've spent years using a cheap microphone, and I completely forgot about the properties a good microphone can have: one that's balanced, designed to only pick up the singer, etc. So, it's probably safe to ignore this part of the question ;)
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Re: The Compression Thread

Postby Jef » Fri Mar 04, 2011 10:18 am

...not to be confused with the 'stomp box' type instrument compressor units. I find most of these units very limiting (pardon the pun) as to what can be controlled. Some have nothing more than an on/off switch and two knobs... one for "gain" and one for "compression".
These may be an idiot proof device for the uninformed, but really, if you have no control of the threshold, ratio, attack, & release, what real good are they?
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Re: The Compression Thread

Postby macrae11 » Fri Mar 04, 2011 11:05 am

Alain Benoit wrote:
Another popular topology is PMW (Pulse Width Modulation) based gain reduction scenario. Examples of this are the famous Cranesong STC-8 and the PYE compressor which I believe was the earliest commercial example of this.


I was trying to figure out what the STC-8 was. Thanks!
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Re: The Compression Thread

Postby macrae11 » Fri Mar 04, 2011 11:46 am

Christian LeBlanc wrote:Before we start talking about noise floors and creative uses of, and differences between, compression/limiting/gating, how about we start with common uses of compression? What instruments typically get more compression, and which get less? And for what reasons?

My understanding is that bass guitar, especially when locked in with the rhythm, will get more than most instruments.
Lead guitar may get a touch, while compression will vary from singer to singer, and from piece to piece. If one singer's volume levels are unintentionally all over the place, then a compressor is the best solution (other than, you know, vocal training and practice), right?

What about singers who appear to sing at almost a whisper, yet manage to get amplified quite loudly without horrible feedback from their microphones? What's going on "behind the scenes" to keep the noise floor at bay? Or is there something else at play? (as an example, I'm thinking of Julie Doiron, but anyone with 'quiet' vocals will do)
EDIT: I've spent years using a cheap microphone, and I completely forgot about the properties a good microphone can have: one that's balanced, designed to only pick up the singer, etc. So, it's probably safe to ignore this part of the question ;)
The first rule is there are no rules. Every situation is different and you have to use your ears. That being said, I have some places that I seem to end up often. First off in the modern digital era, noise floor is not even remotely a concern for me. Like Matt was saying, I was pulling 40 dB on a lead vocal, and there was no noticeable increase in the noise floor in the mix. With 24 bit digital, a mediocre mic and a quiet room, it just doesn't come up.

As far as uses of compression, I've smashed just about everything imaginable at some point or another, but vocals, drums, bass, and percussion always seem to get at least some compression. The majority of compression I use though, is not for volume control, but more for tone shaping. Then any volume control is typically done with automation. The compression for tone does reduce the dynamic range as a byproduct somewhat as well though, so it does make fader rides a little easier.

For drums, most often it's parallel compression so that the transients remain intact, and it's usually on a bus of all or most of the drums. The parallel bus is usually crushed to oblivion, and then just brought up to taste. This creates a type of upward compression, where the transients are left intact, but the lower and middle parts of the sound are brought up, which thickens the drums significantly. Individual drums will sometimes get some comp too, but only for envelope shaping, not volume control. Using attack to let more or less of the transient through, and release to effect the decay. Room mics are the exception to this, as sometimes they will get smashed on their own if I'm looking to add a little "Bonham". Release time is very important to get the breathing right with drums.

Bass really depends on the player. I will use it for volume control on a poor or erratic player, or if doing some slapping and popping. If I need to thicken things up I'll compress quite heavily, but almost always with a very slow attack so lots of point gets through so things stay well defined. release depends on the type of groove of the song. 16th note or 1/8th note grooves will be faster, while if it's primarily 1/4 notes or longer it will be slower.

Percussion in a pop mix will usually be smashed, but with an attack that lets some transient through. Largely for volume control, sometimes I'll use a limiter, or even a brickwall limiter if I really need to lock it in place. In an open mix I'll usually just use a touch to even out any loud parts.

Vocals I'll do anything from no compression to insane amounts. Depends depends depends. If I want a modern rock style vocal it'll get tons. Also if I'm looking for intimacy, bringing up breath and mouth sounds can help that. Not so much as a volume controller, although it usually does help with that. Very often I'll chain multiple compressor together in series. Usually just two, but sometimes 3 or 4. The first is usually to even out any peaks, and add a bit of mojo, the second is for thickening and tone shaping. Having a compressed signal going into a compressor changes how things work in a really fun way.

Acoustic guitar is another one that I'll often put a little on, but it's one that's very easy to screw up. Attack and release times are super critical, and it's very easy to smash things until they're useless. Usually nothing to heavy, but depending on the mix, it could end up with 10ish dB's of reduction. I tread very carefully here though, and spend a lot of time tweaking things.

Everything else(like the above) is on an as needed basis. Crunchy guitars will often get some tone shaping, particularly of the of the pick attack, but not usually as an RMS averager. Background vocals will depend on the lead, typically if there's lots on the lead, there will be a bunch on the BG's. Pads almost never any, unless there's some dynamic issues. Piano is very similar to acoustic guitars. Often will get a little bit, but it's very touchy.

That's a few thoughts for now, I'm sure a ton of stuff will come to my mind in a day or two. On that note, I'm really enjoying this conversation in particular, as well as the renewing dialogue on the board, but I won't be able to participate for the next few weeks. So if I don't respond, I'm not being rude, just having spotty internet connections. Will try to catch up when I get back.
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Re: The Compression Thread

Postby Mathieu Benoit » Fri Mar 04, 2011 12:11 pm

Christian LeBlanc wrote:Before we start talking about noise floors and creative uses of, and differences between, compression/limiting/gating, how about we start with common uses of compression? What instruments typically get more compression, and which get less? And for what reasons?


It's really not that binary, at least not for me. I compress differently based on what I'm working on. But if I was to generalize: Bass sometimes gets compression mostly for tone. In some cases the drums get buss compression, and I love crushing a far room mic on a drum kit in some cases too. Vocals are the only one that I have to say pretty much always get compression with me. They need to be up front IMHO and that's not always just a matter of bringing up the fader. I want the vocals in your face. Although that's just my sense of esthetics.

Christian LeBlanc wrote:My understanding is that bass guitar, especially when locked in with the rhythm, will get more than most instruments.
Lead guitar may get a touch, while compression will vary from singer to singer, and from piece to piece. If one singer's volume levels are unintentionally all over the place, then a compressor is the best solution (other than, you know, vocal training and practice), right?


A vocal ride is what I'd use in that case. Automation in DAWs makes this so much easier. You can literally get every syllable exactly where you want it mix wise before even getting into compression. That being said I usually automate into compression. The two work hand in hand but I let automation do the bulk of the heavy lifting in terms of making the vocal performance "even". Same thing goes for lead guitar, start with automating a ride, then use compression if you still think you need it or if you want to use compression to acheive a sound, then automate into the compressor.

I understand the desire to use compression to "fix" dynamic differences in performances but it's low on my list of uses for it these days. Now 3 years ago... I did no automation and all compression so I get the tendancy. But I like my work a lot better now, so there you go.
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Re: The Compression Thread

Postby Mathieu Benoit » Fri Mar 04, 2011 12:22 pm

Alain Benoit wrote:Another popular topology is PMW (Pulse Width Modulation) based gain reduction scenario. Examples of this are the famous Cranesong STC-8 and the PYE compressor which I believe was the earliest commercial example of this.

How does it work? I realise it may be over my head, but try me all the same.
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Re: The Compression Thread

Postby Jef » Fri Mar 04, 2011 12:39 pm

Shouldn't that read 'PWM'? :roll:
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Re: The Compression Thread

Postby Alain Benoit » Fri Mar 04, 2011 12:40 pm

Drumwaiter wrote:
Alain Benoit wrote:Another popular topology is PMW (Pulse Width Modulation) based gain reduction scenario. Examples of this are the famous Cranesong STC-8 and the PYE compressor which I believe was the earliest commercial example of this.

How does it work? I realise it may be over my head, but try me all the same.



Well first you'd have to understand signal modulation which i'm sure you kinda do on a basic level, then you'd have to grasp the basic concept of Pulse Width Modulation which I'm sure you could, then understand how it applies to audio, digital audio and Class "D" amplification. After all that you'd have to study basic semiconductor electronic concepts to understand how transistors are used as switches, then you'd have the tools you would need to get the basics of how compression is achieved through a PWM topology.
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Re: The Compression Thread

Postby Alain Benoit » Fri Mar 04, 2011 12:41 pm

Jef wrote:Shouldn't that read 'PWM'? :roll:


Yup it sure should.

PS- Did Larry get a hold of you re my shipment?
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Re: The Compression Thread

Postby macrae11 » Fri Mar 04, 2011 1:12 pm

Alain Benoit wrote:Well first you'd have to understand signal modulation which i'm sure you kinda do on a basic level, then you'd have to grasp the basic concept of Pulse Width Modulation which I'm sure you could, then understand how it applies to audio, digital audio and Class "D" amplification. After all that you'd have to study basic semiconductor electronic concepts to understand how transistors are used as switches, then you'd have the tools you would need to get the basics of how compression is achieved through a PWM topology.


Well I've got a grade school understanding of those topics. Can you connect the dots for us? And I thought the STC8 was pure Class A?
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Re: The Compression Thread

Postby Jef » Fri Mar 04, 2011 1:28 pm

Alain Benoit wrote:PS- Did Larry get a hold of you re my shipment?

Nope, but I will be talking to him over the weekend. Is there something you need delivered?
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Re: The Compression Thread

Postby Mathieu Benoit » Fri Mar 04, 2011 2:57 pm

Alain Benoit wrote:Well first you'd have to understand signal modulation which i'm sure you kinda do on a basic level, then you'd have to grasp the basic concept of Pulse Width Modulation which I'm sure you could, then understand how it applies to audio, digital audio and Class "D" amplification. After all that you'd have to study basic semiconductor electronic concepts to understand how transistors are used as switches, then you'd have the tools you would need to get the basics of how compression is achieved through a PWM topology.

Well the transistors would have to be opening and closing at a pretty sick rate for that aplication, probably in the 100Khz range. It's pretty crazy to think about...


macrae11 wrote:Well I've got a grade school understanding of those topics. Can you connect the dots for us? And I thought the STC8 was pure Class A?

PWM is used for Class D amplifiers but I think in terms of compression you can still consider it to be a Class A audio path even if PWM is used for gain reduction. Class A just means the amplifier conducts over the whole of the input cycle in a way that the output signal is an exact scaled-up carbon copy of the input with no clipping. I don't know that the two are mutually exclusive in this case. But I can't be sure of that. Alain would known way better than I would. I still need to read the manual every time i fire it up...

From Crane Song's website:

The STC-8's sophisticated side-chain allows both compression and peak limiting to take place simultaneously, using the same proprietary gain circuit. High signal quality is maintained by utilizing a discrete class A audio path, and by eliminating any VCA or optical gain control elements.

Using Pulse-Width Modulation (PWM) to control an FET is what Crane Song uses in both the STC-8 and Trakker compressor/limiters. The technology has been around for about 20 years, having been used by several companies, including EMT. With modern technology, a PWM gain-control circuit can be very fast; from 0 to full gain reduction in 500 nS (0.0005 mS or 0.5 uS) and an audio frequency response to 60Khz or more.

PWM works as a gate, quickly switching the FET from "off" to "more-on" by varying the pulse-width. With careful selection of components and PC board layout it is possible to have very low control feed-through, a very low noise floor and an audio-path frequency response that does not vary with gain reduction. This type of circuit requires costly high-speed components and as a result, there is a fair amount of power consumption (it runs hot). Distortion is very low and does not change with gain reduction.
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Re: The Compression Thread

Postby macrae11 » Fri Mar 04, 2011 3:25 pm

That makes sense. So it's still an FET (which I assumed it was) doing the actual compression in a Class A signal path, but basing it off of the PWM Class D signal. Correct me if I'm wrong there, but it makes sense in my brain.
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Re: The Compression Thread

Postby Mathieu Benoit » Fri Mar 04, 2011 3:33 pm

macrae11 wrote:That makes sense. So it's still an FET (which I assumed it was) doing the actual compression in a Class A signal path, but basing it off of the PWM Class D signal. Correct me if I'm wrong there, but it makes sense in my brain.


That's right, the FET is simply being controlled by PWM. I don't know if you'd necessarily call it "Class D" though just because PWM is involved. I know you would in the case of amplification but in the STC-8 the amplification stages are still Class A. So That's how I see it... The "class" as I understood it related specifically to amplification and I don't know if the description applies in the case of PWM for gain reduction purposes. But I don't know that for a fact.
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Re: The Compression Thread

Postby Jef » Fri Mar 04, 2011 3:34 pm

Alain Benoit wrote:PS- Did Larry get a hold of you re my shipment?

Sent you a pm.
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Re: The Compression Thread

Postby Alain Benoit » Fri Mar 04, 2011 5:00 pm

Andrew, Matt is more right on this.

My understanding is this, sidechain signal modulates the 'on' time of a high speed square wave thus the PWM part. That signal then controls the FET to open and close based on the envelope of the sidechain signal. The audio path that you hear is all Class "A" having nothing to do with the detector circuit. Yes Class pertains to amplifier topology. Class "D" amplifiers use PWM in a similar fashion to amplify signals.
Pros, process is linear and predictable as in there is no change in frequency dependence or amount of distortion based on gain reduction, in the case of class "D" amplifiers efficiency is high due to the low dissipation of the gain element ie: transistor, it is either fully on or fully off there is no heat loss, it is acting like a switch instead of a valve.
Cons, much more complicated design, component cost.....
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Re: The Compression Thread

Postby macrae11 » Fri Mar 04, 2011 5:34 pm

Yes that was my understanding of it, just explained much more clealy.
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Re: The Compression Thread

Postby Malcolm Boyce » Sat Mar 05, 2011 7:39 pm

gamblor wrote:Great topic Matt!

I’ve always known what compression physically does to a sound signal, but I find I have limited understanding of when, why, and how to use it. I find I’m able to grasp applications where I’m trying to clean up the dynamic range of a signal (usually on a vocal track) but I have a hard time relating it back to affecting the tone of an instrument (see the bass thread).

So my question for you experts is: Where compression applies only to the dynamic range of a signal, how does this relate back to trying to achieve a perceived tone?
Until you can recognize compression and it's artifacts, it will be difficult to make decisions just like any other form of processing. I know many musicians and technicians who know what a compressor does, but can't "hear" it. Some will say that not hearing compression means it's doing a good job, but that's not what I'm getting at. I mean when a signal has been obviously compressed, can you hear it? When someone comments, 'Wow is that every compressed.' ...do you really hear it?

Just like learning how to use an EQ, messing around with dynamic range control devices (Compressor, Limiter, Gate, Expander) is one of the best ways to learn. Throw things into extremes as far as settings go. Listen to what changes. You will quickly realize that a compressor does more than just limit the dynamics of a signal. With a compressor, listen to what changes when you EQ before, or after with the same settings. Same with any other process as well as EQ really.

Try different instruments, vocals, and even entire mixes. With instruments, try miced as well as direct sources. Listen to what compression does to the ambiance of a miced track.
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