Stereo Compressors

[biffjunior]

Hi, so I’m looking at possibly getting an outboard compressor at some point in the near future. I will mainly be using it on my Analog Rytm MkII as a send effect from Studio One and also on bass guitar/soft synth/Moog Mavis bass parts and also vocals.

I’m looking at getting a used dbx 160, and my question is, would I be better getting a stereo pair, or would just one be fine (particularly on the Analog Rytm drum bus)? And on the subject of using 2 compressors together in this way, what does it give you that one compressor alone doesn’t?

[MOF]

A stereo compressor can be used as two mono compressors but for stereo use you’ll need to switch on the link switch to prevent the stereo ’image’ moving about e.g. a vocal in the centre will seem to move to one side.

[muzines]

I will mainly be using it on my Analog Rytm MkII as a send effect

A send effect? Do you mean for parallel compression, as opposed to just regular compression?

[resistorman]

If you're working with stereo tracks, then by definition you'll need a stereo compressor or linked mono units. Unless, of course you want to process L and R separately. If you're working with mono, two compressors can work well in series, one set fast, one set slow. But since you're already digital, there are loads of compressor plugins that sound fantastic, and while outboard gear like the DBX are fun to look at and tactile, they won't do a better job. What are you trying to accomplish?

[biffjunior]

A stereo compressor can be used as two mono compressors but for stereo use you’ll need to switch on the link switch to prevent the stereo ’image’ moving about e.g. a vocal in the centre will seem to move to one side.

So say I wanted to run a mono vocal track through a compressor, would it not be pointless to run it through a stereo compressor seeing as the track going through it is a single mono track? I’m struggling to get my head around why stereo compressors are used apart from bus compression when mixing a track and other mixing/mastering applications

[muzines]

I’m struggling to get my head around why stereo compressors are used apart from bus compression when mixing a track and other mixing/mastering applications

Stereo compressors are used when you want to process a stereo signal. If the unit you are using has the feature to work as two mono compressors, then it can also simultaneously compress one or two mono signals.

If you only have a stereo comp that can't run dual-mono, but want to run a single mono signal through it, you can do that, you just ignore the other channel.

[biffjunior]

A send effect? Do you mean for parallel compression, as opposed to just regular compression?

As in I’ll have my AR multitrack running in to Studio One and then I’d run it through the compressor using the sends in the DAW

[biffjunior]

If you're working with stereo tracks, then by definition you'll need a stereo compressor or linked mono units. Unless, of course you want to process L and R separately. If you're working with mono, two compressors can work well in series, one set fast, one set slow. But since you're already digital, there are loads of compressor plugins that sound fantastic, and while outboard gear like the DBX are fun to look at and tactile, they won't do a better job. What are you trying to accomplish?

I’d like a nice colour compressor with some grit and character to put on my drums and bass, and also maybe liven up my vocals. So the sort of thing the dbx and those old Valley People compressors are good at. What good compressor plug ins could you recommend?

[muzines]

As in I’ll have my AR multitrack running in to Studio One and then I’d run it through the compressor using the sends in the DAW

The reason I ask is that compression is generally an insert effect, you don't run compression via sends, unless you are mixing in a parallel version to the dry signal.

If however this is the only way you can do it though (ie, if S1 has no function to run external hardware via inserts on a channel - I'm not too familiar with S1, I use Logic, and it has an I/O plugin to do this), then I guess you could do it but you'd have to be carefully with routing to kill the dry signal and just use the returned compressed signal.

Of course, if you know all this, feel free to ignore me!

[biffjunior]

The reason I ask is that compression is generally an insert effect, you don't run compression via sends, unless you are mixing in a parallel version to the dry signal.

Ah yeah to be fair I had that backwards! Reading it back I realised why you wouldn’t want to run a comp as a send as it’s not a tome based effect, plus there’s no need for a dry signal!
I’ve not run anything through any outboard gear in S1 yet, I just watched a quick youtube tutorial and the guy put a hardware 2 bus compressor running through some stock plugin that handles outboard gear onto the sends on a track.

[Arpangel]

Just get one with a link switch, that way you can use it on two separate mono sources with different settings, when you want to use it in stereo you’re covered too.

[Wonks]

In case there is still and misapprehension, a stereo compressor consists of two mono compressors in a single unit.

It’s not like a stereo chorus or delay, where you can feed in a mono signal and get an effected signal from the Left and Right outputs, where the L and R output signals are now different to each other.

What is fed into the L input only comes out of the L output and what is fed into the R input only comes out if the R output.

If the stereo link button/switch isn’t used, then the parameter controls for the L and R sides are independent and it’s just the same as having two separate mono compressors. You can use just the left side, just the right side or process two mono signals together.

If the stereo link button/switch is activated, then one side’s controls become the master controls (typically the left side but read the manual to check for your unit). This ensures that both signals get processed with the same attack and release speeds, the same compression ratio, the same threshold level and the same amount of make-up gain.

Not having these the same would create a very skewed stereo image.

The side-chains that drive the compressor are also linked so the loudest signal from either side drives the overall compressor operation. All compressors have a side-chain. Sometimes the compressor will make this obvious by having a side-chain filter option (to reduce the effect of certain frequencies on the compressor operation e.g. bass notes), or allowing an external filter to be the side chain.

If the side-chain isn’t an external signal, then it’s a copy of the input signal. The compressor can’t use it’s effected output signal as when the gain reduction kicks in, the output is pulled down and then becomes lower than the threshold, so the gain reduction then stops. The output signal level then rises, gain reduction kicks in again and so the cycle repeats. You’d end up with a tremolo effect on the loud peaks, not the compression effect you were after.

So an internal side chain signal is always taken from either the input signal and is there whether or not it’s made apparent on the unit by having filtering options.

If you use just one side of a stereo compressor with the stereo link switch off, then it’s like any other mono compressor. But with the stereo link switch on, you either need to use the side that has the master controls, or if you use the other channel, then use the controls on the master controls side to adjust the compression parameters, as the ones on the non-master side will have no effect.

Much safer to flick the stereo link switch off if not dealing with a stereo signal!

[Drew Stephenson]

You can use two mono compressors across a stereo signal without getting weird soundstage shifting if you use them as a mid-side combo rather than left-right.

[Hugh Robjohns]

If the side-chain isn’t an external signal, then it’s a copy of the input signal.

Just for clarity, the side-chain signal can be derived from either the compressors audio input (in which case it's called a feed-forward design), or from the output (when it's a feed-back design), or from an external source. Sometimes these options are user-selectable, mostly the designer bakes one option into the design.

Inevitably, there are different pros and cons to feed-forward and feed-back side-chain topologies. Feed-back tends to be used in limiters and compressors that focus on accurate control. Feed-forward tends to be used more in compressors that introduce character.

Regarding stereo-linked compressors, in some models the controls of one channel take over for both, but in my experience this is rare. More commonly it is necessary to set all the controls identically on both channels (threshold, ratio, attack, recovery, make-up gain, etc).

The stereo-link switch sums the two side-chain signals and applies that sum to the gain-reduction devices of both channels, thus preventing image shifts. However, if the channel controls are set differently the side-chain contributions from each channel could be very different, and the applied gain reduction would behave differently to loud signals on each channel.

For example, if the left channel Threshold was set much lower than the right channel you would end up with gain reduction being applied equally to both channels when the left channel gets loud, but nothing would happen when the right channel did!

[Wonks]

I sit corrected.

[Hugh Robjohns]

You can use two mono compressors across a stereo signal without getting weird soundstage shifting if you use them as a mid-side combo rather than left-right.

Again, just to be clear, you still get image shifting. However, instead of the very obvious and weird left-right shifts of unlinked stereo compressors, the result is a variation of stereo width. The image kind of 'breathes' in and out, which can also sound like it's moving backwards and forwards. It is less noticeable to most listeners and definitely less objectionable, but there are still image shifts.

Stereo linking of dynamics channels is usually* important and required when processing stereo or multichannel material, regardless of the LR/MS format.

*There are some producers who intentionally use unlinked channels to provide a desired musical effect, and in multichannel applications it is sometimes beneficial to process rear channels separately from the front channels.

Different situations should be judged on their individual merit, but pay attention to unwanted image shifts.

[Drew Stephenson]

You can use two mono compressors across a stereo signal without getting weird soundstage shifting if you use them as a mid-side combo rather than left-right.

Again, just to be clear, you still get image shifting. However, instead of the very obvious and weird left-right shifts of unlinked stereo compressors, the result is a variation of stereo width. The image kind of 'breathes' in and out, which can also sound like it's moving backwards and forwards. It is less noticeable to most listeners and definitely less objectionable, but there are still image shifts.

::: joins Wonks in the 'corrected' corner. :::

[Hugh Robjohns]

Feeling guilty now...

You're all safe for a while, I'm taking the afternoon off.

[muzines]

::: joins Wonks in the 'corrected' corner. :::

Being corrected by Hugh is a rite of passage that aspiring audio engineers have to go through on their path to engineering greatness.

I consider it a privilege (which I can pay forward with all the people I can therefore correct about the same thing going forward).

It's the circle of (audio) life...

[Drew Stephenson]

::: joins Wonks in the 'corrected' corner. :::

Being corrected by Hugh is a rite of passage that aspiring audio engineers have to go through on their path to engineering greatness.

I consider it a privilege (which I can pay forward with all the people I can therefore correct about the same thing going forward).

It's the circle of (audio) life...

So circular in fact that it happens to me every week!

[Wonks]

If there are ground loops, are there also stupid loops?

[Arpangel]

I can remember the convoluted linking options on various compressors, to the point where nothing made sense, and knob designations were unfathomable.

[muzines]

If there are ground loops, are there also stupid loops?

I can only vouch for fruit loops...