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mixbus compression

Postby awjoe » Thu Sep 29, 2016 7:17 am

Is the main reason for mixbus compression to get the overall mix level higher? Is it just about loudness?
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Re: mixbus compression

Postby The Elf » Thu Sep 29, 2016 7:45 am

Absolutely not, at least not in my case. Buss compression can add drama, cohesion and generally take a mix to a new level. It's certainly not about adding overall volume - the mastering pass can take care of that.

For a long time I was resistant to adding a mix buss compressor during the mix process, and I still don't do it all the time, but it can help to pull a mix together quicker, and tell you a lot about your mix choices as you progress. You get used to hearing the mix push into the compressor and it can tell you when you're stepping over the line, or wimping out.

Ultimately it's just another tool for the toolbox, but I've come to find it a very useful one.
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Re: mixbus compression

Postby awjoe » Thu Sep 29, 2016 8:18 am

The Elf wrote:...it can help to pull a mix together quicker, and tell you a lot about your mix choices as you progress. You get used to hearing the mix push into the compressor and it can tell you when you're stepping over the line, or wimping out.

How does it do that? How do you know?
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Re: mixbus compression

Postby Hugh Robjohns » Thu Sep 29, 2016 9:15 am

awjoe wrote:Is the main reason for mixbus compression to get the overall mix level higher? Is it just about loudness?

No not just about loudness...and sometimes not about loudness at all.

It's more about making different elements in the mix integrate and meld with each other in a more cohesive way, and it's a very interactive thing.

You mix into the bus compressor -- it's not applying compression to a (finished) mix -- that's a mastering process and has very different results.

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Re: mixbus compression

Postby The Elf » Thu Sep 29, 2016 10:05 am

awjoe wrote:
The Elf wrote:...it can help to pull a mix together quicker, and tell you a lot about your mix choices as you progress. You get used to hearing the mix push into the compressor and it can tell you when you're stepping over the line, or wimping out.
How does it do that? How do you know?
If you lift an element of the mix such that its peaks begin to poke above the overall mix level, then that element begins to dictate the overall mix compression - it's very much a feel thing. You need to see the compression amount returning to zero much of the time (and rhythmically, assuming a rhythmic track), or you're just turning your mix down, which is not what you want. I saw one engineer compare it to 'pushing into a cushion', and that's a good description IMHO.

If you want to see how it works then try it: long attack, short release; start with a high-ish ratio (12:1 or higher) and low-ish threshold so you can hear it working overtly (aim for around 10dB of compression to give you an idea of what's happening). Once you know what you're listening for, then back the ratio off - 4:1 is plenty. On a typical rock/pop mix I'm looking for around max 3-4dB of compression.

Thousands of words won't beat 5 minutes of try-out.
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Re: mixbus compression

Postby Martin Walker » Thu Sep 29, 2016 10:54 am

awjoe wrote:
The Elf wrote:...it can help to pull a mix together quicker, and tell you a lot about your mix choices as you progress. You get used to hearing the mix push into the compressor and it can tell you when you're stepping over the line, or wimping out.

How does it do that? How do you know?

As The Elf says, have a quick listen to see what it does to a mix - I tend to use lower ratios (1.5 to 1 or similar) and rarely more than 3 or 4dB on my mix buss).

What it does in essence is glue everything together by responding to any loud bits that 'stick out', so that the rest of the mix has a chance to shine. That's the way I like to think of it anyway ;)


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Re: mixbus compression

Postby James Perrett » Thu Sep 29, 2016 11:05 am

I wonder if we are so used to hearing compression on recordings that we naturally associate a slightly compressed sound with a polished and professional sound?

Like Elf, I used to resist using it but I was converted when a visiting producer used it on a mix in our old studio. The mix was already sounding good to me but the compressor just added that finishing touch. I don't think he was using much more than 3dB of compression most of the time. This was in the early days of DAT so I guess that my previous mixes to analogue tape would have had a little compression already - although I was never into hitting the tape at a high level so the tape compression would have been minimal.
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Re: mixbus compression

Postby awjoe » Thu Sep 29, 2016 3:23 pm

I have to try it. I shall push into the cushion.

Thanks, everyone.
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Re: mixbus compression

Postby alexis » Thu Sep 29, 2016 4:11 pm

Martin Walker wrote:...

What it does in essence is glue everything together by responding to any loud bits that 'stick out', so that the rest of the mix has a chance to shine ...


Martin

Can someone please disabuse me of a notion I know is incorrect but can't get past?

Re: Martin's post above - wouldn't the loud bit, like a kick drum, make the rest of the mix lower in volume, i.e., have less of a chance to shine?

FWIW, I filter out the low end before the signal gets to the master bus compressor for that reason. But I could easily see myself susceptible to some kind of bias and thinking I'm hearing only what I expect to, rather than what's actually there.

Thanks!
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Re: mixbus compression

Postby awjoe » Thu Sep 29, 2016 4:45 pm

alexis wrote:Re: Martin's post above - wouldn't the loud bit, like a kick drum, make the rest of the mix lower in volume, i.e., have less of a chance to shine?

Reduce the peaks, and you can turn everything up. RMS is bigger. The quieter bits are louder relative to the louder bits and get overshadowed less.

alexis wrote:FWIW, I filter out the low end before the signal gets to the master bus compressor for that reason.

Something I heard recently is very useful to know: compressors respond to low frequencies more than other frequencies. So when you put a compressor on the master bus, you're already reducing the low end of the whole mix - even though that compressor is reducing all peaks, it's responding more to the kick than to the snare, sort of thing.
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Re: mixbus compression

Postby Hugh Robjohns » Thu Sep 29, 2016 4:49 pm

alexis wrote:wouldn't the loud bit, like a kick drum, make the rest of the mix lower in volume, i.e., have less of a chance to shine?

For the brief transient moment that something is louder than everything else, everything else will get attenuated... but only slightly -- most people don't push bus compression more than about 4dB, and typically with fairly fast attack and moderately fast recovery times. Which means that all the rest of the time, the quieter elements are being lifted slightly and being allowed to shine a little more in the grand scheme of things.

Moreover, most people filter the bottom end out of the side-chain, so the bass instruments (which tend to be the loudest anyway) don't cause the compressor to react much anyway.

And -- and this is the most critical aspect of this -- you are actively mixing into the compressor so you're instantly hearing how it is reacting to the mix and you can change the balance to compensate and control how the whole thing is working. It's a very interactive aspect of mixing.

As others have already said, it's really about making the separate elements gel and fuse together to form a cohesive mix.

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Re: mixbus compression

Postby alexis » Thu Sep 29, 2016 4:52 pm

awjoe wrote:
alexis wrote:Re: Martin's post above - wouldn't the loud bit, like a kick drum, make the rest of the mix lower in volume, i.e., have less of a chance to shine?

Reduce the peaks, and you can turn everything up. RMS is bigger. Everything is louder.
Ah, using makeup gain on the master bus compressor, I didn't know that, thx.


awjoe wrote:
alexis wrote:FWIW, I filter out the low end before the signal gets to the master bus compressor for that reason.

Something I heard recently is very useful to know: compressors respond to low frequencies more than other frequencies. So when you put a compressor on the master bus, you're already reducing the low end of the whole mix - even though that compressor is reducing all peaks, it's responding more to the kick than to the snare, sort of thing.
I didn't know that either, thx!
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Re: mixbus compression

Postby Hugh Robjohns » Thu Sep 29, 2016 5:05 pm

awjoe wrote:Something I heard recently is very useful to know: compressors respond to low frequencies more than other frequencies.

True... but potentially misleading.

Simple compressors react to all signal frequencies equally. The side-chain may be designed to respond to the average energy (rms) or the absolute level (peak), but in most designs the side-chain's frequency response is nominally flat, and so it makes no difference whether the signal is 4dB over the threshold at 100Hz or at 5kHz, the compressor will attenuate by 2dB (assuming a 2:1 ratio) in both cases.

However, the reason a compressor appears to react more to low frequency content is because that's where most of the energy actually is in natural music. The critical element here is that the typical spectrum of almost all forms of music and natural sounds falls at around 3dB/octave.

That's also why pink noise sounds more natural than white noise: the former sounding like a waterfall, while the latter sounds artificially bright and edgy.

So when you put a compressor on the master bus, you're already reducing the low end of the whole mix - even though that compressor is reducing all peaks, it's responding more to the kick than to the snare, sort of thing.

Well, kinda, yes. More low frequency elements will tend to poke their heads above the threshold level because they are naturally stronger than mid-range and high end elements. So a simple compressor will tend to react more to the low end and thus tend to spend more time attenuating the low-frequency components than anything else.

And that's the reason why most compressors intended for bus-compression duties have a high-pass filter option in the side-chain to effectively de-sensitise the compressor to the higher average energy levels of the bass end of the spectrum. Essentially this kind of side-chain filtering forces the compressor to react more strongly to the (uber-critical to out hearing) mid-range components while paying much less attention to the real low end.

In other words, the side-chain filter makes the threshold frequency-dependent, and the low end has to get much further over the threshold before the compressor reacts and turns things down, compared to the mid-range and high end.

Some bus compressors offer even more extreme and sophisticated side-chain filtering options, too, specifically to address the way the low end is controlled (or over-controlled) relative to the mids and highs. Think API's THRUST mode, and the Kush Tweaker's extremely versatile side-chain Shaper controls, for example, which really allow you to control exactly what elements in the mix cause the compressor to react.

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Re: mixbus compression

Postby Hugh Robjohns » Thu Sep 29, 2016 5:19 pm

alexis wrote:
awjoe wrote:Reduce the peaks, and you can turn everything up. RMS is bigger. Everything is louder.
Ah, using makeup gain on the master bus compressor, I didn't know that, thx.

Yes... you certainly can dial in a few decibels of make up gain on the bus compressor if you really want to, but in my experience few people actually do. It's just not a practical necessary.

The crest factor (RMS-peak ratio) improves anyway as a direct result of the compression process. The actual peak level of the compressor output is irrelevant to the perceived loudness of the mix.

It's not usually unnecessary to raise the peak level of the mix; in fact most people find themselves fighting to keep the peak level within bounds because they're combining the energy of a lot of separate tracks into a final mix.

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Re: mixbus compression

Postby awjoe » Thu Sep 29, 2016 5:30 pm

Hugh Robjohns wrote:And that's the reason why most compressors intended for bus-compression duties have a high-pass filter option in the side-chain to effectively de-sensitise the compressor to the higher average energy levels of the bass end of the spectrum. Essentially this kind of side-chain filtering forces the compressor to react more strongly to the (uber-critical to out hearing) mid-range components while paying much less attention to the real low end.

Compared to a compressor with a side-chain filter, how would a multiband compressor fare doing the same job?
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