Logic behind post fader insert on Master Buss?

[McMasters]

I did a search on this and found most people make a "Master Aux" which then feeds the "Master Fader".

I am just curious to the logic behind post fader inserts on a master buss.

What is the reason for this?

Am I just missing something here, or is there a practical application for this?

If you put any sort of dynamic processing on the master fader, as soon as you start the fade out you change the whole gain structure feeding the processors?

[liffride]

But if it feeds the master fader it would actually be pre the master fader, would it not?
I'm not trying to be a smartass here, just tryin' to follow you.
no, having a post master doesn't make sense to me either...

Stef.

[McMasters]

That is my point.
The signal gets sent to the master fader first, processors second. So if you pull the master fader down you change the way you hit the processors.

I don't get it.

[Mt.Everest]

Yes, there is a reason for this. The Master Fader usually has a dither plugin on it for preparing your BTD as a 16bit file from your usually 24bit session. So when you fade out, you want the dither plugin working with the increasingly lower level during a fade out, especially if you are doing noise shaping or things like that. For example, when the reverb tails get really quiet, you want the dither to be randomizing that singal as much as possible to avoid quantization error. I dont think dithering would work properly if it wasnt affected by a fade out.

At least, that is my understanding of it.

So to get around this, I usally bus all my tracks/auxes to a Master Aux which behaves like my master fader, but where the plugins wont be affected by a fade out. Then that Master Aux goes to a REAL Master Fader which only has my dither plugin on it..

MT

[minister]

hunh. so you put your "mastering" limiter, eq, multiband, etc on a master aux. then you buss that to master fader? which only has dither on it? does your fade happen on the master aux and the master fader stays at unity? or do you fade individual tracks? or do you fade on the master fader? or do you use fade files? or do you just cut off the power?

[Mt.Everest]

hunh. so you put your "mastering" limiter, eq, multiband, etc on a master aux. then you buss that to master fader? which only has dither on it?

Yes. Although you cant actually 'bus' to your actual Master Fader, it just naturally goes there when your Aux Masters output is
1-2, and the Master Fader is set to output 1-2 as well for example.

does your fade happen on the master aux and the master fader stays at unity? or do you fade individual tracks? or do you fade on the master fader? or do you use fade files? or do you just cut off the power?

The fade happens on the real Master Fader (as you would normally do) so the dither works as it should, i.e., being affected by your fade. The Aux Master stays at unity so the mastering plugins dont get affected by any fades or bumps or dips you might do across the song on the real Master Fader, say for kicking up a chorus .5 dB. .

I also usually name a bus (like 31-32 for ex) in the i/o settings as Master Send 1-2. This way I know that all my tracks, or sub groups final ouput is Master Send 1-2, which feed my Master AUX input. be careful tho, cuz when you create new tracks, the output will default to 1-2 and will go right the Master Fader bypassing your Aux Master. So be sure to check them going to Master Send 1-2, or make that the default output in the setting page.

hope this helps
MT

[minister]

Oooohhh... i get it. i'll try it to see how it works for me. thanks for the tip!

[matt connolly]

The other main reason is for mix buss headroom. Let's say for example that you are playing back 20 channels of a band recording. It is very easy to have a mix of signals that peak above 0dB (the digital clip point). However, one of the funky things about the mixer plugins, is that they do not distort at 0dB - they actually have processing headroom. The output of the mix buss however, does clip at 0dB. The master fader allows the output of the mix buss to be attenuated *without distortion* so that the output signal won't overload the next plugin or record channel. This works equally on TDM busses as it does with physical outputs on an interface.

A simple demonstration: create two audio channels both going to the same output, Stereo 1-2, for example. Use the signal generator to create a sine tone at -1dB. (Turn speakers down to be safe) turn the fader all the way up to +6dB, and pan hard left so we don't have any pan attenuation. Now copy the clip onto the next channel. As we know from adding signals, doubling causes a 6dB boost. Plus our 6dB gain on each fader for a total of 12dB gain. The output signal should be at +11dBFS - which obviously doesn't exist, and you will see read clip lights everywhere. Now create a master fader for Stereo 1-2, and turn it down. if you turn it down to -12dB, you should see at the output a -1dB sine tone WITH NO DISTORTION!

So what this means is that mix busses with many channels can be attenuated *before* being run through any further processing, like dither or limiting, etc.

Essentially, this means that the Pro Tools mix busses have mixing headroom like no other console. Although it still requires diligent use of gain strucutre in your recordings to avoid distortion. And some consoles do have post fader insert points, just for reference.

It also helps to think of masters as outputs only: They have no input, they control only an output.

matt

[McMasters]

Thanks for the replies.

So correct if I am wrong. The Master Fader acts like a Group Master, and not so much as a seperate fader return in the conventional way. When I pull the mix down with the master fader I am really pulling down everything that feeds output 1-2.

If master faders only control output then it would be a master fader in the conventional sense. If your mix was clipping the master fader you would need to re-adjust your entire mix. It is as if the master fader controls the input to itself?? If you clip the master fader all you have to do is lower the fader and that corrects the clip to itself.

If that is the case. (A Master fader is nothing more then a group master with post fader inserts)
Why not have master faders with assignable I/O's. That way I could send my
tracks to "master fader 1" (which is good to control signal level)
"master fader 1" to "aux master 1" (houseing level dependent plugs)
"aux master 1" feed "master fader 2" (for dither and fadeout)

I have found when you do the "Master Aux" technique you must be more carefull with the signal feeding that aux. You don't have the headroom that the master fader gives you in protools. So if you clip the "Master Aux" you must pull down all tracks that feed it, just like those old fashioned mixing desk thingy's I used to own.

[matt connolly]

Now you've got it. Just make sure that you think of Master faders at outputs only, and never associate the word input with them.

Example:

(multiple) audio channels -> output set to "bus 1-2"

master fader "bus 1-2" (this is like the group/bus fader on an analog console - with post fader inserts)
{on an analog console this signal would appear on the patch bay or multi-track inputs for example}

aux fader: input "bus 1-2" output "Speakers" { this is like patching that bus1-2 signal from the patch bay back into the inputs on an aux input channel, bringing it back into the console - this channel has pre-fader inserts and pre/post fader sends}

Here's a bit of a more complicated thing: Master faders don't use any extra DSP because they already exist. Any time that a channel outputs a signal (audio channel or aux channel, main output or send), a mixer plugin is created to control that signal path. When another channel uses the same output, the mixer plugin sums the two signals going to that output.

In the above example, there are two outputs used: "bus 1-2" and "Speakers" . So there are two instances of the mixer plugin created, one for each output. And therefore you can have two master faders -> to control those outputs.

Creating the master fader simply puts a fader on the desk (or screen) that allows you to control the level (gain or attenuation) of that summing process done by the mixer plugin; that already exists because you created it by assigning an output. (Output can be a TDM bus or a physical output.)

On the other hand, an Aux channel does routing and level control of a signal. It receives a signal (from a physical input or a TDM bus - a la group/bus output via patchbay on traditional console), applies any inserts, any/prefader sends, gain from aux channel fader, post fader sends and main output - this requires DSP, albeit not much.

Happy mixing,
matt