[Ardour-Users] input levels, master levels

Chris Caudle chris at chriscaudle.org
Wed Oct 11 14:23:45 PDT 2017

On Fri, September 29, 2017 6:12 am, Matt Keys wrote:
> question. What is the best practice regarding normalizing input volume
> levels of unbalanced, balanced, and xlr (or a mix thereof) into ardour?

Following up on what Seablade wrote a week or so ago, the basic principle
is that all electronics have a minimum level of noise always present, and
a maximum signal level that can be handled, both determined primarily by
the circuit configuration (i.e. out of your control if you are not 
designing from scratch).
A basic rule of thumb is that you want to apply as much gain as you can
stand as early as possible for lowest noise build up, but you have to
check that against maximum signal limits to make sure you do not introduce
distortion if there is equipment in the middle or end of the chain which
cannot handle signals as high as the first device in the chain.

> guitar (unb TS) ->

Can cover a wide range of signal levels depending on whether you are
playing single note lines, or hitting chords really hard.  You may have to
play around with this stage:
>  gfx (2x XLR) ->

to find the best compromise of noise and distortion levels.

>  analog mixer (2x balanced TS) ->

Check the maximum output level spec of the guitar effect and the maximum
input level of the mixer.  That may be given in logarithmic units,
decibels. That relates to absolute levels in volts, but the short version
is as long as the subscripts match you can compare directly.  Watch out
for different subscripts, which means different reference levels are used.
 So if the guitar effect gives it maximum output level in dBu, and the
mixer lists the maximum input level  in dBu, compare them directly.  If
for example the guitar unit lists output level in dBV and the mixer spec
gives the maximum input level in dBu, you can't compare the values
directly, you have to convert to the same reference level.

The analog mixer likely has some kind of LED meters showing peak levels,
but might not have on each channel.  In that case you need to make sure
you do not clip an intermediate stage in the mixer channel before it gets
to the channel level control and the sum bus peak meter.

I'll give a concrete example.
Let's say your guitar FX unit says either max output level 20dBu, or
something like nominal output 4dBu, 16dB headroom, which would be the same
(+4 nominal +16 headroom = 20dBu).

Let's say you look at your mixer spec and it says max input for line input
is 18dBu.  You would need to make sure the trim control on the mixer is at
least at -2dB so that after the trim control the maximum signal coming to
the mixer circuitry could be 20dB -2dB = 18dBu (assuming both devices give
their levels in dBu).

> usb audio interface

Same thing, this time with the mixer max output levels and the audio
interface max input levels.  Depending on where the level control of the
audio interface is in the circuit it may be better to run the mixer high
and cut the levels at the audio interface, or cut back the output levels
at the mixer, or maybe your audio interface is good quality and it can
handle whatever level the mixer is capable of generating.

> -> ardour

Internally ardour is going to be a non-issue, the place where you have to
watch is on the master output bus, because the audio data from there gets
sent to the USB audio interface, and it can only handle up to 0dB FS (the
FS meaning full scale, because that point the data is just abstract
numbers, there is no physical reference yet like there is for the analog
level standards that get labelled as dBu or dBV or even just as volts).

Ardour has meters that will show you the peak level reached, make sure it
never goes over 0 and you are OK on the ardour side.  Hitting 0
occasionally can be OK, I like to aim for lower because some equipment
sounds a little strained as the distortion starts increasing if the levels
are close to 0.  Advanced topic for later, mixing to calibrated loudness
level and not worrying about the levels as much.  Don't worry about that
yet, I just bring it up to point out you can ignore anyone who tells you
that you "have to" get your mix to hit 0dB, or -0.1dB, or some other
arbitrary number that is close to maxxed out, that is an anachronism left
over from cassette tape days.

> .. each point has volume adjust. Let's say I bypass any effects and I'm
> 'clean' through to the usb mixer, but in ardour it sounds too
> hot/distorted in master out.

At that point you have to hope that your analog mixer is flexible enough
that you can listen at various points along the signal chain and make sure
it is still clean at each point. If your guitar efx unit has a headphone
output, check to make sure it is clean at the effect unit.  If the mixer
has a pre-fader listen control, use that to route the guitar signal to the
mixer output and make sure it is cleaning coming out of the mixer channel.
 If not, then turn down the output of the guitar unit, or turn down the
input trim on the mixer channel, if that doesn't help you may be clipping
the input of the guitar unit, try turning down the input gain on the
guitar unit until you are sure that the signal coming through the mixer
channel is clean.  After that change from pre-fader listen back to normal
routing and make sure it sounds OK at the output of the mixer (check the
monitor speaker output of the mixer, or check with headphones, something
that listens to the signal before it hits the A/D converter in the audio

If it is clean at that point then you just have the audio interface to
worry about.  When you route the signal to a track in ardour, watch the
signal meters at the track, if the meters keep hitting 0dB then the signal
is too high going into the audio interface, you need to either use the
input level control of the interface, or the output level control of the
mixer to keep peaks at around -3dB or so at max, lower is perfectly fine. 
Be sure to really wail on your guitar when you are checking, you don't
want to just give it a little pluck and think you are good, only to find
out that when you really get into playing you ruined a good take because
you were playing harder than when you first set all the levels and
something started distorting.

Written out like this it sounds more complicated than it is, the process
is just a little tedious the first couple of times, you just have to start
at the beginning (i.e. the guitar connection) and work your way though the
signal chain checking each step along the way.  After you do it a couple
of times it should come more easily.

Chris Caudle

More information about the Ardour-Users mailing list