[ardour-users] Sidechaining with AMS

[Neil Nelson]

Run AMS (alsamodularsynth) with 'ams --jack --in 8 --out 8'. The 'in'
and 'out' numbers can be adjusted to show as many Jack ports as you wish
to Ardour. AMS should be started before Ardour so that Ardour will see
AMS' ports.

Use the 'New Insert' option just under 'New Plugin' in the drop-down
list shown from a right click in the dark square just above the bus
fader. The 'Insert' window will display.

Select the ams tab--mine shows 'ams_130_0'--in the both the output and
input 'Available connections' window.

I am using a stereo bus and will be sending out to ams_in_0 and ams_in_1
and in from ams_out_0 and ams_out_1. The bus has a drum track input on 1
and a bass input on 2. Ardour counts from 1 and AMS counts from 0 such
that Ardour's 1&2 go to AMS' 0&1 respectively.

In AMS load 'Module->New->PCM In'. This module will show the two ports
'In 0' and 'In 1' that are the ports connected to Ardour's 'Insert'
output. You can load the 'PCM In' module any number of times and on each
load it will advance to the next two numbered ports. Load
'Module->New->PCM Out'. The PCM Out module cam be loaded several times
and will increment its ports also. We onlyh need one of each for this
procedure. Be sure to right click on the titles of these modules and
push all three gain-volume controls to the far right. This will result
in the same signal level in AMS as in Ardour, particularly the AMS output.

Compressor plugins SC2 (mono) and SC3 (stereo) provide a Sidechain
connection. Short descriptions of those and many other plugins can be
found at http://plugin.org.uk/ladspa-swh/docs/ladspa-swh.html. Load
'Module->Show Ladspa Browser'. Scroll down the plugins in the browser
and find sc2_1426. Click on the arrow at the left to show 'SC2'. Click
on 'SC2' and then 'Create Plugin' below. The module will appear in the
AMS window.

Connect 'In 0', the drum signal, from the 'PCM In' module to the
'Sidechain' connection in the 'LADSPA sc2' module. This is done by
clicking on the names just mentioned in those modules. A line connecting
those two points will be displayed. Connect 'In 1', the bass signal, to
'Input' on those two modules in the same way.

Connect the single output of the sc2 module to both the 'PCM Out'
connections. (One connection will will work.) The modified bass signal
will appear on both of the stereo channels in the Ardour bus above.

Right click the title 'LADSPA sc2' to show the parameters that may be
adjusted. These include: Attack time, Release time, Threashold level,
Ratio, Knee radius, and Makeup Gain.

In order for the drum signal to affect the compressor more quickly by
boosting the DB input I have inserted an amp between the PCM In and SC2
modules. Adjusting the SC2 Threashold level down may do just as well.

This combination goes in the direction of 'Compression sidechaining'
shown on page 3 of
http://www.computermusic.co.uk/pdf/tutorials/sidechain.pdf.

I have modified Steve Harris' gate_1410 (mono) to go in the direction of
page 4 of the prior pdf, "Creative gating with sidechains". It seems to
work fairly well. If anyone is interested I can make that plugin
available. I should remark that my LADSPA coding experience dates from
yesterday. The setup is the same as above for the compressor. Setting
the Attack parameter down to .01 ms creates a click whenever the gate
kicks in and moving it up to the next number removes that click.

A rather helpful AMS module is 'Module->View->Scope View'. Just connect
any module output signal to the input of the scope module and right
click on 'Scope'. I have used three scopes to see the two input signals
and the output signal.

Neil
MPHMedia