Cascading mixers

Mike Shellim 08 May 2015
Last updated 03 November 2018

Cascading (or chaining) is a simple technique for linking channels, where a channel output is used as a mixer source. This simple method makes it possible to define hierarchical mixer structures. This offers potential benefits in terms of ease of adjustment, reduced mixer count, and easier maintenance.

The simplified schematic below illustrates how snapflap might be implemented in an F3X model. Note there are three levels of mix in the hierarchy:
snapflap -> ailerons/flaps -> servos

Note also the calibration layer in the OUTPUTS stage: in order to use cascading mixers effectively, the servos must be properly calibrated.

Example: flap camber

Let's start with a simple camber mix feeding two flap servos. The amount of camber may be positive or negative, and is controlled via LS. We'll assume that the left and right channels have been calibrated, so that the mixer weights are the same on both sides.

CH5 (left flap)

Src=LS, weight=87 - camber input

 

CH6 (right flap)

Src=LS, weight=87 - camber input

This works fine, however there's a drawback: in order to alter the strength of the LS mix, two adjustments are needed, one in each channel. That's twice the twice the chance of error.

Let's instead move the LS mixer line into a high channel, say CH10, and cascade the output to CH5 and CH6.

CH10 (outputs camber value)
Src=LS, wt=87

 

CH5 (left flap)

Src=CH10, wt=100

 

CH6 (rt flap)

Src=CH10, wt=100

CH5 and CH6 behave exactly as before, except now there's a single point of adjustment for camber, in CH10. We can now reset the weights in CH5 and CH6 to 100% (remember that weights are multiplied cumulatively as OpenTx progresses through the command chain).

Adding extra mixes

Suppose we want to add an elevator-to-flap mix ('snapflap'). Instead of applying the mix to each channel separately, we can simply add it to the camber mix already in CH10:

CH5 (left flap)

Src=CH10

 

CH6 (rt flap)

Src=CH10

 

CH10 (aggregates camber and snapflap mixes)

Src=Elevator, wt=10, NoTrim
Src=LS, wt=87

Again, the snapflap volume can be altered via a single menu point.

Adding a snapflap adjuster

Let's get a bit bolder. Say we want to adjust the snapflap in flight, using knob S1.

As with the two previous examples, all the work can be done in CH10. We do this by adding a MULT mixer line, with source = S1. The MULT line is placed immediately after the Snapflap mix. The effect is to multiply the snapflap value by S1 (-100 to 100%), then adding the LS mix. (For more information see How to make in flight adjusters.)

CH5 (left flap)

Src=CH10

 

CH6 (rt flap)

Src=CH10

 

CH10 (adds the effect of camber and snapflap)
Src=Elevator, wt=10, NoTrim -- Snapflap

Multiplex=MULT Src=S1 -- Snapflap Adjuster

Src=LS, wt=87

We could do the same for crow brakes.

Summary so far

Let's summarise what's been achieved by designing our setup with provision for servo calibration, and employing cascading mixers.

Combining high channels with GVARS

All the examples so far have assumed a single flight mode. Let's say we have three flight modes 'Normal', 'Thermal' and 'Speed'. Suppose that the maximum camber should be different in each flight mode. We could implement this as follows:

CH5 (left flap)

Src=CH10

 

CH6 (rt flap)

Src=CH10

 

CH10 (outputs flap value)

Src=LS, wt=0, Flight mode=Cruise

Src=LS, wt=35, Flight mode=Thermal

Src=LS, wt=40, Flight mode=Speed

The lines marked in red provide maximum camber values of zero, 35 or or 40 depending on the active flight mode. It will work fine, but it requires three mixers.

A neater way is to use a GVAR, as these are already flight mode aware (for a full discussion, see Using GVARs).

For our example, let's set the camber values in GV1. GV1 can then be referenced in the camber mix.

GVARS

GV1 = 0 (Cruise), 35 (Thermal), -10 (Speed)

 

CH5 (left flap)

Src=CH10

 

CH6 (rt flap)

Src=CH10

 

CH10 (outputs flap value)

Src=LS, wt=GV1

High channels and GVARs together are a very powerful combination!

Compared with other operating systems

Support for cascading mixers is generally limited in other operating systems, or else poorly documented to the point of unusability (e.g. Futaba 12FG). To the author's knowledge the only other radio with proper support for cascading is the Multiplex 4000, and that radio is now obsolete. OpenTx rules!

Real world example - F3F setup

For an example of how these techniques are used in a full-fat F3F setup, see F3F Setup for Taranis, in particular the Excel documentation.

Mixer screen

Cascading example from my F3F setup: CH10 is a high channel which aggregates flap camber from four inputs, two of which (CH16, CH17) are themselves high channels.