How to do master control switching in Helm?

In this document we discuss the various approaches to master control switching in Helm. What we do depends on how the automation system controls the master control switcher. There are 2 basic strategies, flip-flop, and cut-back-to-A. These are explained below.

Typical Master Control setup – on Source 1


The Master Control Switcher takes 2 inputs A and B from a router. One of these inputs at any time is on air (red), the other is the next source to go on air (green) and is shown on the preview monitor. So:

  • router output 1 is on-air
  • router output 2 is on preview

Now let’s tell the switcher to do a transition.

After the transition to Source 2



  • router output 2 is on-air
  • router output 1 is on preview

Now the roles of the 2 router busses are swapped over. There are 2 ways to proceed to the next transition:

  1. Cut-back-to-A,
  2. Flip-Flop

Cut back to A – step 1 route on-air signal to both busses.


We can restore the router busses to their initial roles by routing source 2 (now on air) to ouput 1.

Now both A and B inputs have the same signal.

Cut back to A – step 2 “silent” cut to A


After this step router bus 1 is again on-air, and router bus 2 is ready to prepare the next source.

This is the strategy commonly used in vision mixers.



Flip-flop simply alternates between A and B on each transition. So in each case the currently inactive bus is used to prepare the next source.

How to use Helm to control this.


Here’s a typical Helm Master Control panel. The switcher is a Miranda IS-750 and the router is an NVision NV9000 system.

Router bus (1) is the PGM bus – cuts on here will affect the source currently on air.

Router bus (2) is the PVW bus – used to select the next source.

Helm control – cut back to A


In the case of Cut Back to A, the roles of the router outputs do not change so all we need to do is assign each Helm Router Bus object to the appropriate router output.

Helm control – flip-flop


We use a little script to re-route the control of the PGM and PVW busses to the correct router output. This change is trigger by the Mixer_A and Mixer_B events. Let’s look at those scripts in detail.

Mixer A event script


When the mixer is on A we set:

  • Program Bus to output 1
  • Preview Bus to output 2
  • set the Preview mixer to show the source on input B

MIxer B event script


When the mixer is on B we set:

  • Program Bus to output 2
  • Preview Bus to output 1
  • set the Preview mixer to show the source on input A

reversing the roles of router outputs 1 and 2.

So that’s it!

Flip-flop is easy on automation – hard on Helm.

Cut-back-to-A needs more work from automation and is easier in Helm.

Either way you can take manual control of your master control system, using Helm.