NDi Scan converter audio

According to:-

https://discussions.apple.com/thread/251563983?

the scan convertor takes its audio from the Mac input audio selection and so a loopback device is required to use any other audio.

I have set up two systems, a mini running Mojave (need the QT7 support still) and a Book Pro running Catalina), both running NDI Tools 4.5 and the Blackhole loopback device. with audio out sent to Blackhole instead of speakers, and input selected to Blackhole. However, I still do not get any audio down the Scan Converter output.

I can see the input meter dancing in time with the QT Player running but nothing appears on the Scan Converter output. If I use OBS with the NDI plugin I can get audio that way but not with the Scan Converter so that seems to be the sticking point.