405 lines from Raspberry Pi 5

[ppppenguin]

I visited Mike yesterday and saw his Pi and HAT in action. He's done a very good job. Without a scope and, ideally a spectrum analyser, it's difficult to assess exactly how well the modulator is working.The sound level is indeed low. The attached is the entire modulator from the first iteration of he Aurora SCRF. There were several later mods, including a built-in tone generator using a PWM output from the FPGA and the reduced drive to the xtal inputs. Note that audio is fed to the video input of U9, the audio modulator. R49 seems to set the DC bias on that pin. R55 sets audio gain.

To check what's happening you really need to look at the modulated audio on a scope with the vision carrier switched off.

The very first example of a System A modulator using these chips was by David Robinson.  Again I've attached the schematic. There is no gain in the baseband audio path. Frank's Hedghog is similar.

I don't know if Darryl knew of David's work when he designed the SCRF.

One thing that's clear is that the DC bias on the audio modulator is important.