With thanks to
@Poliander - I found "Listing4.bas" on the disc you sent me.
VIDI:
https://cpcrulez.fr/hardware-digit-digitaliseur_vidi.htm?t=VQ==
https://www.cpc-power.com/index.php?page=detail&num=4207
https://www.cpcwiki.eu/index.php/VIDI_digitizer