Thanks Larry. I am already using SeaTTY via the soundcard. It works
OK. But I had expected that there would be a significant improvement in
clarity from weak signals if I went via the PK232 - maybe that is
wrong.

I often use psk31 with ham radio deluxe which works very nicely. It
has a 'superbrowser' mode which can copy dozens of stations at once,
and look for particular strings of text, and do intelligent things with
them - eg plot locators on a world map. I agree PSK31 is very cool!