Jim and others,

Some rigs have DDS sidetone, including the TS-850.  I modified my TS-850 so
that sidetone is available from the rear panel, and is present whenever CW
mode is selected.  I originally wanted it for doing XY scope displays of
sidetone vs received audio, for confirming zero beat with a wobbly circle
display.  Off-air carriers can usually be checked to very good accuracy
using that approach (but noting 1 Hz nominal tuning steps of the TS-850).
However, having the the DDS sidetone available externally is also excellent
for frequency calibration of sound cards.  Now with Argo, 1 Hz steps are
nicely filled in on the PC screen display.

A note of caution though.  Even though I have a close tolerance master
oscillator in the TS-850, I find that the DDS outputs do have residual
errors, probably related to "digital fragments" from the binary processes in
the DDS chips.  There can be tens of millihertz errors, which are far
greater than the master oscillator ppm figure converted to the DDS output
frequency.  These digital fragments are less than ppm errors at HF, but are
quite significant at LF and audio.

I originally used 1000 Hz sidetone for Argo work, but found that the DDS
sidetone error was worse for 1000 Hz than 800 Hz, so I have gone back to
using 800 Hz reference.

73, Bob ZL2CA