Bill,
I was making a general statement, in that the nature of the design of a
direct conversion radio is that its front end has fewer filters between it
and the ultimate conversion to audio than the superhet such as Orion.
A direct conversion is zero IF, and thus you are at the mercy of how the
very front end performs, and then after detection, you depend on audio
filtering technique. Bleed around the front end would seem more likely to
get into the later stages of a radio that is zero IF.
The bandwidth obtained from the direct conversion can be more susceptable to
adjacent channel interference, thus the well publicized vulnerability of
direct conversion receivers other RF impacting on the front end, etc.
Old superhets like the Hammarlund Super Pro varied the tuning of IF slugs to
narrow the shape of filtering, and improve the selectivity. When you
convert directly, you put all your selectivity up at the front end, and
depend on it stopping strong interfering off channel signals. Practical
components limit the Q and hence selectivity of the RF stage.
The quoted tests do look very good. However, it is my understanding from
persons working for SDR that they had major problems from users who expect
every sound card to perform as well as the suggested sound cards, and then
find there are great differences among sound cards. The project to design a
superior sound card was a needed improvement to enable best use of the
processing.
The other limitation at this time, are in the switching character of sound
cards, and thus true break in is a goal in the software radios.
Stuart
K5KVH
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