You're right in saying that the line between hardware and software transmitters is blurred these days. Most of the transmitters use all-in-one chips to generate RF, with separate electronic devices (required for the real world) to filter, suppress harmonics, etc.
However, the functionality of these chips is fixed. You 'program' them by doing various things to the pins. I guess you could say that you have a 'sort of an SDR' with limited programming capabilities via the front panel interface. Maybe the best word is hybrid.
In a true SDR/SDT, you are dealing with the raw RF signals, and shaping them, via software, to what you want them to be. In other words, the user has programming (i.e., software) access to that data. That's not the case with these chip-based transmitters (never mind the all electronic component ones, like the Decade). With that access, you can support multiple modes, do virtually anything to the signals without limits. You'll still need hardware filters, harmonic suppression, etc. but your computer becomes that chip, with far more capabilities.
There's nothing to stop you, for example, from implementing C-QUAM stereo reception and transmission on the AM broadcast band. You wouldn't need that specialized Motorola chip. I know that the KiwiSDR, for example, supports reception of C-QUAM.
I hope that explains my thoughts a bit more clearly.
@artisan-radio Yes, it does clear things up, but going back to my original post about PDtechnologies mention of a new Part 15 transmitter he hopes to have certified within the next 12 months, -- your response was that it's kind of pointless to design a new transmitter unless it incorporated the new SDR technology.... But if I understand correctly (and this is my point) SDR plays no part in design of the actual AM transmitter itself, but only can provide additional capabilities (RDS, processing, etc.), but can't be used as a transmitter itself.
So, if that's the case, how could a new transmitter design be considered pointless without employing SDR? All the additional SDR capabilities could simply be employed in a separate unit to any transmitter design - so there's nothing to be considered as "going backwards" by not having it built in to a new transmitter design. Therefore it's rather irrelevant whether a new transmitter incorporates it or not.. no?
And let's not forget, we have no idea if his design incorporates SDR or not.
Incidentally, I noticed earlier today our site was down, but ironically I was able to access HB again today.. is it working for everyone else again?