[1000mp] PCT III Not Legal USA? - Sound Card Digital/Ritty Modes

[Garry Shapiro]

Hans, PA1HR said, among other things:
>
> When I compare the older TNC's (e.g. PK232 or KAM) with the
> modern TNC's or
> soundcards I see a lot of improvements. With the PK232 I *had to* use the
> IF-filters on crowded frequencies, otherwise it was not possible to keep a
> good connection. With the modern TNC's (like the SCS PTC-IIpro) and
> soundcards the IF-filters *improve* the performance of the
> connection.
>
Possibly the major reason for your observation with the PK232 was that the
so-called "multimode controllers" of the 1980's had hardware front-ends, and
manufacturers reduced complexity and cost by not optimizing each mode. The
PK232 was a limiter/discriminator which really had only two modes: VHF
Packet and Everything Else. That is, there was one front-end configuration
for all HF modes, which, by definition, had to be the widest mode, i.e. HF
packet. The front end BPF was at least 450 Hz wide, the discriminator was a
linear ("S") discriminator, also wide, and the post-detection LPF BW, for HF
and VHF, was 150 Hz, for 300 bps FSK.

All three sections were much wider than needed for narrower modes. Dramatic
improvement on RTTY, for example, was possible by reducing front-end
bandwith to 250-275 Hz, peaking the discriminator filters to take advantage
of the RTTY spectral shape, and scaling the LPF down by a factor of 5 or
more. Even then, narrow IF filters could still improve performance by
reducing QRM.

Current DSP-based TNC's provide mode-specific configurations in firmware, as
do S/W TNC's operating with soundcards, and some use detectors with
performance superior to the limiter/discriminator approach, such as linear
matched filters.

Garry, NI6T