charlesb wrote:

If somebody comes up with a subaudible tone data system, even at a low data
rate, I'd like to hear about it.

Motorola land mobile radios in the pre-trunking days
had an option for something they trademarked as
"Digital Private Line". It was basically 100 baud
data, IIRC, sent as base-band data with the voice
(in other words, 100baud data stream directly
mixed with audio). The radios obviously needed
good DC-response. The sequence sent was something
like a 23-bit pattern was FEC-expanded from a 16-bit
or somesuch digital code. I think there was also
the equivalent of traditional "reverse-burst", a
sequence sent to mute the receiver when the transmitter
unkeyed, to avoid a burst of unsquelched noise.

100 baud data has a base-band bandwidth well-inside
traditional PL if well-filtered or sent use a
raised-cosine/DDS arrangement. Alternating
1s and 0s produce a 200Hz sine wave. Existing
PL filters in receivers and repeaters would filter
DPL out as if it was regular PL. In this sense,
it was quite compatible with existing PL.

Certainly DPL provides a wealth of interesting
background; you can find examples of the hardware
in service manuals for MCX100s and other 1980s
vintage Motorola land-mobile. I'm sure other
OEMs had similar technology.

They key item to successfully sending low-baud
data over an FM transmitter is low distortion,
which means a transmitter with near-DC response;
this is achieved by modulating both the VCO and
reference oscillator in a PLL system. Most ham
radios I've looked at are unsuitable since they
only modulate the VCO and the PLL will suppress
modulation below 50Hz or so.

The other key lesson here is adopting a heavy
FEC approach since you're not in a packet mode,
you're bussing data along with voice. Send short
chunks of data which are robustly FEC-encoded.
It may limit what you can actually send during a
normal voice contact.

Dana