RadioBanter - Packet radio question

What types of data can be sent over packet radio. I just started
reading about HAMs and Packet radio and am anxious to learn more. I
have a few project ideas in mind and was wonder if they are fesiable.
Can video be sent over packet radio? I would use 2400bps.

Consider that 2400 bps is 240 bytes per second. A standard jpg photo
might be 48 kb (kilobytes). (For a nice round number.) So that
would take 200 seconds to transmit.

Assuming some overhead which is why I rounded 2400 bps to 240 bytes
per second. I'm not at all familiar with packet radio but am
familiar with phone line modems. There could be lots more overhead
with packet radio which I'm not aware of.

Thus while you could send video it could take several minutes per
frame.

Yup.

Or, really, it could take very much longer at 2400 bits/second,
because you're going to have to wait for a long time to find another
ham packet-radio user with a 2400-baud modem! As far as I know, there
are no real or de facto standards for 2400-baud operation on the ham
bands... any such modems are likely to be experimental one-offs.

The original standard is 1200 baud, based on the Bell 202 AFSK (audio
frequency shift keying) modem standard. There were a lot of
202-capable modems sitting around (e.g. older Hayes models) when the
hams started playing with this. A lot of packet radio is still using
these, despite their low bandwidth, as they're cheap to implement, and
can be hooked to a 2-meter radio with little more than a bit of
isolation (cheap audio transformers, or capacitors) and a PTT-keying
circuit.

9600 baud is the next common step up, using FSK (shifting the RF
carrier frequency more directly, without an audio-frequency carrier).
The modem itself can be very simple and cheap, but the hookup to the
radio is more involved - you must bypass many of the audio input and
output circuits (preemphasis and deemphasis) and feed the signal to
the modulator fairly directly. Older radios must usually be modified
to support this. Newer ones often have an auxiliary/data jack with
pinouts for both 1200 and 9600, and a switchable control via a menu.

Even higher speeds have been played with, but they tend to require
more-specialized radios, or radios with the modems/TNCs integrated
into 'em.

As to sending the data... you've got several ways of doing it. You
can use the traditional AX.25 packet protocols, which are usually
designed to let you log into a BBS or remote node, and then implement
a file-transfer capability on top of these... that's how packet BBSes
interconnect. Or, you can run TCP/IP directly on top of AX.25., and
use any file transfer protocol you want (HTTP, FTP, etc. - SSH would
probably violate the FCC's rules, though).

Then, there's the traditional analog slow-scan TV, and some new
digital slow-scan TV methods with built-in forward error correction.

There are some interesting soundcard-based modulations such as SCAMP,
which mandate a file-based transfer methodology rather different than
AX.25 packet. SCAMP is designed for use on HF, but could almost
certainly be transmitted over 2-meter FM or 2-meter sideband just fine.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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