5 PTC-Firmware
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One cannot of course expect an asynchrony protocol to reach the same efficiency as a
small synchrony ARQ protocol (e.g. PACTOR), but for some applications a multi user
service with very uncritical transmit/receive switching, as well as almost zero power
holding up a connection when no data passing, brings a real advantage that outweighs the
lower data throughput.
What are the reasons then, that up until now HF-PR works so poorly, and apart from
“forwarding” is hardly ever used? There is a simple answer: The current modulation type
for HF-PR namely uncoded 300 baud FSK is really unsuitable for normal HF channels.
The symbols are much too small even with moderate “multi path effect” (“delay spread”)
to work. Additionally because no sort of error correction code is used, even short troughs
or “statics” will destroy a many seconds long packet. Just one missing bit leads to a repeat
of the whole transmission.
To overcome this problem SCS has developed a new class of robust modulation types
especially for Packet-Radio. As a special feature for all the variants of this “Robust-PR” a
completely new synchronization algorithm with tracking properties that were not possible
before has been realized. Frequency deviations of ±250 Hz are immediately
recognized and compensated without any loss of sensitivity, and this also with signals
that are buried deep in the noise. Because of this it is possible to remove a tuning display.
One can say with good conscience this is “Plug and Play” for shortwave.
The 3.6 firmware makes available a small band (500 Hz) version of the “Robust-PR”. A
wide band variant (2 kHz) with similar characteristics and 4 times the speed is in
generally possible.
The current “Robust-PR” modulations schemes have the following characteristics:
Bandwidth: 500 Hz @ -30dB.
Modulation: Pulse-Shaped OFDM (BPSK, QPSK); similar to PACTOR-3
Average throughput: 200 or 600 Bits/s (Increase to 1200 Bit/s possible)
Crestfactor: 3.0 or 4.2 dB
Delay-Spread: to ±8 msec can be coped with
Coding: High performance error correction code, “full-frame interleaved”,
rate/2 or rate 3/4
5.12 PACTOR Duplex and PACTOR data transparency
To simplify the PACTOR operation mode, that means to ensure compatibility to many
mailbox and terminal programs made for Packet-Radio (PR) while using PACTOR, the
possibility of working without the usage of special control key sequences (e.g. <Ctrl> +
<Y>) had to be created. Programs written for PR do not know the commands for
changeover used in the half-duplex mode on shortwave, because PR reacts in the half-
duplex mode on the user interface more or less like full-duplex – a changeover does not
exit for PR.
To avoid changeover commands using PACTOR the PTC-IIIusb offers a
CHANGEOVER-automatism , the so called PACTOR Duplex.
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