Cq Calls And Broadcasts - SCS PTC-IIIusb User Manual

15 Basics
It has NOT proved to be a disadvantage to continue working with longer packets when
signals are weak or under heavy interference. The transmit buffer naturally rapidly fills up
under these conditions, causing the PTC-IIIusb to switch automatically to longer packets.
Only the waiting time until a new line of text appears on the screen increases with the
longer packet length, under extremely unfavorable conditions. The effective throughput,
however, remains considerably higher than when using shorter DBPSK packets. If the
link threatens to break, then the MAXError parameter (time-out) can be increased to 255,
and the Memory ARQ parameter (MAXSum) may be increased to 60 during the contact.
NEVER tune the VFO by hand with very weak or inaudible signals! With very weak
or noisy signals, the PTC-IIIusb adjusts its tuning very slowly to minimize tuning error.
With good, to very good, propagation conditions, PT-2 has shown itself to be 4 to 6 times
faster than PACTOR-1. A maximum speed of 140 characters per second can be achieved.
This is approximately 30 times the effective AMTOR speed.
Switching between speeds occurs automatically. The operator can influence this a little by
using the MAXUp and MAXDown parameter, as with Level-1 PACTOR. The PTC-
IIIusb not only uses the packet statistics as a switching criterion, but also measures the
average phase offset from the correct value for every packet, and thereby obtains a very
reliable measure of the optimum speed required.
Important: The PTC-IIIusb only switches to a higher speed if there is more data
available to be transmitted, than the actual speed could transmit.
15.4.3

CQ calls and broadcasts

As with PACTOR-1, a CQ call or broadcast is normally carried out with 100 Bd FSK
UNPROTO mode (see the Unproto command (refer to chapter 6.100, page 91). Such
transmissions can be read by all PACTOR users. If, however, only those users with
Level-2 systems should be addressed, then a DPSK-Unproto transmission can be chosen.
We recommend the Unproto-3 mode for a DPSK CQ call. This is DBPSK with short
packets, and has proved to be the most robust broadcast mode under normal conditions.
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