On Jun 9, 2010, at 6/9 5:25 PM, William Smith wrote:
> I saw something awhile back on this subject saying to use 7 bits bit
> length, but that must have been just talk. At the time I saw it I
> thought it was a legit config and I saved a 75 baud profile using 7
> bits for future use.. guess that was stupid.
Yes, that was a separate discussion.
It was to compare ASCII RTTY (7 bits) with Baudot RTTY (5 bits) in
terms of (a) the length of time it takes to send a short contest
exchange that contains both numerical shifts and letters shifts, and
(b) for reducing the confusion between USOS and non-USOS.
For the same baud rate, while getting rid of USOS confusion, ASCII (7
bits) takes pretty much the same amount of time to transmit a
"typical" exchange as Baudot (5 bits) in spite of needing more bits
per character, since it does not have to add in a bunch of extra LTRS
and FIGS shift characters for an exchange such as "N3XL 599 03 03
W7AY". Notice that a USOS system would need to transmit 7 extra shift
characters when using Baudot to send that exchange that consists of 19
characters (about 35%).
ASCII has 7 bits + start + stop (=9 total) while Baudot has 5 bits +
start + stop (=7 total) so the ASCII over Baudot overhead is 29%.
ASCII has the additional advantage of providing lower case and the
backspace character, but those features are of no use to a contester.
Switching between Baudot and ASCII is independent of changing baud
rates. I.e., you can just as well use 45.45 baud for ASCII.
I believe someone had mentioned that at least one software modem out
there does not automatically switch to ASCII encoding when you select
7 bits on it, so not everyone can just make the switch. The two
programs that work on Mac OS do allow ASCII encoding (fldigi switches
from Baudot to ASCII code when you select 7 bits and cocoaModem has a
separate "ASCII RTTY" mode). Anyone can experiment with ASCII at any
time based on the current FCC Part 97 rules; I think the same is
probably true for other countries.
73
Chen, W7AY
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