Re: [RTTY] DESREVER

[Kok Chen <chen@mac.com>]

On Oct 16, 2004, at 6:20 AM, Richard Ferch wrote:
> In particular, I believe recent Kenwood and Yaesu radios assume the 
> opposite polarity to the one assumed by JE3HHT in MMTTY, so on these 
> radios you have to change one of the radio's menu items.

Ah yes, Yaesu and their nutty interface :-)

The FT-1000MP is reversed, but not wrong per se, since it is actually 
documented exactly in the FT-1000MP manual.  In the "RTTY Polarity" 
section of the Operation chapter of the FT-1000MP Operating Manual, 
they state

"Normally a Mark tone (2125 Hz by default) is transmitted during 
keydown in LSB RTTY operation. With reverse polarity, a Space tone 
(2295 Hz) would be generated instead."

The rub is that the normal "standard" calls for the Space tone to be 
transmitted on an assertion of the keying signal, not the mark tone.

My understanding is that the RTTY standard calls for the teletypewriter 
to be latched to the idle position outside of a character where a mark 
tone is transmitted.  A start bit (22 ms of space tone) releases the 
latch and the stop bit again relatches the mechanical system.  I.e., a 
space tone should be produced by the keydown position since the mark 
tone is supposed to be transmitted when you are not sending a character 
(you can hear that mark idle tone from stations that don't use diddle).

The FT-1000MP (ditto FT-990) defines the factory default keydown to be 
the Mark tone.  Hence the need to reverse it for  us who practices RTTY 
for a living :-).

It actually depends on what you deem an "assertion" to be.  To most of 
us, a keydown is a state where maximum current flows through the keying 
circuit (this way, an open circuit on the keying line is a 
deassertion).  Yaesu engineers might be thinking that it is more 
logical to think that a higher voltage at the keying signal is the 
assertion position.

I think it was Andrew Tenenbaum who said "The nice thing about 
standards is that you have so many to choose from."  He might have been 
quoting some sage before him.

73
Chen, W7AY

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