Digital Operations

Discussion of operating activities for our Technician class licensees

Digital Operations

Postby Jamie_WB4YDL » Sun Jan 30, 2011 9:16 am

An interesting thing happened while trying to communicate on 10M PSK31 - communication didn't happen ! :roll: Mike KJ4LTC made the call for a digital exercise on the 10M calling frequency of 28.120 MHz. Glenn N4MJ and I were there and we began to pass transmissions to each other. I noticed during a break that there was this other signal trying to make itself known, but all that was showing up was gibberish. It didn't dawn on me at the time what was happening until Glenn suggested we try things out on RTTY. Now RTTY is my preferred contest mode and I'm used to seeing some strange things occur with it. So when Glenn and I had successfully transmitted in RTTY, we again asked Mike to jump in and join us. When he did so, it was immediately obvious to me that his signal was "upside down". :o That is, his Mark and Space were reversed. Once he asked his software to Reverse the sense of his input and output, he immediately gained both reception and transmit intelligibility. :geek:

So ... how come Mike had to push the reverse button in the first place ? Well, Mike had just built a very nice digital interface complete with optical couplers designed to prevent audio degradation from RFI. An optocoupler or optoisolator can come in different flavors, but for this application, the commonly used bipolar optical transistor is the choice. BUT, when audio goes through a bipolar transistor, its 'sense' is reversed ! :ugeek:

Most commonly used ham digital software will ask you (or pre-fill) if you wish the common audio mode for digital operations to be in USB - upper sideband. This is preferred for PSK31, MFSK, and several others. The software usually understands that old Baudot RTTY is preferably in LSB - lower sideband - so you don't have to manually change settings when using this mode. In instances where audio is separated by transistors, there are a few options. Most ham radios these days have a menu option for RTTY to be Reversed. This is preferred - you don't have to change anything in any software package. If this is not available, in the software, change the preferred sideband to LSB. This way you don't have to worry about Reversing anything. The 3rd option is of course to use the Reverse button on the software screen.

Got it ? Great ! Have fun on the digital modes ! :D

73, Jamie
WB4YDL
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Re: Digital Operations

Postby KJ4LTC » Sun Jan 30, 2011 2:20 pm

Jamie, great explanation of the events that happened. So, if i had switched to LSB when we were using would that have cured the reversed signal or is LSB just the common mode for RTTY operations. Also, thanks for joining me last night i was fun!

73 de KJ4LTC
KJ4LTC
 
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Re: Digital Operations

Postby Jamie_WB4YDL » Thu Feb 03, 2011 12:57 pm

Mike:

Most digital software packages nowadays are 'mode aware'. That is, it will switch sidebands when RTTY is chosen as a mode if USB is designated as the usual digital sideband - that is for modes like PSK31. Placing the 'usual sideband' into LSB would have fixed the problem at that time. 8-)

73, Jamie
WB4YDL
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Posts: 46
Joined: Sun Nov 14, 2010 8:45 am
Location: Union City, TN


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