[aprssig] Re: VOX to key up soundmodem?

Stephen H. Smith wa8lmf2 at aol.com
Wed Apr 13 15:25:30 EDT 2005

gswiebe at mb.sympatico.ca wrote:

> A few years ago when I tried PSK I constructed a radio to sound card 
> interface using two Radio Shack transformers to go between my TS-440S 
> and the pc. Whenever I operated PSK I simply turned the VOX on and it 
> worked fb. Of course my microphone was not plugged in. I really am not 
> crazy about using vox though, who knows what the squeaks and squawks 
> from the pc will be transmitted. And then there is the matter of 
> forgetting to turn the vox off when you're finished.
> 73 de Glenn...VE4GN
> Jason Winningham wrote:
>> A new ham brought up an interesting question: can a VOX circuit be 
>> used to key up the TX on a soundmodem rig, to avoid wiring another 
>> port from the computer?
>> -Jason
>> kg4wsv
*****    Various observations on the use of VOX with sound card 
interfaces *****

1)     If you are going to use any kind of vox operation for soundcard 
operating, go to the Windows Control Panel and disable ALL sounds for 
system events (like incoming mail, error messages, the bootup music or 
sounds etc) so they don't inadvertently get transmitted. 

2)     Or better, install a second sound card to be dedicated 
exclusively to ham operating.  A basic vanilla PCI-slot sound card can 
be had now for USD $20-25. For ham applications, the cheap basic cards 
are actually better than ones with fancy MIDI synthesizers, 5.1 surround 
sound outputs, etc.  

3)    An alternative (especially for laptops and non-expandable 
small-footprint desktops) is a USB-interfaced external sound card.  
These devices, which are usually a cigarette-pack-sized box at the end 
of a USB cable, typically go for USD $60-$100.   Being USB devices, you 
can use an external USB hub and connect several to one PC without even 
opening up the box.  This has interesting possibilities such as:   
Dedicate one sound interface exclusively  to APRS via the AGW packet 
engine, a second one for HF PSK operating, and perhaps a third one to a 
full-time Echolink or VoIP (Skype, etc) setup.

3)     Unlike PSK31, the attack time (i.e. delay between the time the 
tone starts and the time the radio actually keys up) of the VOX in the 
radio becomes an important issue on packet where you want TXD (Transmit 
delay) in the low 10's of miliseconds. Further, for packet operating, 
you want instantaneous turnaround back to receive (or as close to this 
as possible) when the tones are finished.  Voice-oriented VOX systems 
often have minimum delay times, for TX-back-to-RX, that are 
objectionably long.

4)    On many radios, the rear panel auxilliary input (i.e. the 13-pin 
DIN on large Kenwoods or the 6-pin mini-DIN on many other radios) will 
not trigger the radio's VOX system. (Often, the VOX only responds to 
audio entering the front-panel MIC jack.)

5)    The ideal place for sound card VOX is in the soundcard interface 
itself. This way, you can have "no-serial-port-required" PTT keying even 
on FM rigs that don't have an internal VOX, and the VOX can be designed 
for the shortest possible RX-to-TX and TX-to-RX delays. The commercial 
TigerTronics SL-1 SignalLink

and my homebrew "Tone-Keyed Soundcard Interface" at

both incorporate optimized-for-data  VOX-type  circuits internally.   My 
unit will key a radio within 5 milliseconds of hearing a tone if you use 
the relay I specify or an opto-isolator for keying.

Stephen H. Smith             wa8lmf (at) aol.com
Home Page:                   http://wa8lmf.com

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