[aprssig] FRS radios and PTT

Stephen H. Smith wa8lmf2 at aol.com
Mon Mar 14 14:56:08 EDT 2011

On 3/14/2011 11:47 AM, Steve Noskowicz wrote:
> --- Stephen H. Smith wrote:
>> Huh?   What does this mean? What two voices?
> The 4.5 kHz is detected in the VOX circuit just like a voice signal is.  Therefore it is another "voice" hense Bob's caps VOICE instead of quotes.
> Bob,
>    You should indicate why you want to just key the radio without voice...?
>    While this is very clever and I like it, for anyone else trying this, some advice.
> Obviously these have the VOX before the low pass filtering in the audio path.  Other radios may not. (this will also "work" on Ham HTs and other radios.
> 1 - Use the lowest amplitude of tone to reliably operate the VOX.
> 2 - You'll need to do some investigation on the frequency so you use the highest frequency that triggers the VOX, yet it can't make it through the sudio path and into the modulation.
> 3 - #2 and #1 are in conflict with each other. Higher frequencies will be atenuated more by he audio filtering, but it may take a higher level to trigger the VOX.
Hey, it's kind of like an improvised homebrew approximation of the classic 
Motorola tone-remote control for land-mobile radio base stations:   Low level 
tone sent over the TX voice audio channel that is then notched out at the 
remote radio site.    One could even do the loud initial beep for positive 
keying and then drop the tone 20 dB for hold during voice.

> 4 - Remember that once you truly talk, you don't need the tone.  This tone and your audio are additive and could clip in the audio chain creating distorion of the voice.

I find VOX on FM really obnoxious with the carrier constantly popping on and 
off between syllables or words with the accompanying squelch crashes.     
Having the tone hold the radio on TX between words would be a great improvement.

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