[aprssig] GM-300 speaker output

[Dave Baxter]

Yes but...

Many small radios (some from big names) they float the speaker at some
odd voltage, with some DC bias through it (to save component count, and
hence cost) so need a resistance to bias the OP stage correctly.

Yes, you can capacitivly couple audio from one side, but which side,
without getting a 'scope out and testing?

I was musing too, as to if the radio did something with it's speaker OP
stage, like power it down when the squelch is closed (to save power) so
with a bit of low cost analogue trickery, you could get good audio, and
a squelch indication.

I don't know the Moto' radio in question in detail, but I do know other
makers do that sort of thing, as well as the speech recording/playback
IC I used.

Agreed about getting to the discriminator output directly.  If it's
easily available of course.   Likewise, getting to the modulator
directly for the TX path, if posible.   But, that all needs
modifications inside the radio.  Not everyone has the ability, or are
willing to do that sort of thing (even if it is their kit.)

Of course, if you get to the RX discriminator, you may probably have do
do the de-emphasis thing, to remain compatable with the hoards of
operators out there, who just "Plug n Play".  Likewise if you go to the
modulator yourself...

I won't start a flame war as to premphasis or not, that's been thrashed
out ad-nausium many times here and elsewhere.  Yes you need direct
connection, non pre/de-emphasied etc for 9k6, but for 1k2, the standard
mic-in, spkr-out ports work well enough, with care.

Regards.

Dave G0WBX.
 

> -----Original Message-----
> From: Steve Noskowicz [mailto:noskosteve at yahoo.com] 
> Sent: 29 September 2009 17:56
> To: TAPR APRS Mailing List
> Subject: Re: [aprssig] GM-300 speaker output
> 
>   http://k9dci.home.comcast.net/
> 
> 
> --- On Tue, 9/29/09, Dave Baxter <dave at uk-ar.co.uk> wrote:
> 
> > Use a low ohm resistor in place of
> > the speaker to keep it's OP stage
> > happy,
> 
>   Yes, but....
> 
> > and a simple differential input OP-AMP stage to get the 
> audio off the 
> > resistor.
> 
> If it is true differenial (has to be), you only need to 
> couple off one side.  However, the disc output should be 
> better (with proper audio shaping as appropriate - aka 
> de-emphasis or not)
> 
> 73, Steve , K9DCI
> 
> 
>       
> 
> 
>