Hi everyone,

I build this little circuit that makes it possible to connect
a receiver (scanner) to the audio jack of an Android phone/tab.
It is powered by the audio jack itself.
I tried it with three different scanners and several phones and
tabs,so the circuit and software is somehow "tuned" to these
devices.(see results down below)
Maybe someone who finds this interesting can try
this circuit and app to see if it also works on his setup
and can this circuit be improved ?
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The microphone input carries a DC voltage about 1.4 - 2.2 v
(depends on manufacture device),this voltage is used to power
the BC548B which is used to buffer the earphone or the discriminator
output from the scanner.
The limiting resistor R1 and capacitor C1 protects the earphone or discriminator
output from the receiver against overload.Resistor R4 enables external mic in.
To see if this idea really works I thought lets find a transmission protocol
not to fast and not to complicated.
I ended with the POCSAG(1200) protocol it is used to transmit
data to pagers and is well documented.
If POCSAG is decoded most often the unfiltered audio from the
receiver is used (discriminator-output).
Some scanners have a discriminator output build in ,but most don't.
I didn't want to crack open my brand new receiver and
do some "precise" soldering ,so I thought lets try also to decode using
the earphone jack from the receiver,and then there came
some problems.

When audio is heard through the speaker you can hear a 600Hz tone
these are the POCSAG preamble bits ,next follows sync en data.

In the picture above is shown how much the earphone signal
is distorted when compared with the digital discriminator signal.
The receiver amplifier has an analogue filter at its output
causing this distortion,it seems that the signal collapses
or rizes after one bit high or one bit low,this is why it's
so hard to decode with the earphone signal.

Also each type receiver has different filters and amplifier so
a standard setting is almost impossible see picture above.
Another problem is the different specs of the MIC-in circuit
from the different manufacturers of android devices which
results in different amplitude levels when using the same
receiver but different devices.
So after all this how can the right audio level be found.
Well it's been tried with two trigger values one for the
preamble to get the right timing and one for the data and it
gives some promising results.
Playstore :
[url=https://play.google.com/store/apps/details?id=datainterface.com]https://play.google.com/store/apps/details?id=datainterface.com[/url]
More info under the "Help" button in the App.
JP