| Electronics > Projects, Designs, and Technical Stuff |
| Data interface circuit mic-in android decoding POCSAG(1200) |
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| tek2232:
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 ? 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. :palm: 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 |
| vk4go:
I make this but it not works. I try out with 3x Android mobile phones and a Lenovo Tab 4 8. I use all the schematic parts on a vero board. Is here anyone with a clue what I am doing wrong ? 73 Art |
| tek2232:
Hi Art, With four different devices maybe it could run on one of them. I will send you a pm how it could be done . Just for testing if your circuit is ok you could install a Scope app on the device to see if there is any good signal available,check it with the graphs above. (Free Apps are Oscilloscope , SmartScope ) Jp |
| quantenquark:
Hi JP, this is a very interesting post. I have build the curcuit on a bread board and played a little bit with the resistors and my THF-7E using unfiltered audio signal (TNC mode, 9600bps) Finally I can decode the POCSAG beacon but have some issues with the alpha messages. IT happened 2 or 3 times that I could read a part of the message. Regarding the tolerance, which kind of resistors are needed here? I have used carbon ones which have 2-5 % tolerance. Will now try with my oszi to get an idea if the signal is bad. Is there anything else what can be improved? regards Quanti |
| coppice:
It feels so strange that there are still POCSAG transmissions to decode in 2019. :) |
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