Data interface circuit mic-in android decoding POCSAG(1200) - Page 1
Forum section: electronics / projects

author: tek2232
author_rank: Contributor
timestamp: March 08, 2018, 06:44:15 am
content: Hi everyone,I build this little circuit that makes it possible to connecta 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 andtabs,so the circuit and software is somehow "tuned" to thesedevices.(see results down below)Maybe someone who finds this interesting can trythis circuit and app to see if it also works on his setupand 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 powerthe BC548B which is used to buffer the earphone or the discriminatoroutput from the scanner.The limiting resistor R1 and capacitor C1 protects the earphone or discriminatoroutput from the receiver against overload.Resistor R4 enables external mic in.To see if this idea really works I thought lets find a transmission protocolnot to fast and not to complicated.I ended with the POCSAG(1200) protocol it is used to transmitdata to pagers and is well documented.If POCSAG is decoded most often the unfiltered audio from thereceiver 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 anddo some "precise" soldering ,so I thought lets try also to decode usingthe earphone jack from the receiver,and then there camesome problems.When audio is heard through the speaker you can hear a 600Hz tonethese are the POCSAG preamble bits ,next follows sync en data.In the picture above is shown how much the earphone signalis distorted when compared with the digital discriminator signal.The receiver amplifier has an analogue filter at its outputcausing this distortion,it seems that the signal collapsesor rizes after one bit high or one bit low,this is why it'sso hard to decode with the earphone signal.Also each type receiver has different filters and amplifier soa standard setting is almost impossible see picture above.Another problem is the different specs of the MIC-in circuitfrom the different manufacturers of android devices whichresults in different amplitude levels when using the samereceiver 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 thepreamble to get the right timing and one for the data and itgives some promising results.Playstore :[url=https://play.google.com/store/apps/details?id=datainterface.com]https://play.google.com/store/apps/details?id=datainterface.comMore info under the "Help" button in the App.JP

author: vk4go
author_rank: Newbie
timestamp: July 22, 2018, 09:31:16 pm
content: 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

author: tek2232
author_rank: Contributor
timestamp: July 23, 2018, 06:46:00 am
content: 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 onthe device to see if there is any good signal available,check itwith the graphs above.(Free Apps are Oscilloscope , SmartScope )Jp

author: quantenquark
author_rank: Newbie
timestamp: November 21, 2019, 05:01:03 pm
content: 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?regardsQuanti

author: coppice
author_rank: Super Contributor
timestamp: November 21, 2019, 05:11:14 pm
content: It feels so strange that there are still POCSAG transmissions to decode in 2019.

author: quantenquark
author_rank: Newbie
timestamp: November 21, 2019, 05:20:35 pm
content: Yes, it is!But sometimes this technology is still used and need to be maintained.Anyway, in parallel I am building and programming an interface to show the specific alarms send by the fire brigade control center so that my team has a better overview when they reached the wake.Of course, today we use TETRA but the messaging is still processed via POCSAG.It is a funny world we are living in.73 Quanti

author: tek2232
author_rank: Contributor
timestamp: November 21, 2019, 06:47:55 pm
content: Quote from: quantenquark on November 21, 2019, 05:01:03 pmHi,QuantiCarbon resistors are ok ,and a breadboard works fineto try something out.You can check with your scope if the output signal fromyour scanner stays the same when you connect the interface.If you can are already receiving some characters from a messagethen your almost there.I think unfiltered audio signal (TNC mode, 9600bps) is thesame as discriminator output?.It's a little bit strange but the earphone audio signal can give goodresults when a discriminator output isn't working properly or doesn't work.I should try this first and tweak with the prTrig and Trig valuesKeep the peakvalue about 16000 if possible with the volume buttonfrom your THF-7E.What kind of smartphone or tab do you use to decode ?,Try different ones some preform better.JP

author: quantenquark
author_rank: Newbie
timestamp: November 26, 2019, 05:47:31 pm
content: Ok, just have seen this message. Thank you for replying to me.I assume that TNC/9600 is same like discriminator output accoring to the manual as it is used for packet radio.Should be more or less equal.Will compare the results before and after connecting my devices. I use Samsung A5 / A8 and a Galaxy 10 Tab .What is not really handy to me is the peak value, I need to figure it out and will send a screenshot if needed.regardsQuanti