#1 2016-12-09 17:43:43

whemming
Member
Registered: 2016-12-04
Posts: 28

Frustrated with DIY MicroModem

I built a Micromodem from schematic.  First off it didn't decode at all.  I wrote a small sketch to monitor A0 for clipping and noticed my radio was clipping at very low volume.  So I built a variable attenuater to attenuate the audio.  All of a sudden I was decoding almost ever packet without issue.

Since this was a mess on the breadboard, I rebuilt it exactly the same (just cleaner), but now It wont decode anymore.  I have built it 3 more times from scratch (even with different arduino's -1 uno 2 nano's) to no avail.

I turned everything off and went to bed, came in the next day to try again, and I was again decoding almost every packet. 

However the next day, back to zero decodes.

I'm using a Baofeng radio for testing.  I'm using a YAGI pointed at my nearest digi repeater.  The signal is crystal clear.  And the PC decodes just fine using sound card.

A0 is biased to a perfect 2.5v.  Ardunio sees this as a stead 511 (1023/2).  I set audio to peak to peak ~ 120-900 (as seen by arduino) to ensure no clipping.  But I have tried every volume setting imaginable at this point.

I'm about to lose whatever hair I have left.  Does anyone have any clue?  Experienced a similar issue?

P.S I have tried the HEX simple serial and LibAPRS, both work when it was working, neither when it doesn't.  Transmit works fine, however I'm getting unsupported packet at APRS.fi (but that's the least of my worry right now).

William

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#2 2016-12-09 23:32:46

whemming
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Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

I decided today to test my divider resisters to make sure they were exact, they were not.  So I dug through my parts bin and matched two resisters down to 1 ohm (close enough) .  However this made no difference at all.  I hooked up an oscilloscope and it shows the open squelch noise centered at 2.35v (my 5v line reads 4.7v, so this is exactly centered). 

The noise shown in the scope has a higher amplitude than the APRS packets.  The noise is about ~2.5 ppv, while when APRS packets come in they quiet to ~1.5 ppv.  You can clearly see the 1.2khz wave in the noise.  Audibly the packets sound great to my ears, but shouldn't the packets have a higher amplitude than the noise?

For sake of sanity, I had an old mobile dr-135 laying around.  So I hooked it up and adjusted the open squelch volume to prevent clipping.  APRS packets came in strong, but still not a single decode.

For the life of my I have no idea whats happening.  My only guess is something is interfering, and that something gets turned off occasionally, and thats when it starts working.

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#3 2016-12-20 04:49:11

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

Found out why my modem worked some days, and stopped others.  It seems my local digi repeater was down every time my modem stopped decoding packets(which is most of the time).  However I can hear other strong stations but not decode any.  It seems I can only decode packets repeated by my local digi.  I can also decode 99% of all test packets from the TNCTest CD. 

What perplexes me is why I can't decode any other stations?  Only full quieting STRONG signals are decoded.

I have built 3 modems (1 AtMega2560, 1 AtMega32u4, 1 AtMega328p).  I was able to modify the code to work with all three, but all three had the same exact issue.  I could only decodes my local digi.

I have also tried several radios, passive bandpass and op-amp on AF in with no change.

One thing I have not tried is 3.3v ref.  All 3 I have built using 5v.  Would change a modem to 3.3v ref make any difference at all?

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#4 2016-12-20 17:39:50

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

Seems I'm talking to myself, but I'll keep posting smile

I pointed my Yagi due west toward the next biggest city, and was able to decode a few packets  from over 150miles away (as per the digi I received from).  These signals are much weaker, yet I get successful decode.  I was monitoring the oscilloscope, and the far away signals look and sound different than my local signals.  They have a much more pronounced 2200hz tone.  The decodes signals have a strong 2200hz (sound and seen on oscope), but the unsuccessful I only seem to hear and see the 1200hz above the noise. 

I know my radio does DE-emphasis on the TNC AF port.  So maybe the only stations I can decode are stations transmitting using pre-emphasis.   My guess is the stations I fail to decode 99.99999% of them are not using pre-emphasis which is killing my 2200hz tone.  While the few out there that do use pre-emphasis decode fine.

I will report back when I get a radio that you can disable pre/de-emphasis on.

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#5 2017-01-09 03:06:26

KC1AT
Member
Registered: 2017-01-05
Posts: 8

Re: Frustrated with DIY MicroModem

Hi William

I'll tag on here....Just found the schematic for the MicroModem...looks easy enough to breadboard. Gotta get a couple of parts and ran out of 4x20 displays on my last project...I'll be watching your experiences with the de-emphasis circuit.

don

ps; like your QRZ page

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#6 2017-01-10 19:17:31

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

You should look into the i2c or SPI .96" oled displays.  They are tiny and depending on font can display much more than a char display.  They also cost just a few dollars.  Another awesome option are nextion displays which operate off serial.  My project is using a 2.4" nextion touchscreen. I have programmed in on-screen keyboard to send APRS messages/email.   There is only a power switch and a PTT button on the HT.  All other functions are controlled via the touchscreen. 

Last piece of hardware arrives today, so I can button it up and focus on the programming.  I hope to have a short demo to post in the net few days.  I etched my own pcb's for this, so its not near as small as it could be.  But its a functional prototype none the less.  Here are the highlights:

Battery operated HT
Usb chargeable 6600mah
2.4" touchsceen LCD
APRS decode (including mic-e position and status)
APRS send location via onboard gps
APRS Send message via onscreen keyboard
Full voice capable with repeater offset
onscreen volume, squelch, ctss, etc.
SR_FRS-1w VHF transceiver module
9 element Low pass filter
HT to HT SMS messages

As for the above frustration, it was caused by many factors.  Biggest issue was audio quality.  The micromodem needs a fairly high peak to peak to decode property.  In my findings it works best between 1.5-2v ppv. My intial testing was with a boafeng HT.  While I could adjust volume to those levels.  The audio quality was worse at those levels.  I decided to use much smaller volume level (around 100mv ppv) then amplify it 15 times to 1.5v peak to peak.  This produced a much cleaner signal and decoding improved dramatically.

Then once I etched the PCB and used all SMD parts, the decoding improved even more.  Its amazing how much emf noise you get from breadboard wires. 

I still can't decode 100%, but I don't think this is related to my TNC at all.  I hooked the same radio the computer (using soundcard tnc) and had the same decode results.  So at this point I'm satisfied.

Regards,

William

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#7 2017-01-10 20:06:21

KC1AT
Member
Registered: 2017-01-05
Posts: 8

Re: Frustrated with DIY MicroModem

Hi William

Good ya found the problem....Love to see that setup....sounds nice.

I bread boarded the Micromodem up yesterday....slapped the HEX software into the nano via Xloader and it seems to like it. I can send commands via the serial monitor and it reports back. So I think the software is working.

My next challenge is to get the display working. Its an I2C but its address out of the box was 0x3F didn't work so I changed it to 0x27 which most of them are....still no joy. What is your LCD address ????....I can change this one by opening some solder bridges.

I'm also using a Barfing (not a spelling error, I don't think much of them) HT...I had to move the Nano away form the Barfing cuz it was holding the squelch open....so I figure there is some distortion going on even if it doesn't open the squelch...got to move all this stuff into the other room where I have a o'scope and see what the input audio looks like.

don

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#8 2017-01-11 15:48:41

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

KC1AT wrote:

Hi William

Good ya found the problem....Love to see that setup....sounds nice.

I bread boarded the Micromodem up yesterday....slapped the HEX software into the nano via Xloader and it seems to like it. I can send commands via the serial monitor and it reports back. So I think the software is working.

My next challenge is to get the display working. Its an I2C but its address out of the box was 0x3F didn't work so I changed it to 0x27 which most of them are....still no joy. What is your LCD address ????....I can change this one by opening some solder bridges.

I'm also using a Barfing (not a spelling error, I don't think much of them) HT...I had to move the Nano away form the Barfing cuz it was holding the squelch open....so I figure there is some distortion going on even if it doesn't open the squelch...got to move all this stuff into the other room where I have a o'scope and see what the input audio looks like.

don

Use I2c scanner sketch to find the I2c address

http://playground.arduino.cc/Main/I2cScanner

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#9 2017-01-11 15:53:44

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

Here is a quick video more to come

https://www.youtube.com/watch?v=T7G3qyiAtCA

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#10 2017-01-11 19:02:22

KC1AT
Member
Registered: 2017-01-05
Posts: 8

Re: Frustrated with DIY MicroModem

Hi William

Video is a little dark...but I like the display.....

I know the address on the LCD and I can change it...but whats the program looking for ??? its in HEX so I'm not sure where to look ??? I could figure it out if it was a sketch...(I think) hahahaha...going to try and get the modem hooked up to my 706mkIIG later today and adjust the audio in to 1.5 to 2 pp volts of CLEAN audio.

don

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#11 2017-01-12 15:39:07

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

KC1AT wrote:

Hi William

Video is a little dark...but I like the display.....

I know the address on the LCD and I can change it...but whats the program looking for ??? its in HEX so I'm not sure where to look ??? I could figure it out if it was a sketch...(I think) hahahaha...going to try and get the modem hooked up to my 706mkIIG later today and adjust the audio in to 1.5 to 2 pp volts of CLEAN audio.

don


A display can't be hooked up to the micromodem using the hex code.  You should hook the display to the second Arduino, in a 2 Arduino setup.   You could put a display on the 1st arduino if you used LibAPRS, but you will have to write your own code for that.  With the HEX precomplied code, all you get is a modem that spits out APRS in simpleserial format.

For a two arduino setup see Stanley's code on github.  For his code he used the HEX simpleserial on arduino 1 and this code on arduino 2  https://github.com/stanleyseow/ArduinoTracker-MicroAPRS   However you would need to mod it to suite your hardware etc.


William

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#12 2017-01-12 18:54:22

KC1AT
Member
Registered: 2017-01-05
Posts: 8

Re: Frustrated with DIY MicroModem

Thanks William appreciate your time to explain it all.

Going back to work on some other projects and forget this for a while......maybe forever or until I get more code under my belt.

thanks again

don

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#13 2017-01-15 03:10:51

whemming
Member
Registered: 2016-12-04
Posts: 28

Re: Frustrated with DIY MicroModem

KC1AT wrote:

Thanks William appreciate your time to explain it all.

Going back to work on some other projects and forget this for a while......maybe forever or until I get more code under my belt.

thanks again

don


Don't give up.  You will get it!

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#14 2017-01-17 12:47:11

KC1AT
Member
Registered: 2017-01-05
Posts: 8

Re: Frustrated with DIY MicroModem

William

Went to work on another Arduino project....also couldn't get that working so I emailed the sketch author....after about 12 emails back and forth he asked me to call him. I called him he turned out to be a guy that programs C for a living...we worked out my LiquidCrystal.h problem after he dug into the code and modified it. In between working on my problem he was telling me about all the problems he's run into when playing with Arduino stuff...

Needless to say, I guess I was too naive about Arduino...I thought there were standards and software testing that went on before libraries and sketches are posted to github or one of the other "official' sites. He gave me a lot of tips and insight on libraries and how to spot a crappy sketches..

I'm feeling a little better about diving into this stuff....but I got to get more coding under my belt before diving into the deep end.

don

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#15 2017-01-26 15:44:52

Stanley
Member
From: Kuala Lumpur, Malaysia
Registered: 2014-12-01
Posts: 65
Website

Re: Frustrated with DIY MicroModem

I built my open source SVTrackR based on Micromodem here http://9w2svt.blogspot.my/

32494976436_98d95479bf_n.jpgSVTrack PCB Layout


Stanley
9W2SVT/ N5SVT
http://9W2SVT.blogspot.com

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#16 2017-01-27 17:27:21

KC1AT
Member
Registered: 2017-01-05
Posts: 8

Re: Frustrated with DIY MicroModem

Hi Stanley

I was following your stuff for a while but got bogged down by something...I think it was the interface between the 2 boards. Couldn't figure out how one was talking to the other. Tried to find schematics on Github, couldn't find em. Looked through your sketch for instructions...maybe passed by them.... I liked the upgrade to the TFT display but the architecture was a little too much for me to digest. Then I thought someone said that you scrapped the 2 board project for a Mega 2560...or someone did...at that point I got twisted around the axle and put it away.

I'm a real newbie on this but learning quickly...got a few projects under my belt now and feeling a little more confident. Still working on the project thanks to William...Got to finish a couple of nano projects first and then I'll get back to the APRS one....

thanks for chiming in

don

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