[GreenKeys] Bell 103 demodulator?

Sat Sep 13 12:48:24 EDT 2014

All, thank you for having a go at this file.  With the effort you guys have invested, I feel I must now divulge more information on the origin of this file...

I am a collector of military radios (among other things). Several of the radios in my collection are HF "manpacks" radios carried on one's back.  One radio I have is the british military PRC-319 which was standard issue to the SAS (or special forces) in the late 1980s. It was also used by other militaries. It is a CW/USB HF/VHF radio 1.5-40 MHz with an output of 50W.

The standard -319 has an electronic message unit (EMU) that allowed an operator to enter in a digital (numeric only) message to be sent and received.  However, this -319 has a rare alpha numeric message unit.  Pictures of the whole system can be seen here. It is a modular system where the battery can be removed, automatic antenna coupler, etc.

http://www.w2hx.com/x/PRC-319/

This alphanumeric EMU is removable with its own battery. The idea is that someone can remove the EMU, write a message (optionally encode it), connect back into the radio and transmit the radio. Or, having received a message, remove the unit and read it back (again, optionally decode).

What is very interesting is, if you don't want to use the radio for the transmission, and you have access to a telephone, you can remove the EMU and play or receive the audio tones through a speaker/mic on the rear of the unit.  

http://www.w2hx.com/x/PRC-319/IMG_5913.JPG (screwdriver head points to the speaker/mic)

The file you have listened to was created by me typing in a message into this EMU and "transmitting it" to the rear speaker while I recorded the file on my computer's microphone.  In addition, I was able to play the message on my computer with the EMU near the computer's speaker and the EMU successfully decoded the message back to me! (Great fun).

More background: 
The fellow I acquired this radio from was an engineer on this project (design/support of the radio). He also has one with this rare (only 20 ever made) alphanumeric EMU. He was able to supply me with some of the technical details that I have subsequently relayed to the group. I suspect that after 30 years some of the details have gone fuzzy (like shift, etc). 

I sent this audio file to this engineer. He played them on his computer, and sure enough, sent me the decoded message. Of course, we could have used the radios to communicate this message, but we haven't tried that yet.

Present day:
While this EMU sure is very neat, it is of limited usefulness unless I am communicating with someone who also had one of these EMUs. And considering only 20 were ever made by TRW that would be a rare event!  So I began thinking, what if this could be decoded by any HF radio and a soundcard? Hence my quest.

I believe that this is indeed bell 103 modulation.  I am not 100% sure of the baud rate. 300 baud is the bell 103 standard. However, there is precedent on military digital message units using 266.6 baud, so that could very well be correct.  I also believe this to be ASCII date. 7 or 8 bit, I am not sure.  If it could be demodulated into a bit stream, it might be possible to determine that (and stop bits, etc).

I appreciate everyone's help with this. My quest continues!

73 Eugene W2HX

-----Original Message-----
From: epvgk at limpoc.com [mailto:epvgk at limpoc.com] 
Sent: Saturday, September 13, 2014 11:42 AM
To: Dave G4UGM
Cc: epvgk at limpoc.com; W2HX; greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Bell 103 demodulator?

On Sat, Sep 13, 2014 at 09:54:48AM +0100, Dave G4UGM wrote:
> Well those tones about sound right for Bell 103, as I see from
> 
> http://en.wikipedia.org/wiki/Bell_103_modem
> 
> "The answering station used a mark tone of 2,225 Hz and a space tone 
> of
> 2,025 Hz."
> 
> So is 266.6 baud the right number? Might be worth looking in something 
> like spectrum lab but I can't get a decent waterfall display and I am 
> not really used to the tool.
> 
> Dave.

yeah, and also, should the encoded data be expected to be ascii? 7 or 8 bits? 
parity? and even if so, does it contain something that would even look like text? 

> 
> -----Original Message-----
> From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf 
> Of epvgk at limpoc.com
> Sent: 13 September 2014 09:42
> To: W2HX
> Cc: greenkeys at mailman.qth.net
> Subject: Re: [GreenKeys] Bell 103 demodulator?
> 
> On Fri, Sep 12, 2014 at 08:36:00PM -0700, W2HX wrote:
> > Hello folks,
> > 
> > I have an audio file (wav) that I have been told represents data in 
> > the following audio format:  Bell 103 data, 266.6 baud, 850 shift,
> 1300/2100 tones Does anyone know what software can be used to decode 
> this? I am posting it here in case it helps figure this out.  Thank you all.
> > 
> > http://www.w2hx.com/x/PRC-319/
> > 
> > 
> > 73 Eugene W2HX
> 
> I fiddled around with it a little but no luck. The tones appear to be 
> 220 Hz
> 
> apart, not 850, if the file's WAV header is correct and it's sampled at
> 16khz.            
> The tones seem to be 2228 and 2008 Hz, with the higher one as mark.
> 
> Even so, I couldn't get minimodem or fldigi to decode it to anything 
> human
> 
> readable either as ascii or baudot with any of the permutations of 
> options I
> 
> could think of... :(            
> 
> 
> eric
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