Rob's Ramblings

Saturday, 1 September 2018

Modem Emulation - an RC2018/09 prologue

Most of you will know by now that I'm really into preserving the memory of Prestel and Viewdata systems generally.  I run which, while a bit long in the tooth, is going to get a massive update "soon" ...   But today I'm going to talk about hardware.

Some time back, I fired up my old viewdata BBS "Ringworld" - this operated on a collection of BBC Micros - one per connected user - and an Acorn A5000 acting as fileserver.  I connected these to the internet using a motly selection of modems, ATA telephony adapters, and serial terminal adapters.

The long shot was, for a user dialling in, the call was answered either by the exact same modem it always had been, connected to a SIP ATA - the digital data was transformed to analogue, before being turned back to digital by the modem.  This always seemed like a poor idea to me. What would be better is if some bit of software answered that digitised telephone call, looked at the whistles and warbles, and turned it directly into a sequence of ASCII bytes for delivery to a telnet port.

I had found an program called iaxmodem that allowed an asterisk based PBX to emulate a modem, but it was focused on faxing, and I just couldn't get it to work with the V23 dial-up I wanted.  But it was close.   I spent the next few years, off and on, searching for changes to that, or SIP based alternatives, with no luck.

In the meantime, John Newcombe decided to write his own viewdata host service, called Telstar, in Python, and that can be accessed via a raw-socket. (like telnet, but without the features!)  There's not a lot of software out there that can talk both Viewdata display protocols and connect to a socket, however.  Richard Russell wrote a example viewdata client that could do it, and you can connect from BeebEm if you load up a suitable comms package and set the RS423 IP parameters. 

There have also been a couple of projects to produce a "WiFi Modem" that, basically, looks like a hayes-compatible modem that you connect to via RS232, but it in turn connects to your WiFi, and onwards to a telnet port out on the internet.  This is great for things like BBC Micros, Commodore 64s, etc., where you can just swap out your period modem for this new device.   Not so good for dedicated terminals, or e.g. the ZX Spectrum VTX5000 where the modems are built in.

Then, out of the blue, an old friend, Darren Storer, posted on the BBC Micro facebook group (I think it was there..) that he'd set up a dial-up number for Telstar, and could people test it.  It took me a week or two to get there, but I pulled out a terminal, dialled the number ... and it didn't work.  Not at all.  I did, however find out the software he was using ...  asterisk-Softmodem.  This was exactly the sort of project I'd been looking for all those years.  But, it didn't work for him/

I pulled the code and had a look, and could see nothing wrong.  So, firing up an asterisk server, and installing it, I tried to debug.  The first issue was my terminal was not locking onto the carrier, so I added a t(-10) to increase the volume, and that sorted that!

Next problem, I wasn't getting much data on screen - many characters were just missing!   This was somewhat easy to diagnose, as I had an inkling after seeing how you configured asterisk to use softmodem - you specified the number of data bits, being between 5 and 8.  The example had it as 8. Now Prestel, and of course the terminals, all used 7 bits with even parity. What I was seeing was the terminal being sent 8-bit data, and of course interpreting that as most of the characters having an incorrect parity bit, and ignoring those!

Now, I can set a software terminal to 8bit data, but not the termnal - there  is very little you can configure as a user on these things.  Because the project had no support for parity it looked like a dead end, but that wasn't going to stop me - I'd waited years to find this, and wasn't going to give up now!

Delving into the code, it actually turned out to be a nice simple and straightforward bit of programming.  Adding parity support turned out to be fairly easy... I've published the modifications to my own github fork and submitted a pull request to send them back to the original author.

So now, I can dial into Telstar, CCL4, or anywhere I want to set up a number for!

If you want to try it, the number for Telstar is 0333 340 3311 (from outside UK, +44 333 340 3311). Calls cost the same as an 01 or 02 and are included in any inclusive minutes you may have. Call s are free for A&A customers.)

I can't guarantee that number will stay up, and it may not work from time to time if I'm tweaking things, but if it turns out useful to you, please let me know in the comments below!

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