Bell 202 Modem (Demod Section), Implemented in Analog?

Well, the original 202 wasn't DSP! It had a lot of inductors as I recall, as most of the old Bell stuff did.

This is simple 1200 baud FSK, 1200/2200 Hz carrier freqs. I've done it two ways:

1. Fire a dual one-shot, one section on each edge, widths maybe 600 usec. Resistively sum the Q outputs into a 2 or 3-pole S-K lowpass filter, then slice. Classic tach.
2. Exar makes an ancient PLL chip that will demod this. I have a schematic at work somewhere, I think, if you're interested.

John

Ah, good old FSK.

I've built many a ham-band modem out of a pair of tuned toroids and a comparator, either an IC or a bunch of ad-hoc cascaded transistors.

Or in later days, the venerable National 565 PLL, then later the CMOS 4046.

All the above can do an EXCELLENT job of pulling data out of weak very noisy signals bounced to us on the ionosphere from PAP: te Polish news service, or whatever. I recall many a happy evening watching the Reuters "Motorcycle with side-car" race results coming in on the old "1200-pieces of steel wiggling in formation", the Teletype Model 15.

Look at the ooold spec sheet for the National LM565, I think it had several modem designs in there. I've used that design with excellent first-time results.

Only problem is it needs a bit of offset tweaking to keep the comparator biased slightly towards the "mark" channel.

Heh, I see those for a dime a dozen at Value Village.

*starts singing "Memories"*

-- Gregg

*It's probably useful, even if it can't be SPICE'd*

Going to be listening in on Caller ID, Jim?

In the olden days there was an Exar chip, the XR-2211, which was specifically an FSK Demodulator - everything on one

14-pin chip, including a carrier detect output. I doubt the chip is still available, but you might be able to run down the data sheet to see how they did it.

In article , snipped-for-privacy@invalid.invalid mentioned...

Hey, I'll give you the whole modem! We still use one for the link between the CO and our PBX, and I keep a couple spares, but I've never had one fail. I still have the user manuals for 'em, probably from the late '70s or early '80s; naturally they're all analog. All I know is that when I pull the cover off, there are a ton of 1% resistors on the board. Lots of filters, I'd say.

But rather than burden of reverse engineering one, I'd use an Exar XR2206 or XR2211 chip, I forget which is the receiver.

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Caller-ID... Yep! With the ring-killer circuit we discussed awhile back I'm going to not even have a ring inside the house if I don't recognize the caller-ID. General unrecognized CID will roll to voice-mail; CID recognized as telemarketer will be picked-up then hung-up all automatically (maybe even play a fax tone-set before hang-up ;-)

I'm reluctant to use any ancient parts.

I'm trying to remember a scheme I used years ago that emulated the "S-curve" of an FM discriminator using an All-Pass active filter.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Jim, I can send you .pdf files of the '2211 if you like. Includes block diagrams, formulas and an IC schematic.

Thanks, - Win

whill_at_picovolt-dot-com

Thanks, Win, I'd appreciate that.

Is the '2211 readily available?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Thanks to Win and Gregg for the data XR2211 sheets!

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

It seems to be.

Circuit Specialists and Jameco both list them.

CS charges more but you can drive over to Mesa, AZ and pick up one for $4.20 or get one from Jameco for $1.79 if you're not in a hurry.

See:

and

I have thought about making a CallerId/no ring box myself but it is too far down the list.

Will you post your design?

Regards,

Dave

[snip]

Probably. I'll only keep it to myself it there's a ruling that telemarketer calls are "free speech"... then I plan to profit. Otherwise we can use it to blank politicians and charities ;-)

I've already figured out the ring-blanking yet pass the CID onto the house phones (in case you actually pick-up).

A lot of the project will be software, done by my oldest son, Aaron.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I built such a system several years back, but I used the Motorola MC145447 Caller ID receiver chip, which not only does the FSK demodulation and carrier detect, but also removes the lead-in "U"s before the packet starts. And then I used an old Commodore +4 computer for the brains of the circuit. I probably have the only always-on Commodore +4 in the entire world - up continually except for power outages and upgrades since 1991. :-)

I don't think Motorola makes that chip anymore.

I don't recall about the ring-killer discussion, but remember that CID only comes in AFTER the first ring. In my circuit I detect the beginning of the first ring with the Moto chip, and then immediately open a relay that feeds all of the phones with ringers turned on. So all I hear is a little chirp. Then, if it's not a telescum, I close the relay before the second ring starts.

And then I have it announce the caller audibly throughout the house so I don't have to go look at the CID box.

But you know, for the demodulation part I would think you would just use your voice modem. I think most of them have Caller ID built in, and that would keep you from having to build a demodulator at all.

Jim

You might take a look here.... the AMS3144 is a rather simple solution to callerid decoding.... they have been available here:

Ralph Landry PO Box 1478 Lake Junaluska, NC 28745

Aptek AMS3144 Caller ID Hybrid $7.00 each

My 10 year old smart telephone which does the things your planning. It uses a Motorola chip 145xxxxx (obsolete) and a PIC 16C54 feeding an 8051 processor... I can drop the Aptek callerid application note via USPS if you're interested.

Cheers

Ted > >

down

Try the chips from Holtek HT9030 or similar. Uses a single crystal, does all the right bits.

regards, Darcy Roberts (change the what to watt to reply)

Jim Thomps> >

Whereas On Mon, 29 Sep 2003 20:31:25 -0400, Darcy Roberts scribbled: , I thus relpy:

Such as

--
Gary J. Tait .  Email is at yahoo.com ; ID:classicsat

Hi Jim,

You said you want to avoid DSP but if "a lot of the project will be software" then you should re-consider; especially if you plan to market it later. Maybe both the *old* and *young* bull could learn something?

I won't waste your time with suggestions unless you're interested.

Best Regards, Ray

"Software" for the PC, maintaining and comparing incoming CID with a list... not exactly a DSP task.

The circuitry will simply convert CID tones to serial (or USB) format, the PC will do the rest, issuing hang-up/answer commands as required.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

On Sun, 28 Sep 2003 14:27:41 -0700 in sci.electronics.cad, Jim Thompson was alleged to have written:

Please make sure you wait at least two full seconds before hanging up, to be sure they get charged for the call.

David, thanks for the tip, I'll build that feature in.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.