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Old 16th Aug 2018, 1:00 am   #1
KesterLester
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Default Am I a fool to use a No13 transmitter and 2P receiver in series without a DC bias?

I have created a monster: a mobile-phone-ified version of a 332. I expect many others have done the same, but this hasn’t stopped me having fun seeing if I can do it.

At the moment I have a working system created by putting:

* the existing dial unit,
* the existing bells and solenoids,
* the existing receiver, and
* the existing hook switch and “recall” components,

together with:

* a SIM800L unit that can talk to the mobile GSM network, etc.,
* a Micropython PyBoard to talk to the dial and SIM800L,
* a motor controller to manage the HV bell switching,
* a 3.7V LiPoly battery and a few voltage converters to power everything,
* and a bunch of less important other new parts

Why my system is workings to the extent that I can call people on it and talk to them, it has two current issues:

1. it leaves a lot to be desired in terms of call quality, and

2. my new circuitry it is currently all still on a breadboard and so not yet re-organised into a small enough package to fit inside the old 332 bodywork — though it will get there eventually.

By reason for posting here is mostly in relation to “issue 1” (call quality). In particular, I have adopted a strange way of using the three wire handset that I am concerned could give me problems later - which is where perhaps you experts can help me.

Here’s the problem: my SIM800L (that’s my chip that’s talking to the GSM mobile phone network) is expecting me to interface to:

(a) a two-terminal electret microphone with an 20k Ohm input impedance, and

(b) a two-terminal (differential input) 8-30 ohm speaker.

The SIM800L calls these pads SPK+ SPK- MIC+ and MIC- respectively. When in action, MIC+ gets a +1.9V bias, and MIC- hangs within a few mV of ground.

This gives me a few problems interfacing to the three-terminal handset — since I have a self imposed rule that I don’t want to modify the handset if this can be avoided. E.g. I don’t want to convert it to a four-terminal handset, or to replace the carbon granule microphone with an electret. Given this, the red wire in the handset cord prevents me simultaneously connecting BOTH receiver and transmitter to my SIM800L chip. Specifically:

* if I couple just the GPO transmitter to MIC+ MIC- then I can transmit my voice,

* if I couple just the GPE receiver to SPK+ SPK- then I can hear a remote caller,

* but if I connect both at once, the common red-wire connection in the handset kills both abilities - no audio is sent or received.

However, despite the above, I found a curious way of succeeding in getting full duplex transmission (i.e. send and receive) that I don’t really understand. I attach a picture showing my solution. Hopefully the image (if it’s visible) shows that what I do is leave the handset red wire unconnected, and use just the green and white wires — effectively leaving the receiver and transmitter in series — (with green to SPK- and white to MIC-).

HOWEVER .. and this is the odd part (at least to me) … with this configuration neither SPK- not MIC- has a DC bias. Both hang around at ground.

And yet it works!

Weird.

[ I have heard that mic and speakers in series can make crude intercoms if there is a bias, but not unbiased. ]

My guess is that my current system is so noisy and so poorly shielded, and there is (a) enough stray capacitance and track to track coupling on my SIM800L inputs, and (b) a sufficiently sensitive differential amplifier on the SIM800L, that (i) the microphone is being AC biased by “noise” generating a signal that is somehow being picked up on MIC+ or MIC-, combined with some other effects, and (b) that even though SPK+ and SPK- are designed to drive 8-30ohm, the receiver is sensitive enough to be able to cope even though the series mic is adding another 1K, despite the absence of power from SPK+?

But this is just a guess.

My concerns are, however:

1. that I might miniaturise all this to fit it inside the 332 body, only to then find that it doesn’t work as the current success may be down to some lucky noise or geometry that is not reproduced when packed into a small better shielded space, and

2. people I call with this two-wire unbiased handset arrangement hear their own voice softly and with a little delay. I’d like to get rid of that delay, but can’t really do so until I understand better why this system even works, and

3. I don’t want to shorten the lifetime of one or more components by mistreating them. While this current system is not DC biassing the receiver (at least not at more than a few mV) perhaps even that’s dangerous in the long run, or perhaps I’m going to kill something else some other way.


Do any of you have any ideas as to how what I’ve build manages to work?

I also attach a pdf of the only ~5 pages of the SIM800L that tell me anything about its audio interface, and a photo of my un-miniaturised system

Best wishes,

Christopher
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File Type: pdf SIM800HL_Hardware_Design_V2.01.pdf.pdf (411.9 KB, 45 views)
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Old 16th Aug 2018, 2:05 pm   #2
Graham G3ZVT
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Default Re: Am I a fool to use a No13 transmitter and 2P receiver in series without a DC bias

I love it.

Remember to send us a picture of you in your chauffeur's hat and m'lady in the back using the phone.
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