[Coco] Coco Modem phone line connection

Dave Philipsen dave at davebiz.com
Mon Dec 2 15:38:41 EST 2019

You can look here at how TDM fits in with the telephone system:

It's very likely that if you ever used a 300 baud modem back in the day, 
the phone call that you made was multiplexed with other phone calls 
(conversations) and sent over the same pair of wires and then 
demultiplexed at some point.

The original question from Rietveld had to do with whether two-way modem 
communication is achieved over the same pair of wires on the telephone 
line (most homes are wired with two pairs). As Gene Heskett pointed out, 
the answer is "yes" because a telephone only requires two wires to work. 
The other pair of wires is generally only present for an additional 
phone line. My statement was only reinforcing the idea that a phone 
modem also only requires two wires and when two modems are communicating 
with each other, either modem can modulate the signal on the phone line 
simultaneously. The principle is exactly the same as a standard 
telephone with a carbon microphone and dynamic earpiece.

This method is different, for example, than a speaker or a microphone in 
a public address system. Although they, too, only require two wires to 
carry the analog audio signals, they are considered simplex operations 
and not duplex as the system is only designed for the signal to travel 
in one direction. You cannot listen and speak on the same pair of wires.

Joel replied that he thought the ability to do full duplex on a 
telephone line was related to the fact that the telephone system (PSTN - 
Public Switched Telephone Network) uses time multiplexing. And the fact 
is that the PSTN *does* use time multiplexing but the ability to do full 
duplex analog voice conversations on a single pair of wires is not due 
to this fact. It is due to the inherent design of the telephone itself. 
A telephone (or a modem for that matter) will work just as well if not 
better (in full duplex) *without* time multiplexing. All you have to do 
to prove that is to hook up two telephones over a two wire link with a 
few nine-volt batteries and a resistor and you will have a two-way full 
duplex link over a single pair of wires, no multiplexing required.

Alex replied that phones don't use TDM and that is technically correct. 
The phones themselves do not use it but the PSTN (phone system) that 
they are connected to does. A classic 300 baud modem achieves full 
duplex in much the same way as a telephone handset where two people can 
both be talking at the same time and both can hear each other 


On 12/1/2019 11:35 AM, Alex Evans wrote:
> Phones don't use time division multiplexing, and neither do classic
> analog modems. If we look at the simple case of a 300 baud modem, it
> manages full duplex because it uses four different tones, two for the
> answer side and two for the originate side with one tone one each side
> for 0 and a different one for each side for a 1. Originate uses 1070
> and 1270 Hz while answer uses 2025 and 2225 Hz. You could call it
> frequency division multiplexing.
> On Sun, Dec 1, 2019 at 9:42 AM Dave Philipsen <dave at davebiz.com> wrote:
>> Actually you can talk full duplex on two telephones by simply connecting
>> them both to the same relatively high impedance power supply. I think
>> time division multiplexing is what allows multiple signals
>> (conversations) to share the same path or pair of wires.
>> Dave
>> On 11/30/2019 8:43 PM, Joel Rees wrote:
>>> Time multiplexing, as I understand it.
>>> 2019年11月20日(水) 13:39 Dave Philipsen <dave at davebiz.com>:
>>>> Yes, in much the same way in which one line allows you to both speak and
>>>> listen at the same time (full duplex) using a standard voice telephone.
>>>> Dave
>>>>> On Nov 19, 2019, at 10:43 AM, rietveld rietveld <rietveldh at hotmail.com>
>>>> wrote:
>>>>> Does one line do both receive and send of the data?
>>>>> Sent from my BlackBerry
>>>>>    Original Message
>>>>> From: Gene Heskett
>>>>> Sent: Tuesday, November 19, 2019 11:25 AM
>>>>> To: coco at maltedmedia.com
>>>>> Reply To: CoCoList for Color Computer Enthusiasts
>>>>> Subject: Re: [Coco] Coco Modem phone line connection
>>>>>> On Tuesday 19 November 2019 11:17:10 rietveld rietveld wrote:
>>>>>> It looks like the coco only uses the middle two wires(red and green)
>>>>>> on a four line phone cord.   Is this correct?
>>>>> Probably, since thats all it takes to run a telephone.
>>>>>> Sent from my BlackBerry
>>>>> Cheers, Gene Heskett
>>>>> --
>>>>> "There are four boxes to be used in defense of liberty:
>>>>> soap, ballot, jury, and ammo. Please use in that order."
>>>>> -Ed Howdershelt (Author)
>>>>> If we desire respect for the law, we must first make the law respectable.
>>>>> - Louis D. Brandeis
>>>>> Genes Web page <http://geneslinuxbox.net:6309/gene>
>>>>> --
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