# audio over long distance for intercom application

#### bug13

Joined Feb 13, 2012
1,863
Hi team

I am researching for what are the possible options for carrying some audio signal over long distance (say 100m) for intercom application (half/full duplex) , what options do I have?
Here are what I came up with myself:
• good old differential paris
• digitized the audio and send it over RS485??
• what else? (Is that an IC that I can buy to do just that?)
Thanks team.

#### MrChips

Joined Oct 2, 2009
21,691
100m is not extraordinary.
I think you should get acceptable results just driving the loudspeaker down speaker cables.

One alternative is to go to a 70V system. Google this.

#### Audioguru

Joined Dec 20, 2007
11,249
Connect two speakerphone circuits together. Cable length can be one mile over a twisted pair.

Joined Mar 10, 2018
4,057
Last edited:

#### AlbertHall

Joined Jun 4, 2014
10,258
Landline phones don't have a problem sending audio down a twisted pair over much longer distances than that.

#### Danko

Joined Nov 22, 2017
972

#### Externet

Joined Nov 29, 2005
1,531
Connect 2 telephones 100m+ apart from each other with a 9 to 12V battery inserted in series to one of the twisted pair wires.

Code:
    -------------------------------------------------------------------------------
Phone1                                                                            Phone2
---------------------------------(+)12V(-)-------------------------------------
This will transport the audio full duplex, will not provide ringer.

Last edited:

#### dl324

Joined Mar 30, 2015
11,317
I am researching for what are the possible options for carrying some audio signal over long distance (say 100m) for intercom application (half/full duplex)
I have an intercom between my house and the barn that's 150 feet away. It uses the same 4 conductor wire used for the units in the house; 4 parallel conductors with no twisting or shielding.

#### AnalogKid

Joined Aug 1, 2013
8,542
The phone company keeps hum out of long land lines with two techniques, balance and frequency response.

https://en.wikipedia.org/wiki/Balanced_audio

Long lines can act as a very long distributed capacitor and/or a very long 1-turn transformer. Either way, energy from the massive amount of 60 Hz electromagnetic radiation around us gets into the wires. The wavelength is very long, so the twist pitch (number of inches for a single 360 degree twist) can be many inches while still achieving high cancellation. An advantage for the phone company is that the phone set at the end of the line is not connected to earth ground in any way. It is completely floating, so it does not have to maintain tightly matched terminating impedances for each of the two phone line wires.

No cancellation system is perfect, and human hearing is very sensitive, so th way the remaining hum is reduced is to attenuate it- a lot. Neither the internal hybrid transformer nor the earpiece can (handle / process / deliver) 60 Hz energy.

To work this into your system, some details are needed. Is the remote station powered out there, does it receive power over the lines, or is it simply a speaker? Can it control the direction of conversation, or is all control at the local station? Does it have a "Call" button?

ak

#### Audioguru

Joined Dec 20, 2007
11,249
An audio switching circuit is needed in a loud speaking speakerphone or intercom to avoid distant acoustical feedback howling. A press-to-talk switch or a voice activated switching circuit disconnects your speaker. Then it is half-duplex, not full duplex.

Here is a full duplex intercom circuit that cancels acoustical feedback howling:

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#### bug13

Joined Feb 13, 2012
1,863
Connect 2 telephones 100m+ apart from each other with a 9 to 12V battery inserted in series to one of the twisted pair wires.

Code:
    -------------------------------------------------------------------------------
Phone1                                                                            Phone2
---------------------------------(+)12V(-)-------------------------------------
This will transport the audio full duplex, will not provide ringer.
This one is interesting. Make me want to find out how a telephone work

#### bug13

Joined Feb 13, 2012
1,863
The phone company keeps hum out of long land lines with two techniques, balance and frequency response.

https://en.wikipedia.org/wiki/Balanced_audio

Long lines can act as a very long distributed capacitor and/or a very long 1-turn transformer. Either way, energy from the massive amount of 60 Hz electromagnetic radiation around us gets into the wires. The wavelength is very long, so the twist pitch (number of inches for a single 360 degree twist) can be many inches while still achieving high cancellation. An advantage for the phone company is that the phone set at the end of the line is not connected to earth ground in any way. It is completely floating, so it does not have to maintain tightly matched terminating impedances for each of the two phone line wires.

No cancellation system is perfect, and human hearing is very sensitive, so th way the remaining hum is reduced is to attenuate it- a lot. Neither the internal hybrid transformer nor the earpiece can (handle / process / deliver) 60 Hz energy.

To work this into your system, some details are needed. Is the remote station powered out there, does it receive power over the lines, or is it simply a speaker? Can it control the direction of conversation, or is all control at the local station? Does it have a "Call" button?

ak
Hi @AnalogKid, I am just looking at all possible options at the moment.

But to answer your questions about my application. It's most likely will use an cat6 cable, 2-power, 2-rs485, remaing 4 will be use with audio. We haven't decide how to do the audio part yet (we don't have a lot of experience with audio). Hence my research for all possible options.

So to answer your question, it will receive power over the line. It will be two way intercom. the direction will be control at local station. I won't have a (remote) call button, but the enable and disable of the intercom is controlled from the MCU on the remote end.

#### AnalogKid

Joined Aug 1, 2013
8,542
Is there an advantage to having the electronics at both ends of the intercom be identical? Two available pairs means you could have a mic preamp and speaker power amp at both ends, connected and powered all the time. The only control function would be to unmute a mic.

ak

Joined Feb 20, 2016
3,598
Where I worked some years ago, Radio Australia Transmitters, there was an intercom system that ran for way longer that 100M.
One central amplifier was used and a couple of twisted pairs were run. Shielded I think, as there were 100KW AM transmitters there too. Each station has a speaker with a 600ohm transformer. A DPDT changeover switch connected the transformer 600ohm winding to the amp input when pushed. Otherwise, it was connected to the output.
This has worked for years, with many stations across 2 buildings and even a couple of outside places.

#### bug13

Joined Feb 13, 2012
1,863
Is there an advantage to having the electronics at both ends of the intercom be identical? Two available pairs means you could have a mic preamp and speaker power amp at both ends, connected and powered all the time. The only control function would be to unmute a mic.

ak

Haven’t got to this part yet. But I think it’s a good idea to have identical electronis. It makes BOM management easier.

#### AnalogKid

Joined Aug 1, 2013
8,542
Is this for a commercial product or some quantity production?

#### bug13

Joined Feb 13, 2012
1,863
Is this for a commercial product or some quantity production?
It will be a commercial product if it worked out and the boss want it to be designed in

#### AnalogKid

Joined Aug 1, 2013
8,542
There are tons of audio and intercom circuits available, and many small modules from China. Adafruit has at least two microphone preamp modules, one with compression. And there is the LM386 small audio power amplifier. How loud does the audio need to be?

ak