Salah
Posts: 25
Joined: Mon Jul 09, 2012 12:47 pm

Use Pi Zero as an intercom handset

Wed Oct 31, 2018 12:03 am

Hi,

I'm working on connecting my old flat intercom to the raspberry so that I could receive a call on my smartphone when someone rings at the entrance door.

The intercom handset has 3 wires:
  • blue & red connected to the speaker
  • blue & white connected to the microphone
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To set the intercom handset speaker as audio input on the Pi, I soldered 2 wires from blue & red to a jack 3.5mm TRS connector (blue for ground, red for left channel).
Same thing for the handset microphone with blue and white (blue for ground, white for left channel).

Then I connected the 2 jacks to a USB audio sound card plugged into the Pi.
The call to my smartphone is made using SIP software running on the Pi.

When I pickup the call on my smartphone, there is a very loud noise on the communication which you can hear in both ends.
If I disconnect the audio output jack, the receiver can hear me well.
If I disconnect the audio input jack, I can hear the receiver well.

I have no idea where this noisy sound comes from and why does it happen only when both jacks are plugged to the Pi.
Is there something wrong with the way I wired everything ?

Thank you for your help
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Last edited by Salah on Sat Nov 03, 2018 4:13 pm, edited 1 time in total.

Andyroo
Posts: 2594
Joined: Sat Jun 16, 2018 12:49 am
Location: Lincs U.K.

Re: Use Pi Zero as an intercom handset

Thu Nov 01, 2018 12:39 am

You most likely are suffering from audio feedback where any noise picked up on the mic is sent to the speaker where it is picked up by the mic etc etc...

I would think the old intercom turned off the speaker when you pushed the ‘talk’ button.

You will have to check the software to see if it can emulate this or reduce feedback.
Need Pi spray - these things are breeding in my house...

Salah
Posts: 25
Joined: Mon Jul 09, 2012 12:47 pm

Re: Use Pi Zero as an intercom handset

Thu Nov 01, 2018 1:38 am

Andyroo wrote:
Thu Nov 01, 2018 12:39 am
You most likely are suffering from audio feedback where any noise picked up on the mic is sent to the speaker where it is picked up by the mic etc etc...

I would think the old intercom turned off the speaker when you pushed the ‘talk’ button.

You will have to check the software to see if it can emulate this or reduce feedback.
There are no push to talk button on the intercom, it's a continuous audio feed in both directions like a regular phone call.
Since I don't have this audio loop with the handset, I shouldn't have it either with the Pi wired to the handset I guess, right ?

Andyroo
Posts: 2594
Joined: Sat Jun 16, 2018 12:49 am
Location: Lincs U.K.

Re: Use Pi Zero as an intercom handset

Thu Nov 01, 2018 5:19 pm

You could try a set of headphones - not sure if the impedance would be correct but may prove if the sound is feedback.
Need Pi spray - these things are breeding in my house...

Salah
Posts: 25
Joined: Mon Jul 09, 2012 12:47 pm

Re: Use Pi Zero as an intercom handset

Thu Nov 01, 2018 7:59 pm

Andyroo wrote:
Thu Nov 01, 2018 5:19 pm
You could try a set of headphones - not sure if the impedance would be correct but may prove if the sound is feedback.
What do you mean ?
How do you want me to try with headphones ?

Andyroo
Posts: 2594
Joined: Sat Jun 16, 2018 12:49 am
Location: Lincs U.K.

Re: Use Pi Zero as an intercom handset

Thu Nov 01, 2018 8:51 pm

Replace the speaker with headphones as a test.

These will cut out any sound feedback.
Need Pi spray - these things are breeding in my house...

Salah
Posts: 25
Joined: Mon Jul 09, 2012 12:47 pm

Re: Use Pi Zero as an intercom handset

Sat Nov 03, 2018 4:10 pm

Andyroo wrote:
Thu Nov 01, 2018 8:51 pm
Replace the speaker with headphones as a test.

These will cut out any sound feedback.
The speaker is a handset ear speaker so it acts like a headphone already.
I added on my first post a basic scheme of the connections I've made to make things clearer.

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