Wireless intercom

[wood_workur]

I am looking to build a wireless intercom with 4 units.

[audioguru]

Hi Wood,
Welcome to our forum. ;D
You have a few things to think about to make a wireless intercom for 4 units:
1) How to dial one of them and have it receive but also have the others blocked. Do you transmit a different radio frequency for each one? Or do you 1st transmit a tone that "opens" the speaker of the one you are dialing?
2) What will prevent "distant acoustical feedback howling"? You talk into your microphone and your voice is transmitted, received by the other station or all stations and is reproduced by its speaker to be heard by the other person. But its microphone will also hear its speaker and it will transmit your voice back to you. It is reproduced in your speaker, your microphone hears it and transmits it again louder. Around and around goes the sound, making a loud howling sound.
Intercoms use a manual press-to-talk switch or voice-switching to eliminate this problem.
3) What about busy signals? You want to talk to intercom #2 but it is busy talking to intercom #3. Do you want to barge in?

There are a few more things to consider but it is too late here for me to think about them.
The complexity of your project is why you didn't find it on the internet.

[wood_workur]

What if the intercoms all talk to each other, and one unit has a headphone jack and an audio input that will broadcast to all of the units and play music from that unit?

[audioguru]

Then each unit will need 3 radio receivers , wouldn't they? Is that hard?
The one that transmits music to the other 3 intercoms would need 3 transmitters, wouldn't it?

[wood_workur]

if they all operate on the same frequency,

[audioguru]

If they all operate on the same frequency and they are all trying to talk to each other then each will hear a lot of interference between them. Radios can't receive more than 1 transmitter at a time without interference.
If the FM receiver is excellent then its good capture ratio will capture (lock onto) the strongest signal from one intercom unit and reject signals from the others.

You don't say what range (distance) you need.
You don't say if there are obstructions in between.
You don't say what antenna length is acceptable.
You don't say which country you are in to determine if any frequencies are legal.
Should I guess?

I worked with audio equipment. We had transmitters and receivers for performers on stage. Each performer used their own UHF frequency to avoid interference and the circuitry is so complicated that neither you nor me could build it. Where would you get the special parts?

[wood_workur]

What if was a device that is inline with telephones, and allows both intercom and telephone use, and can page a telephone by pushing a button, and could page during a call.  I found schematics for this, but it requires that both of the phones be plugged into it.  I couldn't use that because I would have to rewire the telephone system, because the wire is spliced in the walls.  Could I modify it so that it is plugged in at the base of the phone, or would that not work?

[Shahriar]

Dear wood_workur

As your writing implies, you want to sell these devices to customers. On thing that you should remeber is that simple transmitters Frequency will change by Tempreture, and even after a month or a year. So if you want to get rid of customers calls, you should use a PLL based design.
If you know programing, maybe you can use a Data transmitter to send address to enable or disable Devices, As I know, it is really difficult to use that Data Transmitter to send a digital Voice. (because of Low Baude rate of that transmitters)
But if you do this, adressing will be very easy and you can increase the devices much more than 4.

At last, I should say that RF designing need Exprience that I am sure you have

Hope success
Shahriar