Voice Switching IC

william chan

Hi,

Anybody knows the name and model of IC's commonly used to switch voices in keyphones or telecom switch boards?

Thanks.

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Bruce Bates

William -

Are you looking for cross-point switches such as these:
http://www.maxim-ic.com/solutions/telecom_switches/parts.mvp/scpk/1278/pl_pk/0

If not, could you give us a better description of what kind of switching product you're seeking?

Regards,

Bruce Bates

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"Genius is one percent inspiration and ninety-nine percent perspiration."

Thomas Alva Edison

william chan

Let's say you want to make a simple analog (fixed line telephone) keyphone with 3 incoming fixed lines and 8 analog extensions.
Normally you would need a chip that can handle all the 11 analog ports right?

You can do it with solid state relays, but you will need too many of them, because any 2 ports can talk to one another.

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Bruce Bates

William -

I'm not real sure what you're trying to design here, but if it's anything like a PBX or local switcher, there is a lot more to it than what you've mentioned so far. If that is indeed what you're looking to do, I'd hop on eBay and see what they offer in terms of small PBX units.

If there are any used Merlin (AT&T) units, you can probably buy one for a song, and it would have all the capacity and features you'd probably ever want, and then some.

Regards,

Bruce Bates

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"Genius is one percent inspiration and ninety-nine percent perspiration."

Thomas Alva Edison

william chan

Yeah, but if everybody always takes the easy way out, then nobody would be inventing anything right?
Sometimes, by re-inventing the wheel, you can come up with a better wheel.

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Franklin

There is no 'chip' that will do what you want by itself you will need to design circuits that can sense when a phone is picked up 'off hook' and which lines are available to use. also you need a way to tell the circuit that a phone is trying to answer a ringing line and a way to ring a phone when a call comes in. This is just a start at what you need to design but if you decide it is worth the trouble you can ask spacific questions and we will try to answer.

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- Stephen

william chan

I believe the PBX boxes must have specialized chips to do this switching.

Consider this, for a 6 line, 16 extension PBX, you would have a total of 22 ports.
For any port to be able to connect to any other port to start a voice connection, you would 22 x 22 solid state relays,
which is about 484 two-pole relays !

It is not possible with discreet circuits, you definitely need a telecom switching IC which has the built in 484 relays.

Could it be that these custom ICs are not sold to the public?

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Post Edited (william chan) : 2/22/2008 4:03:41 AM GMT

LoopyByteloose

Yes, it is very likely that the custom ICs are not sold to the public. When I was living in the States, I picked up some solidstate PBX junk at a HAM swap meet/flea market to salvage parts. It was impossible to get any documents from the manufacturer.

Before solid-state, this was all done with relays and all over San Francisco there are big windowless buildings that were centers for relay banks. Then, they removed all the relay equipment and sold it off very cheap to Mainland China and other countries developing a telcom infrastructure. Of course that was before cellular phones made such equipment nearly worthless.

I had friends that worked in the phone company and their whole job was to stay in one of these buildings and replace relays as they failed. Now, 95% of the building is empty and everything is fed by fiber optic. Alcatel-Lucent is a big provider, but they won't tell you the time of day unless you are associated with a major telcom.

I admire your desire to find out, but is has become a specialty field and I suspect USAs National Security Admininstration wants to keep it under their control as part of national security.· Nowadays, anyone that services the equipment is just given replacement boards and a procedure to decide when and how to swap out boards.· We are less able to observe and learn the fundamentals. You might say it is 'black box security'.

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Post Edited (Kramer) : 2/22/2008 6:34:44 AM GMT

william chan

So, instead of relay switching IC's, perhaps we could use a Propeller to sample a voice from one port and
on-the-fly, play it back to another port, to achieve a software connection.

Using such a scheme, do you think the voice quality will be PBX quality?

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Mike Green

You can try it yourself. There's a demo program for the Demo Board that takes sound from the microphone, digitizes it, and plays it back through the headphone amplifier.

It's in the object exchange: obex.parallax.com/objects/57/

Since one cog could handle one connection, you could get perhaps 6 connections per Propeller with one cog running the main "switching" controller and another cog doing FullDuplexSerial for the inter-Propeller communications. You'd need two I/O pins per input and one I/O pin per output. Given that you need bidirectional connections, you'd have 3 conversations per Propeller. You might have to split things up because of limited I/O pins since you need 6 I/O pins per connection. With 28 I/O pins completely available, that's 4 bidirectional "ports" plus a few left over for interconnectivity.

A high speed bus connecting two Propellers has already been demonstrated on this forum. You could probably transfer the digitized sound between Propellers without too much trouble.

Post Edited (Mike Green) : 2/22/2008 7:32:01 AM GMT

Harrison.

You may be interested in the IP04 IP-PBX project: www.rowetel.com/ucasterisk/ip04.html. It utilizes a blackfin dsp processor to sample and playback the analog audio streams. All of the interconnects are done in software, using the open source Asterisk VOIP PBX software.

Regardless of the route you take, it will end up being relatively expensive. I tried making a 2 port pbx once (when I was 13) and had all the dtmf decoding, interconnect switching relays, off hook sensing, and ring generator stuff done. I never finished the project since it turned out to be relatively useless to have only 2 extensions.

I found it easier to just hook up the dtmf decoder chip to my phone line, and have a PIC look for 'extension numbers' (#1, #2...etc). When it detected a predefined pattern, it would turn on a buzzer on the respective phone to tell the person to pick up. We used this at my house for about 4 years until we got cordless phones that supported multiple extensions.

Harrison

william chan

4 analog "ports" per Propeller seems a bit insufficient.
I think I'll have to wait for Propeller II.....

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Bruce Bates

William -

Indeed there are specialized relays (switches) used in the telecom industry. Below is a link to a typical cross-point switch which can be used as 4 x DPDT or 8 x SPDT, as you see fit. The approximate cost is $4.25 US each, per thousand pricing. Here is the link to the Maxim 4670 cross-point switch: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4998.

These cross-point switches were designed for use in the telecom industry. You would have to ask Chip if the Propeller could be used in such an application or not.

Consider however that there are a limited number of I/O ports on each Propeller chip. Once you need more ports (circuits), it's another Propeller chip, another program, until you get the number of ports you need and then the job of somehow chaining them all together in some coherent manner. Additionally, any microcontroller is only going to provide a HIGH or LOW signal on the I/O port. In other words, it can't operate like a switch connecting two ports to each other. Thus, any microcontroller is just going to act like a relay driver (of sorts) and you're back to something like cross-point relays any way.

Unless this is a project that is going to evolve into a salable product, I'd still recommend the purchase of a solid-state PBX. Here is just one of many that is reasonably priced, and is presently on eBay: http://tinyurl.com/24tegq

Regards,

Bruce Bates

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"Genius is one percent inspiration and ninety-nine percent perspiration."

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