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SX Keyphone Project — Parallax Forums

SX Keyphone Project

william chanwilliam chan Posts: 1,326
edited 2005-07-02 03:48 in General Discussion
Hi Genuises,

I am trying to get a SX52 to act as a "soft voice switch" for a small keyphone system project ( 2 lines 6 extensions ).
This SX should run at 75Mhz and run ADCs to sample voice on one ext and send it to play at the other ext by PWM and vice versa.
I plan to allow 3 simultaneous pairs of conversations at any 1 time. ( 3 virtual buses )

That means I need 8 pairs of ADC's and PWMs for all the 8 ports of the keyphone, though not all wiil be running simultaneously.
I am planning to use the usual 2 pin calibrating method for the ADC, no ADC chips.
I am also planning to use op amp low pass filters to smooth out the voice sampling and playback.

My questions are,
1. Is the SX fast enough for this job?
2. Will the sound quality be acceptable?
3. Will there be problems with echo if I don't use hybrid circuits?
4. Is this project overly ambitious?

Thanks.

Comments

  • pjvpjv Posts: 1,903
    edited 2005-07-01 17:36
    Hi William;

    I·don't consider myself to be a genius, but I·can probably answer part of your questions, and maybe give some insight into some other issues.

    Years ago (pre SX) I built a commercial hardware rendition of what can now be more easily done with software and it worked amazingly well.

    1. Yes the SX is fast enough; you may want to design it in a "scalable" form; one SX per line/channel

    2. If the sample rate is high enough (upwards of 8 K samples/sec) the quality is good. To increase sampling speed while reducing the amount of data to be transmitted, we used the "Continuously Variable Differential" encoding method. This was done in a (Philips?) chip, but you should be able to come up with some really good software agorithms. (You lucky guy......I have been wanting to try this for a long time). You also will want a logarithmic amplitude encoding scheme, A-law or U-law.

    3. Don't know, as best as I can recall we used no hybrids yet had good results.

    4. It is very ambitious if you are looking for commercial quality results. If it is just for yourself and non-commercial, then you can eliminate the regulated default "POTS"
    requirement, and put up with whatever "features" you want.

    Great project, have fun.

    Peter (pjv)

    Post Edited (pjv) : 7/1/2005 5:39:41 PM GMT
  • william chanwilliam chan Posts: 1,326
    edited 2005-07-02 03:48
    Dear pjv,

    Thanks for your reply.

    1. When you say the SX is fast enough, did you mean that 1 SX52 is fast enough is run 3 pairs of conversations simultaneously?
    i.e. 6 ADCs and 6 PWMs at the same time?
    What would be the sampling speed when handling 3 buses at the same time? 8K samples / second?

    You mentioned, "one SX per line/channel", which I don't quite understand.
    Are you saying I may want to use 3 SX52s for the 3 pairs of conversations?

    If I do this, I would need a lot of relays to switch the Exts to the correct buses.
    The reason I want to design it with only 1 SX52 is to allow software switching, no relays for switching.
    Only 2 SSRs to offhook the 2 incoming trunk lines.

    One more question, should I mix the 2 ADC returned values together in software before playing the summed value out for both PWMs to reduce echo?

    4. I have some experience in designing Dialers and CallShop systems.
    So I know a bit on DTMF decoding, generation PSTN handling and opamp filters, but this would be the most ambitious so far. : )

    If you like, I could share 1 board with you when it is made. I usually make 5 boards for prototypes.
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