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Re: DSP question (software modem)

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Paul Rubin wrote:
> I'm wondering how feasible it would be to implement a software modem
> (1200 bps would be plenty) on an 8-bit micro or MSP430 without a
> hardware multiplier, the type with enough memory to run a Forth text
> interpreter with very few external parts.  They tend to have a few
> analog i/o pins and be fairly fast (10 mhz or more), but otherwise not
> be very powerful.
>
> The idea is to be able to use an Android or similar mobile phone as a
> console for an embedded Forth, by connecting an audio patch cable
> between the device and the phone's earphone/mic socket (phones tend to
> not have serial ports).  Of course Bluetooth would be really nice, but
> embeddedable Bluetooth modules all seem big and expensive.
>
> Any thoughts?  I figure the phone side is pretty easy, since phone cpu's
> are so powerful.  On the device side, assuming FSK like an old
> half-duplex phone modem, we need to recognize two frequencies.  With 8
> khz sampling and 32 cycles per 16x16 multiplication, we need 256 cpu khz
> for every multiplication, so maybe we can do 20 or so multiplications
> per sample with enough cycles left for bit twiddling, the interpreter,
> and the user application.  Is that likely to be enough?  Is it
> appropriate to use IIR filters for this sort of thing, with 8 bit
> samples?  What is the right kind of filter to use?  I looked at some DSP
> textbooks a while back so I've seen some of the relevant math, but I
> don't have any real experience with this sort of thing.
>
> All thoughts are welcome, including about the wisdom (or lack of it) of
> the whole concept.
>
> Thanks.

Some things that crossed my mind reading your post:

1. You appear to have a potentially large signal with low 
noise environment (layout and choice of connecting cabling 
etc. )
2. Take a look at how some of the early consumer grade PCs 
with built-in cassette recorders for mass storage handled 
the problem (I'm thinking of the 8k Commodore PETs). They 
had similar control of the electrical environment and few 
processor cycles. Don't know if they had some dedicated 
hardware between the cassette and the CPU.
3. Instead of FSK, if SNR good enough, implement a 'crude 
Schmidt trigger' in software. I.E. a 'one'  would be a 
sequential transition from +16 -> +32 -> +64
      a 'zero' would be a sequential transition from -16 -> 
-32 -> -64
4. For DSP questions there is a fairly friendly group over 
at comp.dsp [they respond well you demonstrate you have put 
effort into your question].

HTH

{PS I know only enough Forth and DSP to be dangerous ;}

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post your solution to this problem quiet_lad <gavcomedy@gmail.com> - 2012-06-03 14:33 -0700
  Re: post your solution to this problem "Rod Pemberton" <do_not_have@notemailntt.cmm> - 2012-06-03 20:59 -0400
    Re: post your solution to this problem quiet_lad <gavcomedy@gmail.com> - 2012-06-03 19:06 -0700
      Re: post your solution to this problem "Elizabeth D. Rather" <erather@forth.com> - 2012-06-03 18:27 -1000
    Re: post your solution to this problem quiet_lad <gavcomedy@gmail.com> - 2012-06-03 19:49 -0700
      Re: post your solution to this problem "Elizabeth D. Rather" <erather@forth.com> - 2012-06-03 18:28 -1000
        DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-03 22:22 -0700
          Re: DSP question (software modem) Mark Wills <markrobertwills@yahoo.co.uk> - 2012-06-04 02:19 -0700
            Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-04 03:07 -0700
          Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-04 04:32 -0500
            Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-04 03:15 -0700
              Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-04 05:46 -0500
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-04 12:16 -0700
                Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-04 14:55 -0500
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-04 13:32 -0700
                Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-07 05:20 -0500
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-07 10:01 -0700
                Re: DSP question (software modem) quiet_lad <gavcomedy@gmail.com> - 2012-06-07 11:04 -0700
                Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-07 17:21 -0500
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-07 20:36 -0700
                Re: DSP question (software modem) "Rod Pemberton" <do_not_have@notemailntt.cmm> - 2012-06-08 02:36 -0400
                Re: post your solution, and don't over write my thread, or I'll do something to Paul Rubbin quiet_lad <gavcomedy@gmail.com> - 2012-06-07 23:55 -0700
                Re: post your solution, and don't over write my thread ... BruceMcF <agila61@netscape.net> - 2012-06-08 09:56 -0700
                Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-08 07:52 -0500
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-08 11:14 -0700
                Re: DSP question (software modem) "Rod Pemberton" <do_not_have@notemailntt.cmm> - 2012-06-08 16:29 -0400
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-08 19:59 -0700
                Re: DSP question (software modem) Richard Owlett <rowlett@pcnetinc.com> - 2012-06-08 22:37 -0500
                Re: DSP question (software modem) Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-06-09 04:36 -0500
                Re: DSP question (software modem) Paul Rubin <no.email@nospam.invalid> - 2012-06-09 09:41 -0700
                Re: post your solution and Paul rubin is a stinker for changing it, you bugger paul!! quiet_lad <gavcomedy@gmail.com> - 2012-06-06 15:52 -0700
                Re: post your solution and Paul rubin is a stinker for changing it, you bugger paul!! BruceMcF <agila61@netscape.net> - 2012-06-06 16:09 -0700
                Re: post your solution, please no one change subject, start new thread of your own instead quiet_lad <gavcomedy@gmail.com> - 2012-06-07 11:02 -0700
                Re: post your solution, please no one change subject, start new thread of your own instead "Elizabeth D. Rather" <erather@forth.com> - 2012-06-07 08:28 -1000
                Re: post your solution, please no one change subject, start new thread of your own instead quiet_lad <gavcomedy@gmail.com> - 2012-06-07 23:58 -0700
                Re: post your solution, please no one change subject, start new thread of your own instead Paul Rubin <no.email@nospam.invalid> - 2012-06-07 11:53 -0700
                Re: post your solution, please no one change subject, start new thread of your own instead quiet_lad <gavcomedy@gmail.com> - 2012-06-07 23:54 -0700
                Re: post your solution, please no one change subject, start new thread of your own instead quiet_lad <gavcomedy@gmail.com> - 2012-06-08 00:00 -0700
                Re: post your solution, please no one change subject, start new thread of your own instead BruceMcF <agila61@netscape.net> - 2012-06-08 10:08 -0700
                [OT] About cancellation of quiet_lad's messages, was Re: post your .... of your own instead Albert van der Horst <albert@spenarnc.xs4all.nl> - 2012-06-08 11:34 +0000
                Re: post your solution, please no one change subject, start new thread of your own instead BruceMcF <agila61@netscape.net> - 2012-06-08 10:03 -0700
                Re: post your solution and Paul rubin is a stinker for changing it, you bugger paul!! "Rod Pemberton" <do_not_have@notemailntt.cmm> - 2012-06-06 19:28 -0400
                This thread is a load of s**t Mark Wills <markrobertwills@yahoo.co.uk> - 2012-06-07 00:54 -0700
                Re: post your solution and Paul rubin is a stinker for changing it, you bugger paul!! quiet_lad <gavcomedy@gmail.com> - 2012-06-07 11:03 -0700
                Re: post your solution quiet_lad <gavcomedy@gmail.com> - 2012-06-07 11:10 -0700
          Re: DSP question (software modem) Richard Owlett <rowlett@pcnetinc.com> - 2012-06-04 07:08 -0500
  Re: post your solution to this problem John Passaniti <john.passaniti@gmail.com> - 2012-06-04 09:06 -0700
    Re: post your solution to this problem quiet_lad <gavcomedy@gmail.com> - 2012-06-06 15:51 -0700
      Re: post your solution to this problem "WJ" <w_a_x_man@yahoo.com> - 2012-06-08 16:12 +0000
      Re: post your solution to this problem "WJ" <w_a_x_man@yahoo.com> - 2012-06-09 16:15 +0000
  Re: post your solution to this problem BruceMcF <agila61@netscape.net> - 2012-06-08 10:15 -0700

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