Re: DSP question (software modem)

From	Andrew Haley <andrew29@littlepinkcloud.invalid>
Subject	Re: DSP question (software modem)
Newsgroups	comp.lang.forth
References	(2 earlier) <ca499d7a-c196-438f-aa30-069f0b049f2f@ra8g2000pbc.googlegroups.com> <StSdnc7tfoLjpFHSnZ2dnUVZ_okAAAAA@supernews.com> <7x4nqrodlk.fsf_-_@ruckus.brouhaha.com> <GYidnThRG4kkHVHSnZ2dnUVZ_r6dnZ2d@supernews.com> <7xehpvqt6n.fsf@ruckus.brouhaha.com>
Message-ID	<t--dnQcLHf2CD1HSnZ2dnUVZ_sqdnZ2d@supernews.com> (permalink)
Date	2012-06-04 05:46 -0500

Show all headers | View raw

Paul Rubin <no.email@nospam.invalid> wrote:
> Andrew Haley <andrew29@littlepinkcloud.invalid> writes:
>> I wouldn't use FSK or anything similar.  If a signal has to be carried
>> over an audio channel, it only has to be DC free and have no
>> components above the passband of the channel.  If you really don't
>> need to push the capacity of the channel I would have thought you
>> could just use a group code.  I'm assuming that the channel doesn't
>> have a lot of frequency-dependent phase shift.  You could just use FM.
> 
> I don't know what happens to the signal phase inside the phone.  I
> guess there is ac coupling at both ends, that could introduce some
> shift.  I don't understand about group codes--they are ECC codes,
> right?

No.  http://en.wikipedia.org/wiki/Group_code_recording

> Are they related to modulation somehow?  Would an FM discriminator
> (in software) have less complexity than some bandpass filters?

http://en.wikipedia.org/wiki/Modified_Frequency_Modulation and
http://en.wikipedia.org/wiki/Run_Length_Limited

All you have to do is poll the line at a regular frequency and look
for transitions.  A software DPLL locks onto the stream.  An automatic
gain control would make life easier.  Maybe you could do a little
filtering to remove spuriae.  With such a crude modulation scheme,
you'd be limited to a bit rate of quite a lot less than the bandwidth
of your channel, but it sounds like that'd be enough.

> I figure the audio going to the embedded side is very good (phones
> double as mp3 players) but going to the phone side it will be crap
> (telephony grade voice audio input).  There's tons of cpu power on
> the phone side though, enough to decode fancy modulation schemes if
> necessary.
> 
> I figured old phone modems had some reason for using FSK, and I was
> going to emulate one (use same frequencies) if possible and if it made
> sense to ever want that.

It's too much trouble to do in software.  The old phone modems used
FSK because it's easy to do with tuned analog filters.  If you're
going to use that much computer power you could use QAM or even a
trellis code, but AFAICS you really don't need that much speed.

Andrew.

Thread

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

csiph-web