Yes, you should transmit a continuous signal. At the receiving end you
will need an amplifier with AGC and a limiter in order to recover
jitter-free signals. The continuous signal gives the AGC something to
work on. But you should consider a DTMF tone decoder chip, and
transmitting sine waves with DTMF, I would think.

> ----------
> From:         Herbert Graf[SMTP:hgraf@GEOCITIES.COM]
> Reply To:     pic microcontroller discussion list
> Sent:         Thursday, January 08, 1998 3:46 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: tones via phone line
>
> > -----Original Message-----
> > From: pic microcontroller discussion list
> > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Pedro Drummond
> > Sent: Wednesday, January 07, 1998 19:45
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: tones via phone line
> >
> >
> > Hello, friends.
> >
> > I need some help.
> >
> > In this project I am involved with, I have to send certain numbers
> through
> > the phone line. Since it is just a few numbers each time, I decided
> not to
> > use a modem chip, and just code my digits in 3 frequencies (440 Hz,
> 1400
> Hz,
> > 2300 Hz) right out of the processor.
> >
> > Basically, what I do to send the code "32" is send 3 pulses (100 ms
> each)
> of
> > 1400 Hz, one pulse of 2300 Hz (same duration) as a separator, and
> then 2
> > more pulses of 1400 Hz. Pulses are separated by a 100ms silence.
> Roughly,
> > that's what I am doing. Could not be simpler.
> >
> > Concerning the hardware, I have a strong clear signal going out and
> a
> > beautiful square wave at the receiver's opamp output.
> >
> > I am decoding these frequencies by measuring the pulse width. If the
> result
> > is the same for, say, 5 consecutive measurements, then the
> corresponding
> > frequency is validated.
> >
> > Well, it works great... in some phone lines. And not so great in
> others,
> > specially if both phones are separated by a greater distance. I
> don't know
> > why, but the decoder tends to miss some pulses in this situation.
> >
> > Why this is happening ? Someone told me that it would be better to
> fill
> the
> > 100 ms silences with some frequency, just to keep feeding signal to
> my
> > receiver's amplifier.
> > I don't think it makes much sense, though...
>
>         Just wondering, why aren't you using DTMF? It is easy,
> slightly immune to
> noise, and there are quite a few encoding and decondings chips out
> there,
> true you already have the
> code for generating and detecting your signals, but by using DTMF you
> would
> be compatible with alot of what is out there now. Just a thought. As
> for
> your frequencies, 2300hz mught be your
> problem, it is kinda high, and although almost every phone line out
> there
> will pass it, I am concerned that some lines might attenuate (sp?) it
> a
> little, which might be throwing off your
> amp. Just a though, from a person who knows little about analog
> electronics,
> but who does know something of modem communications. If you want, and
> have
> the time, change to lower freqeuncies, perhaps 600 and 1200hz? That
> should
> give enough seperation and although your maximum baud rate would be
> lower,
> you are sticking with a 100 interfreq delay so it shouldn't really
> have much
> of an effect. Just a thought. TTYL and good luck.
>