I am not familiar with that chip, but how about providing a DC bias to the
level it jumps to? then use a coupling cap to only get the tone.

        cya,    Andrew...

> -----Original Message-----
> From: Nikolai Golovchenko [SMTP:golovchenko@MAIL.RU]
> Sent: Thursday, October 28, 1999 3:51 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: HT9200A problem (DTMF)
>
> Hi Leandro,
>
> You cannot filter the DC jump out of the signal, because the jump has wide
> spectrum including DTMF frequencies band. The only approach I would
> suggest
> is to use some external circuit that would hush the encoder output at the
> beginning and end of transmission. Though, it's complex. Better switch to
> another encoder, e.g. Philips PCD3312C.
>
> Cheers,
> Nikolai
>
> -----Original Message-----
> From: Lea <lu2aoq@YAHOO.COM>
> To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
> Date: Thursday, October 28, 1999 12:25 AM
> Subject: HT9200A problem (DTMF)
>
>
> >        Hi all, I'm in trouble with a DTMF encoder that I'm trying to use
> >in a new proyect, the HT9200A from HOLTEK, it's a serial (2 wire) DTMF
> >encoder very easy to use, only 3 wires needed (CE , SDATA, SCLK), the
> >logic is working fine and the tone is generated but at the begining and
> >the end of each tone I heard a very short sound "TAC" , in the scope
> >I can see that I have a DC jump at the begining and at the end of the
> tone
> >and I have no idea how to avoid that DC jump!.
> >using coupling capacitors I found no difference, the ugly sound persist
> :(
> >the only way that I could avoid that sound is change the tone without
> >stop it, but most dtmf decoders needs at least 30 ms of silence to detect
> >a new tone, so I think that is not the solution.
> >anyone have an idea about how to avoid this DC jump sound?.
> >thanks in advance.
> >   Leandro J. Laporta (LU2AOQ)
> >   mail: lu2aoq@yahoo.com
> >   wrk: Arg. Assoc. for Space Technology.
> >   ham: TCP/IP high speed group HSG
> >
> >