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
>
>