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'HT9200A problem (DTMF)'
1999\10\27@172524 by Lea

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       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: spam_OUTlu2aoqTakeThisOuTspamyahoo.com
  wrk: Arg. Assoc. for Space Technology.
  ham: TCP/IP high speed group HSG

1999\10\28@035705 by Nikolai Golovchenko

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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 removed}

1999\10\28@043254 by McMeikan, Andrew

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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 removed}

1999\10\28@102144 by Lea

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At 10:51 AM 10/28/99 +0300, you wrote:
>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.

Hi Nikolai, I'm very sad to heard this because I already buy 20 parts!!!
mmm I was thinking about use a 4066 switch so I think that it will not increase
the cost too much but I will need an extra pin out of the pic to control that
gate and this is a problem :).
I will inquire about the Phillips Chip here, see if I can find it in argentina
thanks!.


>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...

Hi Andrew , I already did that but does not work, may be because the change
of state generate harmonics in high frecuencies, I don't know why but still
not work :(.
thanks.
  Leandro J. Laporta (LU2AOQ)
  mail: .....lu2aoqKILLspamspam@spam@yahoo.com
  wrk: Arg. Assoc. for Space Technology.
  ham: TCP/IP high speed group HSG

1999\10\28@211455 by Mike Keitz

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> > You cannot filter the DC jump out of the signal, because the jump
> has wide
> > spectrum including DTMF frequencies band.

Choosing the smallest value of coupling capacitor that will still pass
the tones can help a lot.  This forms a high-pass filter so the "thumps"
become more like "clicks".  Most devices that receive DTMF will not be
affected by such clicks.  Clamping the other side of the capacitor to the
ground (or whatever DC bias is normally present) before stopping the tone
would remove the disturbance completely.
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1999\10\29@004323 by Dennis Plunkett

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At 21:11 28/10/99 -0400, you wrote:
>> > You cannot filter the DC jump out of the signal, because the jump
>> has wide
>> > spectrum including DTMF frequencies band.
>
>Choosing the smallest value of coupling capacitor that will still pass
>the tones can help a lot.  This forms a high-pass filter so the "thumps"
>become more like "clicks".  Most devices that receive DTMF will not be
>affected by such clicks.  Clamping the other side of the capacitor to the
>ground (or whatever DC bias is normally present) before stopping the tone
>would remove the disturbance completely.
>___________________________________________________________________
>Get the Internet just the way you want it.
>Free software, free e-mail, and free Internet access for a month!
>Try Juno Web: dl.http://www.juno.com/dynoget/tagj.
>
>



Lets take one step back,
Where do you hear these noises?
Is it on the line?
or
Is it at the handset?
As long as the levels of the "Pops and clicks" is not higher than the
maximum line level, then there is NO PROBLEM.

At the handset of a phone is different!
Here you have to think like a phone! Which by the way at this point is
somewhat different than you may contemplate. (What the !W#$% is he talking
about?)
OK
When you dial, what do you here?
1/      Dial tone
2/      First digit goes out and you here this and hear it break dial tone corre
ct?
                       BUZZT go straigt to jail do not pass go do not collect $
200
When you dial you do not here what goes to line or what is even on the line.
The phone taps off the DTMF at a LOWER (Spelt softer) level than that, that
goes to line (Sometimes up to -4dBm and -7dBm is typical (This is too high,
should be -10dBm), and MUTES the receive audio from the exchange. Then
after the digit is complete, you get connected to the line again, all pops
and clicks removed!

OK?
So now you ask about stored digits.
Again the same applies, except that you are NOT connected back to the line
between digits.
In such a case the output from the DTMF chip should be constent between
digits.

Now I have some more questions
How do you have this connected? i.e. Is it one its own? Is the DTMF chip
driving a nominal load or buffer amplifier?
We need some more information. I suspect that the implementation may be
incorrect, thus changing the chip to some other device would be like the
Kirham unit (It serves no purpose accoring to Veger)

Dennis

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