DTMF Filtering

Started by christopher41877 July 31, 2004
Hi everyone,

I have a problem with DTMF decoding. I am sending DTMF tones to an
MT8880 from a 16F876. Everything works ok when I send it directly
from the pic to the 8880 but it has to be wireless. I had two
T5620's (Motorola two-way radios)and only when the volume was turned
all the way down did it work. For some reason if it was turned up a
little bit it would not work. I can't use those radios so I bought
another pair, T4525's. I can't get the volume down as low as the
T5620's. My question is, is there a way to filter the incoming DTMF
tones to an acceptable level so the 8880 can decode them?

Thanks,

Chris



If its the amplitude of the signal that is the problem (clipping?),
the first things I would do is measure the the signal (or look at it
with a scope) and then create a voltage divider (2 resistors) to get
the signal amplitude down to a reasonable level. Or, you could use a
pot to figure out what the values of the two resistors should be.

It could also be an impedence mismatch.

--- In , "christopher41877" <CHRIS@R...> wrote:
> Hi everyone,
>
> I have a problem with DTMF decoding. I am sending DTMF tones to an
> MT8880 from a 16F876. Everything works ok when I send it directly
> from the pic to the 8880 but it has to be wireless. I had two
> T5620's (Motorola two-way radios)and only when the volume was
turned
> all the way down did it work. For some reason if it was turned up
a
> little bit it would not work. I can't use those radios so I bought
> another pair, T4525's. I can't get the volume down as low as the
> T5620's. My question is, is there a way to filter the incoming
DTMF
> tones to an acceptable level so the 8880 can decode them?
>
> Thanks,
>
> Chris




Hi Chris,

I've played a lot with these DTMF decoders, and can say that they will work
with quite a wide range of input levels - you can check on the datasheet,
then measure the waveform coming out of the jack on the Moto radio with an
oscilloscope, that will give you a peak-to-peak level. Usually, speaker
outputs on these radios can give 4 to 5 volts peak-to-peak. This would be
too much for the 8880, but you can easily attenuate that with a resistor
bridge (ugly and non-purist, but works).

The other problem you may be facing, is audio mangling by the radios.
Typically, a two-way radio adds filtering and audio shaping to the signal
that goes out the speaker, so that it is pleasant to the human ear. Now,
what is pleasant to us may look like crap to a decoding circuit, and you may
find that the tones come out distorted. Usually, audio fed to detector
circuits is taken before any filtering, at the detector / discriminator. At
low volumes, it is possible the audio is proportionally not so affected, and
tones pass through.

Good luck,

Mike ----- Original Message -----
From: "christopher41877" <>
To: <>
Sent: Saturday, July 31, 2004 6:16 PM
Subject: [piclist] DTMF Filtering > Hi everyone,
>
> I have a problem with DTMF decoding. I am sending DTMF tones to an
> MT8880 from a 16F876. Everything works ok when I send it directly
> from the pic to the 8880 but it has to be wireless. I had two
> T5620's (Motorola two-way radios)and only when the volume was turned
> all the way down did it work. For some reason if it was turned up a
> little bit it would not work. I can't use those radios so I bought
> another pair, T4525's. I can't get the volume down as low as the
> T5620's. My question is, is there a way to filter the incoming DTMF
> tones to an acceptable level so the 8880 can decode them?
>
> Thanks,
>
> Chris >
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> Yahoo! Groups Links



Mike,

Thanks for the reply. When you say resistor bridge, are you talking
about a voltage divider? I tried that with a pot and it didn't
work. I don't have a scope available so I can't measure the output
but I can say that with a T5620 fed directly into the 8880 it worked
perfect(with the volume turned all of the way down. Untortunately,
these radio you can't do that with and I was hoping a resistor in
series would work but that doesn't either. If you have any other
ideas I can try please let me know.

Thanks,

Chris --- In , "Michael Puchol" <mpuchol@s...>
wrote:
> Hi Chris,
>
> I've played a lot with these DTMF decoders, and can say that they
will work
> with quite a wide range of input levels - you can check on the
datasheet,
> then measure the waveform coming out of the jack on the Moto radio
with an
> oscilloscope, that will give you a peak-to-peak level. Usually,
speaker
> outputs on these radios can give 4 to 5 volts peak-to-peak. This
would be
> too much for the 8880, but you can easily attenuate that with a
resistor
> bridge (ugly and non-purist, but works).
>
> The other problem you may be facing, is audio mangling by the
radios.
> Typically, a two-way radio adds filtering and audio shaping to the
signal
> that goes out the speaker, so that it is pleasant to the human
ear. Now,
> what is pleasant to us may look like crap to a decoding circuit,
and you may
> find that the tones come out distorted. Usually, audio fed to
detector
> circuits is taken before any filtering, at the detector /
discriminator. At
> low volumes, it is possible the audio is proportionally not so
affected, and
> tones pass through.
>
> Good luck,
>
> Mike > ----- Original Message -----
> From: "christopher41877" <CHRIS@R...>
> To: <>
> Sent: Saturday, July 31, 2004 6:16 PM
> Subject: [piclist] DTMF Filtering > > Hi everyone,
> >
> > I have a problem with DTMF decoding. I am sending DTMF tones to
an
> > MT8880 from a 16F876. Everything works ok when I send it
directly
> > from the pic to the 8880 but it has to be wireless. I had two
> > T5620's (Motorola two-way radios)and only when the volume was
turned
> > all the way down did it work. For some reason if it was turned
up a
> > little bit it would not work. I can't use those radios so I
bought
> > another pair, T4525's. I can't get the volume down as low as the
> > T5620's. My question is, is there a way to filter the incoming
DTMF
> > tones to an acceptable level so the 8880 can decode them?
> >
> > Thanks,
> >
> > Chris
> >
> >
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and follow the
> instructions
> > Yahoo! Groups Links
> >
> >
> >
> >
> >




Hi Chris,

Indeed, I mean a voltage divider. It all comes down to impedance matching,
and you can start playing with transformers, etc. etc. but then it also gets
messy. Try with a pot, but make sure you wire it right, i.e. the 'slider'
going into the 8880, and each end one to ground and the other to the signal
input. Something like this:

RADIO ---| R1 |--X--| R2 |--- GND

Take the signal at X into the 8880, and play with the values of R1 and R2.
If you go for pots, try different values, 1k, 5k and 10k for example. I can
vouch that this setup works. Are you configuring the 8880 in it's standard,
datasheet mode, or have you varied any of the parameters?

Regards,

Mike ----- Original Message -----
From: "christopher41877" <>
To: <>
Sent: Saturday, July 31, 2004 8:58 PM
Subject: [piclist] Re: DTMF Filtering > Mike,
>
> Thanks for the reply. When you say resistor bridge, are you talking
> about a voltage divider? I tried that with a pot and it didn't
> work. I don't have a scope available so I can't measure the output
> but I can say that with a T5620 fed directly into the 8880 it worked
> perfect(with the volume turned all of the way down. Untortunately,
> these radio you can't do that with and I was hoping a resistor in
> series would work but that doesn't either. If you have any other
> ideas I can try please let me know.
>
> Thanks,
>
> Chris > --- In , "Michael Puchol" <mpuchol@s...>
> wrote:
> > Hi Chris,
> >
> > I've played a lot with these DTMF decoders, and can say that they
> will work
> > with quite a wide range of input levels - you can check on the
> datasheet,
> > then measure the waveform coming out of the jack on the Moto radio
> with an
> > oscilloscope, that will give you a peak-to-peak level. Usually,
> speaker
> > outputs on these radios can give 4 to 5 volts peak-to-peak. This
> would be
> > too much for the 8880, but you can easily attenuate that with a
> resistor
> > bridge (ugly and non-purist, but works).
> >
> > The other problem you may be facing, is audio mangling by the
> radios.
> > Typically, a two-way radio adds filtering and audio shaping to the
> signal
> > that goes out the speaker, so that it is pleasant to the human
> ear. Now,
> > what is pleasant to us may look like crap to a decoding circuit,
> and you may
> > find that the tones come out distorted. Usually, audio fed to
> detector
> > circuits is taken before any filtering, at the detector /
> discriminator. At
> > low volumes, it is possible the audio is proportionally not so
> affected, and
> > tones pass through.
> >
> > Good luck,
> >
> > Mike
> >
> >
> > ----- Original Message -----
> > From: "christopher41877" <CHRIS@R...>
> > To: <>
> > Sent: Saturday, July 31, 2004 6:16 PM
> > Subject: [piclist] DTMF Filtering
> >
> >
> > > Hi everyone,
> > >
> > > I have a problem with DTMF decoding. I am sending DTMF tones to
> an
> > > MT8880 from a 16F876. Everything works ok when I send it
> directly
> > > from the pic to the 8880 but it has to be wireless. I had two
> > > T5620's (Motorola two-way radios)and only when the volume was
> turned
> > > all the way down did it work. For some reason if it was turned
> up a
> > > little bit it would not work. I can't use those radios so I
> bought
> > > another pair, T4525's. I can't get the volume down as low as the
> > > T5620's. My question is, is there a way to filter the incoming
> DTMF
> > > tones to an acceptable level so the 8880 can decode them?
> > >
> > > Thanks,
> > >
> > > Chris
> > >
> > >
> > >
> > >
> > > to unsubscribe, go to http://www.yahoogroups.com and follow the
> > instructions
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> > >
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the
instructions
> Yahoo! Groups Links



Mike,

I configured it like the standard datasheet setup. I'll try it with
a 1,5 and 10K pot....I was using a 100K pot. Thanks again for your help,

Chris

--- In , "Michael Puchol" <mpuchol@s...>
wrote:
> Hi Chris,
>
> Indeed, I mean a voltage divider. It all comes down to impedance
matching,
> and you can start playing with transformers, etc. etc. but then it
also gets
> messy. Try with a pot, but make sure you wire it right, i.e.
the 'slider'
> going into the 8880, and each end one to ground and the other to
the signal
> input. Something like this:
>
> RADIO ---| R1 |--X--| R2 |--- GND
>
> Take the signal at X into the 8880, and play with the values of R1
and R2.
> If you go for pots, try different values, 1k, 5k and 10k for
example. I can
> vouch that this setup works. Are you configuring the 8880 in it's
standard,
> datasheet mode, or have you varied any of the parameters?
>
> Regards,
>
> Mike > ----- Original Message -----
> From: "christopher41877" <CHRIS@R...>
> To: <>
> Sent: Saturday, July 31, 2004 8:58 PM
> Subject: [piclist] Re: DTMF Filtering > > Mike,
> >
> > Thanks for the reply. When you say resistor bridge, are you
talking
> > about a voltage divider? I tried that with a pot and it didn't
> > work. I don't have a scope available so I can't measure the
output
> > but I can say that with a T5620 fed directly into the 8880 it
worked
> > perfect(with the volume turned all of the way down.
Untortunately,
> > these radio you can't do that with and I was hoping a resistor in
> > series would work but that doesn't either. If you have any other
> > ideas I can try please let me know.
> >
> > Thanks,
> >
> > Chris
> >
> >
> > --- In , "Michael Puchol" <mpuchol@s...>
> > wrote:
> > > Hi Chris,
> > >
> > > I've played a lot with these DTMF decoders, and can say that
they
> > will work
> > > with quite a wide range of input levels - you can check on the
> > datasheet,
> > > then measure the waveform coming out of the jack on the Moto
radio
> > with an
> > > oscilloscope, that will give you a peak-to-peak level. Usually,
> > speaker
> > > outputs on these radios can give 4 to 5 volts peak-to-peak.
This
> > would be
> > > too much for the 8880, but you can easily attenuate that with a
> > resistor
> > > bridge (ugly and non-purist, but works).
> > >
> > > The other problem you may be facing, is audio mangling by the
> > radios.
> > > Typically, a two-way radio adds filtering and audio shaping to
the
> > signal
> > > that goes out the speaker, so that it is pleasant to the human
> > ear. Now,
> > > what is pleasant to us may look like crap to a decoding
circuit,
> > and you may
> > > find that the tones come out distorted. Usually, audio fed to
> > detector
> > > circuits is taken before any filtering, at the detector /
> > discriminator. At
> > > low volumes, it is possible the audio is proportionally not so
> > affected, and
> > > tones pass through.
> > >
> > > Good luck,
> > >
> > > Mike
> > >
> > >
> > > ----- Original Message -----
> > > From: "christopher41877" <CHRIS@R...>
> > > To: <>
> > > Sent: Saturday, July 31, 2004 6:16 PM
> > > Subject: [piclist] DTMF Filtering
> > >
> > >
> > > > Hi everyone,
> > > >
> > > > I have a problem with DTMF decoding. I am sending DTMF
tones to
> > an
> > > > MT8880 from a 16F876. Everything works ok when I send it
> > directly
> > > > from the pic to the 8880 but it has to be wireless. I had
two
> > > > T5620's (Motorola two-way radios)and only when the volume was
> > turned
> > > > all the way down did it work. For some reason if it was
turned
> > up a
> > > > little bit it would not work. I can't use those radios so I
> > bought
> > > > another pair, T4525's. I can't get the volume down as low
as the
> > > > T5620's. My question is, is there a way to filter the
incoming
> > DTMF
> > > > tones to an acceptable level so the 8880 can decode them?
> > > >
> > > > Thanks,
> > > >
> > > > Chris
> > > >
> > > >
> > > >
> > > >
> > > > to unsubscribe, go to http://www.yahoogroups.com and follow
the
> > > instructions
> > > > Yahoo! Groups Links
> > > >
> > > >
> > > >
> > > >
> > > >
> >
> >
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and follow the
> instructions
> > Yahoo! Groups Links
> >
> >
> >
> >
> >