dtmfout problem

[lerameur]

that did not work,
everytime a DTMF comes out, its like someone was blowing air at the same time.. from what I can hear from the speaker.
if I choose to do a low pass filter, what would you suggest my cut off frequency be?

[skimask]

if I choose to do a low pass filter, what would you suggest my cut off frequency be?

Well, since DTMF freq's run from 697Hz to 1633Hz (reference Wikipedia), shooting for 1634 would be a good bet, but probably not practical. I'd tend to think that a 'brick wall' above 2KHz would do it.
And I'd also be willing to bet that the 'air' you are hearing is in fact aliasing of the fundamental tone due to the numerous harmonics caused by the square wave output of the PIC.

[Jumper]

Hi,

What do you plan to design the filter? RC? OP-amps? LC? go for a filter chip?

[lerameur]

I dont know yet, but I remember some chip requires +15 and 15- supply , I only have about 6 or 7v supply.
I tried a single stage RC, with 0.2uF and 400ohms, that did not do anything..

[skimask]

I dont know yet, but I remember some chip requires +15 and 15- supply , I only have about 6 or 7v supply.
I tried a single stage RC, with 0.2uF and 400ohms, that did not do anything..

.2uF & 400 ohm gives you an RC time constant of 12.5Khz. (.2/1000000 * 400 = .00008, 1/.00008 = 12,500)
It might work a bit better if you had a bit more cap's and a bit less resistor, say 40 ohms instead of 400, which would give you an RC time constant of 1.25Khz. And since you're trying a single stage RC filter, the roll-off would be fairly shallow, so 1.25Khz may work out fairly decent.
My suggestion...get a copy of 'The Art of Electronics'. It's one of my 'Good Books', take it with me on every trip I take, page thru it all the time. A few good sections on filters, op-amps, etc.etc.etc.

[lerameur]

ok look at this pdf:
AN6_DTMF_gen.pdf
search for it on google, last page shows the calculation i did.

aslo, when I used my 0.2uF cap. I decided to switch to a variable 2k pot, went through all the resistance values pretty quicky. The only thing this did is to increase or decrease the overall sound level, kind of a volume control.

[Jumper]

Hi,

The things you have seen is just what I have seen too. RC filters are not that successful in making Low Pass filters since they dont this nice sudden cutoff FQ.

these links will get you some help designing an active filter.

http://www.microchip.com/stellent/id...cName=en010007

http://www.national.com/appinfo/amps...h_filters.html

There are many Op-amps that can be used with a single 5V supply. This way you get an active filter with very sharp cutoff FQ.

/me