DTMF decoder

CBFalconer wrote:

> Joerg wrote:
> 
>>CBFalconer wrote:
>>
> 
> ... snip ...
> 
>>>There is a simple cure for availability of 1% capacitors - incoming
>>>selection.  All the 5% caps can be used by choosing the appropriate
>>>resistors, which were relatively cheap.
>>
>>That is an expensive proposition, plus you have to sell all the ones
>>between 1% and 5% on the gray market. Selecting resistors is a labor
>>intense job, I would have never been allowed to spec in something like
>>that. What we did with Hilbert shifters and stuff like this was passive
>>(and sometimes active) laser trim. But we tried our darndest to avoid
>>any of that.
> 
> 
> I was talking about selecting caps, not resistors. ...


Sure, but selecting appropriate resistors as you mentioned is a process 
that requires labor and some skills. Read a scope, meter or digital test 
rig. Then select the resistor from a table or as displayed and solder it 
in. I didn't do industrial design in the 70's since I was still at 
college but in the 80's when I did this was very much frowned upon.


>                                  ... For those you
> could (this is 1970, remember) buy the appropriate values. 
> Besides, IIRC, the active filter calculations for the
> band-splitting filter didn't require better than 5% components
> anyhow.
> 

Yes, the bandsplit would be easier.


> Cap selection requires a jig.  Place cap on jig, read, dump in
> appropriate bin.  Even with manual labor it is hard to see more
> than 10 seconds per cap, triple that to 30 sec to allow for breaks,
> bathroom time, chatting time, etc.  
> 
> Building the jig requires some ingenuity.
> 

My boss would have gone ballistic if that procedure were in my ECO ;-)

-- 
Regards, Joerg

http://www.analogconsultants.com

Hello Rafael,

> 
> Not much fun doing a DTMF-decoder on a controller.
> 
> a) One can use a bandsplitfilter seperating lowgroup and highgroup.
>    Getting then the signal in both bands to 1 bit and deciding on the
>    frequency is the easy part. Virtually all old SC-ICs like the
>    MT8870 do it that way.
>    The bandsplitfilter on a controller is difficult.
>    TI has a applicationnote for the MSP430 using WDFs ( "wave digital
>    filter" ). But getting WDFs up is a lot of work, i can
>    scan one or more papers on DTMF bandsplits via WDF if you want 
>    to have a go at it ( Joerg has given up by now i guess ).   


No, I haven't but after finishing the technical expose this project is 
in the VC discussion phase. Those can take a while. If it results in a 
green light I'll have to calculate all that by hand since decent design 
SW for WDF doesn't seem to exist. Except for a collection of routines 
with scant to no documentation.


> b) One can use a filterbank with up to 16 narrow bandpass filters.
>    * using IIRs out of the textbook is impractical. 
>    * Goertzel is the practical IIR-variant and for some controllers 
>      like the Mitsubishi M16 there are implementations. But usually 
>      it is limited to DSPs.


You can also do it on an MSP430. But only with HW multiplier (F427?). 
For some reason TI has decided to introduce new F2xx devices with a HW 
multiplier but taken out the 16bit ADC. Don't know why, banging my head 
on the table here...

-- 
Regards, Joerg

http://www.analogconsultants.com

ivan wrote:

> Hi Vladimir,
> 
> I have another doubt now... I have 10 freqs (2 more for the handshaking), if
> I add all the periods for each freq I get about 9ms, this is the time needed
> to scan all tones once for one period only. Each DTMF tone last 50ms, this
> means that I can scan each tones max 5 times. But I have two square waves to
> scan (sin/cos), hence I can only scan each tone for 2 periods only. Would
> this be enough to get sensible results?

This doesn't seem to be a working solution.
You should do all of the frequencies at the same time.

VLV

"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> ha scritto nel messaggio
news:Zr3Mg.16590$%j7.3220@newssvr29.news.prodigy.net...
>
>
> ivan wrote:
>
> > Hi Vladimir,
> >
> > I have another doubt now... I have 10 freqs (2 more for the
handshaking), if
> > I add all the periods for each freq I get about 9ms, this is the time
needed
> > to scan all tones once for one period only. Each DTMF tone last 50ms,
this
> > means that I can scan each tones max 5 times. But I have two square
waves to
> > scan (sin/cos), hence I can only scan each tone for 2 periods only.
Would
> > this be enough to get sensible results?
>
> This doesn't seem to be a working solution.
> You should do all of the frequencies at the same time.
>
> VLV


I am generating all the freqs by using arrays and calculating values to
reduce the timing errors.. Is this the best way to do this?

ivan

ivan wrote:


> 
> I am generating all the freqs by using arrays and calculating values to
> reduce the timing errors.. Is this the best way to do this?
> 
> ivan
> 

You can have the complete DTMF decoder in C if you like.
Please write to vlv@abvolt.com.

VLV

"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> ha scritto nel messaggio
news:2HeMg.9396$yO7.1549@newssvr14.news.prodigy.com...
>
>
> ivan wrote:
>
>
> >
> > I am generating all the freqs by using arrays and calculating values to
> > reduce the timing errors.. Is this the best way to do this?
> >
> > ivan
> >
>
> You can have the complete DTMF decoder in C if you like.
> Please write to vlv@abvolt.com.
>
> VLV
>
>
>
>

I have it in C, but it uses Goertzel and Z8Encore is not fast enough. I want
to try this other method and see if it fits into Z8Encore... otherwise will
have to choose a faster mcu.

Do you mean pay for it?