CM8870, DTMF Receiver

[walid1]

(1) I read carefully the CM8870 datasheets but I can't conclude that it can
    receive the FM signal directly or it must be connected to a separated FM
    receiver please tell me
(2) I hardly got the datasheets of CM8870/70C, but i really have CM8870PI
    Is there any difference?
thanx

 

[audioguru2]

Hi Walid,
It is just a DTMF tones decoder. It has nothing to do with a radio.
Get a good datasheet from www.datasheetarchive.com .

If you use an ordinary FM broadcast band radio then it has de-emphasis and the transmitter that you use must have a matching pre-emphasis.

 

[walid1]

It is just a DTMF tones decoder. It has nothing to do with a radio.

i understand that it require a separate FM receiver

Get a good datasheet from www.datasheetarchive.com .

i use this link but i have not any results
but in this          http://www.alldatasheet.com/              i faound many

If you use an ordinary FM broadcast band radio then it has de-emphasis and the transmitter that you use must have a matching pre-emphasis.

i'm not understand this statement


thank you

 

[audioguru2]

Hi Walid,
To reduce hiss, FM stations and TV stations use pre-emphasis for their audio transmission, and FM receivers have a corresponding de-emphasis.
Pre-emphasis boosts high audio frequencies and the de-emphasis cuts them back to normal and also cuts hiss.
DTMF needs a frequency response that is fairly flat, but if you don't use pre-emphasis in its transmission then a receiver's de-emphasis will cut high tones frequencies too much.

 

[walid1]

OK Mr audioguru
I may understand the general aspect of what u said but
can u give me an example about this especially on your FM transmitter circuit.
where the subcircuit or the parts that  do the pre-emphasis
also in the receiver where it be placed in the whole circuit
thank u audioguru