DTMF questions in FPGA

[macakolo]

Hail,

I have a question that on detect DTMF from a telephone analogue signal.

I have a AD to convert analogue audio from a telephone line into digital 17bit 8Khz into a FPGA, and we would like to analysis the digital signal and for DTMF use.

One thing I am not sure is how we could tell the different frequency from the anaogue world when it has been converted int digital??

Anyone can give me some help please!!

 

[penrico]

I think you must implement a DSP core in your FPGA. Then, you must implement frequency detect algorithms, such as Goertzel algorithm that is best suited for DTMF detection. You could even implement a Goertzel algorithm directly to a FPGA but could be a little bit difficult.

 

[AmitNahar]

Implementing the Goertzel Algorithm even without DSP is not a difficult task, provided you have an FPGA with in built multiplier. e.g. Xilinx Spartan III and up

We have done it for 14 bit input and consumes only 15 % of the resources in Virtex II.

Amit

 

[macakolo]

Hail, Thank you so much for your advice, will you happen to give me some examples on your previous work which I can follow on implementing on my Xilinx 3E??

And regarding to the algorithm, do you have a link of the info I can check on!!

Thank you!!

 

[Iouri]

also you can try to do FFT, there are app notes on xilinx web site

 

[Gunship]

Goertzel algorithm is a special kind of FFT algorithm that use for
DTMF decoding. Here is a quick link about it

https://en.wikipedia.org/wiki/Goertzel_algorithm
https://ptolemy.eecs.berkeley.edu/papers/96/dtmf_ict/www/node3.html

You should also take a look at analog devices
ADSP-21XX Signal processing application handbook

The printed copy has a very detail explaination of this algorithm.
Unfortunately I don't seem to be able to locate it at the website,
maybe you will have better luck than me.

The Goertzel algorithm is pretty much universal frontend of
a DTMF decoder. But you will find variations regarding how
to determine the frequency and also different kinds of windowing
function. I used to use an AT&T DSP16
to do DTMF and their application note has a very simple detection
backend compared to analog device. Unfortunately I no longer
has either app note. (It was almost twenty years ago. I am no
longer in telcom industry.) You may want to also take a look at
other DSP implementation application note (such as Motorola,
analog devices, TI) just to get an idea of various style of
detection.

I don't believe it is difficult to implement this in an FPGA, especially
one with hardware multiplier (Spartan3 etc.).

Gunship