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Fax/modem detection

Started by Jadran December 20, 2009
Hello,

I m implementing fax/modem detection. So far it is based on CNG and CED
tones. Goertzel's algorithm is used for tones detections and seems to work
fine. However I would like to increase reliability, specialy for cases when
such tones are not present or missed. Idea is to do it by detecting
presence of V.21 FSK sequence. This could be done with solution based on
FSK demodulator. But as my goal is only reliable fax/modem presence
detection, not FSK decoding, I would like to keep my FSK detection code
simple as possible. 

Do you have any suggestions how can I improve my fax/modem detection code
based on Goertzel without adding unnecessary complexity.


On 20 Des, 20:36, "Jadran" <jsiro...@yahoo.com> wrote:
> Hello, > > I m implementing fax/modem detection. So far it is based on CNG and CED > tones. Goertzel's algorithm is used for tones detections and seems to work > fine. However I would like to increase reliability, specialy for cases when > such tones are not present or missed. Idea is to do it by detecting > presence of V.21 FSK sequence. This could be done with solution based on > FSK demodulator. But as my goal is only reliable fax/modem presence > detection, not FSK decoding, I would like to keep my FSK detection code > simple as possible.
To detect a fax, you will have to decode the fax signal. Only if the signal can be decoded to a valid fax transmission can you be reasonably certain that it is a fax signal. Of course, you don't have to decode *all* the signal; a couple of seconds worth of data ought to suffice, and you don't have to print the contents of the fax. So in that sense your decoder might be a bit simpler than the regular decoder stuff. Rune

Jadran wrote:

> Hello, > > I m implementing fax/modem detection.
You are implementing fax/modem detection to do what?
> So far it is based on CNG and CED > tones. Goertzel's algorithm is used for tones detections and seems to work > fine. However I would like to increase reliability, specialy for cases when > such tones are not present or missed. Idea is to do it by detecting > presence of V.21 FSK sequence. This could be done with solution based on > FSK demodulator. But as my goal is only reliable fax/modem presence > detection, not FSK decoding, I would like to keep my FSK detection code > simple as possible. > > Do you have any suggestions how can I improve my fax/modem detection code > based on Goertzel without adding unnecessary complexity.
Fax/modem signal = wideband noise-like signal with high energy. It is very distinctive from anything else. You just have to detect such noise without attempting decoding. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com

Rune Allnor wrote:

> On 20 Des, 20:36, "Jadran" <jsiro...@yahoo.com> wrote: > >>I m implementing fax/modem detection.
> To detect a fax, you will have to decode the fax signal. >
That depends on why would they need fax/modem detection. BTW, in Russia, they tried to impose special tax on fax/modem owners. They even created line circuitry which blocked data transmission and allowed voice communication only. Fortunately, that didn't happen. VLV
> > >Jadran wrote: > >> Hello, >> >> I m implementing fax/modem detection. > >You are implementing fax/modem detection to do what? > >> So far it is based on CNG and CED >> tones. Goertzel's algorithm is used for tones detections and seems to
work
>> fine. However I would like to increase reliability, specialy for cases
when
>> such tones are not present or missed. Idea is to do it by detecting >> presence of V.21 FSK sequence. This could be done with solution based
on
>> FSK demodulator. But as my goal is only reliable fax/modem presence >> detection, not FSK decoding, I would like to keep my FSK detection
code
>> simple as possible. >> >> Do you have any suggestions how can I improve my fax/modem detection
code
>> based on Goertzel without adding unnecessary complexity. > >Fax/modem signal = wideband noise-like signal with high energy. It is >very distinctive from anything else. You just have to detect such noise >without attempting decoding.
Its only wideband once the image modem starts. I assume he is trying to detect if its a FAX near the beginning of a call for some selection reason. All he will have at that stage is the initial 1100Hz or 2100Hz tones, or the V.21 FSK, which isn't awfully wideband. Demodulating the FSK, and looking for the initial flags of the HDLC data is a fairly simple thing to do. Steve

steveu wrote:

>> >>Jadran wrote: >> >> >>>Hello, >>> >>>I m implementing fax/modem detection. >> >>You are implementing fax/modem detection to do what? >>
>>Fax/modem signal = wideband noise-like signal with high energy. It is >>very distinctive from anything else.
> > Its only wideband once the image modem starts. I assume he is trying to > detect if its a FAX near the beginning of a call for some selection reason.
It's the answer side that starts fax/modem negotiation. Utter evil things like automatic telemarketing come to mind. He'd better do something peaceful, like missile guidance system. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
> > >steveu wrote: > >>> >>>Jadran wrote: >>> >>> >>>>Hello, >>>> >>>>I m implementing fax/modem detection. >>> >>>You are implementing fax/modem detection to do what? >>> > >>>Fax/modem signal = wideband noise-like signal with high energy. It is >>>very distinctive from anything else. > > >> >> Its only wideband once the image modem starts. I assume he is trying
to
>> detect if its a FAX near the beginning of a call for some selection
reason.
> >It's the answer side that starts fax/modem negotiation. Utter evil >things like automatic telemarketing come to mind. He'd better do >something peaceful, like missile guidance system.
For things like unified messaging people want to answer the phone, see if a FAX machine is calling, and either handle the call as a voice mail or a FAX. For T.38 FAX over IP people want to detect either FAX tone (CED or CNG) to see if the call should be renegotiated as a T.38 FAX call. There are several perfectly innocent reasons to want to do FAX detection. On the other hand, I hope all telemarketers have severe long term problems in the toilet. Regards, Steve

Hello,

do you have idea how reliable is tone matching alone as fax detection,
there are sone cases when none of tones is transmitted?

>Demodulating the FSK, and looking for the initial flags of the HDLC data >is a fairly simple thing to do.
Anyway, this seems as simple way to ensure fax is actually detected. Please can you suggest which of FSK demodulator/decoder design shemes seems most suitable only for this purpose, having in mind low computational complexity. Short scheme description would be helpfull. Dont worry you are not helping someone in telemarketing bussness, I m not bothering people for living :). Thanks for support Jadran
"Jadran" <jsirotko@yahoo.com> writes:

> Hello, > > do you have idea how reliable is tone matching alone as fax detection, > there are sone cases when none of tones is transmitted? > >>Demodulating the FSK, and looking for the initial flags of the HDLC data >>is a fairly simple thing to do. > > Anyway, this seems as simple way to ensure fax is actually detected. > Please can you suggest which of FSK demodulator/decoder design shemes seems > most suitable only for this purpose, having in mind low computational > complexity. Short scheme description would be helpfull. > > Dont worry you are not helping someone in telemarketing bussness, I m not > bothering people for living :). > > Thanks for support > > Jadran
Jadran, There's a new technology out called the Internet, and a company called Google makes this wonderful search tool for ti. Why don't you try utilizing this tool to answer your questions? -- Randy Yates % "And all you had to say Digital Signal Labs % was that you were mailto://yates@ieee.org % gonna stay." http://www.digitalsignallabs.com % Getting To The Point', *Balance of Power*, ELO
>"Jadran" <jsirotko@yahoo.com> writes: > >> Hello, >> >> do you have idea how reliable is tone matching alone as fax detection, >> there are sone cases when none of tones is transmitted? >> >>>Demodulating the FSK, and looking for the initial flags of the HDLC
data
>>>is a fairly simple thing to do. >> >> Anyway, this seems as simple way to ensure fax is actually detected. >> Please can you suggest which of FSK demodulator/decoder design shemes
seems
>> most suitable only for this purpose, having in mind low computational >> complexity. Short scheme description would be helpfull. >> >> Dont worry you are not helping someone in telemarketing bussness, I m
not
>> bothering people for living :). >> >> Thanks for support >> >> Jadran > >Jadran, > >There's a new technology out called the Internet, and a company called >Google makes this wonderful search tool for ti. Why don't you try >utilizing this tool to answer your questions? >-- >Randy Yates % "And all you had to say >Digital Signal Labs % was that you were >mailto://yates@ieee.org % gonna stay." >http://www.digitalsignallabs.com % Getting To The Point', *Balance of
Power*, ELO
>
Randy, well I already did that, problem is there are too many alternative solutions, so I m here asking help of experienced proffesional to recommend me good starting point, or in concrete case recommend most propriete FSK demodulation approach for my not so common problem. So I m trying to discuss posibilities and pick optimal, that's what you have to do when wonderful search tool Google returns too many results and you dont have experiance in field. Maybe your wonderful Google does it for you, unfortunately mine is not working like that... Anyway, I realy dont understand why you have such hostile attitude toward normal question and want to flame.