I'm finishing my graduation in computer engineering. My background in electronics is quite poor though. I have some good experience with digital circuits, but a huge lack of analog skills.
I'm willing to implement a device to connect in my phone line to encrypt voice. My idea is to convert the analog signal to bits and use a simple algorithm (running in a dsPIC) to do the math.
The encryption stuff are almost done and tested, but I'm so lost in the analog part. I have no idea how to get the signal and process it. I've been reading some docs in the web but I couldnt manage to find anything useful [perhpas because I can't even do a good search].
Can you guys give me a hand? Any help such as advices, tips, circuit schemas, references, ideas, etcetera, would be very appreciated.
I'm not worried about performance. The only requirement is to have a non-expensive circuit, cause since I'm a student founds are really short.
If I didnt make myself clear, please, let me know. I can provide more information about the project.
thanks a lot in advance,
--
sav
(english is not my main language so the message might be full
of typos and weird expressions. Im sorry about that.)
Nothing specific to your needs, but maybe one of these will have something that will help you get started.
Simple phoneline interface
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Various phone circuits
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Btw, your English is lot better than most English speaking people!
Mike
------------------------------------------------------------------------ The odds of a single modest length protein randomly forming is approx
1 in 10^260. By comparison the number atoms in the known universe has been estimated at 10^80 atoms. Figues are from the writings of Nobel Prize winner Francis Crick in 1981.
You need to first interface your digital part to the telephone line with a analog telephone interface circuit. It is better to isolate the telephone line being connected to the digital interface since telephone line voltages are quate high (spoecially when ringing). The link below has many information related to line interfacing.
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Then you need to extract the voice data (of course analog voice signal). Usually telephone bandwidth is limited to 3.4 kHz as I remember and this signal has to be converted to a digital signal with a ADC. (Analog to digital converted) I am familliar with PICs but not DSPIcs. PICs has analog to digital conversion built in to the chip. Even DSpics might be. Then you do not have to do that much. You can sample your voice signal at arround 7 KHz. (Refer : Niquist theorem - Signal should be sampled atleast at twice the frequency). This is very possible with even slower micro processors. Sound quality depends not only on the sample rate but the bit resolution as well. I think DSpics shoulf have this capability. Look at those voice extraction circuits at below link.
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When u want to sent a signal then your digital signal has to be converted to a analog signal.For this u need DAC (digital to analog). You can find those chips at
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for free. (free samples) depending on the bit resolution you can select a one.Most of the time same circuit used to extract voice signl can be used to feed as well.
For dialing purposes you need to generate DTMF signal using the DSpic. The first link should have information about the frequecies to be used in DTMF or you can use DTMF generating ICS. (E.g : MT8889CE)
[in this example the good looking guy is receiving a message]
(1) interface to telephone line, responsible for extracting data. this part will also make sure noise is removed, voltage is not above 48 V nominal, et cetera. (2) cryptography interface; it will convert the cipher to plain text and send to (3). (3) DAC; converts the decrypted string of bits to 'voice' and send to user.
Sending a message follows these steps, but insted of a DAC, it uses a ADC (inside the PIC), and instead of decrypting, we encrypt.
My first question is: what happens when I send a digital signal over the telefone line? Since we are talking about voice, losing a block of information doesnt seem that bad. But what if I can't avoid losing too much? I'm not sure how good is data transmission over the line. I can't use a modem for example, that would make the project too complex.
The second is related to (1). Unfortunatly, I won't be able to afford a Basic Stamp, as suggested by Mike. The links Pubudu gave me are good and I believe I could build my own interface circuit, but isnt there any other cheaper product I could buy to do this job? It could save me some time. Also, how good are these interface examples? The first reference showed me that there are rules (eg. FCC) to connect a device to the telephone net so, I'd have to implement at least the essential [assuming these rules exist for a good reason]. What about noise? Would I have to handle that too? The doc says one solution to noise is to invert the phase of one line and sum to the other, since they are supposed to have the same infomation, that would cancel the noise [bad explanation but I sure you guys know what I'm talking about].
Finally, I think the PIC will do the job. The bw is 3.2 kHz, which means the sample rate will be something around
6.4 kHz (epanorama.net suggests 8 kHz). Also, the block cipher is 64-bit long, so the micro controller would have to wait 8 samples to process data. I really think this is slow enough.
once again, thanks a lot for all help.
cheers,
--
sav
(its so cool, electronics = lots of fun...)
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