Re: help with analog electronics

On 2006-06-24, sav@xxxxxxxxxxxxxxx <sav@xxxxxxxxxxxxxxx> wrote:
Hi,

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 d***) to do the math.

[snipped for the sake of sanity]

Mike, Pubudu, thanks a _lot_ for the answers. After reading
some of the references two doubts came across.

I'm posting a follow-up to my own post in reply to both
answers. (is this messy?)

I realised that the project will be something like this:
(please correct me if I'm missing something)

device _O_
tel network ---> [ (1) | (2) | (3) ] ---> |
/ \

[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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