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- Path: sparky!uunet!elroy.jpl.nasa.gov!lll-winken!novax.llnl.gov!urbina
- From: urbina@novax.llnl.gov
- Newsgroups: sci.electronics
- Subject: Secure voice telephony: technical questions
- Message-ID: <1992Dec18.201341.1@novax.llnl.gov>
- Date: 19 Dec 92 04:13:41 GMT
- Sender: usenet@lll-winken.LLNL.GOV
- Lines: 41
- Nntp-Posting-Host: novax.llnl.gov
-
-
- I'm building my own secure voice telephony (digital speech scrambler ) and I
- would like to use it over the telephone. I've some questions for any of you
- Telcom or DSP type people or anybody that's done this before.
-
- 1.) My design right now incorporates a 14 bit serial output ADC. I sample the
- speech at 7khz which is double the bandwidth of telcom speech. The output from
- the ADC is XOR'ed with a 31 stage linear feedback shift register that generates
- several hours of pseudorandom "noise" before it repeats itself. (It's clocked
- at 7khz to give me that long turn-around time.) This encrypted data stream gets
- sent over the telco line (in theory) and the receiving party whose PN generator
- is synced with mine, decodes the encrypted data stream and the remaining voice
- bits are routed to his/her DAC. My question is 7khz too fast for telcom ?
- I've heard of speeds of 9600 baud with modems. Appreciate any help with this.
-
- 2.) The quick and dirty way I've synced the pseudonoise generators of the
- calling and receiving parties is that I've 2 LM567 tone decoders that monitor
- the telco line for say...the digit 7 (852hz + 1209hz.) When it detects that
- tone, the 567's both send a low out to a NOR gate 74LS02 which sends out a high
- which enables the 74LS164 shift registers that create the pseudonoise.
- The high from the 74LS02 also gets routed through an inverter which then goes
- to a low which enables the chip select lines on the ADC's and DAC's.
-
- Ideally, both calling and receiving parties will detect the #7 digit at the
- same time and therefore be synced up. I haven't built up this part of the
- circuit yet, but it looks good on paper. Any of you guru types see any problems
- with this please let me know.
-
- 3.) I've heard of all this talk about 1 bit ADC's DAC's, Sigma Delta
- converters, Pulse code modulation. Are these converters more appropriate for my
- design ? Do I still have to sample at twice the bandwidth with these converters
- to get understandable speech or, are they 1:1 linear type converters ? What is
- meant by pulse code modulation ?
-
- 4.) I'm more of an analog Pulsed power type of guy, but I'm having a lot of fun
- trying to prototype this scrambler. I'll appreciate any help and ideas from
- anybody that could save me some time. Thanks.
-
- Guy
- Urbina@novax.llnl.gov
-
-