Re: Real-time sound cyphering algorithm

From: Jan Panteltje (pNaonStpealmtje_at_yahoo.com)
Date: 05/31/05 

Date: Tue, 31 May 2005 13:54:03 GMT

On a sunny day (Tue, 31 May 2005 12:43:30 +1000) it happened "David Eather"
<eather@tpg.com.au> wrote in <429bd59f@dnews.tpgi.com.au>:

>Jan Panteltje wrote:
>> On a sunny day (Tue, 31 May 2005 00:28:50 +1000) it happened "David
>> Eather" <eather@tpg.com.au> wrote in <429b2367@dnews.tpgi.com.au>:
>>
>>> http://en.wikipedia.org/wiki/Sound_unit
>>>
>>> Forty dB is a factor of 100.
>> For voltage only.
>
>No. Your own entry into the reference stakes:
>
>B1 "Power equals volts multiplied by current"
>
>B2 "Yeah great. What's the car battery voltage?"
>
>B1 "Five thousand watts"
hey! Not completely fair.
I merely stated that doubling the voltage in the same load, results
in 4 x the power.
Since our ear is logarithmic (in volume) we do not percieve 4 x more volume.
For a car battery the energy content is measured in ampere-hour.
So 100 Ah is 100 ampere for 1 hour (or 50 for 2).
There is a practical limit as the battery internal resistance will limit Imax.
So, I dunno, maybe I misunderstood your original question or was it statement.
Here is a better, more clearly written link:
http://www.phys.unsw.edu.au/~jw/dB.html#definition

>>> OP wrote>
>>> I need the cyphered sound to be audible, but not understandable
>> I think this can also be done by using a say 5kHz audio bandwidth
>> (more then enough for speech), and AM modulating a carrier at
>> 20kHz. The lower sideband will go to 20 - 5 = 15 kHz.
>> You will hear some high peaked noises.
>> An other alternative is to use a FM carrier, and narrow band FM (so
>> use only one sideband), and use 10 kHz carrier :-)
>> This requires some filtering at the rx side, but any dsp board
>> should
>
>I like that idea. I would just suggest that it will not suit the OP
>in terms of resources and ease of implementation
Dunno, for FM 2 4046 PLL chips, some opamps....
I have done that.
PC with soundcard and some soft would do too.

I was wondering where OP was getting this idea from, then I found
an article on nytimes.com about a device that makes telephone
conversations un-intelligible for other people (standing next to it, so
they cannot hear what you say).
It does that by making some bogus sounds if I did read it right.
Maybe that is what made him ask?

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