Invisible wrote:
...
> My first instinct is "how the hell do you compute 
> signal correlation without a computer?" but now I'm thinking maybe you 
> can do it with some op-amps.
...

http://google.com/search?&q=(analog+OR+analogue)+computer

-- 
Tor Olav
http://subcube.com

Post a reply to this message

Tor Olav Kristensen wrote:
> Invisible wrote:
> ...
>> My first instinct is "how the hell do you compute signal correlation 
>> without a computer?" but now I'm thinking maybe you can do it with 
>> some op-amps.
> ...
> 
> http://google.com/search?&q=(analog+OR+analogue)+computer

Oh my God, check it out:

http://en.wikipedia.org/wiki/File:AKAT-1.JPG

Funkalishous, baby!


But yeah, I was thinking that splitting a signal into pieces and 
computing a step-wise correlation would seem to require digital 
electronics. But it doesn't really. You could probably wire up a system 
that has a meter showing the instantaneous correlation between two 
inputs without much difficulty. (And the spring in the meter will 
probably smooth it out quite nicely.)

Post a reply to this message

Invisible wrote:
> Darren New wrote:
> 
>> http://denbeste.nu/special/cdma_spreading.html
>>
>> Not magic, but certainly very difficult to understand intuitively.
> 
> Hey, neat.
> 
> So it's kind-of like this?
> 
> http://en.wikipedia.org/wiki/Visual_cryptography

Not too different, except you do it with dozens of different masks. :-)

> I'm still trying to figure out exactly how it works, but it's quite neat.

Once you figure it out, then realize you can talk at two different speeds at 
the same time on the same frequencies (which is what the phone does for 
voice vs signaling), you can talk to two different carriers on the same 
frequency at the same time, and you have to account for round-trip speed of 
light delays, including predicting how much that delay will change before 
the next packet based on the fact that you're driving towards or away from 
the tower...   And the tower has to modify the timing of the chips for each 
phone based on the round-trip delays.

> And now I'm wondering... can you implement this stuff using only 
> analogue electronics? 

No idea. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   There's no CD like OCD, there's no CD I knoooow!

Post a reply to this message

Invisible wrote:
> Really? That's interesting... I got to the CDMA article *from* the GSM 
> article. o_O

Sure. The old analog phones used FDM (Frequency Division Multiplexing), and 
GSM uses (I think) TDMA - time division multiple access.  There's also 
phase-division multiple access, as well as various combinations thereof.

-- 
   Darren New, San Diego CA, USA (PST)
   There's no CD like OCD, there's no CD I knoooow!

Post a reply to this message

On 23-4-2009 16:33, Invisible wrote:
> Tor Olav Kristensen wrote:
>> Invisible wrote:
>> ...
>>> My first instinct is "how the hell do you compute signal correlation 
>>> without a computer?" but now I'm thinking maybe you can do it with 
>>> some op-amps.
>> ...
>>
>> http://google.com/search?&q=(analog+OR+analogue)+computer
> 
> Oh my God, check it out:
> 
> http://en.wikipedia.org/wiki/File:AKAT-1.JPG
> 
> Funkalishous, baby!
> 
We had one that filled one wall of the room at our practical instruction 
lab (beginning 1980's). With a patch panel of 4 by 2 meters or so. It 
used to be a section of an even larger one used for ship behaviour 
simulation. We got it as a gift when they converted to digital computers.

Post a reply to this message

Darren New wrote:
> Invisible wrote:
>> Really? That's interesting... I got to the CDMA article *from* the GSM 
>> article. o_O
> 
> Sure. The old analog phones used FDM (Frequency Division Multiplexing), 
> and GSM uses (I think) TDMA - time division multiple access.  There's 
> also phase-division multiple access, as well as various combinations 
> thereof.

Hmm, OK. Apparently,

"The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a 
kind of continuous-phase frequency shift keying. In GMSK, the signal to 
be modulated onto the carrier is first smoothed with a Gaussian low-pass 
filter prior to being fed to a frequency modulator, which greatly 
reduces the interference to neighboring channels (adjacent channel 
interference)."

So not CDMA at all. Weird...

Post a reply to this message

Le 25/06/2009 10:27, Invisible nous fit lire :
> Darren New wrote:
>> Invisible wrote:
>>> Really? That's interesting... I got to the CDMA article *from* the
>>> GSM article. o_O
>>
>> Sure. The old analog phones used FDM (Frequency Division
>> Multiplexing), and GSM uses (I think) TDMA - time division multiple
>> access.  There's also phase-division multiple access, as well as
>> various combinations thereof.
> 
> Hmm, OK. Apparently,
> 
> "The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a
> kind of continuous-phase frequency shift keying. In GMSK, the signal to
> be modulated onto the carrier is first smoothed with a Gaussian low-pass
> filter prior to being fed to a frequency modulator, which greatly
> reduces the interference to neighboring channels (adjacent channel
> interference)."
> 
> So not CDMA at all. Weird...

Not at all... but 3G (successor of GSM, in term of end-user technology/availabilty) is
using CDMA.
European use of UMTS (3G) is based on W-CDMA (W- : wideband), but not only.

GSM is 2G (quite old), playing with sub-channels (124 of 200kHz each, with 8 time
slot) in
TX & RX separated bands. Payload per timeslot is about 24.7 kbit/s (and no link
aggregation).
Of the 124 sub-frequencies, for a given point in space, they must be split between, at
least and optimaly, 3 ground-antenna (think plane-coloring... with some antenna having
different forms of cover, and some covers being influenced by the actual topology and
buildings)

GSM transmission is coded in the phase of a constant amplitude signal, allowing to
elect
the best antenna rather easily for the receiver (pick the strongest received
amplitude);
Phase modulation being one of the best modulation for issue with moving apparatus and
echo
of signal on buildings.

But it does not allow to share timeslots.

Between GSM and UMTS, GPRS and EDGE are extending the GSM system.
GPRS allow link aggregation (upto 8 timeslots, if the ground antenna is idle enough to
allow that), and then EDGE use a higher-density coding in the phase modulation to
extend
the bandwidth of GPRS (with limitation to the range... there must be a trade-off).

It's all like the old modem technology, giving new legs to an old horse.
(9600/28k/56k...)

CDMA (UMTS/...) is to GSM what ADSL is to modem. A totally different beast. The end
user
still surf the internet (resp. make phone calls), the technology in the middle is just
not
the same. And yet, it's only an electrical (resp. a radio) signal on a telephonic wire
(resp. on the air).

Post a reply to this message