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Speech coding for security and defense apps, Why do we need very low bitrate?
QuantumKnot
post Jan 18 2006, 03:13
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Hi everyone,

I've heard that ultra low bitrate speech coding (< 800 bps) that use simple modifications to the LPC vocoder (such as segmented LPC vocoders), are of interest mainly for security and defense applications. At such low bitrates, I assume that intelligibility is the only criteria here. My question is why do we need to go to such low bitrates for defense and security? Is it because we want to reserve bandwidth for error-correcting redundancy? Or perhaps encryption? The only thing that I've read which sort of remotely justifies the low bitrate is a lesser chance of being intercepted. huh.gif

If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid. smile.gif

Thanks,

QK
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clintb
post Jan 18 2006, 04:12
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Possibly because they (the men in black) can store more of your communications when encoded at a lower bitrate? Oh crap, I've figured it out...they'll probably have some aliens abduct me now. rolleyes.gif
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boojum
post Jan 18 2006, 04:32
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I would ask the people who tell me this stuff. If they know that much, they must know the whole story, eh?? cool.gif


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Shade[ST]
post Jan 18 2006, 04:48
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Why not steganography? At such a low bitrate, you can practically hide information in powerline level fluctuations and no one would notice...
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saratoga
post Jan 18 2006, 05:42
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Bandwidth is a good thing to save whenever you are doing radio communications. It lets you use way narrower frequency bands, less power, etc.
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HotshotGG
post Jan 18 2006, 05:46
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QUOTE
Why not steganography? At such a low bitrate, you can practically hide information in powerline level fluctuations and no one would notice...


When I started reading about Steganography I was amazed with what you could do with it. ohmy.gif

QUOTE
If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid.


Why do we need GPS to begin with? laugh.gif. GPS was mainly a government thing, until we turned it over for commercial use.


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QuantumKnot
post Jan 18 2006, 06:24
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QUOTE (Mike Giacomelli @ Jan 18 2006, 02:42 PM)
Bandwidth is a good thing to save whenever you are doing radio communications.  It lets you use way narrower frequency bands, less power, etc.
*


Oh, that makes sense. My initial thoughts were that bandwidth was rarely a problem these days (if we can pump streaming video over the cellular network, I'm sure we can handle the CELP and MELP bitrates of 4.8 kbps or 2.4 kbps, respectively). But the communication channels used by defense and security agencies may have much narrower bandwidth than the ones for civilian use, hence the need for lower bitrate. Yes, that makes sense. smile.gif
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Shade[ST]
post Jan 18 2006, 06:30
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QUOTE (QuantumKnot @ Jan 17 2006, 11:24 PM)
QUOTE (Mike Giacomelli @ Jan 18 2006, 02:42 PM)
Bandwidth is a good thing to save whenever you are doing radio communications.  It lets you use way narrower frequency bands, less power, etc.
*


Oh, that makes sense. My initial thoughts were that bandwidth was rarely a problem these days (if we can pump streaming video over the cellular network, I'm sure we can handle the CELP and MELP bitrates of 4.8 kbps or 2.4 kbps, respectively). But the communication channels used by defense and security agencies may have much narrower bandwidth than the ones for civilian use, hence the need for lower bitrate. Yes, that makes sense. smile.gif
*


Having such narrow channels makes them much harder to detect, also, if you don't already know their frequencies. (try finding a pin in a haystack)
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audio_geek
post Jan 23 2006, 06:49
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Do you need lossless eompression for speech communication between two high class delegations or for government purposes or security agencies ??
bandwidth is of course the BIG issue but do u see anything good in lossless compression of speech, though lossless compression is there in audio.
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HbG
post Jan 23 2006, 16:44
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The less bandwidth the easier to transmit and thus the easier you can guarantee that the message arrives, even under tough circumstances, i guess that's underlaying thought.

Submarine and subterranean (mines) wireless communications for example rely on low bandwidth VLF/ULF and even ELF radio.


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pepoluan
post Jan 23 2006, 20:10
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QUOTE (HbG @ Jan 23 2006, 10:44 PM)
The less bandwidth the easier to transmit and thus the easier you can guarantee that the message arrives, even under tough circumstances, i guess that's underlaying thought.

Submarine and subterranean (mines) wireless communications for example rely on low bandwidth VLF/ULF and even ELF radio.
*
Ah you beat me to it tongue.gif

Ever see the movie "Crimson Tide"? The VLF, ULF, and ELF waves are the only ones capable of penetrating the ocean. It requires antennae as long as a submarine, because the wavelength is very long.

Unfortunately at such low frequency, the bandwidth is very very limited, which is why in the film they got the message only partially. Think of a 300 bps modem. Even slower.

And don't forget that you need robust error correction for military orders. So everything that can save bandwidth is very helpful.


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HbG
post Jan 24 2006, 01:11
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Actually the global submarine communications transmitters at 82 and 7-something HZ only do a few symbols per minute, so all you can really do is tell a sub to rise and seek contact via other means.

But there's plenty of other military and secret situations where data rates of 800bps are possible and voice preferred.

I don't think i've seen Crimson Tide (or read Tom Clancy for that matter), but wikipedia is a wonderful resource.

This post has been edited by HbG: Jan 24 2006, 01:12


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QuantumKnot
post Feb 9 2006, 23:55
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Thanks for the replies everyone. smile.gif

Another reason I read in a paper is that with low bitrate, only low power transmitters are required, which I assume are harder to intercept.
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Garf
post Feb 10 2006, 00:20
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I would say another advantage is that low bitrate = short messages = short transmission time = less time for tracking sender?
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jmvalin
post Apr 10 2006, 16:24
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QUOTE (QuantumKnot @ Jan 18 2006, 12:13 PM) *
If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid. smile.gif


My guess is actually that they want to use huge error-correcting codes to make it robust to jamming. With a very small bit-rate, you can afford a lot of redundancy and eventually (with lots of jamming/interference), it may actually sound better than a higher-bitrate codec.
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HbG
post Jun 28 2006, 01:49
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802.11 based wireless networks extend range by switching to lower bitrates, when reception gets poor. Currently this scales from 55 down to 1mbps, yielding a 23db increase in receive sensitivity in typical implementations.

Military applications may do the same, and a 800bps voice channel may be at the extreme end of this.

Another way to look at it is that this flexibility effectively allows you to trade bandwidth for transmission power, hence it may also have a purpose for transmitting when low on power.
Besides the military this makes it useful for intelligence purposes where bugs have to be small and thus have small batteries, it may be more energy efficient to compress a voice signal and transmit it at low power. Question is whether such a bug's microphone can provide a clear enough signal for a 800bps stream to remain intelligable.


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pepoluan
post Jun 28 2006, 19:00
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QUOTE (Garf @ Feb 10 2006, 06:20) *
I would say another advantage is that low bitrate = short messages = short transmission time = less time for tracking sender?
Uh, Garf, shouldn't that be low bitrate = long transmission time?

I'm more to the error-correction reason.


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Garf
post Jun 28 2006, 19:07
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QUOTE (pepoluan @ Jun 28 2006, 20:00) *
QUOTE (Garf @ Feb 10 2006, 06:20) *
I would say another advantage is that low bitrate = short messages = short transmission time = less time for tracking sender?
Uh, Garf, shouldn't that be low bitrate = long transmission time?


No.
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HbG
post Jun 28 2006, 19:40
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QUOTE (pepoluan @ Jun 28 2006, 10:00) *
QUOTE (Garf @ Feb 10 2006, 06:20) *
I would say another advantage is that low bitrate = short messages = short transmission time = less time for tracking sender?
Uh, Garf, shouldn't that be low bitrate = long transmission time?

I'm more to the error-correction reason.


He means the low bitrate of the voice codec, not the data link. When, for example, you would transmit a 800bps voice message over a 1Mbps wireless link it would go at 1250x realtime, meaning a 10 second message takes a mere 8ms to transmit. This makes detection extremely difficult.

I know for a fact that such very short transmission time communications were used during the cold war to communicate with spies.


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maxonen
post Aug 9 2006, 14:58
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QUOTE (QuantumKnot @ Jan 18 2006, 04:13) *
If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid. smile.gif


When using low bitrates you can use spread-spectrum algorithms to distribute the signal power over a very wide range - even to a degree where the signal power will be lower than the background noise - and thus "hide" the signal in the background noise and make it practically impossible for any other party to even detect that you are transmitting something.
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Woodinville
post Aug 9 2006, 19:01
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Because the channels they use for communications are beyond what we would regard as "really, really awful".

It's a channel issue mostly.


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kwwong
post Aug 13 2006, 08:26
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QUOTE (maxonen @ Aug 9 2006, 08:58) *
QUOTE (QuantumKnot @ Jan 18 2006, 04:13) *

If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid. smile.gif


When using low bitrates you can use spread-spectrum algorithms to distribute the signal power over a very wide range - even to a degree where the signal power will be lower than the background noise - and thus "hide" the signal in the background noise and make it practically impossible for any other party to even detect that you are transmitting something.


Can you enlightened me further in this area? How about some detail equations presentations? rolleyes.gif
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Garf
post Aug 13 2006, 10:49
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QUOTE (kwwong @ Aug 13 2006, 09:26) *
QUOTE (maxonen @ Aug 9 2006, 08:58) *

QUOTE (QuantumKnot @ Jan 18 2006, 04:13) *

If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid. smile.gif


When using low bitrates you can use spread-spectrum algorithms to distribute the signal power over a very wide range - even to a degree where the signal power will be lower than the background noise - and thus "hide" the signal in the background noise and make it practically impossible for any other party to even detect that you are transmitting something.


Can you enlightened me further in this area? How about some detail equations presentations? rolleyes.gif


http://en.wikipedia.org/wiki/Spread_Spectr...ecommunications
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HbG
post Aug 13 2006, 11:30
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I recall bluetooth is a good example of spread spectrum communications, hopping the entire 2.4GHz ISM band.

How you can recieve something when the signal is below the noise level? Doesn't make sense to me.


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beto
post Aug 13 2006, 15:07
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It's against common sense but it works.
See CDMA for a more technical explanation (and if you are familiar with mobile telephony.


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