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Radiotriggers underwater


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#1 DuikKees

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Posted 20 October 2006 - 03:28 AM

Has anyone ever tried radiotriggers underwater, to trigger the strobes?
Life could be so much easier if the hassle with synccords would be eliminated.

I am thinking of small units that can be screwed on the bulkheads.

Edited by DuikKees, 20 October 2006 - 03:29 AM.


#2 Giles

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Posted 20 October 2006 - 05:31 AM

Might want to test transmitting through water, I know some radio signalling devies have a huge hard time. I remember someone testing Bluetooth underwater and it not working so well also.
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#3 acroporas

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Posted 20 October 2006 - 05:34 AM

Has anyone ever tried radiotriggers underwater, to trigger the strobes?
Life could be so much easier if the hassle with synccords would be eliminated.

I am thinking of small units that can be screwed on the bulkheads.


I think part of the reason is that radio signals travel very poorly underwater so it would probably not be as reliable as a wired connection. This is why airplanes use radar, but subs use sonar. Too bad sound would be too slow.
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#4 mattdiver

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Posted 20 October 2006 - 06:47 AM

I think part of the reason is that radio signals travel very poorly underwater so it would probably not be as reliable as a wired connection. This is why airplanes use radar, but subs use sonar. Too bad sound would be too slow.


Not so. Sound travels 5x faster underwater than in air, say around 1,750m/s. Assuming a strobe to housing distance of 1m for the sake of discussion, the signal would be transmitted in about 0.5ms. That's pretty damn fast!

#5 DuikKees

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Posted 20 October 2006 - 07:15 AM

I did some research this afternoon and it looks like that radio isn't any good underwater indeed: http://www.qsl.net/v...UwaterComms.htm

edit: Sorry, link isn't working anymore

So it is time to develop a Wireless Soundcontrolled Flash Triggerdevice

Edited by DuikKees, 20 October 2006 - 07:17 AM.


#6 Giles

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Posted 20 October 2006 - 07:20 AM

The speed of light is 195 814.799 times faster than even sound in sea water the densest and hence fastest of all water, I don't know the math .. but I am pretty sure that that won't work. Sound is at the most about 4.5 times faster in salt water than it is in air, this still isn't a patch on light which we are dealing with.

However if the distances are small enough it may be fast enough ... besides .. i don't think there are any Sonar remote flash transmitters on the market .. i could be wrong though !
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#7 wagsy

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Posted 20 October 2006 - 08:26 AM

VLF will penetrate the water. They use it here to send messages to the subs, however they are transmitting at 2 million watts...how many batteries it that.

The sine wave is like more than 10 km long as well, need a long aerial.

I think sonar would be to slow.
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#8 james

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Posted 20 October 2006 - 08:35 AM

I think Matt has shown that ultrasonic transmission IS fast enough.

Cheers
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#9 ikelite

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Posted 20 October 2006 - 09:02 AM

Ikelite researched everything trying to eliminate sync cords, but only light was fast enough to communicate both sync and the TTL quench signals................

#10 herbko

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Posted 20 October 2006 - 09:12 AM

Ikelite researched everything trying to eliminate sync cords, but only light was fast enough to communicate both sync and the TTL quench signals................


So when are you replacing those cables with optical fiber?
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#11 Giles

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Posted 20 October 2006 - 10:20 AM

I think Matt has shown that ultrasonic transmission IS fast enough.

Ok this interested me enough to do some learning and some math and ask for some help from Powerful Seagul, Wikipedia, and Google. But I had heard about people trying this before told it wasn't possible so I thought I would work it out.

I pumped all sorts of different variations into this formula at wikipedia I went from 0.005Mhz (the start of Ultrasound) and went up to 100Mhz .. and I used the aaenuation coefficient of regular water (maybe distilled) and you always get the same answer .. no matter how powerfull the signal (dB) over the range i chose of 60cm i thougt being a good housing to strobe distance for WA, you always get 100% attenuation, as in the wave will not have any power.

Speed isn't everything, said the turtle to the hare.

the cool thing to do would be to have a bulk head that fired a laser grid out like you get at the super markets for scanning barcodes .. it would have to be sort of directional, and it hits a sensor on the side of the strobe (yes new strobe design) and that slaves the strobe basically, no wires ... ooh i need a napkin and a leaky biro .. i need to sketch.
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#12 mattdiver

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Posted 20 October 2006 - 06:24 PM

Giles,

I think you come to these conclusions because you're mixing up 2 formula linked to 2 different concepts.

In you Wikipedia link, the formula for attenuation of sound waves is correct, and the definition of attenuation in optics is also correct.

The problem is that the latter does not apply to acoustics. In acoustics, the definition of attenuation is a(dB) = 20 x log (p0/p)

When used with values of alpha=0.0022, l=100cm, and f=1MHz, you come up with an attenuation of about 2.5% only. Even squared to account for the loss of signal in two directions (camera-to-strobes-tocamera), 5% attenuation is totally acceptable.

Now I look like a proper nerd :D

Put in simpler terms, think about a boat on the surface. You can hear it from 1 mile away underwater and still think it's overhead. Now, shift the frequency up into the ultrasound (increase the frequency by a factor of say 1,000), and cut the distance by the same factor, you can see from the attenuation formula that the net effect of the absorption of sound waves underwater will be the same in both cases. The original sound pressure may be vastly different, but it will also be hardly reduced while travling in water.


With regards to speed, as far as I can tell, trigger and TTL signals are in the tens of millisecond range. When compared to say 0.5ms travel time for a sound wave, it seems a conceptually viable option, even though Ike may have not found it workable in practice...

#13 herbko

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Posted 20 October 2006 - 06:51 PM

Now I look like a proper nerd :D


Welcome to the club. :D

I think you're right. Sound travels too well. If everybody uses this scheme you'll set off strobes over the entire divesite every time you shoot. It's hard to build small acoustic filters to make enough channels to avoid that.
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#14 DuikKees

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Posted 18 November 2006 - 03:01 AM

It looks like that there are not many options availible. :guiness:

But how is the communication done with the hoseless divecomputers, such as the Uwatec smart Tec?