Just came across this- wireless Tx and Rx

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Just came across this- wireless Tx and Rx

Postby mvenkatd » Fri Nov 30, 2007 8:42 am

I really don't have a clue about the technicalities of the whole project. I was browsing for info about wireless recievers and transmitters and came across this site... I thought I'll put it thru here if it can be of any use for the Strobist Trigger...
http://www.e-dsp.com/how-to-build-your- ... plication/

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Postby JonSenior » Sun Dec 02, 2007 8:42 pm

I'd looked at these originally, having played with similar units in the past. The major problem is their baud rate (2400 - 1200 in tests according to the article you linked). This means we get to transmit 2 full bits before we've passed the threshold for 1/1000s triggering. We need to get at least 8 bits across (preferably more), with room for expansion in order to maintain the triggering rate.

Interesting page though. Shows how simple the interfacing modern micros can be.
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Postby mvenkatd » Mon Dec 03, 2007 4:45 am

So, we need a transmitter with a higher baud rate...say in the range of 4000+ is it?
Pardon my ignorance..but am an absolute novice at electronics :)

This means we get to transmit 2 full bits before we've passed the threshold for 1/1000s triggering

Does that mean you're looking at a unit that should be able to sync at 1/1000s??

Shows how simple the interfacing modern micros can be

Haha...still pretty much greek and latin to me :D

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Postby mvenkatd » Mon Dec 03, 2007 6:22 am

I googled around a bit and found these...Not sure if these would help when Seaton is already working on the prototypes...but you guys here will be able to make more sense of this than me...



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Postby seaton » Wed Dec 05, 2007 2:29 am

I'm not a RF person, but from research I've found that the cheaper wireless modules use ASK as their modulation, this basically means that a binary 0 is reprsented as no trasnmission and a 1 is represented with a RF transmission of a specific amplitude. So these are more suseptible to interference, i.e. if noise is picked up when a 0 is being transmitted, then it will probably be picked up as a 1, so you have to have some sort of packet encoding going on. Also they have a pretty slow baudrate, 300 1200 baud I think

The modules I'm using (hopeRF) use FSK as their modulations, which means it varies the frequency of the transmitted signal for a 0 and 1. These are less suseptible to noise and can achieve higher speeds. Their datasheets have a max speed of 115kbps using the internal digital filter, and more if you use an external one, however I would suggest these are ideal conditions.

Another thing I've found is that speed and range are opposite, i.e. the faster the speed then range is affected, and visa-versa, also speed/range is also a factor of what band you use. i.e. the 433MHz will have good range but lower speed, while 915MHZ will have great speed but less range. The HopeRF modules do state a range of 250m while they also have a model that will do 2000-3000m at 433MHz and 915MHz While these operate at 5V they may be worth investigating, when I ordered mine I tried to get some of the extended range ones, but they were on back order production, and the sales person didn't know when they would be in. Also if we use frequency hopping then we can legally up the power of each bust of transmission, which should allow us even more reliable reception and disance.

I feel that the 1/1000th shutter speed could be reachable but will be distance limited, so therefore I'm just concentrating on standard sync speed at the moment, then try and see how far it can be pushed in terms of speed and distance once we get something working. Really we need to transmit 5 Bytes for a successfull trigger request (based on the draft protocol I've put up in the wiki).

I've just done a quick calculations and to achieve 1/1000sec the bitrate will need to be 57600bps to transmit and receive 5 bytes before 1/1000 of a second, (works out to be 696uSec) so really by the time the flash fires the shutter will be about 2/3 of the way across, this does not account for any overheads so in theory it may need more speed. while the wireless modules are certianly capable of it, I don't know what the range will be like at that speed, but I would hazard a guess that it will be less than say 1/250. While at a lower shutter speed the bitrate can be something like 19200bps.

I guess once we get an idea of range and speeds we could have the units switch to long range mode via the firmware etc if we want the extra distance, but at a lower sync speed. But at close proximity say in a studio situation the high sync speedand range will be fine.

Here is a table of shutter speeds and calculated bitrate (please correct me if I'm wrong)

    1/1000 57600 bps
    1/500 38400
    1/250 19200
    1/100 9600
    1/60 4800

Anyway I'm hoping to have a beta version of the RF library online by the end of today (depends how busy I'm at work today) which should allow me and those I've spoken to that have also purchased the hoperf modules to start playing with the RF module more, so we can get some ideas asto range and bitrate.

Also things are getting busy for me leading up to christmas :( but will be trying to squeeze in some development time.
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