Garmin Descent MK2

[dmaziuk]

I suppose if you're only looking at your computer when holding hands in front of you, and you're only holding hands out when you're shooting pictures with the strobe, there is a very high chance of having the strobe's EMP messing up the pressure transmission every time you look...

 

[tursiops]

@ATJ I agree. I don't see how a strobe flashing could do more than make it miss one transmission. 5 seconds later, it should get the next transmission. Unlesss.... If the strobe somehow creates a disruption that has an effect that lasts longer than the actual strobe burn time. Say, maybe it's something about the EM radiation that is put out when the strobe is recharging after it fires. Then it could have an effect that lasts 2 seconds or more (depending on the strobe and at what power you fired it). If that is happening, then you wouldn't even have to get THAT unlucky to disrupt the AI signal multiple times in a row.

That said, I personally am more inclined to think that @Peter Bomberg has been unlucky enough to have a flaky Aqualung 770 AND a really old transmitter (that came with his Atom 2) and it's really those two specific devices that are having the issues.

Like you, I have been diving regularly with a camera with 2 x Inon Z240. I usually take 100 - 200 photos on each dive, and I have never noticed any interruption in my AI. Other than an occasional momentary dropout that seemed to correlate to having the computer at a bad angle/distance to receive the AI transmission.

I concur with this. I've only used the PPS MH8A transmitters, and only listened to them with an Oceanic OC1 and two Shearwaters (Perdix AI and Teric), but see only very rare dropouts, never blackouts for consecutive 5s transmissions, despite taking 200-300 pictures, mostly with two Sea&Sea YS-D1 strobes. The spike EM signal from the strobe has a whitenoise spectrum, so covers the 38kHz band where the MH8A operates. I can see how there might be interference for just that moment, and I suppose it might interfering with both or either the transmitter and the receiver. But I don't see how it lasts beyond the moment of the EM spike.

 

[stuartv]

I suppose if you're only looking at your computer when holding hands in front of you, and you're only holding hands out when you're shooting pictures with the strobe, there is a very high chance of having the strobe's EMP messing up the pressure transmission every time you look...

Because... there's a "very high chance" that the strobe's EMP that lasts around 1/1000 of a second will overlap in time with the AI transmission that happens once every 5 seconds? How long are you asserting that the AI transmission actually lasts?

For the record, my anecdotal experience is that the AI transmissions are picked up by the receiver the vast majority of the time - even when my hands are not out in front of me. If that were not the case, then when I did hold my computer up to read the cylinder pressure, I would often have to wait 1 - 4 seconds for it to receive a transmission before it would show me a reading. In reality, it almost always is showing me a pressure at the moment I hold it up to read it. And, by "almost always", I mean 99+% of the time.

 

[dmaziuk]

Because... there's a "very high chance" that the strobe's EMP that lasts around 1/1000 of a second will overlap in time with the AI transmission that happens once every 5 seconds? How long are you asserting that the AI transmission actually lasts?

It doesn't need to jam the whole transmission, only to garble it enough. I'm guessing your major wasn't computer communications? -- it's what the 8th ASCII bit, checksums, and the rest if is for in the digital transmission world. I've no idea how the WAI transmissions are coded, of course, and how much interference it would take to invalidate one, but from my vague recollection of Comms 101 it may not take much.

You're absolutely right that it should recover on the next re-transmit. That was the punch line: if you're only checking your tank pressure while firing the strobe...

 

[stuartv]

It doesn't need to jam the whole transmission, only to garble it enough. I'm guessing your major wasn't computer communications? -- it's what the 8th ASCII bit, checksums, and the rest if is for in the digital transmission world. I've no idea how the WAI transmissions are coded, of course, and how much interference it would take to invalidate one, but from my vague recollection of Comms 101 it may not take much.

You're absolutely right that it should recover on the next re-transmit. That was the punch line: if you're only checking your tank pressure while firing the strobe...

I'm guessing your major wasn't actual communications? I didn't say anything that even implied that it had to jam the whole transmission. I specifically said "overlap" - meaning to garble some part of it.

If the strobe EMP is 1/1000 of a second and the AI transmission is 4.9 seconds long (out of every 5 seconds), then I might could see how your assessment of "very high chance" of messing up the AI transmission could be a reasonable statement. But, if the AI transmission also only takes 1/1000 of second, then I don't think I would agree with calling it a "very high chance" of stepping on each other. But, that's just me and my subjective opinion. I don't consider 1 in 5000 a "very high chance".

Thus why I asked how long you are asserting the AI transmission to take...

I note that even if the computer lost communication and didn't re-establish it until you hold your hand up, that has no effect on when the transmitter transmits. It's not like putting your hand up and taking a photo means that the AI transmission is somehow more likely to happen just at that time.

 

[tursiops]

I've never found a spec on the digital encoding for the MH8A. Just the 38kHz RF signal. But, we can make some estimates....Without being the least bit clever (pun), we can say that it takes no more than 12 bits to represent the pressure, and perhaps 5 bits for the battery voltage (which is also trasnmitted). Give it a few bits for a checksum...so maybe 20 bits. Bit rate? certainly 32 bps, more likely higher than that. So, transmission time is at most 1 sec, and i'll bet it is slower than that. Now, we don't even know if it is a digital transmission, and I'll bet it is not. I'll bet it is an analog burst of varying frequencies that encode the pressure and the temp, or perhaps two bursts. It is not even necessary to send the battery voltage on each transmission. Again, no reason to need more than a second. Pure speculation.

 

[stuartv]

I've never found a spec on the digital encoding for the MH8A. Just the 38kHz RF signal. But, we can make some estimates....Without being the least bit clever (pun), we can say that it takes no more than 12 bits to represent the pressure, and perhaps 5 bits for the battery voltage (which is also trasnmitted). Give it a few bits for a checksum...so maybe 20 bits. Bit rate? certainly 32 bps, more likely higher than that. So, transmission time is at most 1 sec, and i'll bet it is slower than that. Now, we don't even know if it is a digital transmission, and I'll bet it is not. I'll bet it is an analog burst of varying frequencies that encode the pressure and the temp, or perhaps two bursts. It is not even necessary to send the battery voltage on each transmission. Again, no reason to need more than a second. Pure speculation.

Don't forget it has to encode the 6 digit transmitter ID in there, too.

 

[Peter Bomberg]

@ATJ and @stuartv, Aqualung seems to agree on the flaky 770 as they are replacing it and the transmitter. The explanation I got (and I will admit I don't understand much of it) was that the Signal/Noise ratio was outside the specs with the combination I have, when the strobe fired it would disrupt the link, normally a single lost transmission should have no impact, but it took the combo several cycles to re-establish the link and so it would flash.

I am not complaining as the service has been great and other than loosing the link everything works perfectly, and I would recommend their products any day.

 

[dmaziuk]

I'm guessing your major wasn't actual communications? I didn't say anything that even implied that it had to jam the whole transmission. I specifically said "overlap" - meaning to garble some part of it.

Yes. A single-bit error is enough to invalidate the whole packet if you don't have error-correcting-fu in the protocol.

The computer may be tracking your tank pressure just fine the rest of time, but if you aren't looking, how would you know? If a tree falls in a forest...

 

[ATJ]

I did a test today with my Perdix AI and Atom 3.0.

Regs with transmitter on a tank. Computers in range and displaying tank pressure. Move computers out of range.

For the Perdix, it is around 30 seconds before you get the yellow "No Comms" with the most recent pressure showing and around 90 seconds for the red "No Comms" with ---. That means it has to miss 5-6 broadcasts from the transmitter before the warning and 17-18 for full error.

For the Atom 3.0 it was around 20 seconds for the most recent pressure to start flashing and around 60 seconds for it to display "-----", i.e. 3 to 4 missed broadcasts to warning and 11-12 for full error.

It is so unlikely that firing strobes could cause a problem that a lottery ticket is probably a good idea.

Now, I have no idea what the Aqualung does bit I'd be surprised if it dropped out with one missed broadcast.