I occasionally check out the Tern Micro website. They are manufacturers of controllers and expansion boards for embedded applications. Their controller boards use IAPx86 class CPUs and are programmed in C. A few years ago, I had checked with them about whether they had components suitable for a field acoustic recorder, and given the short time schedule we had, we decided to go with off-the-shelf components instead for that. Things have changed, though, as I found an expansion board of theirs called the GR4 on their page.
Let me set some context. Some years ago, Whitlow Au and Marc Lammers put together a four-element hydrophone array that allowed them to perform acoustic localization. If I recall correctly, their recording system was based upon a National Instruments DAQ card for CardBus hosted in a laptop computer and was capable of 500 kilosamples per second. When multiplexed across four channels, that’s a max of 125 kilosamples per second per channel. With a multiplexed system, you have to account for time offsets between channels as you analyze the data for time-of-arrival estimates of signals. If there is crosstalk at the high acquisition rates, you might have to drop the total sampling bandwidth to give the multiplexing circuitry time to settle to the next channel’s input level. That at least is how I had to work with an NI PCI-MIO-16-E DAQ card back in 1999. The solution to this problem is simultaneous-sampling, where all the channels of interest get their own sample-and-hold circuitry and the conversion is triggered off the same clock input. Simultaneous-sampling hardware is more expensive, since the main sets of circuits have to be multiplied for the number of channels. Around 2001, a project I was involved with bought a couple of simultaneous-sampling DAQ cards for the PCI interface, at a cost of a couple of thousand dollars each.
Of course, lugging a full-up desktop system into the marine environment is not a thing to be undertaken lightly. If one could instead reduce the field recording part to something that could be effectively shielded from the elements and work instead off of straight DC battery power, it would be all-around more convenient. The more remote the field work, the more convenient that gets.
So let’s get back to the Tern GR4. This analog-to-digital expansion board is small, just a bit longer and wider than a business card. It can be provisioned with two ADC chips and 4 MB of memory (and that full configuration is what I’m talking about). The base price is $129, but with the additional features added the cost is $259. The GR4 boards are stackable. There are pin headers that form a communication and data bus with a controller card. Each GR4 permits simultaneous-sampling of two input channels. Each GR4 with two ADC chips aboard can record to its own CompactFlash card continuously by switching between ADC chips and FIFO memory, allowing the just-converted data from one FIFO to be streamed to the CF card while the other is collecting newly-converted data. Because the GR4 units are stackable, you can run several together at once. The Tern page shows a stack of four GR4s and a controller card. The maximum sample rate for the GR4 is 500 kilosamples per second. This means that each simultaneously-sampled channel can be recorded at that 500 kilosamples per second rate. It does 16-bit conversion, which gives good dynamic range to the recordings.
So the technical problem of getting to a four-channel field-deployable data recorder capable of capturing most of the acoustic information from a dolphin click has gotten both easier and cheaper with Tern’s GR4. I had a chat with a technical representative at Tern going over what would be needed for this application, and basically got a recommendation for a couple of different controllers that could do the job with the addition of two GR4 units. Tern offers an evaluation package of a controller board plus the interface hardware and software needed for system development at $249. Add-on options are additional cost. For one of the boards, I’d be interested in an LCD 16×2 readout, RTC clock, CompactFlash interface, and switching regulator, which would add another $100 to the $249 evaluation kit price. So for $349 + 259 + 259 = $867, I’d have that part of the data recorder in hand. Of course, I’d still be looking at a variety of additional costs in development, but this makes contemplating the task that much more feasible.
There are some additional concepts that ought to be broached. For two GR4s, one has to provide CF cards for each. It is pushing the hardware to get continuous sampled data out to the CF card on each expansion card. Trying to move the data over the bus to the controller and out to its CF card just isn’t feasible. There is no file system involved on the CF cards; the data is written to absolute sectors. This makes it a bit more interesting pulling that data off for analysis. In development, it will be up to the programmer to track which sectors go with which recording if multiple recording sessions are used. The signal input range for the ADC circuitry is 0-5V, which means that the output of many amplifiers will have to be conditioned to fit in that range. When recording two channels at 500 kilosamples per second, the total data bandwidth is 2 million bytes per second. So each CF card will receive about 7 gigabytes of data per hour of recording operation. A 32 GB card should be good for over four hours of data recording before needing to be swapped out. The Tern rep estimated that my stack of a controller plus two GR4s would pull around 500 mA of power at 5V while recording. The A-86-P controller at least has on-board power regulation so that it handles DC input from 8.5V to 24V and delivers regulated 5V power to its stack. I figure something like a motorcycle 12V battery would likely provide enough juice for a day’s worth of recording. When not actively recording, though, the controller and its stack can go into a sleep mode that draws only a few mA, which saves a lot on battery power.
I was told by the Tern rep that the GR4 was developed for the needs of a research group doing field work on bat biosonar. It’s no wonder that it caught my eye when I ran across its description.
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