How Many Channels at the Same Time?

Subcutaneous Transmitters (SCT), Implantable Inertial Sensors (IIS), Head-Mounting Transmitters (HMT), Implantable Stimulator-Transponders (IST)
Kevan Hashemi
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How Many Channels at the Same Time?

Postby Kevan Hashemi » Fri Jul 14, 2023 10:24 pm

Yunan Gau writes with the following question.

"I have another question: up to how many channels can we record at the same time in the same rack and still have good signals? Neela and I have 7 of dual channel transmitters recording now and plan to do 5 more dual channel transmitters next week, and Luiz will start 24 single channel transmitters recording from next Monday, and there'll be others recording as well. These transmitters will be spread into 2 racks, with 50 or more channels recording at same time. But I'm concerned with signal quality when there are so many channels recoding together. Many thanks!"

That is a good question and a justifiable concern. The short answer is, "Its' going to be fine." I will post later with a more detailed answer.
--
Kevan Hashemi, President
Open Source Instruments Inc.
www.opensourceinstruments.com

Kevan Hashemi
Site Admin
Posts: 52
Joined: Fri Nov 11, 2022 1:21 pm

Re: How Many Channels at the Same Time?

Postby Kevan Hashemi » Mon Jul 17, 2023 11:57 am

Dear Yunan,

There is no simple answer to this question, other than my usual, "It will be fine." Here is the complicated answer.

Total Sample Rate

When we want to determine if our telemetry system can record from all our transmitters, we start by calculating the total sample rate in units of samples per second (SPS). Suppose we have twenty of a type of two-channel transmitter providing 256 SPS per channel, plus ten single-channel transmitters providing 1024 SPS. Our total sample rate is (20 * 2 * 256) + (10 * 1024) = 20 kSPS. There are two phenomena that limit the total sample rate that our telemetry system can accommodate.

Collision Limit

When two transmitters transmit a sample at the same time, we say they "collide". A collision can lead to the loss of one or both of the transmitter samples, depending upon the arrangement of our antennas. We discuss collisions in detail in SCT Collisions. The collision limit is the maximum sample rate for which telemetry reception remains robust. With one receiving antenna, we obtain robust reception for up to 6 kSPS. We say the collision limit is 6 kSPS. When we have two receiving antennas, each in a separate Faraday enclosure, our collision limit is 12 kSPS. These two receiving antennas must, however, be independent. That is: each antenna must be connected to its own receiver input. If we combine two antenna cables before connecting to a receiver input, the two antennas are no longer independent, and our collision limit is only 6 kSPS. When we have eight independent antennas in eight independent Faraday enclosures, our collision limit rises to 48 kSPS.

Suppose we have eight independent antennas in a single Faraday enclosure. We might have our animals in an IVC rack (individually ventilated cage rack) covered with a Faraday canopy (FE5A). All eight antennas are in the same enclosure, so when two transmitters collide, they could, in principle, collide at every antenna. Our collision limit could be as low as 6 kSPS for the entire rack. But this pessimistic calculation assumes that every antenna is always able to receive from every transmitter. In practice, there are always one or two antennas in the enclosure that fail to receive from any given transmitter. These antennas are in dead spots with respect to the transmitter. These dead spots are caused by destructive interference of radio frequency waves arriving at the antenna. There are many paths for a transmission to arrive at an antenna within a Faraday canopy: five of the six walls are perfectly reflecting. When two transmitters collide in our large enclosure, there will always be one or two antennas that receive only from the first transmitter, and one or two that receive only from the second, so the collision causes no loss. We might hope for the collision limit to be (8 * 6 kSPS) = 48 kSPS. As a rule of thumb, however, we recommend 24 kSPS as the collision limit within an IVC rack with eight antennas.

The Animal Location Tracker (ALT) provides sixteen detector coils, but they are not independent. Any collision that takes place over an ALT results in the loss of one or both of the original samples. The ALT's collision limit is 6 kSPS. If two ALTs occupy the same Faraday enclosure, they share this 6 kSPS, so we can place 3 kSPS over each ALT platform and obtain robust reception.

Readout Limit

Once the telemetry samples have been received, they must be stored in the receiver's local memory and downloaded to our data acquisition computer. Each telemetry receiver places its own limit on the number of samples we can download per second, which we call the readout limit. The readout limit is affected by the hardware that communicates with our computer, but also by the way that the receiver manages duplicate messages. If eight antennas receive the same message, we have eight copies of the exact same message, or eight duplicates. Ideally, the receiver will eliminate all but one of the duplicates, reducing the number of samples it must save to memory by a factor of eight. Before duplicates can be eliminated, however, they must first be read out of the receiver's detector modules and compared by the receiver's logic. In some receivers, duplicate elimination is more efficient than in others.

The Octal Data Receiver (ODR) eliminates duplicates efficiently. Its readout limit is set by communication with a LWDAQ Driver, and between the LWDAQ Driver and our data acquisition computer. This limit is approximately 125 kSPS.

The Telemetry Control Box (TCB) readout limit is 160 kSPS, but this limit includes duplicates. In a Faraday canopy with eight antennas, we are likely to receive four copies of every transmitted sample, so our readout limit will be 40 kSPS. We note that it is not a good idea to put all sixteen of a TCB's antennas in a single Faraday canopy, because doing so would increase duplicates without improving reception. Indeed, in our own measurements of reception versus the number of antennas in an IVC rack enclosure, we find that four antennas are sufficient to provide robust reception, in which case our readout limit will be roughly 80 kSPS.

The Animal Location Tracker (ALT) produces twenty-byte messages: four bytes for the channel identifier, sample value, and timestamp, plus another sixteen bytes for the sixteen power measurements provided by its sixteen detector modules. The ALT's maximum readout rate is 1000 Byte/s, so its readout limit is 50 kSPS.

Examples

(1) In your case, "Neela and I have 7 of dual channel transmitters recording now and plan to do 5 more dual channel transmitters next week, and Luiz will start 24 single channel transmitters recording from next Monday, and there'll be others recording as well. These transmitters will be spread into 2 racks, with 50 or more channels recording at same time." Looking at your recent orders, most of your channels are 256 SPS, but some of them may be 512 SPS. The total sample rate for 50 channels will be less than 20 kSPS. When divided between two IVC racks, each with their own ODR, your sample rate per rack will be less than 10 kSPS. Your collision limit will be 24 kSPS per rack and your readout limit is 250 kSPS per rack. In short: it will be fine.

(2) We have a selection of transmitters with a total of 20 kSPS implanted in ten mice that live in two cages. We put the cages in an IVC rack and load the rack up with sixteen antennas connected to a TCB. We cluster the sixteen antennas around the two cages because we really want to make sure we get superb reception. We do such a good job that all sixteen antennas receive all signals from all transmitters. Our collision limit is 6 kSPS. Reception will be poor. Our readout limit is 10 kSPS (160 kSPS divided by sixteen for duplicates), so we won't even be able to retain all of the samples that we receive. We disconnect 8 of the 16 antennas and distribute the remaining 8 antennas throughout the enclosure. Our collision limit rises to 24 kSPS and our readout limit rises to 40 kSPS (160 kSPS divided by four for duplicates). In short: it will be fine if we arrange the antennas correctly.

(3) We have 32 transmitters that each produce 256 SPS implanted in 32 mice. The mice are in eight cages. We place each cage over an ALT. We have two ALTS in each of four bench-top Faraday enclosures. Each Faraday enclosure contains 8 of the 256 SPS transmitters for a total sample rate of 2 kSPS. The collision limit in each Faraday enclosure is 6 kSPS. The readout limit for each ALT is 50 kSPS. In short: it will be fine.

Best Wishes, Kevan
--
Kevan Hashemi, President
Open Source Instruments Inc.
www.opensourceinstruments.com


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