How can i get Live Amp Impedance Data? #303
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This is pretty complicated but everything you need to know is in the API
to LiveAmp (Amplifier_LIB.h) and the class defined in LiveAmp.h/c.
There is a function called ampGetImpedanceData which will grab the data
from the enabled channels for you (it demuxes the same way that data
grabbed via ampGetData does) and you can check electrode type with
ampGetProperty using the CPROP_I32_Electrode property. Ground is defined
in the hardware and the reference channel can be set with ampSetProperty
and the CPROP_B32_ReferenceChannel property (you also have to tell it
which channel is the reference channel).
"Read the source, Luke!" and good luck!
…On 06/14/2018 02:30 PM, antonello87 wrote:
Hello,
I am trying to get the impedance data with the Live Amp amplifier. I
have seen in the library (Amplifier.h) that there is a method for
doing this (getImpedanceData) but till now I wasn't able to get what I
want.
As I understand from the description of the library the data might be
returned differently compared to the getData method (there are
additional values, such as GND and REF for example) and maybe the
buffer that receives and demuxes these data should be built differently.
Am I wrong? Has someone successfully tried to do the same thing? Do I
have to enable channels differently? How can I tell the library what
kind of electrode (active or passive) I am using?
Thanks in advance,
Antonello
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Dear dmedine,
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The impedance grabbing functionality of Amplifier_LIB.h is not
implemented in LiveAmp.c/h. If you want to use it, you need to call the
functions in Amplifier_LIB.h yourself. I haven't actually done this
myself yet, so I might be inaccurate, but the comments in
Amplifier_LIB.h are pretty clear and this is how I interpret them (could
be wrong, though...).
* Am I right? Is the packet composed like this?
Yes.
* Are the impedance values returned by the method actually INT_32 or
are they FLOAT_something?
Should be float and in Ohms according to the library comments. So your
buffer that you pass to getImpedanceData will have to be of type float
and the size of the number of channels you are checking, i.e. the number
of enabled EEG channels. Triggers channels should not have anything to
do with this..
* Do I have to take other not enabled channels into account for the
sample size and then the offset?
Check the sample size to be sure, but my guess would be that you need to
enable the channels you want to read and the amp will give you the data
for those channels alone.
* Do I have to set the reference channels even if I have the REF and
GND channels placed on the earlobes and i want to use them?
GND is always set in hardware. I seem to remember the same is true for
REF also. So probably you don't need to do anything special with them.
* More specifically, can I access GND and REF channels to set the
reference channel to the REF clips I have?
Should always be set to the black one (GND) and the blue one (REF).
* What are the "always enabled" Digital channels for?
Not sure what your referring to here. Is "always enabled" in the code
comments somewhere? Please point me to this.
* Thanks a lot and sorry for all these questions,
No problem! I hope my answers are actually correct. When I am back at my
desk I can check to make sure, but that won't be before a couple of
weeks. Hopefully this is enough to get you started, though.—
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Thanks dmedine for your prompt answer, following some comments based on what I tried today after your answer:
I created a method pullImpedanceData in the LiveAmp.h and LiveAmp.cpp which grab the impedance in the same way pullAmpData does.
The receive buffer cannot be a vector of float because I believe, but I may be wrong, that ampGetImpedanceData returns a flow of bytes the same way ampGetData does. This means that I need, also in this case, to call the pushAmpData method which scans the received byte flow and demuxes it correctly based on the channel data type.
Code from LiveAmp.cpp, almost the end of enableChannels function
P.s. I will be very grateful if you can check when you will be back at your desk :) |
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pushAmpData is merely a convenience function for transforming the data
buffer needed by ampGetData (void*) and LSL's push_data
(std::vector<T>). It has nothing to do with getting impedance data and
is undoubtedly doing the wrong thing if you are using it to demux.
I don't clearly understand what you are trying to do, design wise. Do
you want to transmit impedance data via LSL? Do you need to check
impedances intermittently during a recording? If you do want to do this,
I would recommend making a completely separate process and a separate
outlet just for impedances. The data is quite different than the EEG
data (structure-wise, different units, etc.), so it doesn't make sense
to combine the two somehow.
I will play around with this in the coming weeks and come up with a
simple working example. Ping me if you don't hear from me after a while.
As for the CT_DIG channels, I will have to work back through what that
is for when I have an amp in front of me. I think you can probably just
ignore that for now.
…On 06/19/2018 05:39 PM, antonello87 wrote:
Thanks dmedine for your prompt answer, following some comments based
on what I tried today after your answer:
The impedance grabbing functionality of Amplifier_LIB.h is not
implemented in LiveAmp.c/h. If you want to use it, you need to
call the functions in Amplifier_LIB.h yourself.
I created a method pullImpedanceData in the LiveAmp.h and LiveAmp.cpp
which grab the impedance in the same way pullAmpData does.
Are the impedance values returned by the method actually INT_32 or
are they FLOAT_something?
Should be float and in Ohms according to the library comments. So
your buffer that you pass to getImpedanceData will have to be of
type float and the size of the number of channels you are
checking, i.e. the number of enabled EEG channels.
The receive buffer cannot be a vector of float because I believe, but
I may be wrong, that ampGetImpedanceData returns a flow of bytes the
same way ampGetData does.
This means that I need, also in this case, to call the pushAmpData
method which scans the received byte flow and demuxes it correctly
based on the channel data type.
But doing as described the problem still remains, as you can see from
the attached image, I keep receiving inconsistent value every column
in the image refers to an electrode. In this case I run the program
with REF and GND on my earlobs and just one electrode out of six (the
number 1) touching the head.
This led me to think that maybe the problem is in the pushAmpData
function which in this case should demuxes the data differently from
when I want to acquire EEG data.
impedancecheckoutput
<https://user-images.githubusercontent.com/32123994/41606133-44a9ff1c-73e3-11e8-843c-3c983a5a812c.png>
What are the "always enabled" Digital channels for?
Not sure what your referring to here. Is "always enabled" in the
code comments somewhere? Please point me to this.
Code from LiveAmp.cpp, almost the end of enableChannels function
|if(cnt!=enabledChannelCnt+2)|
|// add two because we are ignoring (for now) the CT_DIG channels,
which are enabled|
P.s. I will be very grateful if you can check when you will be back at
your desk :)
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Basically I would like to check the impedance just once, before starting the
Thank you very much for that :) |
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Dear dmedine,
As promised I ping you :) Thanks a lot. |
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Dear dmedine, Thanks in advance and please forgive my stubborness. |
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I'm not really in a position where I have time to implement this right
now. However, here is how to do it:
1. stop recording with ampStopAcquisition
(https://github.com/sccn/labstreaminglayer/blob/master/Apps/BrainProducts/LiveAmp/Amplifier_LIB.h#L447-L450)
2. put the amp into impedance mode with ampSetProperty
(https://github.com/sccn/labstreaminglayer/blob/master/Apps/BrainProducts/LiveAmp/Amplifier_LIB.h#L447-L450)
3. in a loop request the impedance data with ampGetImpedanceData
(https://github.com/sccn/labstreaminglayer/blob/master/Apps/BrainProducts/LiveAmp/Amplifier_LIB.h#L447-L450)
4. let it run for 30 seconds or so, then take a few seconds and average
5. push the average impedances out of an LSL marker outlet
6. reset the amplifier into recording mode with ampSetProperty
7. resume recording with ampStartAcquisition etc.
Anyway, if I were to send impedance values as a marker every so often,
that is how I would do. The data could also be sent as a signal.
If you know C++, everything you need as an example is in the source code
(especially in Amplifier_LIB.h and the calls to that library in
LiveAmp.h/cpp). But, you can see that implementing this is fairly
complicated from a design standpoint---particularly since in order to do
it, you need to stop recording for about a minute in order to get
accurate impedance measurements. It is not possible to acquire EEG data
and impedance data at the same time.
…On 8/27/2018 12:33 PM, antonello87 wrote:
Dear dmedine,
I'm sorry to bother you again but I have to try to get these impedance
data as soon as possible.
Did you manage to come up with some working examples?
Thanks in advance and please forgive my stubborness.
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Hello,
I am trying to get the impedance data with the Live Amp amplifier. I have seen in the library (Amplifier.h) that there is a method for doing this (getImpedanceData) but till now I wasn't able to get what I want.
As I understand from the description of the library the data might be returned differently compared to the getData method (there are additional values, such as GND and REF for example) and maybe the buffer that receives and demuxes these data should be built differently.
Am I wrong? Has someone successfully tried to do the same thing? Do I have to enable channels differently? How can I tell the library what kind of electrode (active or passive) I am using?
Thanks in advance,
Antonello
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