Amplitude if I use 16ch weaker than 8ch

Hi all,
I did simple experiment, comparing amplitude of waves between 8ch and 16ch. I used a default electrode and Gain was 8x because I also use ThinkPulse active electrodes. I used only electrode at Fp1. Other settings were default. The first image shows a wave with 8ch.

Similarly, the second shows a wave with 16ch.

Although amplitude was around 15uV when I used 8ch, amplitude was only 7~8uV when I used 16ch. I only changed whether Daisy board was active or not. I did not modify any other software and hardware settings. Do you know why it happens? Intuitively, I guess that amplitude with 8ch was fine but when I used 16ch, amplitude was nearly half the size of it.

Comments

  • wjcroftwjcroft Mount Shasta, CA
    edited May 27

    The annotations at the lower right of the time series may sometimes be confusing. The number represents a short-term average (RMS = root mean square) of the amplitude at the moment. If you have a noise spike, as you do (two of them both left and right) in the 8ch image, it will artificially inflate the number shown. I'm not clear what the averaging period is, it's likely just a second or two, since it is designed to reflect a 'present time' sense of the average voltage -- not long term.

    The only way such a comparison could work, is that if you look at your raw data (filtered say .5 Hz to 45 Hz, and mains notch), and compare the quiet periods average values. My understanding of the two ADS1299 chips involved in the 16ch system, is that they operate identically in terms of voltage reference values and scale factors.

    William

  • hirohiro Japan
    edited May 27

    Hi William,
    Thank you for your reply. I may solved this issue. I tested again with two electrodes, Fp1 and Fp2. I changed active electrodes from default ones. I activated Daisy module. I mediated to avoid causing spike. Other settings were the same as former experiments. The results will show below.

    I got wave has around 15uV amplitude like 8ch. I believe that at least these two electrodes worked well. I will do a next experiment with all of active electrodes.

    Hiro

  • wjcroftwjcroft Mount Shasta, CA

    @hiro said:
    ...
    When we use 16ch system, Cyton+Daisy Boards send average of two electrodes, like Fp1 and Fp2. So, if we use only Fp1, data will be average of value of Fp1 and 0. This data is obviously failed one.

    No, your reading of the 16ch section is not correct. In 8ch mode the sample rate is 250Hz and that is the rate of the radio packets sent, each packet has 8 channels. In 16ch mode, sample rate effectively drops to 125Hz and it takes two radio packets to send all 16 channels. The averaging you mention is only applied WITHIN the same channel. It does not average across channels.

    https://docs.openbci.com/Cyton/CytonDataFormat/#16-channel-data-with-daisy-mdule

  • hirohiro Japan
    edited May 28

    Hi William,
    Thank you for your reply. OK. I deleted a part of thread about this point because someone will be confused. (Do not need to do it?)

    The averaging you mention is only applied WITHIN the same channel. It does not average across channels.

    I understand. I felt so weird when I read the section but it was just misunderstandings. Then, do you think I did still something wrong? I just added up the number of electrodes and the problem disappeared. Using only one electrode is exceptional situation. This is just a test phase to check whether electrodes work correctly or not. I thought if I can observe enough strength wave of amplitude when I used 16 (or 8) electrodes, that is OK though I feel a little bit anxious because I do not know why amplitude decrease only if I use a single electrode. Do you think some serious issue happens? If not, I do not take care it but if yes, I should remove the reason.

    Hiro

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