Impedance measurement

LR__07 · October 2025

Hello,
I'm currently trying to measure the impedance of my Cyton board channels in Python using Brainflow. However, I consistently get this error:
'utf-8' codec can't decode byte 0x9e in position 2: invalid start byte
My plan is to iterate over all channels and compute the impedance. Therefore, I implemented the following workflow:
1. Select Channel i
2. Send "z{i}01Z" via config_board() to the board
3. Starting the stream via start_stream()
4. Get the data via get_board_data()
5. Stop the stream via stop_stream()
6. Send "z{i}00Z" via config_board() to stop the LeadOff Impedance
7. Perform preprocessing on the data (Bandpass (8,30) and Notch(50))
8. Calculate the impedance: impedance = ((math.sqrt(2.0) * preprocessed_data[i]*1e-6)/6e-9) - 2.2
9. Repeat for all channels
10. Set the gain to 12 for EEG measurement

The problem appears already at step 6 for the very first channel—calling config_board("z{i}00Z") throws the exception above.
Could someone explain what I’m doing wrong or what I should change to avoid this decoding error?
Thanks in advance!

wjcroft · October 2025

LR, hi.

There should NOT be any Unicode characters whatsoever, in the command string. All the bytes you send to the board are ASCII only, in the low 7 bits of each byte; top bit zero. The top bit 0x80 (10000000 binary byte) can never be set. Your error message says you sent a byte 0x9e (10011110), which has the top bit set.

https://docs.openbci.com/Cyton/CytonSDK/#leadoff-impedance-commands

William

wjcroft · October 2025

Please look through some of these search results, relating to user app impedance measurements:

https://www.google.com/search?as_q=impedance+commands&as_sitesearch=openbci.com

LR__07 · November 2025

Hi William,
thanks for your answer. I'm still a bit confused, because I never send any non-ASCII code and I also turn off the stream and empty the RX-Puffer before sending the next config command. For clarification, here's the exact snippet I'm using:

    for i in BoardShim.get_eeg_channels(self.controller.board_id):
                config_code = (f"z{i}01Z").encode("ascii") 
                command_reply = self.board.config_board(config_code)
                print(f"Impedance Command reply: {command_reply}")

                if command_reply is not None:
                    time.sleep(1)
                    self.board.start_stream()
                    time.sleep(2)
                    data = self.board.get_board_data()
                    self.board.stop_stream()
                    time.sleep(1)
                    print(f"Data: {self.board.get_board_data()}")

                    time.sleep(4)

                    config_code = (f"z{i+1}00Z").encode("ascii")
                    command_reply = self.board.config_board(config_code)
                    print(f"Impedance Command reply: {command_reply}! Turning off the impedance mode for channel {i}")

And that's the console output I get:
Start impedance checking ...
Impedance Command reply: Success: Lead off set for 1$$$
Data: []
[ERROR]: _check_impendance() crashed due to: 'utf-8' codec can't decode byte 0xc2 in position 2: invalid continuation byte

I don't really see where the Unicode should come from. Do you have any further ideas?

Best regards
Lukas

wjcroft · November 2025

Hi Lukas,

Something in the way you are generating the config_code string, is setting the 8th upper bit as I mentioned before. Brainflow is handling the function calls you are using. Have you tried posting on the Brainflow Slack channel? You can sign up via the home page:

https://brainflow.org/

I suggest posting there and including a link to this Forum thread.

Regards, William

Impedance measurement

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