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Audio Delay Testing

Tie-Clip microphones are available to help measure the audio delay from a sound being triggered to hitting the participant's ear or ears.

Two are identical and were purchased (October 2022), and the third is the original one provided during MEG training (February/March 2018).

Specifications

Original 2 x New
Transducer Principle Condenser Condenser
Frequency Response 20 - 16000 Hz 50 - 18000 Hz
Pickup Pattern Omnidirectional Omnidirectional
Sensitivity -64dB +/- 3dB -62dB
Impedance 1 kΩ 1 kΩ
Battery Power 1.5V DC, "AAA" 1.5V DC, LR-44 Button
Output Connector 6.35mm / ¼" mono Plug 6.35mm / ¼" mono Plug
Microphone Connection/lead length 3.5mm mono Plug, 4M 3.5mm mono Plug, 4M
  • Connect Stim PC or the PROPixx Controller (DATAPixx) audio output, to whichever sound system is being tested - MEG Audio Connections
  • Decide which microphone/s to use and insert the relevant battery. Remove battery after use (plastic forceps available to help remove LR-44 battery from new microphone/s).

Info

  • If using two microphones, best to not mix the Original and New microphones - to avoid possible inconsistencies - see Specifications above.
  • New microphone/s require/s microswitch on microphone case to be switched ON.
  • Uncoil the microphone cable/s and lay it/them out neatly/safely on Control Room floor, to avoid tripping use the provided Cable Protection Cover Mat.
    Insert the 3.5mm mono Plug/s into the microphone case/s.
  • Insert the ¼" mono Plugs attached to the microphone case/s to the relevant ¼" socket-to-socket adapter/s.
    The socket adapter/s should already be attached to the correct ¼" Plug-to-BNC cable/s already connected to the MISC input box.
    • Check MISC Channels for the correct BNC connection/s to use on the MISC Input Box.
      • Microphone Input - Left Response - MISC #8
      • Microphone Input - Right Response - MISC #9
  • Carefully push the microphone/s (minus the Tie Clips) halfway into the provided small piece/s of silicone tubing.
  • Roll up the relevant eartip/s and push into the opposite end/s of the tubing.
    • NATUS:
      • Attach the Natus cabling to the gantry side panel as usual.
        Lay out the Transducers on the gantry chair, on the floor, or on a chair outside the MSR.
        Attach the microphone/s + eartip/s to the Left or Right, or both, Transducer/s as required.
    • SOUNDPixx:
      • Unravel the clear SOUNDPixx tubing from the MSR shelving.
        Attach the black Participant tubing correctly.
        Attach the microphone/s + eartip/s to the Left or Right, or both, red-coiled connector/s as required.

If only listening from one audio channel, to avoid possible spurious noise being picked up attach the relevant "squashed"/blocked eartip to whichever audio channel not recording from.

Don't close the MSR door, or don't close it fully, to avoid cutting cable!

  • In Acquisition Settings, select the relevant MISC Channels as well as the trigger channels associated with the audio test stimulus.

To reduce the resulting file size, turn off the MEG channels.
Increase the "Sample freq (Hz)" to 3000 or 4000 to increase file resolution (more points) to make subsequent cursor line start point selection easier/more defined.

  • Press "GO", "Record", and start audio test stimulus.

  • Recording the triggers, the audio output/s (used for reference, and should be almost instantaneous but delays may be due to how the test sound is being generated in PTB), and the microphone output/s - should now show the audio delay between the sound-onset trigger and the microphone response.

  • Save the resultant .FIF file and read into Graph to find the delay/s.

To check the delay using "Graph"

  1. Start Graph.
    • Applications -> Neuromag -> Graph.
    • Dismiss the "Welcome" splash screen, click "OK".
  2. File -> Load settings.
    • Example settings files/.fif files, to try, are shown in blue.
  3. In the Directories window, double-click on the settings required e.g.
    • /home/meguser/lisp/setup/examples.
  4. In the Files window, select basic.setup.
  5. Click "OK".
  6. File -> Open Diskfile.
  7. In the Directories window, double-click /neuro/data/sinuhe.
  8. In the next Directories window (left-click on the RH window edge and drag to open the window wider if necessary), scroll down and double-click on the Project e.g.
    • /neuro/data/sinuhe/demo.
  9. In the next Directories window (left-click on the RH window edge and drag to open the window wider if necessary), scroll down and double-click on the relevant sub-directory e.g.
    • /neuro/data/sinuhe/demo/221013.
  10. In the Select disk file to open window, chose the .fif file to view e.g.
    • Hyojin_audio_test_code_new_Stim.fif.
  11. Click "OK".
  12. Graph will respond with "No data available" - if the MEG Channels were turned off during data collection to reduce file size.
  13. Displays -> Control Panel.
  14. Double-click the widget labelled "Stim" to add in the MISC and STI channels used during the Acquisition.
  15. In the "Common to widgets", "names" window, type in each channel name, in CAPITAL letters, one channel per line, pressing the Return key after each entry e.g.
    MISC 6
    MISC 7
    MISC 8
    STI 2
  16. Click "Apply", “OK”
  17. Right-click on the "Stim" widget, and drag/draw a line to the "display" widget.
    Release the mouse button.
  18. The four channels should now appear.
  19. In the "Start & length" window, type a reasonable value in the "length" (lower) box, e.g. 10
  20. Left-click on each channel in turn (channel turns "white"). Click on Autoscale for both "Scale" and "Offset".
  21. Adjust the bottom slider to choose the best section to measure.
  22. Right-click on a position near e.g. the STI trigger and drag right to open a section to view (selected section turns "white").
  23. Displays -> zoom in
  24. Repeat as necessary from Step 22.
  25. Once happy with the zoomed view...
  26. "Shift + right-click" on a channel at a relevant starting point e.g. the start of the STI trigger.
    A vertical white line is displayed, along with a cursor box showing the Port number and which channel selected e.g. one from ...
    Port: 0 MISC006
    Port: 1 MISC007
    Port: 2 MISC008
    Port: 3 STI002
    and the "X" and "Y" axis values at the point selected e.g. X: 13.688 (Time scale in Seconds to 3 decimal places).
  27. Make a note of this "X" time value.
  28. Release the "Shift" key and whilst still right-clicking, drag the mouse to the right to the start of the desired time-delayed channel e.g. the microphone response.
    Move the line to the start of the response and note the time e.g. X: 13.711.
  29. The time difference is the delay (in msec) from trigger onset to microphone response
    e.g. in this example, 23msec.
  30. Repeat as necessary from Step 21 to take more readings from different triggers/different sections of the file, to ensure a reliable/repeatable value is collected for the delay.
  31. “Exit”, “Exit”, “Yes” when finished.