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Audio Device Calibration

Description

An audio device calibration document is used to precisely match the gain structure of the four microphone channels when making 3D measurements with IRIS.

As specified here, the audio interface or Dante network must have at least four digitally controlled microphone preamplifiers with channels matched to within 0.1 dB. Although we set each channel to e.g. 30 dB, it's possible they vary more than the required matching. This calibration ensures each channel is sufficiently matched.

The audio device calibration document also specifies which channels to use for making IRIS measurements and enforces these in the AUDIO SETTINGS window.

A ready-made IRIS kit is delivered with an audio device calibration document which is created when we assemble that kit. This will be copied to the correct location during installation.

Note

The audio device calibration only applies when the audio interface is set to one gain setting, e.g. +30 dB. For an audio interface with digitally controlled gains, the small differences between channels at +30 dB might be different when the unit is set to +36 dB, for example.

Note

This is purely an electrical calibration and should not change with time unless your equipment is damaged.

Audio Device Calibration Procedure

If you wish to measure your own audio device or repeat the calibration, please follow these steps. This is written for calibrating the input channels of the Dante network from the Revision 3 Kit. You may need to adapt these instructions to your specific set of hardware.

  1. Partially set up your IRIS Kit, but don't connect the microphone or loudspeaker.

  2. Disable +48 V on the microphone preamplifier channels. Since we will connect the line output directly to a microphone preamplifier input, we don't want to accidentally damage the line output by applying a DC offset to it.

    For the Revision 3 kit, open the Shure Web Service Discovery program, double click on your Shure preamp, and then disable +48V on all four channels.

    ADC Shure Preamp Config

  3. While this window is open, check that all microphone channels are set to the same gain, e.g. 30 dB.

    Note

    You can use a different gain setting as long as it is consistent on all channels. The gain setting you choose must be used when applying the corresponding audio device calibration document.

  4. Open and IRIS and create a new IRIS project.

  5. In STIMULUS SETTINGS set a 10 second sweep and 2 second impulse response time.

  6. Open AUDIO SETTINGS.

    • Set the Receiver to Mono
    • Set the Source to your line output e.g. 1: Dante tx 1
    • Set the one Receiver channel to the first channel of the preamp e.g. 1: Dante rx 1.
    • Click OK to save changes

    ADC Audio Settings

    Note

    If you are using a USB audio interface instead of a Dante network, you should also use a loopback cable to measure latency.

  7. Connect the line output of your IRIS system to channel 1 of the preamplifier using a standard XLR cable.

    ADC loop connection

  8. Add a measurement to the IRIS project and call it Channel 1

  9. Click RECORD to open the Record window.

  10. Click TEST TONE set the output gain sufficiently high so that the In channel level is around -12 dB.

    ADC Test Tone

    Select STOP to stop the test tone.

  11. Push RECORD and wait for the measurement to complete.

  12. Repeat steps 6. to 11. for preamplifier channels 2, 3 and 4. Leave the output level the same i.e. don't repeat step 10.

  13. You should now have an IRIS session containing 4 measurements, like shown:

    ADC IRIS Project

    Note

    You might notice that the source-receiver distance is not zero, meaning there is some latency unaccounted for in the system. If you are using a Dante network or a USB audio interface with loopback connection, this value is purely the latency of the digital-to-analog and analog-to-digital converters in the system (typically 33 samples and independent of sample rate). This value is taken into account when you create an audio device calibration document.

  14. Select Project menu, then Audio Device Calibration

  15. Enter the necessary details:

    • Audio Device Name (can be anything).
    • A serial number (can contain numbers or text). This value will be displayed in the Audio Device calibration dropdown box in AUDIO SETTINGS.
    • The preamplifier gain you used in step 3. above.

    The values at the bottom of the page show the relative gain differences compared to the first channel and the fixed latency of the system if it could be measured.

    ADC Window

  16. Click SAVE. The calibration document will be available to select in AUDIO SETTINGS window.

  17. Disconnect the line output from the preamplifier and re-enable +48V on all preamplifier channels for future measurements.