Correction

If you in 6. Reference have selected Generator as reference, the sound card delay and frequency response is not automatically corrected for. But this can be done here.

The sound card delay and frequency response is not automatically corrected for. But this can be done in point no. 7 as shown below.

If  is checked before the measurement is performed, it will correct your measurements for the influence of the measurement system. With the term ‘measurement system’ we usually mean your sound card, but it is possible to include an amplifier as well. We also sell a special attenuator to be used to measure electrical signals that is too high for the sound card, this can also be included in the correction. The output of a power amplifier usually gives too high voltage so the sound card cannot be directly connected to it.

In order to do this correction, your measurement system first has to be measured. The procedure for this is described below.

We can measure the response used for correction simply by connecting a cable from the output to the input of the sound card. Signal processing theory tells us that this correction can be used to "subtract" the influence of the sound card from our measurements.

(But if we want to record a signal only, e.g. to measure the background noise, it is more problematic to correct for the sound card. This is because when we measure the influence of the sound card, we measure both the output and the input. WinMLS currently does not have a method for such a correction, except the microphone compensation found in Plot->Plot Type Settings->Frequency Response/Spectrum....)

Measure the measurement system using the following procedure

1.  Connect a cable from the input to the output of the sound card.

2.  Make sure the measurement system correction check box  is turned off:

3.  Perform a measurement.

4.  Turn the measurement system correction on, .

5.  Save this measurement as the measurement system correction measurement by clicking the  button.

6.  Perform a measurement again. This should now be flat in the frequency domain. Inspect it in time domain also, it should be a very short peak (Dirac pulse) at time 0 ms.

7.  Now connect the usual equipment and perform measurements without and with the system correction and observe the difference.

(If you measure using synchronization loop-back, you might experience a small tail at the very end of the impulse response. This is a result of the correction filtering procedure. If this tail is more than –50 dB below the maximum of the impulse response, it can be neglected for most applications.)

When  is checked, the relative amplification will be measured. If the amplification of the system you are measuring has increased by 5 dB after you performed the reference measurement, the peak of the impulse response will therefore be at 5 dB. The linear amplification may be displayed by plotting for example the first part (it is given by the maximum) of the impulse response using linear axis. To plot the amplification in dB, do the same using logarithmic x-axis, or plot the magnitude frequency response.

You should now and then verify that the measurement system correction is valid. Especially if you are unsure about your sound card delay. This can be tested by performing a loop-back measurement of your sound card (output connected directly to input).

Clicking the  button will display the  Measurement->Sound Card... dialog.

Clicking the  button will display the Measurement->Delay… dialog.