With the mouse pointer somewhere over the background area of the meter window, click the RIGHT mouse button to open this window:
Enter a figure in the calibration box. You may do this 'live' and see the effect straight away. If the entry is invalid the box turns red.
A value of zero means that 0 on the scale is equvalent to 32768, the maximum value that can be stored in a WAV file, or 'Full Scale Digital'.
However, a real soundcard analogue to digital converter may limit at a lower level than this. To test this ideally needs a steady tone input. Slowly increase the tone level until the meter does not increase any more, or the rate of increase appears to fall off, indicating that limiting is occuring. Take the meter reading and enter it in the box. Something like - 1.0 to -3.0 would be typical. The meter should now read zero. It would be best to allow an extra db for safety. Press OK to save the value.
An alternative method is to use a high grade sound recording program such as the excellent Cool Edit. You can download a demo from www.syntrillium.com (this has a good level meter built-in, but my program is quicker to get going). Record a short piece of sound with the level at maximum. Hit it really hard, to make it limit. Expand the view of the recorded waveform horizontally until you can see individual cycles. If it's limited there will be a flat top to the waveform. Set the scale on the right hand side to 'Decibels' and read off the level of the flat top. It might be say -1db. Give it an extra db for good measure and enter -2 into my program. Now you can use the maximum dynamic range without fear of overload.
Adjusting the PPM
Calibrating the PPM is a little
different. Enter the value which is represented by '4' on the BBC scale or
0db on the db scale.
Since there is 8db between this point and the red segment the highest value
you should enter here is -8, but this would give you no headroom at all. -12
gives you 4db headroom, -20 gives you 12db of headroom. With these settings
it doesn't matter if you go a bit into the red.
Although this setting theoretically requires whole dB steps only, you may enter decimal values to make fine adjustments if need be.