THX Optimode Audio Calibration The Trials and Tribulations... By Stuart M. Robinson.

Recorded Output Levels

The THX Optimode on-screen instructions state that the output levels achieved in a correctly calibrated system when using Optimode should be 75dB SPL. This is the same volume achieved when calibrating using the internal test noise of a THX receiver/processor or any Dolby Digital hardware whose volume control operates in relation to 'reference' level.

For test noise to produce 75dB SPL, it must be recorded at -30dBFS (that's thirty decibels below full-scale, the maximum output of a digital system). As Dolby Digital's maximum output is 105dB for all five main channels, -30dBFS equates to 75dB SPL.

Unfortunately, with 'Fight Club' being the only exception, the THX Optimode calibration noise has been recorded at -20dBFS (twenty decibels below full-scale) which results in an SPL output of 85dB, 10dB above what the on-screen graphics state should be the case.

THX assure me that they will correct the amplitude of all future THX Optimode signals to bring them into line with the 'standardised' calibration output level of 75dB SPL.

The users of some processors/receivers have not been reporting 10dB discrepancies however, 6dB is often the figure quoted. This can be explained by examining how their decoders may be reacting to the Dolby Digital bitstream used to convey the Optimode calibration signals.