THX Optimode Audio Calibration .

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THX Optimode THX Optimode Audio Calibration
The Trials and Tribulations...

By Stuart M. Robinson.

Artificial Level Adjustments

In recent years, DTS has become more popular and it was inevitable that DTS soundtracks would be compared to their Dolby Digital counterparts (ad infinitum). No matter which you prefer, one thing is clear: DTS soundtracks are almost always louder than Dolby Digital soundtracks. This is because the DTS bitstream does not carry an equivalent of Dolby Digital's dialog normalization bit and as we have learnt, in the majority of cases this instructs the Dolby Digital decoder to apply 4dB of attenuation to its outputs.

In an effort to standardise the output levels of both Dolby Digital and DTS, some THX receiver and controller manufacturers (Denon being one example) are introducing a global 4dB gain to their Dolby Digital decoders, perhaps under instruction from the THX department of LucasFilm. This directly counters the 'standard' -4dB Dolby Digital dialog normalization bit and results in matched output volume levels between Dolby Digital and DTS source material. As manufacturers can reduce their internal calibration noise (against reference) by the same amount, output levels remain at the correct level unless the Dolby Digital signal carries a -0dB dialog normalization bit, in which case the outputs will have an artificial gain of 4dB.

It would appear that the THX Optimode calibration noise has been recorded with the small number of decoders that employ this 4dB boost/cut scheme in mind. This is why some users typically those with recently introduced hardware report that the Optimode signals produce predictable output levels, even though as we have discussed, with the exception of 'Fight Club', they will be 10dB too loud.


The moral of this story is clear. Heed the warnings from THX and do not use the Optimode calibration signals to evaluate the output levels of your system... unless all you want is to differentiate one channel from another. Not only are the signals recorded at -20dBFS in error, but based on my own experiments using a number of THX and non-THX receivers and controllers, are also adversely affected by the use of a -4dB dialog normalization bit, contrary to the standards employed by Dolby Laboratories.

As an alternative, I would recommend 'The Ultimate DVD: Platinum'.

Stuart M. Robinson
© - 2000

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