||THX Optimode Audio Calibration
The Trials and Tribulations...
By Stuart M. Robinson.
The Dialog Normalization Factor
Within all Dolby Digital bitstreams (carried by the SI header of each frame) is a data instruction called 'dialog normalization' sometimes known as the 'reference offset' value. As the consumer version of Dolby Digital was originally designed for HDTV applications, the dialog normalization function was included so that broadcasters could control the relative volume level of programme material, especially when different material was shown back-to-back. Don't let the name fool you, the entire soundtrack is affected, not just dialog.
Dialog normalization instructs the Dolby Digital decoder to reduce the level of its outputs by the amount specified within the bitstream. The typical amount of attenuation applied is 4dB, the 'standard' for Dolby Digital motion picture material. This figure can vary however, for added headroom 'Air Force One' (an isolated example) has a -0dB dialog normalization value while mono material, such as the re-releases of old James Bond movies can have up to -9dB dialog normalization.
As mentioned earlier, the maximum output from the five main channels of a Dolby Digital bitstream (in a calibrated system) is 105dB SLP, but if that bitstream contains a -4dB dialog normalization value, the outputs will be attenuated by the decoder resulting in a maximum output of 101dB SPL.
When producing calibration software, it is therefore vital that the volume level of the calibration noise and the bitstream dialog normalization value are considered as a whole. If calibration noise is recorded at -30dBFS, for it to produce 75dB SPL from a standard Dolby Digital decoder the dialog normalization must be -0dB so that the outputs of the decoder are not attenuated. All of Dolby's own calibration material has a -0dB dialog normalization value.
The THX Optimode calibration signals have a dialog normalization value of -4dB and as is clear, this reduces the output of the decoder and leads to the 6dB discrepancy. This is because we begin with a signal that is 10dB too loud (85dB SPL instead of 75dB) and there is 4dB of decoder attenuation. +10dB + -4dB = 6dB. Therefore, Dolby Digital decoders will correctly produce 81dB SPL when evaluated using the THX Optimode signals. The decoders are not to blame, at fault are the THX Optimode signals themselves.
There are situations in which the Optimode signals do produce the correct results (albeit with a 10dB discrepancy), and this we'll tackle in the final section of this article.