the 10dB LFE offset stems from the original 5.1 channel 70mm magnetic soundtracks. The LFE channel was placed on the two tracks for the surround channels. The surround channels were then high pass filtered at 250 Hz. To provide additional headroom, the LFE channels were recorded -10dB below reference to a) avoid distortion and, b) to avoid partially erasing the HF of the surround channels it shared through mag head-gap saturation. The LFE channel on playback would be passed through a low pass filter and then boosted by 10dB to restore it's original level. This would give one a max SPL of 105dBC for each of the 5 channels and 115 dBC for the LFE channel. To avoid calibration errors and confusion, the practice was continued in the cinemas for Dolby Digital, DTS, and SDDS sound tracks as they came into use.

Calibration of each channel in the home (using the internal test signals) is to the same SPL, assuming a 2 octave band limited signal (REF=-30dBfs); 20-80 Hz for the LFE and 500-2 KHz for all other channels. The +10 dB adjustment is applied by the surround decoder for Dolby Digital sources. DTS may actually use something different. When a Discrete digital input on a Meridian controller is used, the controller provides no +10 dB offset as this is anticipated to have been applied by an upstream surround decoder. In the case of a Meridian source, the decoding and offset (if required) would be done by the DVD player.

Dolby Digital (and one hopes DTS) have flags in the originating bit-stream which would identify film material from other sources such as music. This flag tells the decoder whether or not to apply the +10 dB offset or not.

As for music tracks, it's up to the producer to use the LFE track on Dolby Digital or not. Most will simply use 5 full range channels, as we rarely have the need for 115 dBC SPL in music.

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Last edited by prm1177; 2008-06-11 23:09.
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