User manual

miniDSP Ltd, Hong Kong / www.minidsp.com / Features and speci fications subject to change wi thout pri or noti ce 44
6.4 LFE ALIGNMENT GAIN
“Known as the LFE channel (for Low Frequency Effects), it handles bass created specifically for
subwoofer boom effects and may also carry low frequency information from the other channels in
order to enrich the overall soundtrack.”
5.1-Channel Production Guidelines, Dolby Laboratories Inc.
“The LFE channel is recorded with a level offset of −10 dB. This offset is compensated for in the
reproduction system, where the LFE loudspeaker has an acoustic output (within its low frequency
passband) of +10 dB with respect to the other channels.”
Recommendation ITU-R BR.1384-2 : Parameters for international exchange of multi-channel sound recordings
with or without accompanying picture, International Telecommunication Union.
6.4.1 The LFE channel and the DDRC-88A
As noted above, the LFE channel is recorded 10 dB lower than the speaker channels. On replay, a signal in the
LFE track must therefore produce 10 dB more acoustic output than the same signal in one of the speaker tracks.
If we were to assume that the monitoring system (amplifiers, speakers, and subwoofer) were initially calibrated
so that the same electrical input signal delivered the same acoustic output from all channels, then the channel
that plays the LFE content (i.e. the subwoofer channel) must have 10 dB of electrical gain added to it,
somewhere between the bitstream decoder and the subwoofer driver.
This additional 10 dB gain we will refer to as the “LFE alignment gain,” since it is there to align the level of LFE
acoustic output with the output from the speakers. (Bass management is discussed below.)
In a studio environment, where calibration of the monitoring system is likely, the mixing desk typically includes
an additional 10 dB of gain between the recording output and the monitoring output. With that said,
subwoofers designed for studio use often include an “LFE +10 dB” gain switch, specifically to cater for those
situations in which the mixing desk does not have this facility.
In a home theater environment, a priori calibration of the amplifier/speaker system is unlikely. Nonetheless, A/V
receivers and processors typically apply 10 dB of gain to the subwoofer output. In other cases, such as players
with analog output, the source does not apply the LFE alignment gain and the subsequent electronics (receiver
or subwoofer amplifier) is expected to provide it instead. SPL calibration is done for the whole replay chain, and
may be a manual process involving test signals and adjusting subwoofer gain, or an automated process whereby
onboard software adjusts the subwoofer trim level.
The DDRC-88A caters to all of these situations. If the DDRC-88A is to “ignore” the whole question of LFE
alignment gain, choose the Custom System configuration. Every channel will be treated as equal (in terms of
gain) by Dirac Live, and the onus is on external equipment to provide 10 dB of alignment gain – usually in the
source equipment, but it can also be done in the subwoofer amplifier after Dirac Live calibration has been
completed.