Dolby Laboratories DP564 - Manual (Page 89)

DP564 Multichannel Audio Decoder Appendix A: Metadata
A-19
Surround 3 dB Attenuation
The Surround 3 dB Attenuation parameter determines whether the surround
channel(s) are attenuated 3 dB before encoding. The attenuation actually takes place
inside the Dolby Digital encoder. It balances the signal levels between theatrical
mixing rooms (dubbing stages) and consumer mixing rooms (DVD or TV studios).
Consumer mixing rooms are calibrated so that all five main channels are at the same
sound pressure level (SPL). For compatibility reasons with older film formats,
theatrical mixing rooms calibrate the surround channels 3 dB lower in SPL than the
front channels. The consequence is that signal levels on tape are 3 dB louder.
Therefore, to convert to a consumer mix from a theatrical calibration it is necessary to
reduce the surround levels by 3 dB by enabling this parameter.
Surround 3 dB
Attenuation Setting
Enable
Disable
Surround Phase Shift
This parameter causes the Dolby Digital encoder to apply a 90-degree phase shift to
the surround channels. This allows a Dolby Digital decoder to create an Lt/Rt
downmix simply. For most material the phase shift has a minimal impact when the
Dolby Digital program is decoded to 5.1 channels, but provides an Lt/Rt output that
can be Pro Logic decoded to L, C, R, S, if desired. However, for some phase-critical
material (such as music) this phase shift is audible when listening in 5.1 channels.
Likewise some material downmixes to a satisfactory Lt/Rt signal without needing this
phase shift. It is therefore important to balance the needs of the 5.1 mix and the Lt/Rt
downmix for each program. The default setting is Enable.
Surround Phase Shift Setting
Enable
Disable
A.5.2 Extended Bitstream Information Parameters
In response to requests from content producers, Dolby Laboratories recently modified
the definitions of several metadata parameters from their original definition as
described in ATSC document A/52. The original parameters were rarely, if ever used.
The revised definitions allow more information to be carried about the audio program
and allow more choices for stereo downmixing. When the metadata parameters
carried in Dolby Digital were first described, they were generically called Bitstream
Information or BSI. We refer to the alternate parameter definitions as Extended BSI.