Set-top boxes, used to receive terrestrial, cable, or satellite digital television, typically offer an analog mono signal modulated on the RF/Antenna output, a line-level analog stereo signal, and an optical or coaxial digital output. DVD players offer an analog stereo and a digital output, and some offer a six-channel analog output (for a 5.1- channel presentation). Portable DVD players offer analog stereo, headphone, and
digital outputs. DVD players in computers and game consoles offer a digital output as well as analog stereo, headphone, and possibly six-channel analog outputs. 5.1- channel amplifiers, decoders, and receivers have six-channel analog outputs and possibly six speaker-level outputs.
In all of these cases, a Dolby Digital decoder creates the analog audio output signal. In the case of the set-top box or DVD player, the analog stereo output is a downmixed version of the Dolby Digital data stream. The digital output delivers the Dolby Digital data stream to either a downstream decoder or an integrated amplifier with Dolby Digital decoding.
In each of these devices, the analog stereo output is one of two different stereo downmixes. One type is a stereo-compatible Dolby Surround downmix, of the multichannel source program that is suitable for Dolby Surround Pro Logic® decoding. This kind of downmix is also called Pro Logic or Left total/Right total
(Lt/Rt). The other type is a simple stereo representation (called Left only/Right only, or Lo/Ro) suitable for playback on a stereo hi-fi or on headphones, and from which a mono signal is derived for use on an RF/Antenna output. The difference between the downmixes is how the Surround channels are handled. The Lt/Rt downmix sums the Surround channels and adds them, in-phase to the Left channel and out-of-phase to
the Right channel. This allows a Dolby Surround Pro Logic decoder to reconstruct the L/C/R/S channels for a Pro Logic home theater. The Lo/Ro downmix adds the Left and Right Surround channels discretely to the Left and Right speaker channels, respectively. This preserves the stereo separation for stereo-only monitoring and produces a mono-compatible signal. In all downmixes, the LFE channel is not
included.
On most home equipment, the consumer can use the product’s user interface to choose the appropriate stereo output for his playback system. The mono signal feeding the RF/Antenna output is usually derived from the Lo/Ro downmix. There are separate metadata parameters that govern the Lo/Ro and Lt/Rt downmixes. Certain metadata parameters allow the engineer to select how the stereo downmix is constructed and which stereo analog signal is preferred, but Lt/Rt is the default selection in all consumer decoders. See Section 5, Parameter Definitions, for more information on individual parameters.
During downmixing, as we have seen, the adjustment of Dynamic Range Control parameters is limited. Broadly speaking, the stereo outputs use the Line mode compression profile while the mono signal uses RF mode compression. As with dynamic range control, downmixing is ultimately dependent upon each consumer’s unique listening environment.
While the engineer must optimize the multichannel mix for reproduction in an ideal listening environment, it is also important to preview the mix in downmixing conditions to ensure compatibility with different playback systems when selecting the downmixing metadata parameters. These previews can be achieved in real time using the DP570 Multichannel Audio Tool.
5 Parameter Definitions
This section explains both professional and consumer metadata parameters in greater detail.
Metadata parameters include:
• Universal parameters
• Extended Bitstream Information (Extended BSI) parameters
Extended BSI parameters are active only when both the producer chooses to use them and the consumer’s decoder is capable of reading them. All decoders can successfully decode a metadata stream without Extended BSI parameters, and Extended BSI parameters translate seamlessly to decoders that read only universal parameters. Note: Universal parameters include both professional and consumer metadata.
5.1 Universal Parameters
All universal parameters are supported by Dolby E encoders and decoders; all except Program Configuration and Program Description Text are supported by all Dolby Digital encoders and decoders.
Program Configuration
This parameter determines how the audio channels are grouped within a Dolby E bitstream. Up to eight channels can be grouped together in individual programs, where each program contains its own metadata. The default setting is 5.1 + 2. Program Description Text This parameter is a 32-character ASCII text field that allows the metadata author to enter a description of the audio program. For example, this field may contain the name of the program (Movie Channel Promo), a description of the program source (Football Main Feed), or the program language (Danish).
Dialogue Level
The Dialogue Level parameter is discussed in Section 2, Dialogue Level.
Channel Mode
This parameter (also known as Audio Coding mode) indicates the active channels within the encoded bitstream and affects both the encoder and consumer decoder. This parameter instructs the encoder which inputs to use for this particular program; it tells the decoder what channels are present in this program so the decoder can deliver the audio to the correct speakers. The setting is described as X/Y, where X is the number of front channels (Left, Center, Right) and Y the number of rear (Surround) channels. The availability of certain channel modes depends on the Dolby Digital encoder data rate and whether the LFE channel is present. For example, you can’t have a mono stream with an LFE channel (1.1!) or a 3/2 stream at 96 kbps. Appropriate data rates are shown in the definition of each setting.
Note: The presence of the LFE channel is indicated through a different metadata parameter (see LFE Channel).
Channel
Mode
Setting Definition and Data Rate 1+1 Dual mono (not valid for DTV broadcast or DVD production)
1/0 Mono From 56 kbps, usually 96 kbps
2/0 Stereo From 96 kbps, usually 192 kbps
3/0 From 256 kbps
2/1 From 256 kbps
3/1 From 320 kbps
2/2 From 320 kbps
3/2 From 384 kbps, often 448 kbps
LFE Channel
The status of the LFE Channel parameter indicates to a Dolby Digital encoder whether an LFE Channel is present within the bitstream. Channel mode determines whether the LFE Channel parameter can be set. You must have at least three channels to be able to add an LFE channel.
LFE Channel Setting Enabled Disabled
Bitstream Mode
This parameter describes the audio service contained within the Dolby Digital bitstream. A complete audio program may consist of a main audio service (a complete mix of all the program audio), an associated audio service comprising a complete mix, or one main service combined with an associated service. To form a complete audio program, it may be (but rarely is) necessary to decode both a main service and an associated service using a maximum total bit rate of 512 kbps. Refer to the Guide to the Use of the ATSC Digital Television Standard, Document A/54 (see
www.atsc.org) for further information. Although a detailed description of each option follows, in practice, most programming uses the default setting, Complete Main. An example of an exception to this rule is a special karaoke DVD, or an emergency service within digital television.
Bitstream Mode Setting Definition
Complete Main
(CM)
CM flags the bitstream as the main audio service for the program and indicates that all elements are present to form a complete audio program. Currently, this is the most common setting. The CM service may contain
from one (mono) to six (5.1) channels. Main M&E (ME) The bitstream is the main audio service for the program, minus a dialogue channel. The dialogue channel, if any, is intended to be carried by an associated dialogue service. Different dialogue services can be associated with a single ME service to support multiple languages.
Assc. Visual Imp.
(VI)
This is typically a single-channel program intended to provide a narrative description of the picture content service. The VI service may also be a complete mix of all program channels, comprising up to six channels. Assc. Hear Imp. (HI) This is typically a single-channel program intended to convey audio that has been processed for increased intelligibility and decoded along with the main audio service. The HI service may also be a complete mix of all program channels, comprising up to six channels.
Assc. Dialogue (D) This is typically a single-channel program intended to provide a dialogue
channel for an ME service. If the ME service contains more than two channels, the D service is limited to only one channel; if the ME service is two channels, the D service can be a stereo pair. The appropriate channels of each service are mixed together (requires special decoders).
Assc. Commentary (C)
This is typically a single-channel program intended to convey additional commentary that can be optionally decoded along with the main audio service. This service differs from a dialogue service because it contains an optional, rather than a required, dialogue channel. The C service may also be a complete mix of all program channels, comprising up to six channels.
Assc. Emergency (E)
This is a single-channel service that is given priority in reproduction. When the E service appears in the bitstream, it is given priority in the decoder and the main service is muted.
Assc. Voice Over
(VO) This is a single-channel service intended to be decoded and mixed to the Center channel (requiresspecial decoders).
Main Sv Karaoke (K)
The bitstream is a special service for karaoke playback. In this case, the Left and Right channels contain music, the Center channel has a guide melody, and the Left and Right Surround channels carry optional backing vocals.
Line Mode Compression Profile
Line mode is discussed in Section 3, Dynamic Range Control.
RF Mode Compression Profile
RF mode is discussed in Section 3, Dynamic Range Control.
Dolby Laboratories, Inc. Metadata Guide
RF Overmodulation Protection
This parameter is designed to protect against overmodulation when a decoded Dolby Digital bitstream is RFmodulated. When enabled, the Dolby Digital encoder includes
pre-emphasis in its calculations for RF Mode compression. The parameter has no effect when decoding using Line mode compression. Except in rare cases, this parameter should be disabled.
RF Overmodulation Protection Setting Enabled Disabled
Center Downmix Level
When the encoded audio has three front channels (L, C, R), but the consumer has only two front speakers (left and right), this parameter indicates the nominal downmix level for the Center channel with respect to the Left and Right channels. Dolby Digital decoders use this parameter during downmixing in Lo/Ro mode when Extended BSI parameters are not active.
Center Downmix Level
Setting Definition
0.707 (–3 dB) default The Center channel is attenuated 3 dB and
sent to the Left and Right channels.
0.596 (–4.5 dB) The Center channel is attenuated 4.5 dB and
sent to the Left and Right channels.
0.500 (–6 dB) The Center channel is attenuated 6 dB and
sent to the Left and Right channels.