About Dolby Digital Professional (AC-3) Export Options

In addition to the general options for all export formats (see General Export Options), there are additional options when you export as Dolby Digital Professional (AC-3) files. The Dolby Digital Professional (AC-3) export options are divided into three separate panes: Audio, Bitstream, and Preprocessing.

Figure. Export dialog with AC-3 options.

About Options in the Audio Pane

The following export options are available in the Audio pane:

Figure. AC-3 Audio Pane.
  • Enable Low Frequency Effects checkbox: Select this option to include the LFE channel in the encoded stream.

    Note: This option is not available for 1/0 mono or 2/0 stereo.

  • Dialog Normalization field: Specifies the average volume of the dialogue, using decibels of full scale (dBFS), in your sound files relative to full modulation. The playback device uses this information to maintain similar volume among different AC-3 streams. The goal is to make all AC-3 encoded audio files have the same listening level, regardless of the source file.

    It is especially important to properly use the Dialog Normalization feature if you have different audio files (with different volume levels) going on to a DVD. If you know the average level for each of them, enter that number in the Dialog Normalization field for each file. For example, if the dialogue in your project averages around –12 dB on the level meter, enter –12 in the Dialog Normalization field.

    Note: This normalization is between the audio streams on the DVD, not within an individual audio stream itself.

    You can enter values from –1 dBFS to –31 dBFS. The louder the source file, the smaller the value you enter, and the more the audio in the encoded file is attenuated during playback.

    The difference between the value you enter and 31 dBFS (which represents the normal dialogue listening level) is the amount that the source audio is attenuated. For example:

    • If you enter –31 dBFS: The attenuation is 0 dB (31 dBFS–31 dBFS), and the source audio levels are not affected at all.
    • If you enter –27 dBFS: The attenuation is 4 dB (31 dBFS–27 dBFS).
    • If you enter –12 dBFS: The attenuation is 19 dB (31 dBFS–12 dBFS).

    Important: It is critical that you have set the Dialog Normalization setting correctly if you intend to use any of the Compression Preset settings. The Compression Preset settings assume that the audio, after the dialogue has been normalized, is at the normal listening level of 31 dBFS. Levels that are consistently louder than that result in distorted sound and erratic levels.

    If you want to ensure that the AC-3 encoder does not affect your sound levels, enter –31 for the Dialog Normalization and choose None from the Compression Preset pop-up menu (located in the Preprocessing pane).

About Options in the Bitstream Pane

These settings are stored in the finished stream for use by the playback device. Leave them at their default values unless you have a specific technical reason for changing them. The following export options are available in the Bitstream pane:

Figure. AC-3 Bitstream Pane.
  • Copyright Exists checkbox: Select this option to specify that a copyright exists for this audio.
  • Content is Original checkbox: Select this option to specify that this audio is from the original source and not a copy.
  • Include Audio Production Information: Select this checkbox and fill in the fields below to specify how the encoded audio content was mixed. Playback devices may use this information to adjust output settings.
    • Peak Mixing Level field: Specifies peak sound pressure level (SPL) (between 80 dB and 111 dB) in the production environment when this mix was mastered.
    • Room Type pop-up menu: Specifies information about the mixing studio.

About Options in the Preprocessing Pane

Preprocessing options are applied to the audio data before encoding. With the exception of the Compression Preset setting, leave these settings at their default values unless you have a specific technical reason for changing them. The following export options are available in the Preprocessing pane:

Figure. AC-3 Preprocessing Pane.
  • General: Select the Digital Deemphasis checkbox to specify whether input audio data is pre-emphasized and needs to be de-emphasized before encoding.
  • LFE Channel: Select the Low-Pass Filter checkbox to apply a 120 Hz low-pass filter to the Low Frequency Effects (LFE) channel. Turn off this filter if the digital signal fed to the LFE’s input does not contain information above 120 Hz.
  • Full Bandwidth Channels: You can select two options:
    • Low-Pass Filter checkbox: Select this option to turn on a low-pass filter with a cutoff near the available audio bandwidth that is applied to the main input channels. If the digital signal fed to the main input channels does not contain information above the available audio bandwidth, you can turn off this filter. Compressor automatically determines the available bandwidth.
    • DC Filter checkbox: Select this option to turn on a DC high-pass filter for all input channels as a simple way to remove DC offsets. Most mixed audio material is already free of DC offsets.
  • Surround Channels: You can select two options:
    • 90° Phase-Shift checkbox: Select this option to generate multichannel AC-3 streams that can be downmixed in an external 2-channel decoder to create true Dolby Surround-compatible output.
    • 3 dB Attenuation checkbox: Select this option to apply a 3 dB cut to the surround channels of a multichannel film soundtrack being transferred to a consumer home theater format. Cinema surround channels are mixed 3 dB “hot” (higher) relative to the front channels to account for cinema amplifier gains.