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Tutorial: -Why does my DVD audio level differ from the level in FCP?

January, 2009


Why does my DVD audio level differ from the level in FCP?

by Jon Chappell

Have you noticed a discrepancy between the audio in your Final Cut Pro timeline and the audio of the finished DVD when using Compressor's DVD presets?

That's because the Dolby Digital 2.0 audio preset in Compressor has several functions to improve unsuitable audio, but if you have already mixed your audio they might do more harm than good in some cases. Rather than just telling you which settings to use, I'm first going to explain what these functions are designed to do because there are a lot of misconceptions about them.

Compression alters your dynamic range (the difference between the loudest and quietest sounds in a soundtrack) by compressing the level of sounds to fit within a certain range. The loudest sound in a Dolby soundtrack can be 105dB. Considering the fact that prolonged exposure to 90 dB audio can result in hearing loss, most people do not play their audio at 105dB. When you set up a Dolby decoder with a reference tone, you are telling it the volume level it should play 105dB audio at. So if this is lower than 105dB (which is likely), you will lose some of the very quiet sounds on the soundtrack. The compression presets allow you to regain these sounds by sacrificing some dynamic range in the process.

It is set to Film Standard by default but if you have already performed a mix with a moderate dynamic range, further compression is not likely to be necessary and so it is worthwhile to switch this to None.

Dialog normalization is a feature on several Dolby decoders that ensures all content plays back at the same level. The listener sets their preferred playback level and the dialogue in every DVD and TV show plays back at the same level, meaning you don't need to keep reaching for the remote in order to adjust the volume. Contrary to popular belief, it only adjusts the overall volume level when the source changes (i.e. when you begin playing the DVD or when a new TV program begins) so dynamic range is preserved throughout the movie. It does not dynamically adjust the volume as the movie is playing.



The normalization value indicates the difference between the overall level of dialog and the maximum audio peak of 0 dBFS. The default is set to -27 dBFS, which is the established level for film soundtracks. It is also the level most decoders are set to by default. If you have your decoder set to -27 dBFS and then, for example, try to play a -25 dBFS movie, the decoder will lower the overall volume by 2 dBFS so that the general dialogue level remains the same. If you do want to switch it off when compressing your audio files (if you've already leveled your sound mix for example), set it to -31 dbFS.

So if you would like your audio to be exactly how it was in Final Cut Pro, set Dialog Normalization to -31 dBFS and set Compression in the Pre-Processing tab to None. However, these functions do serve useful purposes for the end user and it is worthwhile to bear their needs in mind.

Copyright 2009 Digital Rebellion, LLC.

This article was first published on Digital Rebellion and is reprinted here with permission from the author.

Jon Chappell is an editor, VFX artist and software developer originally from the UK. He is the owner of Digital Rebellion LLC and is a regular contributor to the Final Cut Pro community. He is well known for developing the popular troubleshooting applications FCS Remover, Preference Manager and Compressor Repair. His film credits include the award-winning feature Perfect Sport.

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