Making a DCP: Audio

Aaron Owen

Author
May 2
    -    
7 minutes
Audio waveforms as shown in Adobe Premiere.

Sound is a huge part of how we experience films in the theater, so it’s really important to have some understanding of how audio works in digital cinema if your film will be playing in a theater. Audio in cinema is a huge topic, but we’re only going to take a look at two parts of it: Cinema Sound Formats and Volume / Loudness.

Cinema Sound Formats

Before we get into the various sound formats, it’s important to know that there’s difference between audio channels and speakers. An audio channel is a channel in the DCP that is routed or mapped to a speaker cluster in the theater. Each theater is different in how many speaker clusters are installed and how the audio channels are mapped between various clusters for different sound formats. It should also be mentioned that when we talk about assigning different types of content to various channels, that is usually just a starting point. Mixing cinematic audio is an art form unto itself. Audio engineers make lots of artistic choices in terms of placing sounds in space, and employ lots of “tricks” in bringing a theatrical mix to life. With those caveats out of the way, let’s dive into generally accepted cinematic sound formats.

There’s an array of formats that have been popular over the years. Some are more commonly used than others. According to the Academy of Motion Picture’s submission guidelines for the Oscars:

The audio in a Digital Cinema Package (DCP) is typically 5.1 or 7.1 channels of discrete audio. The minimum for a non-mono configuration of the audio shall be three channels as Left, Center, Right (a Left/Right configuration is not acceptable in a theatrical environment).

The most common sound formats in use in cinema today are 5.1 and 7.1 surround sound. We’ll get back to these surround formats in a bit, but if you’re going to be playing your film in a digital cinema, creating a surround mix with a professional engineer will lead to the best results.

You may also have noticed that the Academy doesn’t consider stereo audio to be an appropriate format for Cinema. So what does this mean for us? Well if you only have a stereo version of your film, won’t be trying to win an Oscar from the Academy, and won’t be playing your film in huge theaters, and don’t have the budget to get a professional mix done, you’re probably fine. But there’s a chance that your film will feel will feel unbalanced or strange sounding, as the left and right speakers are installed at the edges of the screen, which can be quite far apart from each other in a large theater.

There’s two other options available for those who can’t afford (or don’t have the time) to create a surround sound mix for their film:

  1. Mono
  2. Left, Center, Right (LCR)

With mono audio, all of the sound is placed onto a single channel which is mapped to the center speaker in the theater. Because all sound is coming from a single source which is located in the middle of the screen, there’s no chance for weird phasing effects or echos to take place that can ruin the viewing experience for your audience.

Generally speaking, for LCR audio, music and effects go on the Left and Right channels and are panned like a normal stereo mix, while dialog is mapped to the Center channel. This is preferable to a Stereo mix because the dialog only comes from one speaker in the middle of the theater instead of from either side. To execute this, output three discrete channels from your audio environment. One for each of the Left, Right and Center channels, each containing the correct content type.

Surround Sound Mixes

There are a ton of different surround formats: 5.1, 6.1, 7.1, 9.1 , 7.2, 9.2, 11.2, Dolby ATMOS, etc. It’s hard to know where to begin. The first step is to understand what the numbers mean. In the standard “dot” notation, the first number is the number of speakers in the configuration, while the number after the dot is the number of subwoofer channels. To get the total number of channels, add both sides together. So 5.1 means five speakers and one subwoofer for a total of six channels.

The vast majority of theaters in the world are set up for 5.1 surround sound and for most films, this is the best way to present your film in the theater. 5.1 surround mixes use 6 channels, and are laid out like so:

  1. Left — Music & Effects
  2. Right — Music & Effects
  3. Center — Dialog
  4. Low Frequency Effects (LFE) — Subwoofer
  5. Left Surround — Surround Effects
  6. Right Surround — Surround Effects

The best result will come from mixing in a properly calibrated 5.1 studio with a professional audio engineer. But even when mixing in a 5.1 studio environment, it is important to understand that the theater where you’ll be screening is a much larger room, and physical proximity to the speakers does play a role in how you’ll perceive the mix. One of the most common complaints that I’ve heard from directors is that they feel like their mix is only coming from the front speakers when they listen to it in the theater. They don’t “feel” the surround sound channels coming from the rear of the room. Normally that’s because in the studio, they were sitting much closer to the surround speakers, and in the theater the distance that sound must travel is much greater. This means the rear speakers are harder to hear over the front speakers, or are less clearly perceived.

There’s a ton of information about Cinema Sound Formats here, if you want to go down a rabbit hole.

Volume & Loudness in the Cinema

Nothing is worse or more distracting to an audience than having your film’s sound be too loud or too soft in the theater. So how can you be sure that your film is mixed to the right level?

Sound mixing for cinema is not like mixing for TV, Radio, or the Web. In those mediums digital audio meters are a useful tool to understand your audio levels. If you use meters like a sample peak or loudness meter as the indicator if something is too loud or soft in your film, you’ll experience poor results. In digital cinema, these meters are much less useful. The exact reasons are quite complicated and out of the scope of what we’re talking about here. The main thing to understand is that the cinema sound environment is much louder on average than TV, Radio or the Web.

Calibration at the Cinema

Cinemas around the world are calibrated to a standard called 85dB SPL. This means a -20dB pink noise signal is played out of a single front channel, the amplifier powering that speaker cluster is turned up until a sound pressure level meter (SPL) reads 85dB. The process is repeated for each of the front speaker sets. The surround speakers are calibrated the same way, except to 83dB. Playing a calibration signal at -20dB means that there’s an extra 20dB available per channel as “ headroom,” or loudness which can be accessed before the signal clips or distorts. This incredible dynamic range is how the audience can hear very quiet scenes where whispers or footsteps are audible as well as scenes like plane crashes where the sound is very loud.

Calibration at the Mixing Stage / Studio

In order to replicate the cinematic environment, the audio engineer must also calibrate the mixing studio in the same manner. Hollywood mixes their films on what’s called “mixing stages” (sometimes called “dub stages”) which are essentially full-sized theaters calibrated exactly like your local cinema. Anyone who isn’t producing a large budget film mixes in a much smaller studio which is calibrated using the same techniques, but sometimes to lower volume more appropriate for the smaller space.

Once the calibration / reference levels are set, the best way to see if something is too loud or too soft is to use your ears. Audiences need to be able to hear dialog over the underscore music or sound effects. Because your speakers are turned up quite loud, the volume levels on the sample peak meters or loudness meters might appear really soft. In reality, your film will sound correct when played back in a theater. I’ve said this a few times in this article, but it’s importance deserves to be repeated: You’ll get the best results if you work with a professional audio engineer who is experienced in mixing films for digital cinema in a calibrated environment.

Films usually require different mixes for different destinations. A television or home-video mix will be quite different than the mix for a DCP which could also be quite different from a mix you’d upload to YouTube. Knowing and communicating what deliverables you’ll need to produce before you start mixing with a professional will make your budget go further.

For more information about loudness in cinema, here’s a link to the ISDCF white paper from 2016. For more information about calibration including reference files and instructions, here’s a link to BlueSky’s blog post.

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