Dual mono audio

Mono or monophonic audio describes a mix in which all sounds are mixed together into a single channel. To prevent the listener from hearing only 1 channel (Example: out of only one headphone), the single mono channel is duplicated onto the second channel. The duplication of the mono audio channel is intentional and is the correct use of dual mono audio. This allows support for sound systems with 2 or more channels, when playing a mix that was created as a single mono channel.

Stereo or stereophonic audio incorporating 2 unique channels allows for a "field" in which sounds can be panned to some degree left or right of center.

Example: Imagine a scene in a movie where a car moves from left to right on the screen. The stereo audio mix will typically pan the sound of the car from left to right to follow the action of the vehicle. As the sound of the car transitions from the left to right channel, the volume of the sound effect essentially decreases in the left channel and increases in the right channel.

The key point to keep in mind with a true stereo mix is that there is some amount of audible or measurable difference between the left and right channels.

Click any section below for info about dual mono audio:

Stereo vs. dual mono
There is often confusion with partners when we reject a title for having dual mono audio. When viewing the properties of the rejected file, 2 channels are seen. On some tools, the 2 channels are actually flagged by default as “Stereo":

However, the difference between stereo and dual mono audio is determined by the actual audio content within the 2 channels and not simply by the number of channels.

  • A true stereo mix will have subtle or in some cases quite noticeable difference between the 2 channels.
  • In a dual mono mix, the audio in both channels will always be identical.

Below is a comparison of what true stereo and dual mono look like when played through an audio meter:

Stereo mix
Dual mono mix


Notice the meter levels are different for left and right channels in the stereo mix but are the same in the dual mono mix. Also for the stereo mix, the stereo field is seen in the scope as a wider 2-dimensional trace, while the dual mono mix shows up as a vertical line, indicating the absence of a stereo field.

Dual mono rejection
Google rejects dual mono audio while other platforms don't reject this issue. We reject dual mono when we feel it was introduced as an error.
Example:
 
A particular title was mixed in 1 of the following audio configurations:
  • Stereo
  • 5.1 and stereo

But the stereo portion of either of these 2 configurations was incorrectly mixed as dual mono instead of true stereo.

Per the audio guidelines in Delivery requirements, audio should be delivered in native format. If mono is delivered for a stereo or 5.1 title, your program will be rejected.

Ultimately, if a mix was intended by the director or sound mixer to be stereo but is delivered as dual mono, this is grounds for a rejection. Back in the day, dual mono audio was the standard. Audio recording technology at the time (pre 60s/70s), only allowed a single input of mono audio recording. Due to the age of this older content typically being dual mono audio, there isn't really any sort of fix that will make the content have stereo audio.

Fix dual mono
Most of the time, unintentional dual mono audio is the result of an incorrect export of a master or mezzanine file. A source can be ingested as stereo but outputted as dual mono. This is a common issue with exports done in Final Cut Pro if the "Stereo Pair" function isn't properly set for stereo tracks. Importing a stereo track into the timeline, both channels are panned to the center instead of panned left and right accordingly. This panning of both channels to the center will render the outputs as dual mono.

Stereo source ingestion

If the source ingested is stereo (see example below), where the left and right audio track meters move independently, settings can be fixed for a proper stereo export.

In Final Cut Pro, to set the "Stereo Pair" function, select the stereo tracks:

Then go to Modify → Stereo Pair.

Dual mono source ingestion

If the source ingested is dual mono (see example below), where the left and right audio track meters move alongside each other, the issue will have to be resolved by the partner. Dual mono source issues can't be fixed by changing the export settings. This issue needs to be flagged during QC review, so the partner can be notified for redelivery.

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