Requirements for audio files delivered to Google

Audio files delivered to Google should be free of artifacts, including but not limited to:

• Ticks
• Pops
• Dropouts
• Phase errors
• Clipping
• Peaking
• Distortion

The duration of audio track(s) should be within a half-second of the duration of the video track. If produced for your program, 5.1 audio should be included. 5.1 should always be accompanied by stereo, as 5.1-only deliverables won't be accepted. The audio should be frame accurate in sync, and audio channels should be appropriately labeled within the container, with the exception of MPEG-2.

When possible, a "near-field" mix intended for home entertainment should be delivered for your program. Furthermore, audio should be delivered in the original-in format. If mono is delivered for a stereo or 5.1 title, your program will be rejected. 

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.

Audio should be delivered in the original 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.

Audio Channel Assignments

Audio channel assignments are essential for creating stereo and 5.1 stereo playback. Google requires that all video content have correctly assigned audio channels in the file's metadata before delivery.

When audio channels aren't assigned before delivery, the encoding process can't be performed properly as Google can't fix files once they've been delivered. If this happens, the order will be be rejected and assets will need to be redelivered.

Common reasons Play rejects audio channel assignment

When audio channel assignments are delivered with out of spec configurations, the files can't be encoded properly. The most common causes for this are:

• Some or all tracks are unassigned: Tracks are delivered without audio assignments. While tracks or audio streams might have a file name that indicates a channel assignment, this does not create a channel assignment. Good channel assignments should be part of the container metadata within the file.
• Audio files have been named but are not assigned: Naming an audio file identifies which track is in a file, but this won't assign the file to a channel. 
• 5.1 audio file was delivered with no stereo streams: Files were delivered for L, R, C, LFE, Ls, and Rs streams, but are missing Lt and Rt streams. Stereo audio must be delivered with the files for 5.1 audio.
• The channel assignments are in the wrong order: For example, if a file is delivered where stream 1 is assigned as "Right Total, Left Total" but the correct assignment should be "Left Total, Right Total", then the file can't be encoded correctly. 
• Duplicated audio channels: If audio stream 1 is assigned as "Left Total, Right Total", and audio stream 2 is also assigned as "Left Total, Right Total", then audio stream 2 is a copy of audio stream 1. Duplicated audio channels can't be processed correctly.
• Audio channel assignments were delivered in incorrect layout: Google Play accepts a variety of different audio profiles for order delivery, which all use different layouts. Each layout has unique specifications and aren't interchangeable. To learn more about audio channel assignment specifications, go to "Format Options" in this Encoding article to find the layouts for accepted audio profiles.
• Audio channels use invalid assignments: The audio delivered uses channel assignments that are not supported by Google Play.

How to fix audio channel assignments

If your title was rejected for issues with audio channel assignments, it will need to be fixed before redelivery. The easiest way to fix this issue is to use QuickTime Pro 7.

1. Open QuickTime Pro 7.
2. Open the source file that needs to be fixed, then select Window→Show Movie Properties.
   
   
    
3. The "Channels" column should be present. If you don't see the column, right-click anywhere in the columns and select "Channels" from the dropdown menu.
   
   
    
4. Select the track you want to work on, then click Audio Settings.
   
   
5. Use the dropdown menu in each audio channel to choose the correct channel assignment.
   
   
    
6. Repeat steps 3 and 4 until all tracks have been corrected.

   Under "Format Options" in this Encoding article, you can find the correct layout formats for all audio profiles supported by Google Play.

7. Make sure all tracks have the correct channel assignments, then save your work.
8. Redeliver the order with the corrected audio assets.

   Below is an example of what the correct channel layouts for ProRes audio option 2 look like: