During the process of importing and exporting audio and video between several applications it became apparent that the way that each application interprets file types, file names and file extensions can be very different. I came across three very distinct differences when dealing with multichannel audio files. I would say at this stage that these are standards that need to be understood by users of these applications to avoid issues that can result in audio stems being directed to an incorrect loudspeaker or being misinterpreted by a plug-in.
Here are some examples of how these differences manifested themselves.
1. Pro Tools LE
When Pro Tools LE is presented with a 4 channel interleaved file, which in this case is a B-Format file, it interprets it as a Dolby Pro Logic File and appends the extensions accordingly. A Dolby Pro Logic file carries the extensions LCRS. This is fine when working in Pro Tools alone but the problems don't arise until you attempt to bring these into Logic 9. Dolby Pro Logic is an old 4 channel surround format not generally used any more. An explanation can be seen here:
2. Logic 9
Logic 9 does not recognise files wit a '.S' extension and therefore when importing the files as a multichannel interleaved file it places it on what it feels is the correct stem. This is problematic because in order to transcode the B-Format audio into G-Format (5.1) the plug-in must be presented with the appropriate file and channel allocation. Although the channel allocation looks correct I couldn't be sure that the appropriate stems were on the correct channels.
Logic Channel Meter showing LCRS files on incorrect stems
As we can see from the image above Logic is looking for L R Ls Rs files when importing a 4 channel interleaved file. I now had to establish the relationship between the two standards, i.e. what was the equivalent file extension for LCRS in relation to L R Ls Rs? To do this I quickly created 4 files in Pro Tools, 1 that contained audio and 3 that did not. This would allow me to see which stem it would be allocated in Logic. Through repeating this process of creating interleaved files with only 1 channel containing audio I was able to produce the following table:
This table allowed me to keep track of the B-Format stems from the recording stage to the export stage. It has proved to be a very useful tool for keeping track of file names and extensions.
What I was now able to do was go back to Pro Tools, carry out the tasks that needed to be done there, rename the files according to the conventions in the table and confidently bring these into Logic for further processing.
3. B-Format to 5.1 (ITU 775 Standard)
With the channel allocation now correct I was able to convert the B-Format stems into a 5.1 file ready for editing alongside the video. To do this the B2G plug-in was used. This too brought about some initial confusion. When I first played back the audio there seemed to be some incorrect positional information that conflicted with the video, i.e. things were appearing from the wrong spatial locations. To ensure that the 5.1 file was a true spatial representation of the B-Format file then several steps had to be taken to ensure this was possible. Firstly Logic needed to be set-up to ensure that the correct stems were pointing at the appropriate speakers. As there are several options available it was decided that the ITU 775 standard should be used. If this standard was adopted throughout the process then I could be confident that channel allocation and eventual output were consistent.
Logic Preferences showing ITU channel allocation
The next step was to ensure that the physical outputs from the Audio Interface were being routed to the appropriate speaker. This was checked visually. The final step was to test if the outputs on the plug-in were consistent with the ITU standard as there were three options available.
The first option proved to be the one. This is also consistent with the ITU/SMPTE standard track layout of 1 2 3 4 5 6 = L R C Lfe LS Rs.
This article by Bobby Owsinski provided some valuable track assignment information
Footnote...
The final element of confusion emerged when the 5.1 track was imported into Final Cut Express. FCE uses film standard track allocation of L R C Lf Ls Rs....but as I was not using FCE to handle the multi-channel audio I could forget about this for now....
Industry Standards and Regulations
Dolby 5.1 Channel Music Production Guidelines
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