Workflow: From Mix to Finished Product(s)
By Bob Katz
Below is an excerpted from iTunes Music: Mastering High Resolution Audio Delivery (Focal Press, 2012), by Bob Katz.
The diagram below describes one possible workflow where the end goal is to produce master files for LP, CD, iTunes (standard), Mastered for iTunes, iCloud, Internet download, and streaming. It’s quite a comprehensive list! The file-naming conventions are my suggestions—use whatever approach you wish—but I do suggest that file names contain dates or version numbers, wordlength information, and indicate if the file is a master, mix, or other work part, because it’s a tough job to keep files identified. I suggest using the word master only for final products that should not be further manipulated and are ready to be converted directly to the distribution format (e.g. AAC). Use the terms source, mix, work part, or premaster (for example) to name files that will be further manipulated before conversion to final product.
Let’s follow the progress of a mythical song called “You Break My Heart” (not my problem).
Mixing (shown in red)
The original tracks (e.g., drums, trumpet, piano, vocal) are mixed and processed (pictured below). The mix engineer may use one or more reverb chambers (an internal plugin or an external digital or analog unit), internal or external equalizers, compressors and other processes. External processors may be either analog or digital: if analog, then a separate D/A/D path is necessary. He may mix through an external analog console or summing mixer (not shown). The principles are always the same: whenever there is any processing (gain change, EQ, compression, etc.), wordlength grows, and wherever the wordlength must be reduced, dither is necessary on that output. Inputs do not need dithering, only outputs. Dithering inputs can deteriorate the sound. Notice how the aux send to the external reverb chamber must be dithered to 24 bits, and the output of the external reverb must also be dithered within the reverb. If dither is not in the external processor’s menu and you cannot confirm that it’s “built-in,” then you are likely not getting the potential depth and dimension from the processor or chamber.
If a signal is summed digitally (mixed within the DAW), usually you should capture to a 32-bit float file. If you must capture to a 24-bit file, the output feeding that file should usually be dithered, except in the rare cases where the mix engine is internally dithered, as is the case of Pro Tools HD’s dithered mixer. If the signal is mixed via an external analog summing box, each output of the DAW should be dithered on its way to the summing box, unless the mix engineer never changes gains or adds any plugins on the way to the summer (a very rare occurrence these days). Refer to the file names at the right of the mix section for the possibilities of your particular DAW. In some cases it is optimum to mix to a 32-bit file, in others a 24-bit file is adequate. Again, there is no harm in mixing to a longer wordlength than necessary, but the reverse is not the case!
Mastering (pictured in blue)
Like the mixing engineer, the mastering engineer will probably go through the following steps (pictured below):
Use a 32-bit float mastering DAW, along with internal and/or external processors.
Upsample the file obtained from the mix engineer, producing the 3296 file shown at the left, making it possible to proceed at a 96 kHz sample rate. Or (not shown), some mastering engineers play the original mix file through a superior-quality DAC directly to analog processing, followed by capture with an ADC at 96 kHz sample rate (which is a form of “analog sample rate conversion”) or at 44.1 kHz, if there is no digital processing after the analog stage.
Level and/or process the sound, using plugins or internal processes, which work at 32-bit wordlength.
Feed an external processing chain, which has a 24-bit pipeline, so ensure the DAW outputs feeding the processing chain are dithered down to 24 bits.
Confirm that each external digital processor’s output is dithered to 24 bits on its way to the next processor in the chain.
Optionally, capture the raw output from the external processors without limiting to a “prelimiter” file for use in cutting a more dynamic LP. The producer and mastering engineer decide which of the two files they prefer for LP, without limiting or the limited master which became the CD.
Optionally, insert a plugin for peak limiting on the return from the external processors.
Capture the output of the DAW to a “premaster” 96 kHz file (or whatever naming convention suits the workflow), with file name and filetype examples shown at the right side of the mastering section.
Final Files (pictured in green)
The prelimiter file (if chosen), is likely suitable for LP without further work (pictured below). For clarity, the mastering engineer may rename this file as the LP master, as shown. The premaster file must be down sampled, first to a 3244 file. At this point the signal branches off depending on the destination requirement. For example, it needs to be dithered down to 16 bit for CD, for 1644 download, etc. As shown, it also needs to be dithered to 24 bits for Mastered for iTunes, where it will be sent to the record company to log and produce the information for metadata, and thence to Apple for encoding to AAC files and sale at the iTunes store. After this long journey, the music finally makes its way home to the consumer!
Bob Katz played the B flat clarinet from the age of 10, and his lifelong love of sound and music led him to become a professional recording, mixing and mastering engineer (since 1971). Three of his recordings have garnered the Grammy™ award and many others have been lauded in publications such as Stereo Review, Audio, and Stereophile. He has written over a hundred articles for audio and computer publications, and is an inventor and manufacturer with processors and support gear in use at mastering studios worldwide. His most recent patent-pending inventions, the K-Stereo and K-Surround Processors, fill a missing link in the mastering and post-production pantheon. He has been a workshops, facilities and section chairman of the AES and has given lectures in several countries. Currently, Bob runs Digital Domain Studios just north of Orlando, Florida, and is the author of Mastering Audio and the new iTunes Music: Mastering High Resolution Audio Delivery.