Digital Audio Precisions: Sample Rate, Bit Depth, and Audio File Type
The Producers and Engineers Wing of The Recording Academy recommends that digital mix masters be delivered as 24-bit split or stereo-interleaved Broadcast Wave files (BWF), with sample rates equal to or higher than 88.2kHz. Follow these recommendations for the best results. Lower sample rates like 44.1kHz are fine when unavoidable.
It is highly recommended that 24-bit mix masters be delivered to the mastering process. Mix masters recorded at 16-bit can be perfectly fine in and of themselves, but 24-bit mix masters allow for higher resolution processing during pre-mastering. Mixing engineers should generate a separate set of 16-bit reference bounces for their clients.
When in doubt, don’t change. For less experienced mixers, the easiest choice for generating mix masters from DAW’s like Pro Tools and Logic is simply to use the same sample rate and bit depth settings as the source mixing session. In general, sample rate conversion and re-quantizing are best addressed at the end of the pre-mastering process.
Mix Master Levels
It is recommended that 24-bit mix masters be bounced with peak amplitudes between -6.0dBFS and -3.0dBFS. While there is no reason to leave any headroom for the mastering process itself, several circumstances suggest this range as a best practice:
1) Metered peak amplitudes greater than -3.0dBFS can include significantly higher inter-sample peaks, including material beyond -0.0dBFS.
2) Many mixing environments lack the metering and monitoring tools to detect these inter-sample peaks.
3) At approximately 144dBFS, a 24-bit digital audio file has dynamic range to spare.
It is not productive for mixing engineers to use peak limiting on the mix buss to achieve higher average amplitudes (louder mixes). Loudness is most successfully addressed in the mastering process. Rather, mixers should simply use the master fader to adjust peak amplitude. If clients complain about low volume, peak limiting can be used on their 16-bit reference mixes.
For more information about mix loudness, check out the Bouncing Mix Masters FAQ
What To Bounce?
1. In addition to the ‘Final Mix’ or album version of each song, it is often useful for artists, record labels and mastering engineers to have both ‘Instrumental’ and ‘A Cappella’ versions of mixes. These mix reductions can be simply generated by alternately muting all vocal or instrumental content, and re- bouncing the same timeline as the Final Mix.
2. If mix-buss processing like compression, EQ, or peak-limiting is being used on the Master Fader, bounce mixes both with and without the buss processing. These processes can often be more consistently applied during the mastering process than they can be on individual mixes. That said it’s always useful to hear examples of what sounded good during mixing sessions. Be sure to clearly label all mix master files to reflect their variation. Here are some labeling examples:
ArtistName_SongTitle_FinalMix.wav
ArtistName_SongTitle_Instrumental_NoBussProcess.wav
ArtistName_SongTitle_VocalUp.wav
3. Organize digital mix masters (bounces) in folders. Here’s an example:
Further Reading
Recommendations for Delivery of Recorded Music Projects
http://www.grammy.com/Recording_Academy/Producers_And_Engineers/Guidelines/
0dBFS+ Levels in Digital Mastering (AES Paper about Digital Gain Staging)
http://www.tcelectronic.com/media/Level_paper_AES109.pdf