• James Guido

Mixing for Mastering: Part 1 Aliasing & Bit Rate.

A lot of focus is placed upon the Mixdown but a lot of discussion about the stereo render is dangerously left untouched. I wanted to write a short post on adherents for sending your stereo mix to an engineer for mastering.

Bit Rate - there seems to be a little ‘bit’ of confusion about bit rate in the audio world however as most music is created and recorded digitally in 2019 it is increasingly more important. Bit Rate is essentially the speed or rate at which samples or slices of audio (computer samples) are recorded and played back. The industry standard for music is 44.1kHz which is most likely the default on your DAW. In general 44.1kHz is a global setting, which means that audio is usually recorded at this bit rate and then played back at the same rate which gives us no problems.

When there are differences in the recorded bit rate and the exported bit rate is when we encounter problems in the exported audio, this is called Aliasing. Aliasing is a by product of mismatched frequencies and it actually generates a new frequency which is destructive to the exported audio. In most cases this is audible.

The common cause for this is on the bounce, producers make the mistake of thinking “Oh, 48kHz will give me higher quality.” which is firstly incorrect as the audio has been recorded at 44.1kHz and secondly will cause an alias frequency to be embedded into the audio.

However, what is possible is to either double or half the recorded frequency. Doubling the frequency of a 44.1kHz recording to 88.2kHz will not increase the sound quality, but halving an 88.2kHz recording to 44.1kHz will decrease the quality. This is mainly heard in the upper frequencies as a 44.1kHz recording will only playback frequencies of exactly half (22.05 kHz) which you can read more about by checking out the Nyquist Theorem. Likewise an 88.2kHz recording will record 44.1kHz. Interesting…

There is much debate about this and if it is required but in general humans are only able to hear between 20Hz and 20kHz which means that a 44.1kHz recording is fine for a professional release. Unless of course you are making an album for video which uses a 48kHz standard that aligns more smoothly with the video frame rate or for dogs who have a much wider range of hearing, woof.

In either case to avoid problems the Bit Rate of the recording must be consistent with the Bit Rate of the render unless it is by a factor of either 2 or half. More importantly in general this cannot be fixed by the mastering engineer and it is good practice to include a small text file with your master which lists the Bit Rate and Bit Depth which I will aim to post about soon.

If you are looking for more information or to booking a mixing or mastering session get in contact today.

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