What is Mastering?

“Mastering” is the process of preparing a sound track for distribution. It might involve, but is not limited to:

  • Critical listening, to verify that there are no technical errors in the final product.
  • Fixing equalization problems that were caused by poor acoustics in the original production/mixing room.
  • Applying final tone control to match the overall tone of other tracks in the collection, or comparable tracks in the same genre.
  • Applying final loudness adjustments to match the level of other tracks in the collection, or comparable tracks in the same genre.
  • Sequencing of tracks into a collection ( i.e. sequencing songs on a CD ).
  • Applying frequency-limiting and dynamic-range adjustments, to match the media that will be distributed.
  • Creation of metadata (such as track markers, song name(s), performance rights, etc ) for the distribution media.
  • Exporting/packaging the distributable in a format that is suitable for the next stage of duplication/distribution.

Although many of these tasks are outside the scope of Mixbus, Mixbus provides several tools that can help with the mastering process.

Master Bus Processing and Metering

The master bus provides a suite of processing and metering that helps achieve the overall tone and loudness that the artist wants.

Master bus strip tone controls

The master bus provides a 3-band EQ (“tone”) control for the final output. The tone control has a fairly limited range ( +/-6dB) for fine control. The center frequencies are optimized to solve problems that are commonly used in the final stages of mixing:

  • Low: a low-frequency shelf at 90Hz : the Low band is provided to balance the low bass content of the track, without having to re-mix the bass instruments.
  • Low Mid: a wide bell shape at 300Hz : many instruments have strong output in the low-midrange, and they can combine to make too much energy in this range. This range is also a site for tape-saturation harmonics. The “Low Mid” band is provided to increase or decrease the main band of music.
  • High: a high-frequency shelf at 4,000Hz : A high-frequency shelf at 4 kHz allows you to brighten the entire track without unduly affecting individual instruments.

Master bus strip compressor

The master bus strip compressor operates like the channelstrip compressors. ( see : Compression Techniques )

Because the master bus strip compressor is applied before the Master Limiter, this provides a chance to level-match parts of a song (using the leveler mode), or to clip outstanding transients before they reach the master limiter (using the limiter mode).

Typically a very small amount of compression is needed on the master bus, therefore the master bus limiter threshold has a very different “scaling” than the channelstrip compressors. You have to pull the threshold down much lower before it has any effect.

The order of master bus compressor, fader, and EQ may be manipulated to achieve a wide range of desired effects before the signal is finally sent to the master bus limiter.

Master Limiter

The master bus limiter. It has no controls other than an on/off indicator.

The Master Limiter is a lookahead limiter with a very fast attack and release time.

The “lookahead” feature imparts a short 5ms delay on the input signal. This delay allows the limiter to “look ahead” and begin reducing the gain before a loud peak occurs. This dramatically reduces the distortion that is caused by hard limiting.

The limiter’s threshold is fixed at -2dB. This value is intentionally less than digitally clipping so that your final processing (sample-rate conversions, or lossy encoding like mp3) are provided extra headroom for their interpolation. This can result in better sounding mixes. If you want your exported file to peak at a higher value, you can implement that in the “normalization” setting for each of the file formats you are exporting.

Each segmented LED represents 1 dB of gain reduction.


There are many plug-ins that are developed specifically for the master-bus output, for the purpose of increasing the master output level. These may be placed in the channelstrip signal flow, and used instead of, or in conjunction with, the Mixbus master bus processors.

Stereo Correlation (“Phase”) Meter

The stereo correlation meter provides an indication of the “mono compatibility”, or stereo width, of your mix. The indicator uses a bright bar which simultaneously indicates the range of in-phase (highly correlated) and out-of-phase (decorrelated) sounds in your mix.

  • If the indicator is fully to the right, and yellow, this indicates a completely “mono” mix. (the signals in the left and right channels are the same)
    • many signals (such as a spoken-word recording with one microphone) should be mono.
  • If the indicator is fully to the left, and fully red, this indicates a mix that is completely “out of phase”, in other words the left and right channels are equal, but one is inverted with respect to the other. This almost certainly indicates a technical fault which should be investigated.
  • If the indicator is largely to the right, and green, then it indicates a mix with some stereo width, but largely mono compatible. This is optimal for pop music recordings that might be played in a variety of playback systems.
  • If the indicator is in the center, showing a range of red and green, then the mix is very wide stereo. This is probably not optimal for pop/rock recordings, but can be acceptable for natural or orchestral recordings. Mono compatibility of this mix is questionable.

Mono compatibility is largely defined by low frequencies. If your mix indicates poor mono compatibility ( red bar on the left ), then you can probably solve this by reducing the number of bass instruments in the mix, high-pass filtering any non-bass instruments, and high-pass filter any stereo effects, such as reverb, in the mix.

K-14 Meter

The K-14 meter is an RMS (averaging) meter, originally specified by mastering engineer Bob Katz as a means to compare and help control the dynamic range of pop music mixes. (see : http://www.digido.com/how-to-make-better-recordings-part-2.html )

The K-Meter displays the level of the signal directly before the output of the master bus. The K-Meter comes after the fader, EQ, plugins, and final limiter in the master bus.

If the K-14 meter indicator is straight up, at 0, this indicates that the sound’s average level is at -14dBFS. This was selected as a good target for most pop/blues/jazz/rock recordings. At this level, a song will sound quieter than most contemporary pop music, but when the listener turns the volume up to match, the increased dynamic range of this mix will allow the song to sound much better than a typical pop song played at the same volume.

Increasing the volume beyond the “0” indicator will result in a louder mix, at a cost of decreased fidelity and impact when the user turns the volume up.


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