Basic Audio Mixing
This guide offers an introduction to the basic principles of audio mixing within digital audio software.
The principles of mono and stereo audio are summarised here in this short screencast.
Multi-track mixing essentially the blending of a number of audio files or regions to create an overall mix that sounds well balanced. This introductory guide is written for those who wish to conduct very basic mixing of small multi-track projects and who have little or no knowledge or experience of the subject.
A mix is the putting together of audio (on separate tracks or channels in your software) to create a final mix file. With the creation of a new audio resource, this could be in the form of separate audio files of spoken word, adding background music to voice recordings, or blending in special effects sound files with other source files.
It discusses the fundamental aspects of mixing in digital audio software, such as volume, pan and automation, and what mixing down to mono, stereo and surround actually means. This guide also advises on how to finalise your mix, the starting blocks of the mastering processes.
What is mixing?
A simple example of mixing is having to create a mix of an interview, where two microphones were used to record each person speaking. The resulting two audio audio files will need to be mixed together to create a final mix - and the final audio file. A more complex example is a music recording where each instrument in an orchestra is recorded with its own microphone resulting in a number of individual audio files that will need mixing together to produce the final music file. The main aim of mixing is to create the illusion that all the sounds were recorded together, or just that they fit well together allowing each individual element to be herard clearly. Effects and processing techniques are usually applied when mixing to enhance the quality of individual sounds and to the overall mix as a whole.
The result of a mix is a final rendered audio file. There are three main types of resultant file that can be mixed to; mono, stereo and surround formats, which increase in file size respectively. Files within a project may be either mono or stereo. For a definition of mono, stereo and surround file formats, see the advice document File Types and Compression.
A monaural (mono) file consists of one-channel of sound. Upon playback it contains all the audio in the central balance of the sound field. If one were to listen to a mono file on headphones all the sounds would appear together in the middle, with no left or right pan positioning. Monaural sound production was common in early recordings but has since been surpassed by stereo and surround production.
Mono mixing is useful when only one source has been recorded, i.e. a person speaking into a microphone, or when use of stereo perception is irrelevant or not required for delivery purposes. This could be to keep the file size to a minimum for efficient web delivery or when delivering a one-channel recorded spoken word file.
Stereophonic (stereo) production uses two channels which represent the left and right output of a system - such as speakers or headphones. Sounds can be panned (positioned) at locations within boundaries of far-left and far-right. The two channels are then combined to produce a stereo file.
Stereo mixing allows you to create a stereo image - a spread of sounds - which is more similar to how we naturally hear sounds. For example if you were listening to a conversation between two people the sound from each person would come from a different location. Two single channels can be positioned to represent this natural occurrence which helps us distinguish the location of each person in this example.
Surround sound mixing
Surround sound is a multi-channel format to create a 3-dimensional immersive listening experience where the listener perceives sound in a 360 degree sound-field.
There are many formats of surround sound, the most common consumer format being 5.1, where the 5 relates to the number of speaker positions (see diagram 3 below), and the 1 to a sub-woofer which amplifies low frequencies from all of the 5 channels.
To mix in surround sound requires a speaker array as shown in diagram 3 above and only the most sophisticated audio software offers surround mixing features. Further encoding software is then required to create the finalised surround sound files.
Mixing volume levels
The first step in mixing is balancing the volume levels of each track. The volume sliders, known as faders, in audio software allow you to manipulate the volumes. Faders that have a decibel (db) meter show a numerical value of volume. This is shown in diagram 4 below.
A master level meter, which displays the combined output of all the channels should be monitored and adjusted correctly so that this too does not exceed 0db, as this results in distortion.
A major advantgae of digital audio mixing is the ability to adjust the volume within regions. Automation allows you to draw a volume line or curve that the software remembers. This moves the fader during playback to follow the line or curve that you draw and the changes will be applied to the final mix file.
For stereo and surround mixing, it is possible to adjust each channel to a position in the sound-field. Each channel in a DAE commonly has a panning controller which allows you to position the channels output relative to the main output format.
A stereo pan control lets you control how much of a channels signal is fed to the left or right output channel. The amount sent to one channel is relative to the amount reduced in the other. When fully to the left side, the signal is sent only to the left speaker and vice versa.
What is automation?
Automation allows you to draw changes to parameters of a track, such as the volume or pan, on the timeline. These changes are heard during playback, they are remembered by the editing software and are applied to the final mix. Automation comes at different levels of sophistication, but primarily most offer simple volume and pan control over single tracks.
Diagram 6 shows the volume of a track gradually increasing the later decreasing due to the automation curves that have been drawn.
In more high-end DAEs automation offers control over routing levels and plug-in parameters and as such requires a high processing speed and amount of RAM.
What are plug-ins?
Plug-ins are software versions of hardware effects units and dynamic processors, from filters and distortion to reverberation and equalisation. Different formats of plug-ins are compatible with different software. VST (PC and Mac) and Audio Unit (Mac only) plug-in formats are common examples. Some plug-ins often come bundled with audio software whereas more sophisticated plug-ins are available to purchase separately.
Plug-ins can generally be applied to individual channels within a DAE, across a bus or even over the entire mix. Care and thought should be applied when using plug-ins and further advice can be found in the advice documents regarding use of digital effects and processing.
Should I Normalise my audio files?
Normalisation increases the overall volume of an audio file (or region) relaitve to the loudest point contained within. Normalisation increases the volume of file so that the loudest part reaches the maximum value allowed in a digital audio file. This can be useful for increasing the overall volume of files. The downside is that for file that have very loud points as well as a lot of quieter sections the process will have little effect if the file already reaches the masximum volume allowed in one or more places.
I have finished mixing, what next?
When you have perfected your mix you will need to render all of the channels to your chosen output format. This feature may be termed ‘bounce', ‘export', or ‘share'. Note that when you ‘save' or ‘save as...' it is the project file you are saving and not the mix you are creating.
You should select the wrapper format and the format settings such as sample rate and bit depth. See the guide to choosing an audio file format for more information.
There may be options for what you wish to render to the output file. These may include the timeline as a whole, a selection of the timeline, or specific channels only within the arrangement. Make sure you enter the correct range of time that you wish to be exporting. One your mix has been rendered, you should playback and listen to the new file to check for any audible errors.
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