It is common to see the mixer LED indicators turn red and then hear the monitors clipping, but what does this mean? How can it be avoided?
- How an audio signal works
- What is clipping?
- What happens to a clipped audio signal
- The importance of Gain/Trim
- Why redlining is wrong
- Possible reasons why a channel is clipping
- How to avoid clipping
How signal audio works
To best explain clipping it is necessary to review audio signals and how our brain perceives them.
How is the audio signal perceived by our hearing?
The sound enters and passes through the auditory canal, where the sound waves hit the eardrum and the ossicles inside the middle ear, making them vibrate; these vibrations are transmitted to the cochlea causing a movement of the fluid inside. The movement of the fluid is then transformed into neural signals for the auditory nerve, passing them into the brain which interprets these signals into sounds.
These sound waves, which are the ones that make the eardrum vibrate, propagate in a sinusoidal shape, which has 3 main characteristics:
Frequency: this is the number of times the wave is repeated, or ‘oscillates.’ This repetition rate of a wave is measured in cycles per second (called Hertz or Hz). The higher the rate of oscillation, the higher the Hz, the more we will perceive a high-pitched sound. The more a sound propagates with wider, more separated waves, the more low-pitched the sound will be (low Hz)
Frequency and wavelength are inversely proportional: the lower the frequency, the greater the wavelength.
Amplitude: this is the magnitude, and therefore volume of a wave, and is measured in Decibels (dB). The height of each wave peak measured from 0db is what defines the amplitude.
Phase: An angular measure of the time difference between two similar waveforms (expressed in degrees).
What is clipping?
Clipping is a form of (non-linear) distortion of the waveform. It occurs when an amplifier is pushed beyond its maximum limit, causing the amplifier circuity to produce a voltage that is beyond its capacity, causing signal distortion and therefore a noisy and unpleasant output audio signal.
So, simplifying, as far as DJing is concerned, ‘redlining’ or clipping occurs when an audio signal coming from a mixer channel is too strong and the signal is distorted, causing a reduction in sound quality.
What happens to a clipping audio signal
The audio signal, which has a sinusoidal shape, must be carried by an electric current before being output by the speakers.
The magnitude of the electrical voltage is directly proportional to the amplitude of the waveform, and as happens for any electrical device, there is a minimum and maximum value that can be conveyed, as is evident in the image below:
In clipping, the audio signal is overloaded until it reaches a point where the equipment is no longer able to manage the output. What will happen in this case? The sine wave will be cut, or ‘clipped’ from the top and bottom near the maximum values (peaks) resulting in distortion and deterioration of the sound.
It is at this time that you will see the red indicators on the mixer.
Clipping can be of two types: soft and hard. In the first case we have a slight ‘red’ signal in the meter but the sound will still have clarity and quality. In the second case, as seen in the image, it will result in real damage to the sound.
If the mixer, especially during transitions, (that is when there are two open channels with overlapping frequencies) only occasionally goes ‘into the red’ (i.e., it does not remain constantly red) this does not mean that the audio signal is particularly degraded. In fact, clipping is a term for when the signal is constantly deteriorated.
The importance of Gain-Trim
The Gain or Trim control adjusts the input volume of a single channel and usually undergoes very small variations during a DJ set. This parameter is essential for two specific uses; first, to compensate for any volume variations between one track and the other during mix transitions; and second, in order to keep the volume of the master constant.
Generally, the Gain is set by default to 0 dB, but this does not mean that the song will play at 0 dB, rather that we keep the volume of the track at its original level.
The Trim is often the first ’culprit’ of clipping, in fact many DJs tend to abuse it in order to increase the volume of the master, but run the risk of going ‘into the red’ and at that point distorting the sound. This is harmful not only for the audio system, but also for your hearing, as explained in more detail below.
Why redlining is wrong
From the images shown above it is clear why it is important to avoid ’clipping’.
As we have said, one of the most common mistakes made by DJs is to tend to turn up the volume, because to the ear a higher volume is more desirable, but be careful!
There is a limit that balances sound power and quality. In fact, as previously mentioned, you cannot go beyond a certain ceiling.
This limit is dictated by a very important constraint: the power limit of the club sound system. This limit cannot be exceeded and if we try to push our console over, we will do nothing but obtain a distorted and ruined signal.
Thus, upon trying to exceed that limit, you will not gain volume at all, but instead will have the same volume (at the sound system’s maximum limit) just at a poorer quality.
Remember that in a situation where the sound is distorted (redlining/clipping) because you are trying to gain more volume, it is useless to keep turning it up from our console as this will just cause the club sound engineer (or the club’s compressors and limiters) to lower the volume in an attempt to recover quality on an over signal coming from the console.
After all, it will trigger a vicious cycle where the DJ will continue to raise the volume, and the sound engineer will try to lower it, with the result that the sound will come out degraded but at a ’normal’ volume, if not lower. This situation could compromise the success of your evening, ruining the experience of guests who will be listening to “distorted” music.
Possible causes of a channel clipping
It may happen that you buy a track from a retailer, but make the mistake of downloading it in an unsuitable format, such as MP3, or in a worst-case scenario, download it illegally and therefore use a low-quality track. The result of this will be to offer your audience poor quality audio signals.
CHANNEL EQUALISATION (EQ)
We reiterate the importance of checking the console EQ levels before starting to play, even if the following scenario might also happen during your own DJ set.
What can happen is that too many frequencies (low, medium and less often, high) have been added (or removed) to a single channel and this can cause the track to go into clipping. During your set, ensure you use EQ carefully to avoid sums of frequencies that can push the signal ‘into the red,’ even with appropriately adjusted gain/trim.
During a DJ set it is common to use effects, but remember that most of them, especially effects such as delay, tend to multiply the sound, and so it is very easy to end up clipping. It is therefore essential to leave enough headroom so the overlapping sounds can accumulate safely.
Obviously, it is possible to avoid clipping, as it also depends on the type of effect and how much is applied. Just remember to keep sufficient headroom to avoid the redlining zone.
How to avoid clipping
To avoid clipping it is therefore essential to:
- Keep the Trim/Gain at a level that maintains some headroom between the signal output and the maximum capacity of the mixer channel. A good example of headroom can be identified on a mixer showing either just green or green and yellow (or orange) colours when the track is playing.
- Buy high quality songs
- Pay attention to the equalisation of the channels
- Apply any effects appropriately during the DJ set
- Keep a good balance between the volume of the monitors (on which we will soon do an ad hoc in-depth study) and of the headphones (flipping between the CUE and MASTER signal), so that you have a volume suitable for mixing, which does not alienate you from the dancefloor, but at the same time does not damage your hearing due to excessive volume
- Always remember to keep an eye on the meters on your console