In its essence, a linear phase is a property of a filter, where the phase response of the filter is a linear function of frequency. The result is that all frequency components of the input signal are shifted in time (usually delayed) by the same constant amount (the slope of the linear function), which is referred to as the group delay. And consequently, there is no phase distortion due to the time delay of frequencies relative to one another.
When it comes to linear phase equalizers, we can say that they are one of the equalizers with the most transparent sounding types of equalization. A significant downgrade of this type of equalizers is the tendency to create some undesirable side effects. For example, latency and pre-ringing. As mentioned above, a linear EQ is entirely transparent since, as its name implies, is a property of a filter. The phase response of the filter is a linear function of frequency. The result is that all frequency components of the input signal are shifted in time (usually delayed) by the same constant amount (the slope of the linear function), which is referred to as the group delay. And consequently, there is no phase distortion due to the time delay of frequencies relative to one another.
Situations like live performances is where linear phase EQ is not a suitable option since it produces a noticeable latency. We can safely say that the biggest drawback with the linear phase EQ is without any doubt the pre-ringing. The shape of the EQ curve governs the amount of pre-ringing (in fact there is a precise mathematical relationship between the EQ curve and the amount of pre-ringing for linear phase EQ). This means pre-ringing can be controlled by changing the EQ curve. Some plugins attempt to suppress pre-ringing for you. But, as a result, you’re not getting the curve (or phase) you think you are.
EQ curves and Pre-ringing
In the rest of this article, we’ll give you some crucial information about the relationship between EQ curves and pre-ringing. After this, you can apply linear phase EQ like a seasoned pro.
Pre-ringing is essentially a backward echo sound. It sounds like a strange sucking sound. You can hear it most noticeably on transients. Technically speaking the only way to get perfect linear phase is to apply an EQ twice: once forwards in time and once backward in time. The application of an identical backward in time EQ cancels out any phase changes. The latency introduced in the linear phase EQ is the result of the backward in time portion of the EQ nullifying the phase.
As a side note, all EQ suffers from post-ringing, but our auditory senses don’t notice it nearly as much. Linear phase EQ effectively halves the post-ringing and instead provides an equal and opposite amount of pre-ringing. Typically, the longer the pre-ringing, the more noticeable it is.
The following examples are a clear example of where you can use the linear phase EQ as a suitable option.
- Acoustic Instruments – Even at extreme settings, the sound of harmonically-rich acoustic instruments such as guitars and piano remains much more transparent with Linear Phase EQ as opposed to regular EQ.
- Stereo sources: Linear Phase EQ is ideal for stereo-correlated sources such as left and right overheads, or stereo-mic a piano. With regular EQ, adjusting the frequency response on one of the stereo correlated sources will introduce phase differences over parts of the frequency spectrum, which can cause the stereo coherence to collapse. This effect does not happen with Linear Phase EQ.
- Mastering: Linear Phase EQ is perfect for “colorless” matching of sonic character between different songs on an album, without adding the artifacts that non-linear phase EQs can add.