The placement of pickups on the body of a guitar or bass does a great deal to alter its sound. The sound produced by a vibrating string depends greatly on the point along its length at which it was struck. The pole-pieces on the top of a pickup act as little microphones, and the point where they rest under a string determines the kinds of sounds they'll detect.
Straight pickups sit below the strings, running perpendicular to the strings. The idea is to have a pole-piece directly beneath each string and to represent the output of each one equally. These are useful if you use any complex parametric equalization effects since the filters can be applied specifically to certain frequencies. Having a neutral baseline allows you to tailor these equalization settings precisely to all the different ranges of the bass sound.
The idea of slanting a pickup is to maximize the treble or "twang" on the high strings without sacrificing any of the low-end body or weight on the low strings. The pickup closest to the bridge is often angled with the treble side nearer the bridge, which requires the pole-pieces on the pickups to be spaced slightly further apart and the whole pickup to be slightly larger. This means the whole pickup is of a slightly higher output, and both treble and bass frequencies are slightly more pronounced and separated. However, as the pickup is at a set angle, these attributes cannot be adjusted to the player's tastes.
Direction of Slant
Although most slanted pickups have the treble side nearer the bridge, some do go in the opposite direction, which thins out the bass frequencies and mellows the treble, rounding out the overall tone. This configuration can be quite useful for bassists who spend a lot of time playing high up the neck since it can prevent the tone from becoming muddy when over-driven, but for most bassists it produces the opposite of the deep-rich tone they're looking for.
Fender's Precision bass features an innovative split-coil design, seen on numerous other subsequent models. They are essentially two tiny single-coils, each one covering a pair of strings. They can be staggered with the treble coil nearer to the bridge. This produces a similar effect to a slanted pickup, only more cleanly compartmentalized into bass and treble frequencies. It offers the tonal highlights of a slanted pickup while retaining the precise tone-shaping of a straight one.
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