”Overdrive” is a gain-based effect which people often mistake as a distortion effect. It’s pretty hard to draw the line between the two, especially when it comes to guitar effect pedals. One of the simplest ways to describe this effect is to say that overdrive occurs when input gain exceeds the capacity of a device to handle the amount of gain thrown at it; in this case, a tube. What happens is that the smooth waveform that goes into the device gets “clipped”. Sonically, we perceive the result of this clipping as distortion. The higher the amplitude of the wave, the greater amount of distortion we hear. There’s been a lot of confusion about these two terms. This is because they’re used so loosely, and oftentimes interchangeably.
A way to think about overdrive vs distortion is that overdrive happens in the front-end (what you put in) while distortion happens on the back-end (what you hear). But here’s where we get into a bit of murky territory, especially with pedals. If we’re talking about overdrive as simply overpowering the front-end of an amp to make the tubes clip, the only pedal that is technically an overdrive is a boost pedal that takes your guitar’s signal and ups its voltage. But lots of manufacturers call their pedals overdrive pedals. In reality, most of those are technically distortion pedals. The effect is most closely associated with true amp distortion. It literally aims to mimic the effect of amp tubes being overdriven by high gain.
Overdrive guitar pedals break up into two categories. First are transparent, mid scooped boosters and second, compressed mid boosted overdrives.
The transparent, mids scooped boosters were originally designed in the late 60s and early 70s to drive large tube stacks into distortion. These pedals can sound horrible on an uncompressed. However, they sound creamy and smooth on a compressed and mids boosted Marshall. Compressed and mids boosted overdrives, started to appear in the late 70s, with the Tube Screamer and later, with boutique designs mimicking the Dumble amp and other mids oriented amps. These pedals will produce a smoother and overall bigger sounding tone, on amps like the Twin and smaller bedroom amps.
Put simply, overdrive tends to be a milder, less linear distortion, with a more dynamic response. So, the tone will saturate less when played more quietly. On the other hand, it will distort more when played louder/ harder, and apply a mild compression to the tone. In effect, when it starts to overdrive the signal is soft-clipping. Consequently, the signal is distorted and compressed smoothly and gradually according to much it has crossed this threshold by.
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