A day or so ago, I waxed on about birds and distortion specs—the kind that never seem to make it into glossy reviews, even though they matter more than half the numbers that do get published.
A few of you reached out asking about two of the lesser-known villains I mentioned: TIM and SID. These aren’t secret societies or new streaming services—they’re actual forms of distortion. And as a bonus, both are classic TLAs (welcome to engineering humor).
So, what are they?
TIM stands for Transient Intermodulation Distortion. It occurs when an amplifier is hit with a rapid signal—something sharp or fast—and can’t respond in time without glitching. The result is added frequencies (intermodulation products) that weren’t in the original signal. It’s not harmonic distortion. It’s something else entirely, and it can make music sound edgy or harsh in ways that don’t show up in traditional THD measurements.
SID is Slew-Induced Distortion, and it’s closely related. When the input signal changes faster than the circuit’s slew rate can handle, the amp effectively chokes—it distorts because it physically can’t keep up. Think of trying to draw a sharp zig-zag with a thick marker. You can’t make the corners tight, no matter how hard you try.
The problem is these distortions don’t show up in static tests like sine wave analysis. They’re only revealed when the amp is pushed dynamically—exactly what happens in real music with sharp attacks, fast passages, or layered complexity.
You hear these distortions even if they are never discussed in spec sheets or reviews. This is some of the work we've been on about for years and why we spend half the product's development cycle listening and tweaking.
Most spec sheets ignore TIM and SID because they’re hard to measure, harder to explain, and don’t make anyone look good unless the design is truly exceptional. But that doesn’t make them go away.
Sine waves are easy. Music is messy. And good audio design means handling both.
TLAs and all.
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