What’s the deal with inner groove distortion?

Without getting too technical, as a cartridge mounted to a radially tracking tonearm moves across the record, playback distortion characteristics change with changing tracking error.

Generally, the highest distortion is found playing the innermost grooves of a record. There will be two points with the lowest distortion, where the radial arc traced by the arm intersects a radius from the centre of the record. On either side of those two points, distortion will rapidly increase.

What you may not know is that certain stylus profiles and cartridge alignments minimise inner groove distortion. In fact, you can ask me to specifically align your cartridge so that it minimises inner groove distortion, though I generally recommend using the manufacturer’s specified alignment/overhang.

Note that linear tracking tonearms don’t suffer from this variable positional distortion as they don’t trace an arc as they move across the record surface. This is a key benefit of linear tracking arms. It’s also why longer tonearms are preferable, the arc they trace has a greater radius and is, therefore, less curved, leading to lower variability in distortion across the record.