Cartridge tests - Separation

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Channel separation of -27dB recorded by Rohde & Schwarz UPL set to third-octave mode, using Shure TTR-109 test disc.



This is the amount of separation between the two stereo channels, or how much right channel is in the left, and vice versa. It tells us about the degree of stereo that can be achieved. With high separation the cartridge will produce a wide sound stage. As separation decreases the sound stage narrows until at zero separation the channels are fully mixed and a mono central image results.

Cartridges can be relatively poor in this area, and the LP too, often with just 20dB separation, a pathetically low figure compared to other sources (VHF/FM radio 40dB: CD 110dB). Yet the stereo illusion is still subjectively maintained. One reason is that the human head manages only 20dB or so separation between the ears and is used to working with this amount, so around 25dB-30dB of separation is adequate in practice for a good sense of stereo.



We measure separation using Shure test disc TTR-109. It was cut with a 1kHz test tone having correctly aligned modulation axes, specifically to provide an accurate separation figure, and it is also cut on both sides so the affect of a dished disc can be averaged out, and from outside to inside also to give some idea as to the impact of dishing may affect results. We ensure the disc lies flat and measure the signal residual in each channel with a spectrum analyser to remove any contribution from warps, noise etc. The result published is the average of the two channel residues.

Separation is affected by the azimuth of the cartridge in the headshell. Headshells can be often be rotated (tilted left or right) slightly to ensure a cartridge sits perfectly upright. Unfortunately, many cartridges have poorly aligned internal generators and when upright give unbalanced channel separation figures. Adjusting azimuth to get a balanced result shows the degree of generator misalignment and can result in a visibly non upright cartridge. See our Adjust+ measurement system review for more on this.



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