Where it shows up
The same picture governs any link that makes a threshold decision on a noisy analogue waveform. High-speed serial lanes (PCI Express, USB, HDMI) are signed off on eye masks, optical transceivers quote eye margin in their datasheets, and hard-disk and flash read channels equalise until the eye reopens. The folding trick itself is broader than communications: astronomers phase-fold a faint periodic signal onto one cycle to pull a planet out of the noise, and the ISI you see here is just convolution, the same operator behind blur, reverb, and every finite-impulse-response filter.
The knobs
- Roll-off β: how gently the raised-cosine pulse decays. More roll-off spends extra bandwidth to steepen the transitions, so the eye opens wider horizontally; near zero the pulse is a ringing sinc and the eye narrows.
- SNR (dB): the signal-to-noise ratio. Lower values fatten the noise band on every trace, and the eye squeezes shut vertically first.
- Bandwidth: the channel low-pass cutoff in symbol rates. Drop it and neighbouring symbols smear together (ISI), collapsing the eye from the top and bottom.
- Overlaid traces: how many two-symbol slices are stacked. More traces sample more bit patterns, so the persistence band fills in and the true worst-case boundary of the eye becomes visible.
Changing the roll-off, bandwidth, or trace count reseeds the random bitstream, so you always see a fresh sweep of symbol patterns.
Further reading
- Eye pattern (Wikipedia)
- Raised-cosine filter
- Proakis, Digital Communications