I’ll give you a little background on this. The transmit power from the GPU to the display varies. At startup (or when you connect the cable), there’s a training sequence where displayport determines what minimum voltage is required for its digital signals to reach the display reliably.
The tether cable contains many smaller wires inside, and each of those individual wires consists of individual wound strands. What generally happens is some particular spot on the cable keeps getting kinked and flexed over and over again until those strands start breaking from fatigue. Or the cable just plain suffers trauma that damages those strands outright. The cable continues to function as these strands get broken… the remaining strands are able to carry the signal. But each broken strand increases the resistance of the cable by some amount until eventually all strands for one of the wires are broken and the cable doesn’t work at all.
As the resistance of the cable increases from this damage, the GPU needs to raise the voltage in order to avoid getting bit errors… which is what appears as sparkles in the image. The training algorithm is supposed to detect this and raise the voltage automatically, but it only works up to a point.
What Valve appears to have done for the Index is provide alternative training methods that more aggressively ramp up the voltage. Which can get it to work over a damaged cable without sparkles.
Here’s the thing though. If the cable is failing from fatigue, once sparkles start, the cable is not long for this world anyway. More strands will break in whatever spot is getting kinked as you continue to use the headset, and the cable will degrade until it won’t work at all anymore regardless of cranking up the voltage at the GPU. Generally once sparkles start, the cable will reach total failure within days or weeks.
In rare cases, the cable has problems to begin with when it’s new, and the alternate training makes it happy, and so it stays happy indefinitely. But if your cable was previously fine, and it started getting sparkles because it’s failing from fatigue, Valve’s alternate training will at best only delay the inevitable by a little longer.
That said, your cable might just need reseating and cleaning. If you haven’t tried that yet (at both ends), try that first and see if the sparkles go away. Otherwise, you’ll need to buy a replacement cable. And hope it arrives before your current cable fails the rest of the way.