Adding input and output capacitors either side of my 5V DC-DC converter doesn't seem to help much, and I'm wondering if some kind of signal interference might be my problem.
I'm running signal from a Teensy 3.1, through 74HCT245 level shifters, then from there through about 4m of 1.5mm^2 3-core stranded domestic power cabling (alongside ground and +12V from a Meanwell power supply—yes, the grounds of the Teensy, level shifters and power supply are tied together) via a combination of Neutrik PowerCON connectors to strips of WS2812B (aka Neopixel) LEDs.
The LEDs don't seem to be getting a clean signal. Every output pin works perfectly individually, but when I get up to about 6-8 of the strings connected, some pins glitch really badly, some glitch a little bit and some are perfectly fine. When I reduce the speed of the signal (using FastLED's sequential output, rather than parallel output) everything is much more solid.
I'm wondering if there's some kind of signal interference, which somehow gets worse as the power supplies get loaded? (Note: there are four power supplies, feeding four strings each—their grounds, but not the +12V rails, are tied together.)
For clarity this diagram only shows one of the signal paths—there are another seven off the pictured 74HCT245, plus another eight off another 74HCT245 not pictured
Without having a oscilloscope (I'm trying to get hold of one), what diagnostic steps could I take to try and rule this out?
SUPPLEMENTAL: We've now tried plugging the strip directly into the control box (omitting the 4m cable in between) and are still getting glitching. So it doesn't seem to be the distance per se.