Rockbox mail archive
Subject: Survey: how fast is your display?
From: [IDC]Dragon (idc-dragon_at_gmx.de)
I'd like to make a little survey on the variation of the LCDs.
The background is that for halftoning it's desirable to "flicker" the pixels
with the same speed as the internal LCD scan. Only then every flicker phase
is actually displayed, important for the appearance. If we're too slow, a
part of the screen is shown twice, giving that temporal value additional,
unwanted "weight". If we're too fast, a part gets not shown at all, therefore
The grayscales look perfect and stable on a nice match, grainy on mismatch.
Unfortunately the LCD is internally clocked by an RC oscillator, those are
far less stable and reproduceable than a crystal, e.g. depend on temperature.
And there is no vsync, scan position readout or other means of feedback. So
all we can do is blindly do our best shot. I'd like to find out what a good
average value would be.
I made a diagnostic plugin to find out your value, see attached. It should
be good for Recorder (2MB), V2 and FM, requiring you to run a build not older
than 27/01 (when I optimized the LCD code again). Source code is included, if
you build your own. It tells the LCD rate by making a pattern which
interferes with it.
To use it, simply play it. It shows 2 alternating frames, a dark and a light
one. The alternation is done by a precise timer which you can tune now, with
up and down keys. The goal is to get the screen free of flicker, with only a
slow-moving residual artifact remaining that looks a bit like "a school of
fish", small black horizontal stripes that chase each other. The rest of the
screen may look "cloudy" in varying shades, but is stationary. The pattern may
move off-screen for a while, so you may only see the background.
I tried to take a picture of it, but it didn't came out well. When tuning,
if you see a storm of diagonal patterns, you're way off. If the patterns slow
down, you're getting closer. If the patterns move up, you're too fast,
compensate by going down. Vice versa, increase if they move down.
My display does about 67 Hz when freshly started. It decreases as it gets
warmer (charging and prolonged disk activity), can get as low as 61 Hz. How
Thanks for participating,
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Page was last modified "Jan 10 2012" The Rockbox Crew