Mumbling about computers

Measuring keyboard to display latency

I've got fed up with some latency I've been experiencing when using the work-issued VPN (remote desktop over IP), and wanted to get measurements, to see if the latency perceived is real.

The approach is as basic and end-to-end as I could think of: emulate a USB keyboard pressing a button and measure the light levels changing on screen.

Software on the target to measure

Wrote a very basic SDL program to flash the screen white while the spacebar is being held down, effectively it boils down to:

if (key_pressed) {
    SDL_SetRenderDrawColor( gRenderer, 0xFF, 0xFF, 0xFF, 0xFF );
} else {
    SDL_SetRenderDrawColor( gRenderer, 0x00, 0x00, 0x00, 0x00 );
}
SDL_RenderClear(gRenderer);
SDL_RenderPresent(gRenderer);

which means "If the key is pressed, make the window white, otherwise make it black"

Microcontroller software

Wrote a very basic Arduino sketch for its trivial-to-use USB HID support, which will take the mean result of 10 measurements after detecting a button press.

Hardware

The light sensor is a basic LDR (light dependent resistor) which is not great, because these take a "significant" amount of time to settle to new values when their input changes. The typical latencies I've found online are ~8ms for a full transition (0% -> 100%)

Here's a very basic schematic:

Data

Target Measurement Notes
Linux 5.13, Dell P2418D 53ms 60Hz
Linux 5.13, Dell P2418D 36ms 60Hz, Enabled "Game Mode"
Linux 5.13, Dell+VPN 70-87ms High variance
Linux 5.7, Thinkpad T430 51ms 60Hz, no VSync
14" M1 Macbook Pro ?? 60Hz

Source

You can find the repo with the sources + schematic here.

References