No Signal

What no signal does

Test patterns are the reference images broadcast engineers use to check that a picture is being reproduced correctly, and the no signal screen is what a display shows when nothing is connected to it. This page draws both. The patterns are generated at your screen resolution rather than scaled from an image, so the bars have clean edges at any size, and there is an optional 1 kHz tone, which is the audio reference that traditionally accompanied them.

How to use it

1. Choose a pattern. SMPTE bars are the broadcast standard; the no signal screen is the prop.
2. Click the panel, or press F, to fill the display.
3. Turn on the 1 kHz tone if you are checking audio as well as picture.
4. Read the note under the panel, which explains what each pattern is actually for.

About no signal

What SMPTE bars actually check

The pattern is three bands and each does a different job. The wide upper band is seven colour bars at 75 per cent intensity, arranged in descending luminance order, so on a correctly set up monochrome view they form a smooth staircase and any error appears as a step out of sequence. The narrow middle band repeats the colours in reverse with black between them, which is used to check colour phase. The bottom band carries the pluge, three near-black patches at slightly below, at, and slightly above true black: the correct black level is the one where the middle patch is just indistinguishable from the surrounding black while the brightest is just visible.

Why 75 per cent and not full intensity

It looks like an odd choice and it is deliberate. Analogue broadcast systems combined luminance and chrominance into one signal, and fully saturated colour bars produced a combined signal that exceeded what transmission equipment could carry without distortion. Setting the bars to 75 per cent kept the signal inside legal limits while still exercising the full colour path. EBU 100 per cent bars exist alongside them for the opposite purpose: checking whether a modern digital display or capture chain clips the brightest values, which 75 per cent bars can never reveal.

The no signal screen, and why it looks like that

What a display shows when nothing is connected varies by manufacturer, but the conventions are consistent: a dark background, large legible text, and often a prompt to check the input. It exists to distinguish a working display with no input from a broken one, which is a genuinely useful distinction when troubleshooting. As a prop it works as a stream holding screen, as a joke, or as set dressing, and it is honest about being a web page rather than pretending to be a system message.

The 1 kHz tone

A steady 1 kHz sine wave is the standard audio reference that accompanied colour bars on tape and in transmission, used to set levels so that recordings from different sources matched. One kilohertz sits near the middle of the range where hearing is most sensitive, which makes it easy to judge by ear and convenient to measure. The tone here is generated in your browser rather than loaded as a file, and it starts quietly on purpose, because a full level test tone is genuinely unpleasant and there is no reason to surprise anyone with one.

Using the greyscale steps

Eleven even steps from black to white are the fastest way to see whether a display is crushing detail at either end. Each step should be clearly distinct from its neighbours. If the darkest two or three merge into one black, the display is losing shadow detail, usually from a contrast or black level setting pushed too far. If the brightest merge, it is clipping highlights. On a television this is very often a picture mode rather than a fault: dynamic and vivid modes routinely crush both ends in exchange for apparent punch.