2026-09-24 · class-notes, a-plus, comptia, messer, hardware, displays, resolution, refresh-rate, colour-gamut
Full class notes for Professor Messer A+ 220-1201 objective 3.1: pixel density and PPI maths, refresh rate and frames per second, resolution and aspect ratio, and colour gamut.
Class notes · Professor Messer, CompTIA A+ 220-1201 Core 1 · Section 3, objective 3.1 (Display Attributes)
Quick reference: the short version of this lesson is the Display Attributes cheat sheet in A+ Core 1 section 3. It is the companion to the Display Types lesson in the same section: that one covers the technologies (LCD, OLED, mini LED), this one covers the numbers on the specification sheet.
By the end of these notes you should be able to:
Display specifications vary enormously between models — different options, different numbers, different settings. The way to make sense of them is to start from the other end: decide how the display will be used, then read the specification sheet to find one that suits.
The use cases the lesson raises are worth holding in mind throughout:
| Use | What matters most |
|---|---|
| Gaming | High refresh rate, fast response |
| Watching video | Resolution and colour; refresh rate less critical |
| A screen on a lobby wall | Size and brightness; nobody is sitting close |
| Presentations | Resolution and viewing angle |
| Graphics and video editing | Colour gamut above everything |
Only once you know the job do the numbers mean anything. A 144 Hz specification is irrelevant to a lobby screen and essential to a gaming monitor.
Pixel density is how many pixels fit into one inch of display area — PPI, pixels per inch. Outside the United States you may see pixels per centimetre instead. It puts a number on how crisp a display looks: a higher pixel density means a sharper, clearer image, because the individual pixels are smaller and closer together.
+------------------------------------------------------------------------------+
| The same one-inch square at different pixel densities |
| |
| 1 PPI 2 PPI 4 PPI 8 PPI |
| +-------+ +---+---+ +-+-+-+-+ +++++++++ |
| | | | | | +-+-+-+-+ +++++++++ |
| | | +---+---+ +-+-+-+-+ +++++++++ |
| | | | | | +-+-+-+-+ +++++++++ |
| +-------+ +---+---+ +-+-+-+-+ +++++++++ |
| |
| 1 pixel 4 pixels 16 pixels 64 pixels |
| PPI counts across AND down, so doubling PPI quadruples the pixels |
+------------------------------------------------------------------------------+
If the image will only ever be viewed on screen, PPI is the number that matters. If it is going to be printed, check the printer's DPI (dots per inch) as well, and whether it can reproduce what you are seeing on the display. Screen and paper are measured separately.
Note (beyond this lesson): PPI and DPI are often used interchangeably but describe different things — PPI is pixels on a display, DPI is ink dots a printer lays down. Printers typically quote far higher DPI figures than any monitor's PPI, because several ink dots combine to make one perceived colour.
The calculation is simply pixels divided by inches. The catch is that a display's advertised size is its diagonal, so you need the width in inches, not the diagonal.
A 27-inch 4K monitor. 27 inches on the diagonal works out at roughly 24 inches wide, and 4K means 3,840 pixels across:
A 65-inch 4K television. Same 3,840 pixels, but about 57 inches wide:
I checked both the width figures and the division. For a 16:9 display, a 27-inch diagonal gives a width of 23.5 inches and a 65-inch diagonal gives 56.7 inches, so the lesson's rounded 24 and 57 are right. The divisions come out at 160 and 67.4 PPI.
Exam tip: identical resolution does not mean identical sharpness. Spread the same 3,840 pixels over a bigger screen and each pixel gets bigger, so the image is less dense. Resolution alone tells you nothing about clarity without the physical size.
Scenario: a user wants a 32-inch monitor and is choosing between 1080p and 4K. They sit about 70 cm away.
A display is not showing one image; it is redrawing the screen continuously. Hertz (Hz) is the generic term for cycles per second, and a display's hertz figure is how many times it can refresh the screen in one second.
Frames per second (FPS) is a related but not identical idea — it counts the images in the content being shown. You can configure a system so that hertz and FPS match, and some display configurations update only part of the screen on a given cycle, which breaks the equivalence.
V-sync (vertical sync) is the setting that ties them together: with V-sync enabled, the frame rate matches the display's refresh rate.
+------------------------------------------------------------------------------+
| Refresh rate (Hz) how many times the DISPLAY redraws per second |
| Frame rate (FPS) how many images the CONTENT provides per second |
| |
| With V-sync on, the two are locked together: Hz = FPS |
| |
| FPS higher than Hz -> frames the display never shows (or tearing) |
| FPS lower than Hz -> the same frame shown twice; motion stutters |
+------------------------------------------------------------------------------+
| Content | Typical frame rate |
|---|---|
| Films (United States) | 24 FPS |
| Television shows and online video | 30 FPS |
| Sport and gaming | 60 FPS or higher |
The lesson demonstrates the difference with falling circles updated at 1, 15 and 30 FPS: at 1 FPS the motion is visibly choppy, while at 30 FPS it falls smoothly. That is the whole argument for a higher refresh rate — with fast-moving content, too low a refresh rate makes the image appear to stutter as it moves across the screen.
Note (beyond this lesson): US broadcast television is actually 29.97 FPS rather than exactly 30, a legacy of the transition to colour. The 30 figure is the right one to remember for the exam.
The display is only one link. To actually get a high refresh rate you also need:
The lesson gives two figures: HDMI 2.1 supporting 4K at up to 144 Hz, and DisplayPort 2.1 supporting dual 4K displays at up to 144 Hz each. Buy a 144 Hz monitor and connect it with a cable or a card that cannot manage it, and you will run at a lower rate without any obvious warning.
Caution: HDMI 2.1's headline, uncompressed specification is 4K at 120 Hz; 4K at 144 Hz is reached using DSC (Display Stream Compression), which the standard also defines. Treat 144 Hz as achievable over HDMI 2.1 rather than as the plain uncompressed maximum. The DisplayPort 2.1 dual-4K-at-144 Hz figure matches VESA's own published configuration.
In the real world: cables are the most common culprit. An HDMI cable rated for an earlier version physically fits an HDMI 2.1 port and works — just at a lower resolution or refresh rate. If a monitor will not offer its top mode, check the cable before anything else.
Scenario: a new 4K 144 Hz monitor only offers 60 Hz in the display settings.
Resolution is the number of pixels horizontally and vertically. 4K is 3,840 × 2,160; HD (1080p) is 1,920 × 1,080. More pixels give a sharper image, so at the same physical size a 4K screen looks considerably better than an HD one.
Note how much more work that is: 4K has exactly four times as many pixels as HD — twice as many across and twice as many down, which is 8,294,400 against 2,073,600. That is why a card that runs a game comfortably at HD may struggle at 4K.
Most standard resolutions share a 16:9 aspect ratio, which is why the numbers look like a family: 1,920 ÷ 1,080 and 3,840 ÷ 2,160 both reduce to 16:9. Some displays deviate from the standard resolutions or use different aspect ratios entirely, so pick the shape that suits the use — ultrawide for side-by-side working, 16:9 for video.
+------------------------------------------------------------------------------+
| 16:9 family, to scale |
| |
| +---------------------------------------------+ |
| | | |
| | 4K 3840 x 2160 | |
| | | |
| | +---------------------+ | |
| | | HD 1920 x 1080 | | |
| | | (one quarter the | | |
| | | pixels of 4K) | | |
| | +---------------------+ | |
| +---------------------------------------------+ |
+------------------------------------------------------------------------------+
Note (beyond this lesson): to get the aspect ratio from a resolution, divide both numbers by their greatest common divisor. 1,920 × 1,080 divided by 120 gives 16 × 9. The same works for 2,560 × 1,440 (QHD) and 3,440 × 1,440 (a 21:9 ultrawide).
The human eye sees a far wider range of colours than any display can reproduce. The range a display can show is its colour gamut. For general use this hardly matters; for graphics and image work it may be the deciding specification.
Gamut is plotted on the CIE 1931 colour space — a diagram of everything the human eye can see, onto which each standard is drawn as a triangle covering the portion of that space it defines:
| Standard | Note |
|---|---|
| sRGB | Standard Red Green Blue; the common baseline for web and general use |
| Adobe RGB | Adobe's wider standard, aimed at print and photographic work |
| ITU standards | From the International Telecommunications Union, such as Rec. 709 |
| DCI-P3 | A wide gamut also quoted on monitor specification sheets |
You will find this broken down in a monitor's technical specifications as percentages. One display in the lesson quotes 100% Rec. 709, 100% sRGB and 98% DCI-P3. You do not need to know each standard's details — the percentage tells you how closely the display meets it.
Comparing two otherwise similar monitors, one covers 100% of sRGB and the other 95%. Which is right depends entirely on the job:
Both of those displays are LCDs using IPS, and the better one uses a variant of IPS that achieves deeper blacks, which is likely why it reaches a higher percentage of sRGB. Panel technology and colour performance are connected.
The best colour gamut generally comes from OLED, which tends to have the best colour representation of all and usually matches these standards more closely than a traditional liquid crystal display.
Exam tip: for colour-critical work, the ranking that emerges across both display lessons is OLED first, then a good IPS LCD. Match this to the panel-type lesson: IPS is the LCD answer for colour, OLED is the overall answer.
Scenario: two 27-inch 4K monitors at similar prices, for a marketing team that does light photo editing and a lot of video calls.
Display specifications are not a security topic by nature, but several of them have defender-relevant consequences.
| Term | Meaning |
|---|---|
| Pixel density | How many pixels occupy one inch of display area |
| PPI | Pixels per inch; the unit of pixel density |
| DPI | Dots per inch; the printer equivalent of PPI |
| Diagonal | How display size is advertised, measured corner to corner |
| Hertz (Hz) | Cycles per second; a display's refresh rate |
| Refresh rate | How many times per second the display redraws the screen |
| FPS | Frames per second; how many images the content supplies |
| V-sync | Vertical sync; setting that locks frame rate to refresh rate |
| Stutter | Visible jerkiness when the refresh rate cannot keep up with motion |
| Resolution | The number of pixels horizontally and vertically |
| HD | 1,920 × 1,080 |
| 4K | 3,840 × 2,160; four times the pixels of HD |
| Aspect ratio | The proportion of width to height, commonly 16:9 |
| Colour gamut | The range of colours a display can reproduce |
| CIE 1931 | The colour space diagram gamuts are plotted on |
| sRGB | Standard Red Green Blue; the common baseline colour standard |
| Adobe RGB | Adobe's wider colour standard |
| Rec. 709 | An ITU colour standard quoted on monitor specifications |
| DCI-P3 | A wide colour gamut standard quoted on monitor specifications |
| DSC | Display Stream Compression; how HDMI 2.1 reaches 4K at 144 Hz |