Draft:ZX Spectrum graphic modes (US English)

Simulation of ZX Spectrum graphics output on a PAL TV set

The original ZX Spectrum computer produces a one bit per pixel, bitmapped color graphics video output. A composite video signal is generated through an RF modulator, and was designed for use with contemporary 1980s television sets.

The image size of the framebuffer is 256x192 pixels, with a palette of 15 non-modifiable colors, where the entire color palette is extremely saturated. The resolution of the color output is 64 times lower than the resolution of the pixel bitmap. The extremely low color resolution was used to conserve memory, totaling just 768 bytes for color attributes. Color is stored separate from the pixel bitmap, as a 32x24 cell grid, using one byte per each of the character cells. One character cell is composed of 8x8 pixels. In practice, this means any character cell can only use two selected colors for coloring the contained 64 pixels.

Since the machine was designed for usage with a standard television set, the 256x192 pixel area is surrounded by a wide border that fills up the remaining space of the standard 384x288 low-resolution PAL TV visible screen area. Usually, the border area assumes a single color, but using software tricks it is possible to display some low resolution graphics there.

The ZX Spectrum lacked dedicated hardware for scrolling and sprites, or a dedicated hardware blitter. To facilitate the display of color graphics, the original ZX Spectrum employs 16 KiB of discrete graphics RAM. The latency of the graphics RAM is 150 ns; the peak bandwidth is 2.1875 MB/s (calculated as 224x5/8 bytes per 64 μs).

Frame Rate and Timings

The original ZX Spectrum doesn't conform strictly to the PAL standard frame rate of 50 Hz. ZX Spectrum outputs one video line in exactly 224 CPU clock cycles, where the CPU clock rate equals 3.5 MHz. This exactly matches the PAL standard 64 μs line time. However, the ZX Spectrum produces only 312 lines to form one display frame, while the PAL standard recommends 312.5 lines. As a consequence, the frame rate of the ZX Spectrum is approximately 50.08 frames per second.

This discrepancy was not known to many programmers, who assumed the frame rate of 50 frames per second. As a result, some programs, including the built-in BASIC interpreter, had the time running little too fast. Later ZX Spectrum models have a slightly different frame rate and the CPU clock rate, which causes some older software to run slightly too slow if it uses vertical synchronization, or too fast if it relies on CPU clock rate.

At the start of each display frame, the maskable interrupt signal is sent to the ZX Spectrum's Z80 CPU, enabling programs to easily detect and measure the passage of time. It also allows the programs to perfectly synchronize the graphics output with the display of video frames. It is exactly the same mechanism as the vertical synchronization of a modern computer, which can be employed to prevent display tearing.

Display timings on PAL

	ZX Spectrum 16K, 48K, and ZX Spectrum+	128K, +2, +2A, +3
CPU cycles per PAL line	224	228
PAL line time	64 μs	64.28 μs
PAL lines per frame	312	311
Frame time	19.968 ms	19.9915 ms
Frame rate	50.08 frames per second	50.02 frames per second
Display Vertical Synchronization	available, by the Z80 maskable interrupt line (INT)	available, by the Z80 maskable interrupt line (INT)

Graphics memory structure and pixel coordinates

The 16 KiB discrete graphics RAM is directly accessible to the CPU at addresses 16384 to 32767. The main pixel bitmap is stored at the very beginning of the graphics RAM, while the attributes array follows immediately behind it.

The video controller circuit is built into the semi-custom Ferranti ULA integrated circuit, consisting of approximately 480 configurable cells (depending on the model). The cells of the ULA were factory-configured in various ways to produce the ULA for ZX Spectrum, where one cell has the functionality of approximately two NAND gates.

An CPU access to the graphics RAM is called a contended access, because the video controller has a higher priority then the CPU. On an access to the graphics RAM, the CPU commonly incurs a slight delay while waiting for the video controller to complete the data reads.

The addresses in the video DRAM are interleaved, which produces an unusual coordinate system for the pixel bitmap. If coordinates of a pixel are (x, y), and the bits of the x coordinate axis are labeled x7, x6, x5, x4, x3, x2, x1, x0, and the bits of the y coordinate axis are labeled y7, y6, y5, y4, y3, y2, y1, y0, then the pixel is stored in a byte with the address bitpattern 0, 1, 0, y7, y6, y2, y1, y0, y5, y4, y3, x7, x6, x5, x4, x3.

The address bitpattern contains two more bitpattern transpositions than necessary. The fastest and the simplest bitpattern would have been 0, 1, 0, x7, x6, x5, x4, x3, y7, y6, y5, y4, y3, y2, y1, y0, which makes the pixel addresses increase by the column first. Unfortunately, the ZX Spectrum designers were working under extreme time pressure, and failed to notice this simple improvement.

While the original ZX Spectrum 48K model has 32 KiB of main RAM, the 16K model has just the 16 KiB graphics RAM. This is the reason why the color attributes array was designed to fit in just 768 bytes. The color attributes could have easily had a double or a quadruple resolution in the vertical axis, but that would have reduced the remaining free memory space for programs by another kilobyte or two, especially in the 16 KiB model.

To facilitate the display of color graphics, the original ZX Spectrum employs 16 KiB of discrete graphics RAM, unlike the shared graphics RAM architectures of most other microcomputers, including the Commodore 64. The resolution of the pixel bitmap is similar to that in other contemporary microcomputers, where the popular Commodore 64 commonly uses the 160x200 resolution at two bits per pixel.

The system of color cells, called "the attributes", drew a lot of criticism, because it significantly complicated the programming of color applications. Still, the color attribute system had sufficient capabilities for supporting the common applications and games of the era, and even the very limited color capabilities were welcomed by the users.

Color palette

ZX Spectrum screen showing the available colors

The ZX Spectrum (and compatibles) computers uses a variation of the 4-bit RGBI palette philosophy (also used on CGA, Thomson MO5, Sharp MZ-800, Mattel Aquarius, etc.), resulting in 8 basic colors with brightness variations. On the 128, +2 and +3 models, the ULA outputs TTL level red, green, blue and sync signals that are used by the TEA2000 Video Encoder for composite video signal generation.

Each of the colors of the 3-bit palette has a basic and a bright variant. The bright half of the palette is generated using the video displays' maximum voltage level for the luminance signal. The basic half of the palette is generated by simply reducing the voltage of the luminance signal.

Both colors of a character cell cell must share their brightness property, because there is only a single brightness bit for the entire cell. Besides that limitation, any combination of the palette colors can be freely selected as the two colors of a character cell. In the following table, all the "bright" colors are given in the right column.

Color number	Binary value				Color name		Binary value				Color name
	G	R	B	I			G	R	B	I
0	0	0	0	0	#000000	Black	0	0	0	1	#000000	"Bright" Black
1	0	0	1	0	#0100CE	Blue	0	0	1	1	#0200FD	Bright Blue
2	0	1	0	0	#CF0100	Red	0	1	0	1	#FF0201	Bright Red
3	0	1	1	0	#CF01CE	Magenta	0	1	1	1	#FF02FD	Bright Magenta
4	1	0	0	0	#00CF15	Green	1	0	0	1	#00FF1C	Bright Green
5	1	0	1	0	#01CFCF	Cyan	1	0	1	1	#02FFFF	Bright Cyan
6	1	1	0	0	#CFCF15	Yellow	1	1	0	1	#FFFF1D	Bright Yellow
7	1	1	1	0	#CFCFCF	White	1	1	1	1	#FFFFFF	Bright White

• Some ZX Spectrum clones or NTSC machines might display "bright black" as dark grey.
• The luminance of colors in the table is relative, not absolute. The common television sets usually had much higher luminance than the standard sRGB LCD luminance of 80 cd/m². In absolute terms, ZX Spectrum's colors are much brighter then the sRGB colors in the table. ZX Spectrum's "White" color is as bright as the brightest sRGB white color, while the "Bright White" exceeds the limits of the standard sRGB color luminance.
• The given sRGB values for green, yellow and cyan are only best approximations, because sRGB displays are unable to produce all the colors of the PAL TV standard. The green PAL phosphor is out-of-gamut in the sRGB color-space of the modern common LCD displays. For most purposes, this color inaccuracy is small.
• Colors simulated as sRGB assume non-bright as 84.615% voltage (0.55 V) and bright as 100% (0.65 V). Each ZX Spectrum model used different voltages for colors, so the values here are only indicative. The colors were computed by a conversion of the PAL TV color-space into the sRGB color-space. The PAL gamma of 2.8 was applied as recommended by the BT.470 PAL standard; the conversion of primary colors to sRGB primaries (by IEC 61966-2-1 sRGB standard), and the standard sRGB gamma correction (by inverse EOTF) was applied.
• The given colors are probably not the real ZX Spectrum colors. The color approximations were computed by assuming the maximum possible saturation that a ZX Spectrum can produce on the PAL TV output. The real ZX Spectrum colors are currently unknown, and they are probably less saturated. To compute the real ZX Spectrum colors, a measurement of the phase-amplitude shift of the chroma sub-carrier has to be performed, for each color, by an oscilloscope on the ZX Spectrum's PAL output.

In the ZX Spectrum encoding, the color components are in the GRB (Green, Red, Blue) order, rather than the more common RGB order. The GRB order has the advantage that the color numbers become ordered by increasing luminance, so if viewed on black-and-white display the ordered sequence forms a gradient from black to white.

All the color properties of a cell are stored in memory as one byte called the attribute. Counting from least to most significant bit, an attribute byte dedicates three bits for color of the pixels valued 1, three bits for color of the pixels valued 0, one bit for the brightness flag, and one bit for the flashing effect. The flashing effect causes the displayed foreground and background colors to alternate every 0.64 seconds.

Most current ZX Spectrum emulators are displaying inaccurate and possibly over-saturated colors. Those colors were computed by simplistic approximations that don't take into account many subtleties of the PAL-to-sRGB color-space conversion. Similarly, on the ZX Art website the usage of inaccurate colors is very common. The theoretically impossible sRGB color #00CD00 is commonly used as the ZX Spectrum's green color (i.e. the Spectrum's real green color is probably more bluish).

BASIC commands for colors

The two colors of a character cell are called the foreground color and the background color. For any value of n from 0 to 7, the following Sinclair BASIC commands can be used to set or alter the colors of a cell:

• PAPER n, the background color for the character cell; applied to all pixels of value 0 in the cell
• INK n, the foreground color for the character cell; applied to all pixels of value 1 in the cell
• BRIGHT i, selects the value of the brightness bit of the character cell, where the value can be either 0 or 1

Additionally, the BORDER command selects a color for the screen area surrounding the pixel bitmap. It does not use a brightness flag, thus only the eight basic colors are supported for the border color.

Standard mode

Standard ZX Spectrum screen, with attribute blocks clearly visible

The original ZX Spectrum has a screen resolution of 256x192 pixels. In all cases, colors are extremely saturated. Color information is overlaid onto this as a grid of 8x8 pixel regions known as attribute blocks. All color properties of an attribute block are stored in memory as a single attribute byte, and each attribute block matches one character cell. Within each attribute block, only two colors may be used out of a palette of 8 colors. Additionally, the entire attribute block may be designated as 'bright', resulting in a total of 15 possible colors (black has no 'bright' variation). In many programs, this limitation of only two colors per attribute block is evident as the unwanted effect of attribute clash.

A screen in this mode occupies 6144 bytes for the pixel bitmap, totaling 6912 bytes together with the color attributes.

Details:

Pixels: 256 x 192

Attributes: 32 x 24

Colors: 15 (counting non-bright and bright)

Machine: All

Standard mode gallery

• 
  Wikipedia logo
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  New Year tree
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  Flowers

Monochrome TVs and monitors

ZX Spectrum on a monochrome display

By using a monochrome monitor or black and white TV (or reducing the saturation settings of a color TV), it is possible to take advantage of the differences in intensity over the Spectrum's color range to generate a 15-shade greyscale image at 256x192 resolution.

Details:

Pixels: 256 x 192

Attributes: 32 x 24

Colors: 15 greyscale

Machine: All, using a monochrome display

Hi-Color attributes

Hardware implementation

Hi-Color / Multitech

ZX Spectrum "8x1" attributes / Hicolour display

Several third-party Spectrum clones, including the Timex Sinclair machines, the Pentagon, the eLeMeNt ZX, and the MB03+ Ultimate interface support a screen mode named Hi-Color, in which attribute blocks are 8x1 pixels in size rather than the usual 8x8. A screen in this mode takes 12 KB RAM. In the case of the Timex, this mode is activated through the command OUT 255,2. At least one editor for Timex machines supports this mode. In addition, this screen mode can be generated through the use of the MB-02 disk system's DMA hardware (where the technique is known as Multitech), and is also available as Mode 2 (with a linear byte order) on the SAM Coupé.

Details:

Pixels: 256 x 192

Attributes: 32 x 192

Colors: 15

Machine: Timex Sinclair models, ZX Spectrum Next, Pentagon, eLeMeNt ZX, SAM Coupé

Interface: MB03+ Ultimate

Software implementation

8x1 attributes / Hicolour / Multicolour / FLI / BIFROST*2 / Rainbow Generator / Rainbow Processor

On other Spectrum models, this effect can be replicated by exploiting the fact that the ULA re-reads the attribute information on every pixel row when generating the video output; it is possible to write a new value to the relevant memory location in between successive lines, and thus cause a different pair of colors to be shown. However, the Spectrum's processor is not fast enough to write to an entire row of attribute bytes in one scanline, so 8x1 attributes can only be achieved over 20 columns. This technique is variously known as Hicolour, Multicolour, FLI, BIFROST*2 "SuperColour", "Rainbow Generator" or Rainbow Processor mode.

Details:

Pixels: 256 x 192

Attributes: 32 x 192 (limited to a 18x192 area)

Colors: 15

Machine: All (with specific code tailored for each machine's timings)

8x2 attributes / Bicolour / Nirvana+

ZX Spectrum "8x2" attributes display

A variation on this method is to change the row of attributes over the course of two scanlines, resulting in 8x2 pixel attribute blocks over a wider region of the screen. This mode is known as Bicolour, and can be applied to the full width of the screen through the use of the Nirvana+ engine.

Details:

Pixels: 256 x 192

Attributes: 32 x 96

Colors: 15

Machine: All (with specific code tailored for each machine's timings)

4x1 / 4x2 / 4x4 attributes

A special case involves alternating between the two available colors per attribute cell for 4 pixels each, allowing each 4x1 region to be treated as an independently colourable 'pixel' (although the limitation of one brightness level per 8x1 cell is still in effect). Again, a 4x2 variant of this mode can be applied to a wider region of the screen. And a 4x4 variant can be achieved on 128K machines by timed switches between the two video RAMs (rather than re-writing the attribute data), to display the upper half of the character cells from one screen and the lower half from the other.

Details:

Pixels: 64 x 192; 64 x 96; 64 x 48

Attributes: 8x1 brightness limitation

Colors: 15

Machine: All (with specific code tailored for each machine's timings)

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  4x1 attributes example image
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  4x2 attributes example image
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  4x4 attributes example image

ULAplus / HAM256 / HAM8x1

ULAplus is compatible with the standard ZX Spectrum and Timex Hi-Res and Hi-Color modes, and adds the ability to redefining of the palette. If only used to slight modify the basic 16 colors, ULAplus software can be displayed on a standard Spectrum. Use of the full 64 colors is incompatible, as it will trigger the "flash" attributes of the original Spectrum. Amiga HAM inspired modes are also possible (HAM256 and HAM8x1), displaying up to 256 colors on screen.

Details:

Pixels: 256 x 192; 512 x 192 (Hi-Res mode)

Attributes: 32 x 24; 32 x 192 (8x1 Hi-Color mode)

Colors: 2 from 256 (Hi-Res mode), 64 from 256; 256 (HAM256 and HAM8x1)

Machine: ZX Spectrum SE, ZX Spectrum Next, ZX-Uno, Chloe 280SE, Chloe 140SE, eLeMeNt ZX, zx128u+

Interface: MB03+ Ultimate

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  ULAplus example image
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  ULAplus 8x1 Hi-Color example image
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  ULAplus HAM256 example image
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  ULAplus HAM8x1 example image

Interlace/Switched modes

Simulated interlaced ZX Spectrum image on a CRT screen

Interlace

By alternating between two screens on every frame interrupt (50 Hz), it is theoretically possible to simulate a doubling of the vertical display resolution from 192 to 384 lines.

The Timex clones and ZX Spectrum 128K implement a 'shadow' screen area which can be switched into place through the use of a single OUT command, and this is often utilised to rapidly switch between two images for this purpose (although this can also be achieved with a standard block copy, albeit not over the entire screen).

When viewed on a CRT television screen (the standard at the time), the flicker is less noticeable than on a modern monitor. And example image is linked here (warning: the linked image flickers very rapidly, which could potentially cause seizures in people with photosensitive epilepsy).

Interlaced highres monochrome ZX Spectrum image. Real picture taken from a Sinclair ZX Spectrum +3 using composite output and a PCI capture card.

The technique does not in fact achieve a true interlaced display, as the Spectrum lacks the ability to synchronise with the display hardware at such a low level. On a CRT television screen, the effect is more akin to anti-aliasing, with certain pixels appearing at half intensity.

On modern LCD TVs, capture cards, or other devices that convert the original analog signal into digital (thus partially ignoring the original timing signals), this mode can be interpreted as true interlace, as shown on the image featuring a cat, at the right.

Details:

Pixels: 256 x 384

Attributes: 32 x 24

Colors: 15

GigaScreen / DithVIDE / BZither

GigaScreen ZX Spectrum screen

The attributes (colors) of an image are alternated at 50 Hz on the screen. This way, the palette is increased to approximately 36 colors. It's possible to switch screen per scanline and thus mix the screens, but this is very CPU intensive and needs exact and steady timings.

With real tstate precise software its possible to simulate a GigaScreen on an 128k "toastrack" original 128k ZX Spectrum.

For Pentagon machines, a hardware modification is available which directly combines the two alternate screen areas into the video signal, thus eliminating the flicker associated with this method. The MB03+ Ultimate interface and the eLeMeNt ZX computer can display three hardware modes of non-flickering GigaScreen (mixed video RAMs, mixed video frames and autodetection mode) in 256 x 192 and 512 x 192 resolutions.

Furthermore, the GigaScreen and Hi-Color techniques may be employed together to produce even richer-coloured images; this format has been named DithVIDE and BZither, both names referring to the dithering methods employed when converting true-color images to the format.

Details:

Pixels: 256 x 192

Attributes: 32 x 24

Colors: ~36

Machine: All (with flickering)

Hardware GigaScreen: Pentagon, eLeMeNt ZX, MB03+ Ultimate (no flickering)

KeyLayer

The MB03+ Ultimate interface and the eLeMeNt ZX computer provide this graphic mode which allows to display image data from the second video RAM at the place of one selected color in the video RAM no.1. This adds a third color to an attribute.

Details:

Pixels: 256 x 192, 512 x 192

Attributes: 32 x 24, 64 x 48, 3 colors per cell

Colors: 15

Machine: eLeMeNt ZX

Interface: MB03+ Ultimate

3colour / Multichrome / RGB-3 / Interchrome

3colour ZX Spectrum screen

Three separate images, comprising a red, green and blue layer, are displayed on the screen rapidly, one after the other, relying on persistence of vision effects to merge the three layers into a single coloured image. The result is an 8-color image where each pixel may be coloured independently.

Details:

Pixels: 256 x 192

Attributes: 256 x 192

Colors: 8

Machine: All

Compatible machines and interfaces

Later ZX Spectrum compatible machines offered extra video modes. These are based on the standard 256x192 mode but incompatible with the original Spectrum. Also interfaces, the Spectra interface and the MB03+ Ultimate interface extend the Spectrum’s display to support more colors or/and extra video modes.

16c

ATM Turbo / Pentagon 16 color mode

Video mode where each pixel can have one of 16 colors.

Details:

Pixels: 256 x 192

Attributes: none, no limitations

Colors: 16

Machine: ATM Turbo, Pentagon

Sam Coupé 256x192x16 color mode

256x192x16 / Mode 4

A Sam Coupé mode, where each pixel can display one of 16 colors from a 128 color palette.

Details:

Pixels: 256 x 192

Attributes: none, no limitations

Colors: 16 (from 128) per line

Machine: Sam Coupé

384x304x16

A mode supported by the Pentagon computer.

Details:

Pixels: 384 x 304

Attributes: 48 x 38

Colors: 16

Machine: Pentagon

HGFX Graphics

The eLeMeNt ZX computer and the MB03+ interface provide a planar-based 256x192 (LowRes), 512x192 (HighRes) and 512x384 (SuperHiRes) resolutions. HGFX graphics are handled through planar layers, but can be also accessed as "chunky" RAM pages. The highest, chunky-only, is the "PAL" resolution of 720x546 pixels. The HGFX consists of ZX-screen compatible (non-linear or linear) layers-bitplanes to achieve up to 256 colors both in LowRes and HiRes modes, 16 colors in SuperHiRes and 4 colors in the highest resolution. It occupies only part of the memory, similar to the ZX-screen memory, has own internal buffers and can be combined with the original ZX-graphics, in a so-called transparency mode. The HGFX provides a 24-bit true-color palette (HiRes indeXedColour compatible) with a 256 indexed colors. HGFX screen data can be also displayed in HAM (Amiga-like) and FILL (Apple2GS -like) modes,

The HGFX is based on a more powerful HGFX/Q system, which was originally expandable in a border area to 320 x 240 or 640 x 240.

Details:

Pixels: 256 x 192, 512 x 192, 512 x 384, 720 x 546

Attributes: none, no limitations, plus FILL and HAM modes

Colors: up to 256 (from 16777216), up to 16 (SuperHiRes) and 4 (PAL 720 x 546)

Machine: MB03+ Ultimate, eLeMeNt ZX

Layer 2

Details:

Pixels: 256 x 192 (256 colors from 512), 320 x 256 (256 colors from 512) and 640 x 256 (16 colors from 512)

Attributes: none, no limitations

Machine: ZX Spectrum Next

Radastan

This mode displays 128x96 double sized pixels. Each pixel holds one of sixteen colors, with no attribute limitations. Pixels are stored in linear buffer as 4-bit nibbles (i.e. 2 pixels per byte). The buffer is 6144 bytes long and occupies same memory as 256 x 192 pixel screen.

Details:

Pixels: 128 x 96

Attributes: none, no limitations

Colors: 16 (from 256)

Machine: ZX-Uno, ZX Spectrum Next, MB03+ Ultimate, eLeMeNt ZX

LoRES

This mode displays 128x96 double sized pixels. Each pixel holds one of 256 colors (from 512), with no attribute limitations. Pixels are stored in linear buffer as 8-bit. The buffer is 12288 bytes long and occupies same memory as 256x192 pixel primary (DFILE1) and shadow (DFILE2) screens.

Details:

Pixels: 128 x 96

Attributes: none, no limitations

Colors: 256 (from 512)

Machine: ZX Spectrum Next

Spectra (+128)

The Spectra has 31 display formats. These allow up to 64 unique colors to be shown simultaneously, and at a variety of color resolutions, with attribute heights of 1, 2, 4 and 8 pixels, and widths of 4 and 8 pixels.

Hi-Resolution modes

512x192x2 / Hi-Res

This mode was mainly used to display 64x24 or 85x24 columns text screen and originally only Timex Sinclair computers (where its named Hi-Res) and some Russian clones can display it. It also takes 12 KB RAM. In the case of the Timex, this mode is activated through the command OUT 255,1. Two graphics editors ("Draw 512" and "Tech-Draw") support this mode, along with BASIC64 and some CP/M implementations.

Details:

Pixels: 512 x 192

Attributes: none, no limitations

Colors: 2 (Four palettes: Black & White, Blue & Yellow, Red & Cyan, Magenta & Green) (out of 512 on the ZX Spectrum Next)

Machine: Timex Sinclair, ZX Spectrum Next, ZX-Uno, MB03+ Ultimate, eLeMeNt ZX, Pentagon

• 
  512x192x2 example image with Black & White palette
• 
  512x192x2 example image with Blue & Yellow palette
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  512x192x2 example image with Red & Cyan palette
• 
  512x192x2 example image with Magenta & Green palette

Sam Coupé 512x192x4 colors example

512x192x4 / Mode 3

The Sam Coupé has this mode, where each pixel can use one of 4 colors from palette of 128.

Details:

Pixels: 512 x 192

Attributes: none, no limitations

Colors: 4 (from 128) per line

Machine: Sam Coupé

Pentagon 512 x 384 video mode example

512x384x2

A mode supported by the Pentagon computer.

Details:

Pixels: 512 x 384

Colors: monochrome

Machine: Pentagon, Timex Sinclair 2128 (TC 2048 based modification)

HiRes Color

Pixels are stored in a linear order. Attributes are doubled in the vertical direction, the composition of the attribute is identical to the original ZX Spectrum mode. HiRes indeXedColour can choose 32 colors from 16777216 colors. Two video RAMs can be mixed in both Color and indeXedColour modes through the Keylayer.

Details:

Pixels: 512 x 192

Attributes: 64 x 48

Colors: 16 (or 32 indexed, from 1677216)

Machine: MB03+ Ultimate, eLeMeNt ZX

Emulated

256x192, 256 colors, "256 color mode", no attributes

The SPEC256, EmuZWin and Es.pectrum emulators have a screen mode 256x192 pixels where each pixel can be in one of 256 colors. This is achieved by extending the word size of the emulated Z80 from 8 bits to 64, making eight bits of data available for each pixel; the screen thus takes 48 KB of memory. This mode only exists on the emulator and software graphics must be modified to use it.

Details:

Pixels: 256 x 192

Attributes: none, no limitations

Colors: 256

Dithering and Blur

Simulated TV blur on a 256x192 ZX Spectrum image with a regular dithering pattern. Patterns intentionally blur generating new colors (ex: orange, grey, etc). Regular dithering reduces the effective resolution of this image to 128x96.

Standard ZX Spectrum palette with dithering, in pixel art

The low per-pixel bit depth of only one bit per pixel has urged the graphic artists (e.g. at ZX-Art) to resort to various dithering techniques. Dithering works very well with CRT TV sets of the era, because analog televisions employ a high amount of blur in both the horizontal and vertical axis. As the focus of the common CRT TV electron beam is intentionally low, the resulting high blur produces color mixing of neighbouring pixel colors. Blur reduces the unwanted visible noise added by dithering in displayed images.

To faithfully simulate the actual output image of a ZX Spectrum, a modern emulator needs to employ a blur filter of significant strength. Simple bilinear filters introduce too much blur, while the extremely sharp Lanczos filter is inadequate. Therefore, the filter has to be specially constructed to simulate the effect of blur on a common CRT TV set. Most emulators employ filters of very low quality, causing the final image to appear extremely sharp and pixelated. Some emulators, like FUSE, employ available filters in an attempt to reduce the pixilation effect. Those filters produce the final image significantly different than the ZX Spectrum's real video output.

A low mutual contrast of some palette color combinations allows improvement of the dithering effect by further reducing the amount of noise introduced by dithering. The table below demonstrates this effect - for example, the contrast of a white color over a yellow background is extremely low, making the white text virtually invisible.

Color contrast chart

0,00,00	0,00,01	0,00,02	0,00,03	0,00,04	0,00,05	0,00,06	0,00,07
0,01,00	0,01,01	0,01,02	0,01,03	0,01,04	0,01,05	0,01,06	0,01,07
0,02,00	0,02,01	0,02,02	0,02,03	0,02,04	0,02,05	0,02,06	0,02,07
0,03,00	0,03,01	0,03,02	0,03,03	0,03,04	0,03,05	0,03,06	0,03,07
0,04,00	0,04,01	0,04,02	0,04,03	0,04,04	0,04,05	0,04,06	0,04,07
0,05,00	0,05,01	0,05,02	0,05,03	0,05,04	0,05,05	0,05,06	0,05,07
0,06,00	0,06,01	0,06,02	0,06,03	0,06,04	0,06,05	0,06,06	0,06,07
0,07,00	0,07,01	0,07,02	0,07,03	0,07,04	0,07,05	0,07,06	0,07,07

External links

• ZX-Spectrum graphics modes
• ZX-Spectrum extra graphics modes Archived 15 June 2006 at the Wayback Machine
• Image to ZX Spec - An open source utility for image conversion from PC to ZX Spectrum (including 14 dither modes)
• Retro-X - A utility for image conversion from PC to old machines (includes non standard ZX Spectrum modes)
• ZX GIMP plug-in A GIMP plug-in to make any RGB image to look like a ZX Spectrum screen.
• BIFROST*2 Engine (Multicolour "8x1" attributes 20 columns)
• NIRVANA+ Engine (Multicolour "8x2" attributes full screen width)
• ZX-Spectrum Next Video Modes
• ZX-Spectrum graphic art collection