The image size of the framebuffer is 256×192 pixels, with a palette of 15 non-modifiable colours, where the entire colour palette is extremely saturated. The resolution of the colour output is 64 times lower than the resolution of the pixel bitmap.[1][2] The extremely low colour resolution was used to conserve memory, totaling just 768 bytes for colour attributes. Colour is stored separate from the pixel bitmap, as a 32×24 cell grid, using one byte per each of the character cells. One character cell is composed of 8×8 pixels. In practice, this means any character cell can only use two selected colours for colouring the contained 64 pixels.[3]
Since the machine was designed for usage with a standard television set, the 256×192 pixel area is surrounded by a wide border that fills up the remaining space of the standard 384×288 low-resolution PAL TV visible screen area. Usually, the border area assumes a single colour, but using software tricks, it is possible to display some low-resolution graphics there.[4]
The ZX Spectrum lacked dedicated hardware for scrolling and sprites, or a dedicated hardware blitter. To facilitate the display of colour graphics, the original ZXSpectrum employs 16KiB of discrete graphics RAM. The latency of the graphics RAM is 150ns; the peak bandwidth is 2.1875MB/s (calculated as 224×5/8 bytes per 64μs).[5]
Frame rate and timings
The original ZX Spectrum does not conform strictly to the PAL standard frame rate of 50Hz. ZX Spectrum outputs one video line in exactly 224 CPU clock cycles, where the CPU clock rate equals 3.5MHz. This exactly matches the PAL standard 64μs line time. However, the ZXSpectrum 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.[6]
This discrepancy was not known to many programmers, who assumed the frame rate of 50 frames per second.[citation needed] As a result, some programs, including the built-in BASIC interpreter, had the time running slightly too fast. Later ZXSpectrum 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.[6]
At the start of each display frame, the maskable interrupt signal is sent to the ZXSpectrum'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.
available, by the Z80 maskable interrupt line (INT)[9]
available, by the Z80 maskable interrupt line (INT)[9]
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.[10]
The video controller circuit is built into the semi-custom Ferranti ULAintegrated 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.[11]
A CPU access to the graphics RAM is called a contended access, because the video controller has a higher priority than 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.[12]
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.[13]
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.[11]
While the original ZX Spectrum 48K model has 32 KiB of main RAM, the 16K model has just the 16KiB graphics RAM. This is the reason why the colour attributes array was designed to fit in just 768 bytes. The colour 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 colour 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.[citation needed] The resolution of the pixel bitmap is similar to that in other contemporary microcomputers, where the popular Commodore64 commonly uses the 160×200 resolution at two bits per pixel.[14]
The system of colour cells, called "the attributes", drew a lot of criticism, because it significantly complicated the programming of colour applications.[citation needed] Still, the colour attribute system had sufficient capabilities for supporting the common applications and games of the era, and even the very limited colour capabilities were welcomed by the users.[citation needed]
Each of the colours of the 3-bit palette has a basic and a bright variant.[15] The bright half of the palette is generated using the video display's maximum voltage level for the luminance signal. The basic half of the palette is generated by simply reducing the voltage of the luminance signal.[20]
Both colours of a character 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 colours can be freely selected as the two colours of a character cell. In the following table, all the "bright" colours are given in the right column.
The luminance of colours 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, ZXSpectrum's colours are much brighter then the sRGB colours in the table. ZXSpectrum's "white" colour is as bright as the brightest sRGB white colour, while the "bright white" exceeds the limits of the standard sRGB colour luminance.
The given sRGB values for green, yellow and cyan are only best approximations, because sRGB displays are unable to produce all the colours of the PAL TV standard. The green PAL phosphor is out-of-gamut in the sRGB colour-space of the modern common LCD displays. For most purposes, this colour inaccuracy is small.[citation needed]
Colours simulated as sRGB assume non-bright as 85% voltage (0.55V) and bright as 100% (0.65V).[20] Each ZXSpectrum model used different voltages for colours, so the values here are only indicative. The colours were computed by a conversion of the PAL TV colour-space into the sRGB colour-space. The PAL gamma of 2.8 was applied as recommended by the BT.470 PAL standard; the conversion of primary colours to sRGB primaries (by IEC61966-2-1 sRGB standard), and the standard sRGB gamma correction (by inverse EOTF) was applied.
The given colours are probably not the real ZX Spectrum colours. The colour approximations were computed by assuming the maximum possible saturation that a ZXSpectrum can produce on the PAL TV output. The real ZXSpectrum colours are currently unknown,[clarification needed] and they are probably less saturated. To compute the real ZXSpectrum colours, a measurement of the phase-amplitude shift of the chroma sub-carrier has to be performed, for each colour, by an oscilloscope on the ZXSpectrum's PAL output.[citation needed]
In the ZX Spectrum encoding, the colour components are in the GRB (green, red, blue) order, rather than the more common RGB order. The GRB order has the advantage that the colour 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 colour 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 colour of the pixels valued 1, three bits for colour 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 colours to alternate every 0.64 seconds.[21]
Most current ZX Spectrum emulators are displaying inaccurate and possibly over-saturated colours.[citation needed] Those colours were computed by simplistic approximations that do not take into account many subtleties of the PAL-to-sRGB colour-space conversion. Similarly, on the ZXArt website the usage of inaccurate colours is very common.[citation needed] The theoretically impossible sRGB colour #00CD00 is commonly used as the ZXSpectrum's green colour (i.e. the Spectrum's real green colour is probably more bluish).[citation needed]
BASIC commands for colours
The two colours of a character cell are called the foreground colour and the background colour. For any value of n from 0 to 7, the following Sinclair BASIC commands can be used to set or alter the colours of a cell:[15]
PAPER n, the background colour for the character cell; applied to all pixels of value 0 in the cell
INK n, the foreground colour 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 colour for the screen area surrounding the pixel bitmap. It does not use a brightness flag, thus only the eight basic colours are supported for the border colour.[22]
Standard mode
Standard ZX Spectrum screen, with attribute blocks clearly visible
The original ZX Spectrum has a screen resolution of 256×192 pixels.[23][24] In all cases, colours are extremely saturated. Colour information is overlaid onto this as a grid of 8×8 pixel regions known as attribute blocks.[25][26] All colour 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 colours may be used out of a palette of 8 colours. Additionally, the entire attribute block may be designated as "bright", resulting in a total of 15 possible colours (black has no "bright" variation). In many programs, this limitation of only two colours 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 colour attributes.
Details:
Pixels: 256 × 192
Attributes: 32 × 24
Colours: 15 (counting non-bright and bright)
Machine: All
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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 colour TV), it is possible to take advantage of the differences in intensity over the Spectrum's colour range to generate a 15-shade greyscale image at 256×192 resolution.
Details:
Pixels: 256 × 192
Attributes: 32 × 24
Colours: 15 greyscale
Machine: All, using a monochrome display
Hi-Colour attributes
Hardware implementation
Hi-Color / Multitech
ZX Spectrum "8×1" attributes / Hicolour display
Several third-party Spectrum clones, including the Timex Sinclair machines,[27][28] the Pentagon, the eLeMeNt ZX, and the MB03+ Ultimate interface support a screen mode named Hi-Color,[28] in which attribute blocks are 8×1 pixels in size rather than the usual 8×8.[29][30] A screen in this mode takes 12KB RAM. In the case of the Timex, this mode is activated through the command OUT 255,2.[27][28] At least one editor for Timex machines supports this mode.[31] In addition, this screen mode can be generated through the use of the MB-02 disk system's DMA hardware[32] (where the technique is known as Multitech[33]), and is also available as Mode2 (with a linear byte order) on the SAM Coupé.[34]
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 colours 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 8×1 attributes can only be achieved over 20 columns.[35] This technique is variously known as Hicolour, Multicolour, FLI, BIFROST*2 "SuperColour",[36] "Rainbow Generator" or Rainbow Processor[37] mode.[30]
Details:
Pixels: 256 × 192
Attributes: 32 × 192 (limited to a 18 × 192 area)
Colours: 15
Machine: All (with specific code tailored for each machine's timings)
8 × 2 attributes / Bicolour / Nirvana+
ZX Spectrum "8 × 2" attributes display
A variation on this method is to change the row of attributes over the course of two scanlines, resulting in 8×2 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.[38][39]
Details:
Pixels: 256 × 192
Attributes: 32 × 96
Colours: 15
Machine: All (with specific code tailored for each machine's timings)
4 × 1 / 4 × 2 / 4 × 4 attributes
A special case involves alternating between the two available colours per attribute cell for 4 pixels each, allowing each 4×1 region to be treated as an independently colourable "pixel" (although the limitation of one brightness level per 8×1 cell is still in effect). Again, a 4×2 variant of this mode can be applied to a wider region of the screen. And a 4×4 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 × 192; 64 × 96; 64 × 48
Attributes: 8 × 1 brightness limitation
Colours: 15
Machine: All (with specific code tailored for each machine's timings)
4 × 1 attributes example image
4 × 2 attributes example image
4 × 4 attributes example image
ULAplus / HAM256 / HAM8x1
ULAplus is compatible with the standard ZX Spectrum and Timex Hi-Res and Hi-Colour modes, and adds the ability to redefining of the palette.[40] If only used to slight modify the basic 16 colours, ULAplus software can be displayed on a standard Spectrum. Use of the full 64 colours is incompatible, as it will trigger the "flash" attributes of the original Spectrum.[41]Amiga HAM inspired modes are also possible (HAM256 and HAM8x1), displaying up to 256 colours on screen.[42][43][44][45]
Simulated interlaced ZX Spectrum image on a CRT screen
Interlace
By alternating between two screens on every frame interrupt (50Hz), 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 ZXSpectrum+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.[30][47]
The attributes (colours) of an image are alternated at 50Hz on the screen.[48] This way, the palette is increased to approximately 36 colours.[49] 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 ZXSpectrum128K.[50]
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 eLeMeNtZX computer can display three hardware modes of non-flickering GigaScreen (mixed video RAMs, mixed video frames and autodetection mode) in 256×192 and 512×192 resolutions.
Furthermore, the GigaScreen and Hi-Colour 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-colour images to the format. Details:
Pixels: 256 × 192
Attributes: 32 × 24
Colours: ~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 colour in the video RAM no.1. This adds a third colour to an attribute.
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.[51] The result is an 8-colour image where each pixel may be coloured independently.[52][53]
Details:
Pixels: 256 × 192
Attributes: 256 × 192
Colours: 8
Machine: All
Compatible machines and interfaces
Later ZX Spectrum compatible machines offered extra video modes. These are based on the standard 256×192 mode but incompatible with the original Spectrum.[30] Also interfaces, the Spectra interface and the MB03+ Ultimate interface extend the Spectrum’s display to support more colours or/and extra video modes.
16c
ATM Turbo / Pentagon 16-colour mode
Video mode where each pixel can have one of 16 colours.[54]Details:
The eLeMeNt ZX computer and the MB03+ interface provide a planar-based 256×192 (LowRes), 512×192 (HighRes) and 512×384 (SuperHiRes) resolutions.[56] 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 720×546 pixels. The HGFX consists of ZX-screen compatible (non-linear or linear) layers-bitplanes to achieve up to 256 colours both in LowRes[57] and HiRes modes, 16 colours in SuperHiRes and 4 colours 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.[58] The HGFX provides a 24-bit true-colour palette (HiRes indeXedColour compatible) with a 256 indexed colours. 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 320×240 or 640×240.[59]
This mode displays 128 96 double-sized pixels. Each pixel holds one of 16 colours, 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 256×192 pixel screen.
This mode displays 128 × 96 double-sized pixels. Each pixel holds one of 256 colours (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 256×192 pixel primary (DFILE1) and shadow (DFILE2) screens.
The Spectra has 31 display formats. These allow up to 64 unique colours to be shown simultaneously, and at a variety of colour resolutions, with attribute heights of 1, 2, 4 and 8 pixels, and widths of 4 and 8 pixels.[61]
High-resolution modes
512 × 192 × 2 / Hi-Res
This mode was mainly used to display 64 × 24 or 85 × 24 columns text screen, and originally only Timex Sinclair computers (where its named Hi-Res) and some Russian clones[62] can display it. It also takes 12KB RAM. In the case of the Timex, this mode is activated through the command OUT 255,1.[27][28] Two graphics editors ("Draw512"[63] and "Tech-Draw"[64]) support this mode,[65][66] along with BASIC64 and some CP/M implementations.[30]
Details:
Pixels: 512 × 192
Attributes: none, no limitations
Colours: 2 (4 palettes: black & white, blue & yellow, red & cyan, magenta & green) (out of 512 on the ZXSpectrum Next)
Pixels are stored in a linear order. Attributes are doubled in the vertical direction, the composition of the attribute is identical to the original ZXSpectrum mode. HiRes indeXedColour can choose 32 colours from 16777216 colours. Two video RAMs can be mixed in both Colour and indeXedColour modes through the Keylayer.
256x192, 256 colours, "256 colour mode", no attributes
The SPEC256,[67][68] EmuZWin[69][70] and Es.pectrum[71]emulators have a screen mode 256x192 pixels where each pixel can be in one of 256 colours.[30] 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 48KB of memory. This mode only exists on the emulator and software graphics must be modified to use it.[citation needed]
Details:
Pixels: 256 x 192
Attributes: none, no limitations
Colours: 256
Dithering and blur
Simulated TV blur on a 256 × 192 ZXSpectrum image with a regular dithering pattern. Patterns intentionally blur generating new colours (ex: orange, grey, etc). Regular dithering reduces the effective resolution of this image to 128×96.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 colour mixing of neighbouring pixel colours. 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.[citation needed] Some emulators, like FUSE, employ available filters in an attempt to reduce the pixilation effect.[citation needed] Those filters produce the final image significantly different than the ZXSpectrum's real video output.
A low mutual contrast of some palette colour 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 colour over a yellow background is extremely low, making the white text virtually invisible.
Colour 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
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1 2 Smith, Chris (2010). The ZX Spectrum ULA: How to Design a Microcomputer (1sted.). United Kingdom: ZXDesign Technology and Media. pp.224–227. ISBN978-0-95-650710-5.
1 2 Smith, Chris (2010). The ZX Spectrum ULA: How to Design a Microcomputer (1sted.). United Kingdom: ZXDesign Technology and Media. p.74. ISBN978-0-95-650710-5.
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