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The original ZX Spectrum computer produces a one bit per pixel, bitmapped colour 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 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 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 colours for colouring the contained 64 pixels.[3]
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 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 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).[5]
- ^ Vickers, Steven (1982). "Introduction". Sinclair ZX Spectrum BASIC Programming. Sinclair Research Ltd. Archived from the original on 24 September 2006. Retrieved 23 August 2006.
- ^ Vickers, Steven (1982). "Colours". Sinclair ZX Spectrum BASIC Programming. Sinclair Research Ltd. Archived from the original on 24 September 2006. Retrieved 23 August 2006.
- ^ EP patent 0107687, Richard Francis Altwasser, "Display for a computer", issued 6 July 1988, assigned to Sinclair Research Ltd
- ^ "Database - ZX-Art". zxart.ee. Retrieved 14 April 2023.
- ^ "48K ZX Spectrum Technical Information". worldofspectrum.org. Retrieved 28 December 2023.