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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 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 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 224 × 5/8 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. Retrieved 14 April 2023.
- ^ "48K ZX Spectrum Technical Information". comp.sys.sinclair FAQ. Retrieved 28 December 2023.