Packed with over 170 illustrations and circuit diagrams, this book takes the reader through the cutting edge technology that was the Ferranti ULA and the design of the ZX Spectrum home computer, illustrating the principles and techniques involved in creating a cost effective computer that required nothing more than a television set and a cassette recorder.
The ZX Spectrum ULA is an essential read for the electronics hobbyist, student or electronic engineer wishing to design their own retro-style microcomputer or anyone with an interest in historical micro-electronic and digital design.
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The machine was the brainchild of English entrepreneur and inventor Sir Clive Sinclair. Referred to during development as both the ZX81 Colour and the ZX82, the ZX Spectrum was designed by a small team in Cambridge. It was designed to be small, simple, and most importantly inexpensive, with as few components as possible. The addendum 'Spectrum' was chosen to highlight the machine's colour display, which differed from the black and white display of its predecessor, the ZX81. Its distinctive case, rainbow motif and rubber keyboard were designed by Rick Dickinson. Video output is transmitted to a television set rather than a dedicated monitor, while software is loaded and saved onto compact audio cassettes.
The ZX Spectrum was conceived and designed by English entrepreneur and inventor Clive Sinclair, who was well known for his eccentricity and pioneering ethic.[7] On 25 July 1961, three years after passing his A-levels, he founded Sinclair Radionics Ltd as a vehicle to advertise his inventions and buy components.[8] In 1972, Sinclair had competed with Texas Instruments to produce the world's first pocket calculator, the Sinclair Executive.[9] By the mid 1970s, Sinclair Radionics was producing handheld electronic calculators, miniature televisions, and the ill-fated digital Black Watch wristwatch.[10] Due to financial losses, Sinclair sought investors from the National Enterprise Board (NEB), who had bought a 43% interest in the company and streamlined his product line. Sinclair's relationship with the NEB had worsened, however, and by 1979 it opted to break up Sinclair Radionics entirely,[11] selling off its television division to Binatone and its calculator division to ESL Bristol.[12]
The distinctive wedge-shaped case and colourful design of the ZX Spectrum was the brainchild of Rick Dickinson, a young British industrial designer who had been hired by Sinclair to design the ZX81. Dickinson was tasked to design a sleeker and more "marketable" appearance to the new machine, whilst ensuring all 192 BASIC functions could fit onto 40 physical keys.[23] Early sketches from August 1981 showed the case was to be more angular and wedge-like, in similar vein to an upgraded ZX81 model. Dickinson later settled on a flatter design with a raised rear section and rounded sides in order to depict the machine as "more advanced" as opposed to a mere upgrade. In drawing up potential logos, Dickinson proposed a series of different logotypes which all featured rainbow slashes across the keyboard.[30]
Designing the Spectrum's rubber keyboard was minimalised from several hundred components to a conventional moving keyboard down to "four to five" moving parts using a new technology.[31] The keyboard was still undergoing changes as late as February 1982; some sketches included a roundel-on-square key design which was later featured on the later Spectrum+ model.[30] Dickinson recalled in 2007 that "everything was cost driven" and that the minimalist, Bauhaus approach to the Spectrum gave it an elegant yet "[non] revolutionary" form.[3] The drawing board on which Dickinson designed the ZX Spectrum is now on display in the Science Museum in London.[32][33]
Clive Sinclair became a focal point during the ZX Spectrum's marketing campaign by putting a human face onto the business. Sinclair Research was portrayed in the media as a "plucky" British challenger taking on the technical and marketing might of giant American and Japanese corporations. As David O'Reilly noted in 1986, "by astute use of public relations, particularly playing up his image of a Briton taking on the world, Sinclair has become the best-known name in micros."[59] The media latched onto Sinclair's image; his "Uncle Clive" persona is said to have been created by the gossip columnist for Personal Computer World.[60] The press praised Sinclair as a visionary genius, with The Sun lauding him as "the most prodigious inventor since Leonardo da Vinci". Adamson and Kennedy wrote that Sinclair outgrew the role of microcomputer manufacturer and "accepted the mantle of pioneering boffin leading Britain into a technological utopia".[61] Sinclair's contribution to the technology sector resulted in him being knighted upon the recommendation of Margaret Thatcher in the Queen's 1983 Birthday Honours List.[62][63][64]
The United Kingdom was largely immunised from the effects of the video game crash of 1983, in due part to saturation of home computers such as the ZX Spectrum.[65][66][67]The microcomputer market continued to grow and game development was unhindered despite the turbulence in the North American and Japanese markets. Indeed, computer games remained the dominant sector of the British home video game market up until they were surpassed by Sega and Nintendo consoles in 1991.[68] By the end of 1983 there were more than 450 companies in Britain selling video games on cassette, compared to 95 the year before.[69] An estimated 10,000 to 50,000 people, mostly young men, were developing games out of their homes based on advertisements in popular magazines. The growth of video games during this period was comparable to the punk subculture, fuelled by young people making money from their games.[70]
Sinclair's claims that his technology would surpass IBM's personal computers were taken seriously. In an interview with Practical Computing magazine, Sinclair was confident that the ZX Spectrum's compact size and fewer chips would set the precedent for future computers around the world.[22] By the mid 1980s, Sinclair Research's share of the British home computer market had climbed to a high of 40 per cent.[71] Sales in the 1984 Christmas season were described as "extremely good" with the ZX Spectrum outselling every other microcomputer, including the Commodore 64, its fiercest rival.[72] In early 1985 the British press reported the home computer boom to have ended,[73] leaving many companies slashing prices of their hardware to anticipate lower sales.[72] Despite this, celebration of Sinclair's success in the computing market continued at the Which Computer? show in Birmingham, where the fifth-million ZX Spectrum was issued as a prize.[72]
The ZX Spectrum's successor, the Sinclair QL, was officially announced on 12 January 1984, shortly before the Apple Macintosh went on sale.[74] Contrasting with its predecessors, the QL was aimed at more serious, professional home users.[75] It suffered from several design flaws; fully operational QLs were not available until the late summer, and complaints against Sinclair concerning delays were upheld by the Advertising Standards Authority (ASA) in May of that year. Particularly serious were allegations that Sinclair was cashing cheques months before machines were shipped. By autumn 1984, Sinclair was still publicly forecasting that it would be a "million seller" and that 250,000 units would be sold by the end of the year.[76] QL production was suspended in February 1985, and the price was halved by the end of the year.[77] It ultimately flopped, with 139,454 units being manufactured.[78]
Video output is channelled through an RF modulator, intended for use with contemporary television sets, to provide a simple colour graphic display. Text is displayed using a grid of 32 columnsĀ 24 rows of characters from the ZX Spectrum character set or from a set provided within an application. The machine features a colour palette of 15 shades, comprising seven colours at two levels of brightness each, along with black.[89] The image resolution is 256192 pixels, subject to the same colour limitations.[90] To optimise memory usage, colour is stored separately from the pixel bitmap in a low resolution, 3224 grid overlay, corresponding to the character cells. In practical terms, this means that all pixels within an 8x8 character block share one foreground colour and one background colour. Altwasser received a patent for this design.[91]
An "attribute" consists of a foreground and a background colour, a brightness level (normal or bright) and a flashing "flag" which, when set, causes the two colours to swap at regular intervals.[90] This scheme leads to what was dubbed "colour clash" or attribute clash, where a desired colour of a specific pixel could not necessarily be selected. This became a distinctive feature of the Spectrum, requiring programs, especially games, to be designed with this limitation in mind. In contrast, other machines available at the same time, such as the Amstrad CPC or the Commodore 64, did not suffer from this limitation. While the Commodore 64 also employed colour attributes, it utilised a special multicolour mode and hardware sprites to circumvent attribute clash.[92]
The ZX Spectrum integrated various design elements from the ZX81. The ROM code, responsible for tasks such as floating point calculations and expression parsing, exhibited significant similarities, although a few outdated ZX81 routines remained in the Spectrum ROM.[96] The keyboard decoding and cassette interfaces were nearly identical, although the latter was programmed for higher-speed loading and saving. The central Uncommitted Logic Array (ULA) integrated circuit shares some resemblance with the ZX81's, however it features a fully hardware-based television raster generator with colour support. This enhancement indirectly provided the new machine with roughly four times the processing power of the ZX81, as the Z80 was relieved of video generation tasks. An initial ULA design flaw occasionally led to incorrect keyboard scanning, which was resolved by adding a "dead cockroach" (a small circuit board mounted upside down next to the CPU) in Issue 1 ZX Spectrums.[42] be457b7860
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