The "flash" attribute of the ZX Spectrum's display, designed to swap ink (foreground) and paper (background) colours in any 8×8 attribute block that enable it, suffers from a timing issue, in that the inversion of the display's bitmap data is not properly synchronised with pixel boundaries. This issue was fixed in later models of the Spectrum. The ULA outputs Y, U and V signals without modulation with a carrier, so this can be avoided entirely, but the pixel clock cannot be adjusted without adversely affecting video timings. This was known about before release, and was a deliberate compromise in the design. The 48K Spectrum suffers from severe dot crawl, as the pixel clock is not synchronised with the PAL colour subcarrier. This issue was partially resolved by means of the "spider" modification, which gates the ULA's IORQ input with A0. This was presumably avoidable, as the ULA uses the Z80's A0 line to determine whether to respond to I/O port requests. It was later noticed that the ULA contended all I/O access, not just that to its own I/O port. The "dead cockroach" was incorporated into later revisions of the ULA, rather than fixing the timing issue. An error in the timings applied by the ULA's I/O contention circuit required a modification, the "dead cockroach", such that all I/O access to the I/O port that the ULA provides is contended as though the access is to the lower 16K of RAM, for which access is shared with the ULA. The ULA suffers from a few oversights in its design and implementation. The ZX Spectrum ULA by Chris Smith describes this device in detail.
Much of the ZX Spectrum's custom logic is contained within an Uncommitted Logic Array ("ULA") and was designed by Richard Altwasser. The angle of the stripes was measured as being at 24-degrees. The exact Pantone colour values couldn't yet be retrieved. One of the iconic elements of the design is the coloured stripes on the right side of its faceplate in the colours Red, Yellow, Green and Cyan. The industrial design for the original rubber-keyed ZX Spectrum and the plastic-keyed Spectrum+ was by Rick Dickinson, who had previously done the industrial design for the ZX80 and ZX81.
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