One example of an NTSC Spectrum has been found in Chile. As far as is known, it is the same as a normal (PAL) Spectrum with the following differences:
- The CPU is clocked at 3.5275 MHz.
- The ULA is a model 6C011E-3 (dated 8501 in the example in question) which generates a NTSC frame size and rate.
- One frame lasts 0xe700 (59136) T-states, giving a frame rate of 3.5275×106 ÷ 59136 = 59.65 Hz.
- 224 T-states per line implies 264 lines per frame.
- The first contended cycle is at 0x22ff (8959). This implies 40 lines of upper border, 192 lines of picture and 32 lines of lower border/retrace.
- The contention pattern is confirmed as being the same 6,5,4,3,2,1,0,0 as on the 48K machine.
Another NTSC Spectrum has been found by in the USA, containing a 5C114E ULA, dated 8248.
Although the clock rate is slightly higher, any software making heavy use of the lower 16 KB of RAM will run slower on the NTSC Spectrum, since the proportion of time during which memory is contended is greater. This is a result of the NTSC Spectrum's need to render the display at a higher frame rate (60 Hz instead of 50 Hz).
A UM1233 modulator is found in both of the NTSC Spectrums described above.
- The ZX Spectrum ULA, Chris Smith, 2010, p. 252
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