The Sinclair 128, as its name suggests is a 128k version of the ZX Spectrum. well this is not quite true as the spectrum was not capable of addressing the 128k so it was basically two computers in one a 48k Spectrum and a new enhance Spectrum ."in 128K mode, a greatly enhanced Spectrum with some new hardware and software that's vaguely compatible with the old machines" taken from YS 1985

The 128k mode brought real sound to the spectrum with three voices, adjustable waveforms and various special effects.

Apparently the manual for the 128 was none existent with only a 14 page booklet and a copy of the Spectrum+ manual.