 In April 2012 Asus revealed a revision of the two original Zenbooks, this time based on the Ivy Bridge microarchitecture. The model numbers were UX21A and UX31A for the 11 and 13 inch models respectively but are more frequently referred to as Zenbook Primes. Changes made to the original included the aforementioned update to Ivy Bridge processors and the introduction of the corresponding HM76 chipset bringing native USB 3.0, 1920 × 1080 IPS displays for both models, an Intel Centrino Wi-Fi controller, an improved trackpad and new plastic keyboard with backlighting. The new screens were highly praised by reviewers when considering brightness, contrast ratio, viewing angle and colour accuracy, the improvements over previous models being put down to the switch from TN to IPS displays. The UX21A has contrast ratio of 939 and the UX31A achieved 1085, giving it the highest resolution and contrast ratio of any ultrabook at the time of release. The improved keyboard also garnered praise for the increased backlighting and improved key travel while the Intel Wi-Fi controller was found to perform better than the Qualcomm used in the first generation of Zenbooks. The Zenbook Primes still received some criticism: the latest version of the trackpad was acknowledged as an improvement over the original Zenbooks, but still irritating, and the sound quality was found to be worse than that with the first generation. Despite these issues, the overall reaction was positive: the UX31A was called "today's best ultrabook" and "the best ultrabook out there" at the time of release.
