Nitroplus

Background
Nitroplus, also known as Nitro+, is a Japanese developer of visual novels, originally founded as an eroge developer. Among their notable works include Chaos;Head, Super Sonico and Soukou Akki Muramasa; however, they're mostly known for the Steins;Gate franchise which is mostly popular for its acclaimed anime adaptation. The company also collaborated with Type-Moon for the Fate/Zero light novel series and in the development of several anime series.

(2000s-)
Visuals: Over a black or white background, we see a symbol consisting of two adjoined semi-circles with the next "nitro+" next to it.

Technique: None.

Audio: None. Availability: Most notably seen on the visual novels Steins;Gate, Soukou Akki Muramasa and Chaos;Head Noah.