Analog and early real time digital synthesizers existed before modern computer 3D modeling. Typical 3D renderers are not real time, as they concentrate on computing each frame from, for example, a recursive ray tracing algorithm, however long it takes. This distinguishes them from video synthesizers, which must deliver a new output frame by the time the last one has been shown, and repeat this performance continuously (typically delivering a new frame regularly every 1/60 or 1/50 of a second). The real time constraint results in a difference in design philosophy between these two classes of systems.