1. A signal processing apparatus for converting a first image signal into a second image signal, comprising: storage means for storing a predetermined number of newest frames of the first image signal being input; playback speed setting means for setting a playback speed of the first image signal stored in the storage means; frame phase calculation means for calculating a frame phase of the second image signal relative to the first image signal, based on the playback speed; spatial parameter setting means for setting a spatial parameter indicating a spatial phase of the second image signal relative to the first image signal; image quality parameter setting means for setting an image quality parameter indicating the degree of image quality adjustment made on the first image signal to obtain the second image signal; classification means for classifying the second image signal into one of a plurality of classes, depending on a level distribution of the first image signal; tap coefficient output means for outputting, for each class, tap coefficients acquired via learning based on the frame phase, the spatial parameter, and the image quality parameter; and calculation means for determining the second image signal by performing a calculation using the first image signal and the tap coefficients of the class determined by the classification means.