  We adapt the spectral viscosity (SV) formulation implemented as a modal
filter to a Spectral Difference Method (SD) solving hyperbolic conservation
laws. In the SD Method we use selections of different orthogonal polynomials
(APK polynomials). Furthermore we obtain new error bounds for filtered APK
extensions of smooth functions. We demonstrate that the modal filter also
depends on the chosen polynomial basis in the SD Method. Spectral filtering
stabilizes the scheme and leaves weaker oscillations. Hence, the selection of
the family of orthogonal polynomials on triangles and their specific modal
filter possesses a positive influence on the stability and accuracy of the SD
Method. In the second part, we initiate a stability analysis for a linear
scalar test case with periodic initial condition to find the best selection of
APK polynomials and their specific modal filter. To the best of our knowledge,
this work is the first that gives a stability analysis for a scheme with
spectral filtering. Finally, we demonstrate the influence of the underlying
basis of APK polynomials in a well-known test case.
