This was a 3rd party rpm repository which contained MythTV rpms. ATrpms is no longer being maintained. Consider using the RPMFusion rpm repository for MythTV.
The original focus of ATrpms laid upon software used in natural sciences, especially in the field of high energy physics, e.g. tools for numerical programming or such for scientific publications. The repository expanded to include many non-scientific software titles, like system tools or multimedia packages, resulting in a far more generic repository.