According to a first aspect of the present invention there is provided a tennis ball having a hollow resilient core to which a needlefelt covering is adhered. In one embodiment, a personal license server is provided comprising a memory and circuitry operative to receive a digital rights management license from a license server, store the DRM license in the memory, and provide the DRM license to a personal license server client, wherein the personal license server client receives the DRM license without communicating with the license server. The control device includes an operation mode determining section, a swing pattern storage area and a control command generator.