31306807communication systemsPT2026-03-30 20:22:34UF31306808communication systemNDUF31306809communications systemsNDUF31306810signal systemsNDUF31306811systems, communicationNDBT31298981systems by functionPTNT31321820television antenna systemsPTNT31321827clock and program systemsPTNT31321831sound systemsPTNT31321834NTSC systemPT