Television systems that have a resolution that meets standards but not considered high definition, this is what Standard-definition television or SDTV refers to. This usually refers to digital television, especially while broadcasting at the same (or similar) resolution as analog systems. In ATSC, SDTV can be broadcast in 704 pixels × 480 lines with 16:9 aspect ratio (40:33 rectangular pixel), 704 pixels × 480 lines with 4:3 aspect ratio (10:11 rectangular pixel) or 640 pixels × 480 lines with 4:3 ratio (and square pixels). The refresh rate can be 24, 30 or 60 pictures per second. Digital SDTV in 4:3 aspect ratio has the same form as the regular analogue TV (NTSC, PAL, PAL2, SECAM) excluding the ghosting, snowy images and static noises but however with poor reception one may encounter various other artifacts such as blockiness and stuttering. Though ATSC and ISDB were originally developed for HDTV, they later proved their ability to deliver multiple SD video and audio streams via multiplexing,