Complex sentence: Countries that have adopted digital terrestrial recently may have a single MPEG4 based system for SD and HD, while countries with more established system may use MPEG2 for SD and MPEG4 for HD.
Split into simpler sentences: Countries that have adopted Digital Terrestrial recently may have a single MPEG4 based system for SD and HD. Countries with more established Digital Terrestrial may use MPEG2 for SD and MPEG4 for HD.