Complex sentence: The receiver locally regenerates the subcarrier by doubling a special 19 kHz pilot tone, but in other DSB-SC systems, the carrier may be regenerated directly from the sidebands by a Costas loop or squaring loop.
Split into simpler sentences: The receiver locally regenerates the subcarrier by doubling a special 19 kHz pilot tone. But in other DSB-SC systems, the carrier may be regenerated directly from the sidebands by a Costas loop or squaring loop.