 Double-blind listening tests in 2004 between DSD and 24-bit, 176.4 kHz PCM recordings reported that among test subjects no significant differences could be heard.DSD advocates and equipment manufacturers continue to assert an improvement in sound quality above PCM 24-bit 176.4 kHz.A 2003 study found that despite both formats' extended frequency responses, people could not distinguish audio with information above 21 kHz from audio without such high-frequency content.