Mceljo - SACD is also created without anti-alias filters that tend to screw-up transient response. SACD is basically the same as output of Delta-Sigma converter before filtering (PWM). Many audiophiles dislike Delta-Sigma converters but the same time enjoy SACD (strange).
HDCD is basically regular CD where the least significant of 16-bits is used to switch dynamic range (technique known as "in-band signaling") of the remaining 15 bits. Theoretically it is playable on standard CDP but it will play 15 bits with strange dynamics plus noise on bit #16.
I don't know how much better it can sound, but often recordings selected for such formats are excellent to start with making impression of new format superiority - I don't have opinion since I've never heard HDCD.
HDCD is basically regular CD where the least significant of 16-bits is used to switch dynamic range (technique known as "in-band signaling") of the remaining 15 bits. Theoretically it is playable on standard CDP but it will play 15 bits with strange dynamics plus noise on bit #16.
I don't know how much better it can sound, but often recordings selected for such formats are excellent to start with making impression of new format superiority - I don't have opinion since I've never heard HDCD.