The patent on HDCD has now expired. The idea behind HDCD was to improve the dymamic range of softer passages in audio.
HDCD is capable of higher quality sound reproduction because HDCD encodes the equivalent of 20 bits worth of data in a 16-bit digital audio signal by utilizing custom dithering, audio filters, and some reversible amplitude and gain encoding. Peak Extend, which is a reversible soft limiter and Low Level range Extend which is a reversible gain on low-level signals. There is thus a benefit at the expense of a slight increase in noise.
Most DVD and BD drives can handle HDCD. FooBar2000 has a decoder for HDCD to 20-bit PCM. FooBar2000 can save the audio tracks to FLAC or ALAC containers among others.
There are roughly the same number of albums available on SACD as there are HDCD’s — around 4,000 although SACD’s are steadily growing while the number of HDCD’s has declined over the last few years. The patent on SACD will expire in May 2023.
It’s disappointing that HDCD has not become more prevalent. Support in consumer electronics is universal. The conventional CD patent expired a long time ago but the RIAA has continued to back the format due to its pervasive nature. With lower costs of manufacture prices are slightly lower. Catalog compilations are a popular choice with consumers.