December 2008

Classic Home Toys Installment #19
The Final CD Format: HDCD

Author: James Russo

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 an increase in noise.

In Classic Home Toys #17 and #18, both SACD and xrcd were examined and their sound reproduction was compared to standard CD. Despite the fact that xrcd’s faithfully reproduce recordings and are playable on standard consumer level compact disc players, the obscure format has become extinct with the exception of some companies which continue to produce CD’s in the format.

The winner in this race to produce the best quality sound was an unlikely candidate --- Sony’s own CD format. Super Audio CD or SACD. Unlike xrcd, SACD’s require a completely different player which contains a special decoder to decode the direct stream digital signal of a SACD. SACD’s are generally more expensive than their standard CD counterparts and can be harder to find.

However, the battle for audio domination is not quite over just yet. Yet another format, the last audio format , is known simply as HDCD or High Definition Compact Disc. HDCD technology was developed between 1985 and 1991 by audio engineers Keith Johnson and Michael Pflaumer of Pacific Microsonics. The first publicly sold HDCD was released in 1995. In 2000, Pacific Microsonics folded and all of the companies intellectual property was acquired by Microsoft Corporation. Seeing the huge potential for HDCD, Microsoft made all version of it’s Media Player starting with version 9.0 capable of playing HDCD with a 24 bit sound card installed.

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 an increase in noise.

HDCD encoding places a control signal in the least significant bit of a small subset of the 16-bit RedBook audio sample (a technique known as in-band signaling). The HDCD decode in the consumer’s CD or DVD player, if present responds to the signal. If no decoder is present, the disc will be played as a regular CD.

In itself, the use of the first bit in the dithered least significant bit stream does not degrade the sound quality on a non-HDCD; it only decreases the signal-to-noise ratio by a miniscule amount. HDCD Peak Extension, if chosen in HDCD mastering, will apply compression to the peaks which will be audible in playback on a non-HDCD system which does not apply the appropriate expansion curve.

HDCD provides several digital features, which the audio mastering engineer controls at his/her own discretion. They include: Dynamic range compression and expansion with which virtually 4 more bits of dyanamic range can be added to the music signal. The other control is precision digital interpolation filtering with multiple modes of operation, which can reduce alias distortion and temporal smearing resulting in a more natural, open and accurate sound reproduction.

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. Nashville superstar Trisha Yearwood has been one of HDCD’s most dedicated supporters and over the years has released several of her CD’s in the HDCD format. However, even she has reverted to releasing her current CD’s in standard CD format. Although there has been no formal declaration as to why some artists such as Trisha Yearwood have stopped releasing CD’s in the HDCD format, many audiophiles have argued that new. conventional CD’s produce a quality of sound that is now equal to or even superior to an HDCD without the cumbersome need for special encoding.

Related links: www.wikipedia.com

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