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CD What Is The HDCD Process?

Pacific Microsonics' HDCD® (High Definition Compatible Digital®) process utilizes a sophisticated encode/decode system that greatly reduces both additive and subtractive distortions in digital audio recordings, while remaining compatible with existing formats. HDCD recordings with wider dynamic range, more natural vocal and instrumental timbre, greater depth and space, and better resolution of low-level detail. The Model One HDCD Processor produces recordings that provide improved fidelity when played on standard compact disc players. With the simple addition of an HDCD decoding chip to CD players, HDCD encoded recordings provide a higher fidelity of audio reproduction than has previously been available. In addition, the HDCD decoder chip significantly improves the reproduction of standard, non-HDCD recordings. The extraordinary fidelity of the HDCD process is achieved by identifying and correcting previously misunderstood [or unknown] sources of distortion in digital audio reproduction. These include both additive artifacts of the analog-to-digital and digital-to-analog conversion processes, and subtractive distortions resulting from insufficient data present in the 44.1 kHz, 16-bit PCM sampling standard of the compact disc format. The HDCD process effectively cancels the additive distortions and simultaneously provides additional data to reduce the subtractive distortions. The resulting recordings are at least equal in resolution and freedom from distortion to the finest analog master tapes, yet are compatible with existing consumer digital playback equipment.

Most of the "intelligence" of the HDCD process has been incorporated in the encoder. Thus, the decoder can be relatively simple and has been reduced to a single, low-cost IC chip.

The professional HDCD encoder/decoder generates and receives industry standard AES/EBU digital code compatible with virtually all 44.1 kHz, 16 bit PCM digital recorders, as well as 20- or 24-bit I/O editor/workstations such as the Sonic Solutions.

How Does It Work?

Technically, the HDCD process can be summarized as follows: an analog audio signal is fed to the HDCD encoder, which converts it into a digital signal having a word length longer than 16 bits, and a sampling frequency higher than 44.1 kHz. This high resolution digital signal contains far too much information to fit onto a conventional compact disc. Therefore, a continuous, real-time analysis of the high-resolution signal is made using DSP techniques [6 Motorola 56007 DSP processors are used] to determine what elements beyond the range of a conventional compact disc are perceptually important. The analysis uses algorithms based upon Pacific Microsonics' research in psycho-acoustics and the mechanisms of auditory perception. Once the perceptually important high-resolution information is identified, it is encoded into a compact disc standard 16-bit, 44.1 kHz, linear PCM signal using two methods. Part of the information is added directly to the PCM signal by optimizing utilization of its linear bits, and part is hidden in the LSB (Least Significant Bit) of the PCM signal. Over time, only 1% to 5% of the LSB is used for hidden code. The hidden control channel is inaudible and causes no loss of perceptible information.

When an HDCD encoded signal is fed to conventional playback equipment, the additional information in the linear PCM signal is perceived as a clear improvement in fidelity. When the same signal is fed to an HDCD decoder, the information in the hidden control channel tells the decoder that the signal is HDCD encoded, and directs the decoding function to precisely reconstruct the high resolution signal, which is then output in a form appropriate for the D/A converter being used [i.e. 20 bits, 8x over sampling]. Because the HDCD decoder replaces the conventional monolithic digital filter used prior to the DAC, it is also designed to function as an excellent and audibly superior filter when reproducing conventional, non-HDCD process recordings.

Why HDCD?

When the compact disc was introduced to the consumer marketplace 13 years ago, its 44.1 kHz, 16-bit PCM, format was dictated by the constraints of hardware available at that time at a reasonable cost. Since then, technology has advanced considerably. However, the industry is locked into the original standard due to the enormous existing base of consumer players, prerecorded discs and recording and mastering facilities. This existing base is simply too large and valuable to be written off and replaced with a new, incompatible format. In spite of this, there is a growing consensus among both recording industry professionals, artists, and consumers that the existing compact disc format doesn't contain enough amplitude or frequency information to be musically accurate. It is a tragedy that many 30-year-old analog tape recordings or vinyl records preserve more of the life and reality of musical performances than today's "state-of-the-art" digital efforts. As a result, many fear that the genius of today's greatest recording artists will remain only as a compromised legacy for future generations. The HDCD process solves this dilemma by providing a digital recording technology that is equal or superior, in terms of subjective fidelity, to the finest analog technology, yet is compatible with today's digital audio equipment. Thus, the HDCD process can be staged into the audio industry easily, with benefits occurring at every step and very little obsolescence of existing equipment.