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HDCD Process Overview

The professional Model Two HDCD processor converts analog signals to digital with unprecedented accuracy, and HDCD decoding integrated circuits (ICs) greatly improve D/A conversion in consumer playback equipment. If you would like more detail on these processes, please see HDCD Articles and Papers.


HDCD Encoding Process


A/D Conversion

HDCD A/D conversion uses extremely accurate, discrete, multi-bit A/D converters that operate at a sampling frequency of 192 or 176.4 kHz. They employ sophisticated digitally subtracted dither to produce low noise and keep distortion below -120 dB or less than one part per million. The 192-kHz or 176.4-kHz signal can be output directly using an HDCD 1FS digital filter or converted to a 96-kHz or 88.2-kHz, 24-bit signal using HDCD-optimized decimation filtering.


Signal Analysis and Delay Memory

To be converted to a 48-kHz or 44.1-kHz signal, an HDCD 96-kHz or 88.2-kHz, 24-bit signal is sent to a memory buffer that delays the signal long enough for the HDCD processor's eight Motorola 56009 digital signal processors (DSPs) to analyze the signal. This powerful "look ahead" design allows the HDCD processor to perform the complex signal analysis and processing described below to produce a 44.1-kHz, 16-bit CD-format recording with the highest fidelity possible.


Decimation Filter Selection, Reduction to 44.1 kHz

The 96-kHz or 88.2-kHz, 24-bit signal is converted to 48 or 44.1 kHz using patented HDCD dynamic decimation filter technology. Conventional 44.1-kHz decimation uses a fixed 22.05-kHz anti-alias filter. However, research has shown that no single 22.05-kHz brick-wall filter can be sonically neutral for all different types of program material, due to its low frequency and required sharp cutoff.

Thus, fixed-filter 44.1-kHz decimation often sounds very different from the original analog or high sampling frequency digital master. In contrast, the HDCD processors' real-time analysis allows the anti-alias filter's transition band characteristics to be dynamically optimized to preserve the program signal's contentóreducing distortion and producing a more sonically accurate 44.1-kHz signal.


Amplitude Encoding/Gain Structure

Two optional HDCD features are available to the recording engineer to reduce the 44.1-kHz, 24-bit signal to 16 bits while retaining many 24-bit sonic benefits. Both of these features can be turned on or off depending on the sonic requirements of the source material.

  • Peak Extend is a reversible soft limiter that allows the engineer to increase the peak signal level by up to 6 dB without "overs." These peaks are precisely reconstructed when decoded, increasing dynamic range by 6 dB. For undecoded playback, Peak Extend provides all the advantages of an excellent limiter.
  • Low Level Range Extend gradually increases the gain on low-level signals (starting at -45 dBfs) by 4 dB over a 20-dB range. Decoded playback reverses the added gain, providing low-level resolution beyond 16 bits. For undecoded playback, the increased gain compensates for the lack of low-level resolution in inexpensive CD playback systems, producing a sound more like the original high-resolution master.


Hidden Code Addition/Output Dither/Quantization

The final step in the reduction to 16 bits is to add high-frequency weighted dither and round the signal to 16-bit precision. The dither increases in amplitude in the frequency range of 16 to 22.05 kHz, leaving the noise floor flat below 16 kHz where the critical bands of hearing associated with tonality occur. As part of the final quantization, a pseudo-random noise hidden code is inserted as needed into the least significant bit (LSB) of the audio data. The hidden code carries the decimation filter selection and Peak Extend and Low Level Range Extend parameters. Inserted only 2–5 percent of the time, the hidden code is completely inaudible-effectively producing full 16-bit undecoded playback resolution. The result is an industry-standard 44.1-kHz, 16-bit recording compatible with all CD replication equipment and consumer CD players.

HDCD Decoding and Digital Filtering

The HDCD decoding process performs precise decoding of HDCD-encoded recordings and also provides a state-of-the-art digital filter for both HDCD and standard recordings. The decoding process begins with the HDCD decoder extracting the hidden code from the LSBs of the audio data and then decoding the commands contained in the hidden code. Signal peaks limited with Peak Extend are restored and low-level gain is undone, resulting in a signal with 20 bits of dynamic range. This signal then is interpolated to 96 or 8 8.2 kHz using a filter that is complementary to the anti-alias filters used in the A/D filter-switching process. The signal can be output at 96 or 88.2 kHz or further interpolated to four or eight times over sampled frequency to drive popular 18-bit to 24-bit D/A converters.



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