| 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
content—reducing 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 2Ð5 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. |
