I've been searching the net concerning this and have had mixed comments. HDCD, is native 16bit with coded dither, how and why the OPPO outputs a 20bits HDCD signal seems puzzling. On one account, I read that the OPPO uses software based decoding, pre-dac to output a non-decode-able "fake" HDCD signal.
This from the PM Model 2 Manual:
... 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 16-bit signal by optimizing utilization of its
linear bits, and part is hidden in the LSB of the 16-bit signal. Over time, less than 5% of
the LSB is used for the hidden code. The encoded information is inaudible and causes no
perceptible loss of information.
16-bit Decoding
When an HDCD encoded signal is fed to conventional playback equipment, the additional
information in the signal is perceived as a clear improvement in fidelity. When the same
signal is fed to an HDCD decoding system, the information hidden in the LSB tells the
system that the signal is HDCD encoded, and directs the decoding to reconstruct the high
resolution signal, which is then output in the correct form for the D/A converter being
used. Decoding can be performed by the Pacific Microsonics PMD-200 HDCD decoder DSP
ASIC, other ASIC’s including a one chip HDCD decoder and 24-bit D/A converter from Burr-
Brown, and HDCD coded DSP based chips from Motorola, Analog Devices, Zoran, Mediamatics,
Crystal Semiconductor and others.
There are two subsets to the HDCD process, 16-bit amplitude domain encoding and time
domain encoding. The Model Two provides the option of turning off decoding of the 16-bit
amplitude domain functions of the HDCD process for monitoring purposes. This is done in
either the Second Operating Menu, Levels Menu, or Monitor Setup Menu. See pages 41 -
43, and 61 for additional information. Time domain decoding is automatic with HDCD
decoder filtering and cannot be turned off.
HDCD Indicator
The HDCD Indicator on the Model Two consists of a blue or amber LED back lit HDCD logo.
For additional information, see the HDCD Indicator description on page 18.
Peak Extension and HDCD Limiting
Peak Extension is normally used when converting analog or high resolution 24-bit or 20-
bit digital signals to HDCD 16-bit amplitude encoded signals. Peak Extension is a digital
soft limiter, with a curve that mimics analog tape saturation, except that on decoding the
peaks are restored. Peak Extension can be used for several purposes: to provide headroom
to capture occasional peaks or as a method to raise the average signal level to create a
“hot” recording. Peak extension allows raising the average level of a recording as much as
6 dB without overload. Of course, like any limiter, if overused it can produce audible
distortion when not decoded. Decoding restores the limited peaks, eliminating any distor-
tion that might be audible. Therefore, whenever peak extension is used, the output should
be monitored with HDCD 16-bit amplitude decoding both on and off to determine how
much peak extension is appropriate. Peak Extension does not need to be used when the
original program material has limited dynamic range.
HDCD Limiting is identical to Peak Extension except HDCD Limiting is not undone by HDCD
playback decoding, and is used for its limiting effect with all types of playback.
The Model Two HDCD Processor has Peak Extension Meters which indicate the extended
dynamic range available when in HDCD Peak Extension or HDCD Limiting modes. The Peak
Extension meters are only active when monitoring HDCD 16-bit amplitude encoded signals
that contain peak extension. With HDCD Limiting, the Peak Extension Meters only indicate
during recording. Peak Extension and HDCD Limiting can be turned On or Off in the Second
Operating Menu or the Digital Output HDCD Setup Menu. The default is Off. See pages 35,
Low Level Extension
Low Level Extension is an average signal level based, low level compression/expansion
system used on HDCD 16-bit amplitude encoded recordings which very gradually raises
gain a preset amount when the average signal level drops below a preset threshold. During
HDCD 16-bit decoded playback the compression curve is expanded back to linear gain by
the HDCD decoder, using a precisely mapped inverse of the curve controlled by a hidden
code, producing a dynamic range and resolution floor beyond 16-bit. Low Level Extension
can be turned On or Off in the Second Operating Menu or the Digital Output HDCD Setup
Menu. The default is Off. See pages 36, 41 and 51 for additional information.
Gain Scaling
HDCD 16-bit recordings using peak extension, when decoded, have an increase of 6 dB in
headroom compared to 16-bit recordings without peak extension. Therefore, it is neces-
sary to scale their playback level up by 6 dB to match the average level of non-peak
extended 16-bit recordings. Gain scaling is accomplished in the digital domain by the
Model Two, which reduces the monitor level by 6dB whenever a 16-bit signal without peak
extension is detected. Gain Scaling can be set to On (automatic) or Off in the Monitor
Setup Menu or the Second Operating Menu. When monitoring 24-bit or 20-bit signals, Gain
Scaling is normally Off. When monitoring 16-bit signals, scaling is normally On (auto-
matic) and monitor gain is increased by 6 dB for HDCD signals with Peak Extend. See pages
42, 57 and 61 for additional information.
Processing HDCD 16-bit Signals
When digital processing of an HDCD encoded 16-bit signal is needed, the Model Two is
first used to convert it to a 24-bit or 20-bit word length. When editing or processing is
completed, the Model Two is used again to convert the signal back to HDCD 16-bit.
Any process or signal path through which HDCD 16-bit amplitude encoded signals are
passed must not affect the bit structure of the signal. Digital processing which alters the
data will destroy the HDCD encoded information located within the LSB of the signal.
Digital processing includes any level changes, EQ, dither, etc. Butt splices or rapid cross
fades are allowable, but certain rules apply. For additional information see page 33.
HDCD Silence
HDCD Silence is automatically generated by the Model Two HDCD Processor when it pro-
duces HDCD encoded output with no input program audio. HDCD silence contains dither
and HDCD code within the LSB of the signal which tells an HDCD decoder that it is an HDCD
signal. For additional information see page 33.
