The superb quality of the Pacific Microsonics Model One
and Model Two (which also function as HDCD processors) A/D
conversion technology makes it a valuable tool in all
phases of audio recording. Mastering studios around the
world use the Model One and Model Two every day to convert analog
or high-resolution 20-bit or 24-bit digital master
recordings to the 16-bit Compact Disc format for
release. Many producers also are using the HDCD
processor to mix down from analog multitrack to 20-bit
or 24-bit HDCD digital masters. These digital masters
are sent to HDCD-equipped mastering studios for
conversion to HDCD Compact Disc. (Some transfers have
also been put on DVD-A release
masters.)
The Model Two HDCD processor has also has proven
extremely convenient for state-of-the-art remote
recording because, in addition to superb A/D conversion,
the HDCD processor's other functions--including D/A
conversion, reclocking, and digital processing-eliminate
the need to take many other products into the
field.
In addition to accepting analog and high-resolution 20-bit or 24-bit
digital inputs, the HDCD processor can be used to
convert 16-bit digital audio tapes (DATs) to 16-bit HDCD
release masters by first converting the DATs (which many
times are at a 48kHz sampling rate) to analog
using the excellent D/A converters in the HDCD
processor. This allows the mastering engineer to perform
high-resolution analog processing, adding information to
the signal beyond 16 bits. After analog processing such
as EQ or reverb is complete, the signal is converted to
20-bit or 24-bit digital using the high-resolution A/D
converters in the HDCD processor whose output is fed to
a digital audio workstation (Sonic Solutions, SADiE,
etc.) for final editing and sequencing. The edited
high-resolution signal then is returned to the HDCD
processor for conversion to a 16-bit HDCD output. For
more information about creating 16-bit HDCD releases
from 20-bit or 24-bit digital sources, see Creating the
16-Bit Release Sound You Want.
Many 24-bit recording storage products can record the high-resolution
A/D output of the Model Two HDCD processor. Current
Digital Audio Workstations (such as a DAW with a Lynx
AES-16 digital audio card and running either Samplitude
or Sequoia software) as well as legacy equipment (such
as the Genex GX-8500
recorder and the Tascam DA-98HR) have 24-bit capability.) Most devices can record the
Model Two's 48/44.1-kHz and 96/88.2-kHz—which
is a single-wire AES-EBU 24-bit output. And there are
also a lot of DAWs and recorders that can record the Model
Two's dual-wire AES-EBU output at 192/176.4 kHz. Once a 176.4 kHz,
88.2-kHz or 44.1-kHz, 24-bit HDCD master recording has
been made, many of its high-resolution qualities can be
preserved in a Compact Disc release by using the Model
Two HDCD processor to convert to 16-bit, 44.1kHz HDCD.
For more information about creating 16-bit HDCD releases
from 20-bit or 24-bit digital sources, see Creating the
16-Bit Release Sound You Want.
The features of the HDCD processor allow you to best preserve the sound
of any source for both un-decoded and HDCD-decoded
playback, whether it has a wide dynamic range with
infrequent short duration peaks (such as most classical
or jazz) or is compressed or hard edged with little
dynamic range (typical of pop or rock).
Amplitude Processing Features
After 24-bit A/D conversion and 24-bit digital
processing, the HDCD processor's amplitude processing
features—Peak Extension, DSP Gain, and Low Level
Extension—can be used together to produce the best
possible 16-bit HDCD-encoded recordings from the 24-bit
source. Descriptions of each amplitude processing
feature are given below and are followed by suggested
operating techniques for different types of source
material.
Peak Extension
Peak Extension is a restorable (with HDCD decoding) soft
peak limiter that allows peak levels of up to 6 dB above
standard full scale peak level (+6 dBfs) on 16-bit
HDCD-encoded recordings without generating "overs." The
limiter has a carefully crafted "easy over" curve,
designed to mimic the sound of analog tape saturation,
which operates over an input signal level range of -3
dBfs to +6 dBfs. This squeezes the top 9 dB of the input
signal's range into the top 3 dB of the 16-bit
recording.
During HDCD-decoded playback, the peak limiting is
undone by the HDCD decoder chip using a precisely mapped
inverse of the limiting curve controlled by a hidden
code. The dynamics of the original material are restored
up to +6 dBfs, thus extending dynamic range by up to 6
dB. During undecoded playback, the effect of Peak
Extension is that of a very high-quality standard
limiter. Use of Peak Extension is optional in the 16-bit
HDCD encoding process.
DSP Gain
DSP Gain is a digital gain control that allows the level
of digital input sources to be varied from unity gain
over a +12 to -31.9 dB range with extremely low
distortion. DSP Gain allows 20-bit or 24-bit input
sources to be "fit" optimally into the more limited
dynamic range of a 16-bit release by setting peak levels
as close as possible to 0 dBfs. Note: If Peak
Extension is used, DSP Gain can be used to set peak
levels as high as +6 dBfs.
If DSP Gain is set below unity (e.g., -0.1 dB), then,
irrespective of input level, the output level can be no
greater than the DSP Gain level relative to peak record
level. In this example, that is -0.1 dBfs without Peak
Extension or +5.9 dBfs with Peak Extension. Input levels
that would drive the output higher than -0.1 dB relative
to peak record level are clipped, and no "overs" can
occur.
Low Level Extension
Low Level Extension is an average signal level-based,
low-level compression/expansion system used on 16-bit
HDCD-encoded recordings, which very gradually raises
gain a preset amount when the average signal level drops
below a preset threshold.
During HDCD-decoded playback, the compression is
expanded back to linear gain by the HDCD decoder chip
using a precisely mapped inverse of the compression
curve controlled by a hidden code, thus producing a
dynamic range and resolution floor beyond 16 bits.
During undecoded playback, low-level information
normally lost by standard 16-bit players is preserved,
providing more accurate timbral and spatial
reproduction.
There are two modes of Low Level Extension: Normal
and Special. Normal mode begins to affect the input
signal 45 dB below peak level, gradually raising the
gain 4 dB as the level drops over an 18-dB range.
Special mode begins to affect the input signal 39 dB
below peak level and gradually raises the gain 7.5 dB
over a 26-dB range. Normal mode is optimized to provide
the best combination of decoded dynamic range and
resolution and undecoded compatibility. Special mode is
designed to provide the best possible decoded dynamic
range and resolution at the expense of undecoded
compatibility.
Typically, Special mode is used only for 16-bit
master tracking with the assumption that the recording
will be decoded by the HDCD processor to a 20-bit or
24-bit word length for digital postproduction before
being re-encoded to 16 bits using Normal mode to produce
a release master.
Use of Low Level Extension is optional in the 16-bit
encoding process. In addition, Low Level Extension can
be turned on and off dynamically during 16-bit encoding
whenever the signal level is above -30
dBfs.
Obtaining the best results with undecoded playback is emphasized,
because certain effects such as Peak Extension limiting
can only be heard undecoded.
Limited Dynamic Range Pop or Rock
The best method to record highly compressed, limited
dynamic range material depends greatly on the results
that are desired with undecoded playback.
Using Peak Extension allows very high average
recording levels without "clipping" or generating "overs."
This approach can be used to get the "hottest" possible
sound (almost no dynamics) during undecoded playback for
airplay, with decoding restoring normal dynamics for
home listening.
However, because Peak Extend limiting has an "easy
over" curve that begins to affect the signal at -3 dBfs,
it usually shouldn't be used with highly compressed
source material that almost always will be in the
limiting curve, unless a highly limited or distorted
sound is desired during undecoded playback.
Typically, Peak Extension recordings do not have the
"crunch" or "edge" produced by hard clipping that is
sometimes desired for certain types of rock material.
To get a hard "crunch" without any "easy over"
limiting, turn Peak Extend off and adjust DSP Gain to a
level just below full scale, usually -0.1 dB. The
digital input signal level then is adjusted using an
external device such as a 20-bit or 24-bit editing
workstation. This allows as much clipping as desired
without generating any "overs."
Also, when a very "dry" or "punchy" low-level sound
is desired with limited dynamic range material that has
little ambient information, Low Level Extension can be
turned off.
Wide Dynamic Range Jazz or Classical
This type of material typically has infrequent,
short-duration peaks such as jazz snare drum rim shots
or classical cymbal crashes, bass drum hits, etc. These
types of short-duration peaks can be limited by as much
as 5 dB or 6 dB using Peak Extension with little audible
effect during undecoded playback. This allows the
average record level to be raised, which improves
overall resolution and fidelity.
The optimum level of Peak Extension is best
determined by turning off HDCD amplitude decoding. The
Operating menu allows you to turn off HDCD decoding
while adjusting the record level (typically by using DSP
Gain) and listening to the amount of limiting.
Low Level Extension, set to Normal mode, almost
always should be used with wide dynamic range material
because it greatly aids in preserving the low-level
ambient and timbral information that gives "life" and
naturalness to recordings.
With certain types of source material such as a very
low-level passage in a noisy analog tape, Low Level
Extension occasionally can cause an audible shift or
"breathing" of the noise floor during undecoded
playback. Low Level Extension can be turned off on the
fly prior to such passages when the signal level is
above -30 dBfs and turned back on afterwards when the
signal level is above -30 dBfs to preserve the best
possible fidelity for the remainder of the recording.
However, with source material that is very noisy, Low
Level Extension should be left off because the noise of
the source will set the resolution floor |
