Goodwin's High End

 

HDCD - Mastering, Mixing, Recording

 

Introduction

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.


Mastering

 

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.


Mixing and Recording

 

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-kHzwhich 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.


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 featuresPeak Extension, DSP Gain, and Low Level Extensioncan 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.


Operating Tips
 

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

 

 

   
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