H D C D
P R O C E S S D E C O D E R
GAIN SCALE APPLICATION NOTE
I N T R O D U C T I O N
Because Peak Extend adds 6dB of dynamic range to the top end, the “average” decoded signal level will be 6dB lower than an HDCD recording without Peak Extend, or a non-HDCD recording. Un-less the decoded level of Peak Extended and non-Peak Extended recordings are matched using Gain Scaling, Peak Extended Recordings will be 6dB quieter than non-Peak Extended recordings and this is not acceptable.
All HDCD decoders must either:
1) lower the gain of non-HDCD recordings and HDCD recordings without Peak Extend by 6dB, - OR -
2) raise the gain of the Peak Extended recording 6dB. This is a requirement of the HDCD license agreement. This gain matching can be done in either the digital domain or the analog domain, as explained later in this application note.
Peak Extend and Gain Scale Basics
1) an HDCD encoded non-Peak Extended recording
- OR -
2) non-HDCD encoded recording (in other words, standard PCM 16 bit recordings)
When any HDCD Process Decoder recognizes an HDCD Peak Extended recording, it reduces the decoded average signal level of this recording only, to allow for the increased head room of the “extra” 6dB of dynamic range. For the other cases - HDCD recordings without Peak Extend, or non-HDCD recordings - the average decoded levels need to be matched to the decoded HDCD Peak Extended recording. This 6dB gain matching could have been built into the HDCD decoder, and done digitally, automatically, however we have given the user the option of doing the gain matching in the analog domain, which may offer some sonic benefits. Table 1 summarizes these average and peak level differences using an example of a -20dB average and 0dBfs peak level in the original material.
HDCD versus Non-HDCD Decode
Model One and Model Two HDCD Encoder
HDCD Process Decoder
Peak Extension Limit Curve
Before HDCD Encode
After HDCD Encode
Decoded Peak Extension encoded recordings will thus sound 6dB quieter than non-Peak Extension recordings, unless the difference in average levels is compensated for after the decoding has been performed. Key to this discussion is that the 6dB average level difference originates from the HDCD process used to encode the material when the recording was produced using the Model One or Model Two with the Peak Extend option enabled.
Analog or Digital Gain Scaling?
Digital Gain Scaling Mode
In Digital Gain Scaling mode, the HDCD Process Decoder automatically reduces the gain 6dB for both types of recordings in order to maintain the same average listening level volume (Peak Extended recordings and non-Peak Extended recordings). Don’t forget that non-Peak Extended recordings can be either standard PCM 16 bit recordings or HDCD encoded non-Peak Extend recordings. Use of Digital Gain Scaling mode reduces the complexity of the subsequent analog circuits. It eliminates the need for switching relays or transistor stages to switch the analog gain in and out. These switching methods usually degrade sonics to the inverse of the care with which the design is accomplished, i.e., the more care taken here will reduce sonic degradation - it will not completely eliminate it.
The Digital Gain Scaling solution requires 1 bit of DAC resolution to properly decode Peak
Extended recordings. This means that on non-HDCD recordings, the PMD-100 or PMD-200 IC loses 1 bit of resolution. In practice, however this is rarely a problem. In fact, this configuration often times sounds better than Analog Gain Scaling, especially for sigma-delta type DACs. Once Analog or Digital Gain Scaling is implemented in a properly designed circuit, the net effect is that any loss of resolution in the DAC is essentially the same for both HDCD and non-HDCD recordings since the average levels are the same (again, in a correctly designed system). Normally an 18 to 20 bit DAC is used to play 16 bit recordings, so the top bit is not a great loss. With sigma-delta DACs, low level linearity is usually good while high level performance may not be.
Analog Gain Scaling Mode
The circuit designer must decide to either increase the analog gain 6dB for Peak Extended recordings or reduce the analog gain 6dB for non-Peak Extended recordings. In either case, there must be a glitch-free 6dB gain stage inserted into the analog circuit for each channel that can be controlled by the Gain output pin of the HDCD decoder. And, as discussed above, the gain of the analog stages following the DAC must be adjusted for correct output levels when the HDCD process decoder is used compared to other digital filters. Also, the analog circuits must be capable of handling the full peak level of the 6dB peaks. We recommend that the gain change stage be capable of switching within 50mSec of the control signal level change to insure proper audio output levels.
S U M M A R Y
To produce this increased dynamic range and because the peak digital signal level prior to decoding is 0dBfs (which cannot increase beyond 0dBfs in the digital domain by definition), the average signal level of decoded Peak Extended recordings must be decreased by 6dB to allow for the increased head room of the “extra” 6dB.
When Peak Extended and non-Peak Extended recordings are decoded using the HDCD Process Decoder, the Peak Extended recording will be 6dB lower in average volume level when compared to non-Peak Extended recordings. Non-Peak Extended recordings originate from either HDCD encoded non-Peak Extend recordings or from standard PCM 16 bit recordings.
Analog or Digital Gain Scaling implementations affect decoded volume levels differently. When using Digital Gain Scaling, the HDCD Process Decoder automatically matches both types of recordings. When using Analog Gain Scaling, the level adjustments are done by an external circuit. With Analog Gain Scaling, the circuit designer must decide to either reduce the non-Peak Extended recordings 6dB or increase Peak Extended recordings 6dB within 50mSec of the HDCD decoder control signal level change.