HDCD 101

[manisandher]

There are 3 HDCD 16-bit amplitude encoding options:
1. Peak Extension
2. HDCD Limiting
3. Low Level Extension

Along with HDCD 16-bit Dithers, these were designed to produce the best possible 16-bit HDCD-encoded release masters from analogue or 20/24-bit sources.

From the Model Two manual:

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Peak Extension is a restorable (with HDCD decoding) soft peak limiter that allows peak levels up to 6 dB above standard full scale level (+6 dBfs) on HDCD 16-bit recordings without generating “overs”. The limiter has a carefully crafted “easy-over” curve, designed to mimic the sound of analog tape saturation that operates over an input signal level range of -3 dBfs to +6 dBfs, in effect squeezing the top 9 dB of the input signal’s range into the top 3 dB of the 16-bit recording.
During HDCD 16-bit decoded playback, Peak Extension peak limiting is undone by the HDCD decoder using a precisely mapped inverse of the limiting curve controlled by the hidden LSB code, and the dynamics of the original material are restored up to +6 dBfs, thus extending dynamic range by up to 6 dB.

HDCD Limiting functions exactly the same as Peak Extension during recording, except that during playback the limiting curve is not undone with HDCD decoding. During HDCD decoded playback, HDCD Limiting has the same effect as during undecoded playback.

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 compression curve controlled by a hidden code, producing a dynamic range and resolution floor beyond 16-bit. During undecoded playback low level information normally lost by standard 16-bit players is preserved, providing more accurate timbral and spatial reproduction.

HDCD 16-bit dither options allow optimizing perceived timbral and spacial accuracy with different types of source material during HDCD 16-bit A/D conversion, or when converting 24-bit or 20-bit signals to HDCD 16-bit.

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Mani.