HDCD & Peak Problems

Sidebar: HDCD & Peak Problems

An interesting aspect of the PMD100 HDCD decoder/filter is that it attenuates all signals by 1dB to reduce the audible effects of the clipping present on many CDs. Mastering engineers set the levels on the digital master (which end up on the CD) as high as possible for two reasons: to get the maximum loudness on the disc, and to avoid throwing away any resolution. Setting the levels so that the loudest peak reaches 6dB below full-scale digital is identical to losing one bit of resolution.

In this quest for maximum level, engineers can set the levels too high and exceed the digital format's limitations. On the Sony PCM-1630 professional mastering processor used to encode CD master tapes, a red "Overload" LED illuminates when several consecutive samples are at full-scale. What this means is that the analog audio signal had a higher amplitude than the highest digital number (the quantization word value) available to represent that amplitude. Consequently, the A/D converter outputs its highest number for several samples in a row. When that digital signal is reconstructed, the peaks in the analog output are clipped, producing distortion.

This type of digital overload is particularly annoying: the musical peaks are chopped off mercilessly. Conversely, an analog tape machine driven into overload, or an amplifier (particularly a tubed amplifier) into clipping, distorts the signal more gently by squashing the peak and rounding off the edges rather than acting as a brick-wall limit.

How many CDs have digital overloads? Before joining Stereophile, I worked with CD master tapes at a CD-manufacturing plant, and looked at the levels on more than 2000 CD master tapes. A seat-of-the-pants guess at that time was that more than half of all CDs have at least one overload, and about 10% have repeated overloads, such as on snare-drum transients. (These estimates may have changed in the six years since I've worked with CD master tapes, as engineers have become more familiar with the digital medium, footnote 1.)

The PMD100 HDCD decoder/filter prevents digital overloads from clipping the digital filter by attenuating all signals by 1dB. The PMD100 therefore lets the filter ring naturally rather than clipping the overshoots. Although the analog output is still clipped, the 1dB of attenuation prevents the filter from saturating and adding its own distortion. The result is reportedly a greater sense of sonic ease on peaks (footnote 2).—Robert Harley

Footnote 1: I regularly log peak levels on CDs, using a Dorrough AES/EBU-input peak/average meter, which reveals levels at –3, –2, and –1 LSBs below 0dBFS by red LEDs illuminating. While almost all modern discs have transients that regularly reach –1dBFS and many light up two of the top three red LEDs, only very rarely do I discover a disc that is actually clipped. In the six years since Bob surveyed recordings, the widespread use of accurate level meters, such as the Sony or Dorrough, and the ubiquitous use of hard-disk editors which also allow the engineer to keep a careful eye on true peak levels, have, I believe, rendered this a non-problem.—John Atkinson

Footnote 2: Michael Ritter of Pacific Microsonics explained that an HF transient, like a cymbal clash, even as low as –9dBFS but typically at –6dB, can overload conventional digital filters. These will then ring, with the ringing clipped. The 1dB attenuation gives better subjective ease with such material.—John Atkinson

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