Today’s recording environment demands that sessions are mixed and mastered as “hot” as is possible, pushing the levels up to the highest tolerable amount, supposedly just short of clipping. Sophisticated digital tools allow music to be highly compressed, then recompressed, compressed even more so with multi-band compressors, limited, normalized, and maximized to get the audio to play as loud as possible out of a consumer’s system. Hence, it is very common for popular music CDs to be full of digital samples that are at, or nearly at full scale.
The problem is realized in that while going through these digital gyrations and utilizing digital tools to amplify the signal as much as possible, both during mixing and during mastering, the “peak value” of the sample points is closely watched to ensure that it does not get to full scale. Since the peak meters in said DAW and digital mixing systems are inaccurate, and do not actually indicate the peak values of the resulting waveform, the result is that while the samples themselves do not exceed full scale and are carefully monitored to ensure this, the resulting waveforms represented by the samples may exceed full scale throughout any standard CD!
While the digital mixing system is not clipping the music or distorting the music, the digital to analog converters that have the task of recreating the audio through digital reconstruction filters are clipping repeatedly throughout most CDs on the market. The result is that most CDs and DVDs end up distorting with regularity when they are asked to reconstruct and play back audio that appears to be completely “legal” because not a single sample actually clipped.
In a recent paper [Nielsen 2003], seven consumer CD players were subjected to tests designed to analyze their ability to reproduce and reconstruct signal levels above full scale (0 dBFS). All of the players experienced difficultly dealing with signal levels this high, further showing that, while all of the samples can be legal, the level can still be hotter than is legal. The result is that a CD player can be unable to reproduce the audio accurately. In some cases, the reconstruction sounds “perfect” to the mastering engineer, because the engineer’s equipment can actually reproduce the waveforms properly.
The Red Book format for CDs and the DVD specs both allow for this illegal content and the mastering engineer is still allowed to put out releases that meet the spec while allowing consumers’ players to distort. With an oversampled peak meter, the engineer will be able to know that the music is clipping, by how much and where. With this knowledge the engineer can then decide with complete information whether or not to accommodate the legal range of digital audio on a PCM sampled system.
The goal of TL MasterMeter is to allow an engineer to use a DSP model of the reconstruction process to monitor the reconstructed waveform for potential clipping at the final mix and mastering stages. Using TL MasterMeter, engineers can compare regular and intersample peaks over time and make appropriate adjustments without sacrificing overall level or dynamic range. Utilizing an oversampled peak meter in the digital audio studio that represents the reconstruction filters in digital to analog converters is the first step toward an improvement in audio quality in music releases.