Ask the Doctors: K-System Metering in the Precision Limiter
By Dave Crane and Bob Katz

How many times has this happened to you: you load up you iPod with your favorite albums from the past 20 years and put it on shuffle mode, and when a circa 1975 Led Zeppelin song comes on, you crank up the volume and rock out-but when that Velvet Revolver song from 2005 comes on, it blows the earbuds right out of your ears? Congratulations, you have just become a victim of the loudness wars!

Not only is music louder than ever before, but the sound quality has diminished and it is more fatiguing to the ears, shortening our listening time. Try popping on a set of good headphones and just sitting and listening to a typical CD of today, and I bet you can't get through an entire album without needing a long break. There's very little dynamic range in today's music. Then, try encoding that music as an MP3 and listening to that for a while. In my experience, music that is mastered very loud sounds really bad when converted to lossy file formats like MP3 and AAC, even at high bit rates. Material with a lower average level comes through the conversion process with fewer artifacts.

“Each client wants his CD to be as loud as or louder than the previous transgressor, and we must spend a lot of time showing them that the sound quality suffers as the average density goes up.”

With the release of version 4.2, we have updated the Precision Limiter to include all three K-System metering modes. The K-System was developed by mastering engineer Bob Katz, and its primary purpose is to create a standard integrated system of metering and monitoring that will encourage more consistent leveling practices among the broadcast, film, and music industries. Right now chaos reigns in the music industry as everyone wants their CD to be mastered as loud as possible. As we move toward high-resolution audio for home systems, Bob Katz asks, "What good is a 24-bit/96 kHz digital audio system if the programs we create only have 1-bit dynamic range?"

In the spirit of saving the sound quality of audio, we have incorporated the K-System metering in the Precision Limiter. If K-System metering were standardized and widely used, your audio CDs, DVDs, and radio would have average levels much closer to each other, and they would all have better sound quality. Rather than explain this all in detail, I have reprinted (with permission) excepts from a white paper, written by Bob Katz, titled "An Integrated Approach to Metering, Monitoring, and Leveling Practices," in which he details the K-System approach. For more information, please refer to the entire paper. Bob writes:

"By 1997, some of my music clients were complaining that their reference CDs were 'not hot enough,' a tragic testimony on the loudness race which has hit the industry. Powerful digital compressors and limiters now enable mastering engineers to produce CDs whose average level is almost the same as the peak level! There is no precedent for that in over 100 years of recording. Each client wants his CD to be as loud as or louder than the previous transgressor, and we must spend a lot of time showing them that the sound quality suffers as the average density goes up. The psychoacoustic problem is that even when two identical programs are presented at slightly differing loudness, the louder of the two appears 'better.' This explains the creeping increment of average program loudness, until sound quality is so bad that everyone can perceive it. When the mechanical VU meter ruled, it was difficult for engineers to ignore the warning sign of the needle banging against the peg, but today there is no such warning. This is why monitor and meter calibration is absolutely essential for the new digital consumer formats."

"With the integration of media into a single system, finally it is in the direct interest of music producers to unite with video and film producers for a more consistent consumer audio presentation. This will eventually happen, but not if music producers experimenting with 5.1 surround pay only casual attention to monitor level calibration. New program producers with little experience in audio production are also coming into our field from the computer, software and computer games arena. We are entering an era where the learning curve is high, engineers' experience is low, and the quality of monitors used for program production may be less than ideal. It is definitely time to educate, and establish a standard, before new chaos reigns. The current lack of VU meters, and the plethora of peak-only meters on every computer, DAT machine and digital console will definitely not help. Engineers must be trained to realize that the peak meter is for one purpose only: to protect the medium."

"A manual for a digital limiter reads: 'For best results, start out with a threshold of -6 dBFS.' This is like saying, 'always put a teaspoon of salt and pepper on your food before tasting it.' This kind of widespread misinformation does not encourage proper production practice. A gain reduction meter is not an indication of loudness."

"If only the designers of the compact disc system had foreseen the chaos that would result from the loss of an average metering standard or a monitoring standard. To avoid the same problem with the DVD and other new high resolution media, we must unite behind a new standard that integrates metering, monitoring and leveling practices, called the K-system. First, we should stop using meters which have 0 dB at the top-this discourages operators from understanding where the message is. This 21st century meter should be tied to a calibrated monitor gain, with the averaging meter's 0 set to 83 dB SPL. The scale must be linear-decibel through at least a 24 dB range, dual characteristic, peak and average, with the average (VU) level being the most important part of the display. The averaging portion of the meter should be the bar, with a moving line above the bar representing the instantaneous (1 sample) peak level."

"The 0 point (always 83 dB SPL) slides depending on the venue of interest, resulting in a change in headroom and the amount of compression required. The K-system is not just a meter scale, it is an integrated system tied to monitoring gain."

"The three K-System meter scales are officially known as K-20, K-14, and K-12 (see Figure 1). The K-20 meter is for use with wide dynamic range material, e.g., large theatre mixes, 'daring home theatre' mixes, audiophile music, classical (symphonic) music, hopefully future 'audiophile' pop music mixed in 5.1, and so on. The K-14 meter is for the vast majority of high-fidelity productions for the home, e.g. home theatre, and pop music (which includes the wide variety of moderately compressed music, from folk music to hard rock). And the K-12 meter is for productions to be dedicated for broadcast."

Figure 1: K-Metering examples

"If console and workstation designers standardize on the K-System it will make it easier for engineers to move from studio to studio. By anchoring operations to a consistent average reference, operators will produce more consistent output, and everyone will recognize what the meter means. The process is simple. Upon beginning a production, the operator chooses his goal: If making an audiophile recording, then he uses K-20; if making 'typical' pop or rock music, or audio for video, then he probably uses K-14. K-12 should be reserved strictly for audio to be dedicated to broadcast, and broadcast recording engineers may certainly choose K-14 if they feel it fits their program material. Audio for video studios who've been working without an averaging meter should convert to the K-System right away."

The Precision Limiter now features the three K-metering modes: K-12, K-14, and K-20 (Figures 2, 3, and 4). The three figures are shown in Zoom mode to give you a better idea of the changes. The zero point is different for each type. As Bob Katz explained, this is not the same zero that we see in Peak/RMS mode. This zero corresponds to 83 dbc on an SPL meter with pink noise.

Figure 2: Precision Limiter in K-12 Mode

Figure 3: Precision Limiter in K-14 Mode

Figure 4: Precision Limiter in K-20 Mode

So you might be asking, "What does that mean? And what is pink noise?" First of all, you're probably familiar with white noise, which is a sound that contains every frequency within the range of human hearing (generally from 20 Hz to 20 kHz) in equal amounts. Pink noise is basically white noise with some filtering. The filtering is designed to reduce the volume at each octave to compensate for the increase in the number of frequencies per octave, and the result is a sound wave that has equal energy at every octave.

If you play pink noise at -20 dbfs (decibels full scale, in which case the volume control of your pink-noise generator is set to -20) though your monitors and set them for 0 attenuation (in other words, on full blast), and you measure the output with an SPL meter, it should read 83 db on an SPL meter. If you had the Precision Limiter set up right after the pink-noise generator, then it would read 0 in all three K metering modes. In a perfect world, mastering engineers would try to keep the average level around that zero point, and they would keep their monitor control at the appropriate attenuation for the particular K-System metering point, that is, around 0 dB for K-20, around -6 to -8 dB for K-14, and so on.

Bob Katz makes a great argument for a standardized approach to audio levels, but goes much further by offering a solution. Whether or not the audio world accepts this remains to be seen. But many other companies are adopting the K-System of metering, and with some discipline, audio quality will survive the loudness wars.

Bob Katz' Web Site