Visual audio-signal monitoring
Feb 1, 2010 12:00 PM, By Michael Kahsnitz
The introduction of HD video in broadcast has triggered an increasing demand for quality control and monitoring of audio signals. Here's what you need to know.
Figure 1. This digital PPM instrument has an analog scale and 9dB headroom.
Today, fast perception and accurate assessment of audio, using visual representation methods, are indispensable in production, post production and broadcast. Suboptimal monitoring conditions, stress and aural fatigue are just a few of a large number of reasons why relying just on one's ears is not enough when it comes to quality control.
This is particularly true for complex 5.1 surround mixing. In the pro audio market, there is an extensive range of specialized tools for any type of audio-signal monitoring — from simple peak meters to highly sophisticated surround analyzers. Ideally, those units allow for quick and intuitive interpretation of what is being viewed and fast reaction as necessary; however, this requires an understanding of at least the most critical technical interrelations and the basics, as well as a reasonable configuration of the available instruments.
Level
Figure 2. Seen here in combination with a PPM bar graph, loudness metering uses an LKFS scale.
In professional audio, the type of visual display required most often is the level meter used for examining signal levels; today, this component is found on every mixing console and countless peripherals and recording devices. A level meter is needed, for example, for visualizing clipping on recorders or transmission lines, or in signal processing. At the same time, level meters reveal too-low levels, which make optimum use of the dynamic range between the noise floor and the clipping threshold difficult. The signal level does not only need to be adapted to the technical conditions of the transmission network; at the latest, when exchanging programs with other studios or broadcasters over links or recording media, adhering to agreed standard levels as well having internationally different standards becomes critical. Here, too, level meters are crucial. In this context, it can be assumed that we mostly deal with digital audio today. The first stumbling block, in particular, for a professional user who might not exclusively — and not even mainly — have to handle audio is the actual recording to a tape, hard-disk or solid-state medium: All audio devices used in practice have some kind of meter — be it a pointer or bar graph instrument or a GUI element on a PC. Unfortunately, only a few of these meters meet professional demands and standards, and are therefore capable of delivering comparable and reliable results. The same is true for digital audio, although there is a clearly and uniquely defined unit: dBFS (decibels relative to full scale). In fact, the task at hand is quite simple: A standardized digital dBFS-scaled peak meter (PPM) is needed to meet professional requirements.
Now, it would be reasonable from a technical viewpoint to define the scaling of such a PPM instrument for digital audio in a way that the zero corresponds to the maximum level of 0dBFS. (For reasons of simplicity, we assume here that levels above 0dBFS do not exist.) On the other hand, there are a number of reasons for including headroom instead of using a fixed zero (i.e. 0dBFS = 0dB on the scale). In practice, this approach would make things simpler and safer.
For example, when examining the level of any commercially available pop-music CD on the digital domain, it almost constantly remains near the digital full-scale level. Modern CDs are purposefully mastered in this way to achieve the highest possible loudness and thus the highest possible attention for the program in question. Unfortunately, many producers seek to achieve a similar level near the full-scale threshold already during the recording phase. Let's consider a sample scenario: An audio engineer prepares for an interview. Right before the recording starts, he quickly checks the recording level and adjusts it with that goal in mind. It is obvious what will happen: The first loud clearing of the throat will result in considerable clipping. This is a serious problem. Unlike analog recording media, digital systems do not have a smooth transition to clipping. Even an upstream limiter will normally not prevent excessive levels that usually result in extremely unpleasant distortion. Recorded digital clipping can be fixed afterward only using disproportionate post-processing efforts — if at all. Therefore, appropriate headroom is a must, in particular, with digital systems. Moreover, this approach presents virtually no drawbacks because modern devices usually have an extensive dynamic range. Even recordings made at a level far below the allowable maximum are not at risk of getting too close to the noise floor. Using modern digital systems, raising low recording levels at the post-processing stage is child's play.
Each production facility or standardization body independently defines a level range as headroom below full-scale level. For example, the EBU recommends 9dBFS; this means that when using a digital scale with 0dBFS at the top, the headroom range would start at -9dBFS. In order to visualize that, the maximum level of the signal should be set at or around -9dBFS, specifying a color or brightness change above that level would make sense.
Many devices implement an analog graph scaled in decibels rather than in dBFS. In our example, the 0dB position would be at -9dBFS, while the maximum scale interval would be +9dB. Of course, this would still correspond to 0dBFS; after all, only the scaling has changed. This approach visualizes the desired maximum recording level even better. (See Figure 1.)
Figure 3. DK-Technologies features a surround-signal visualization method called Jelly-Fish.
Many users from the fields of professional audio have become accustomed to the integration time of peak meters, which is typically 10ms. Therefore, sticking to that integration time when metering digital signals has become common practice. This is to ensure that the familiar viewing characteristics can be retained, although there are actually different principles applicable on the digital domain. To make sure, however, that no digital peaks are missed by the instrument, the display should also include a marker showing the level at sample precision without integration time.
Comparing the display values with and without integration time for different types of programs such as speech, music or test tones reveals partly considerable differences. For example, with speech recordings with proximity effect, the deviation can be more than 6dB. In practice, this means that peaks can actually reach levels at or above -3dBFS even if the recording level has been set with 9dBFS headroom at an integration time of 10ms. Consequently, setting headroom on such a scale is realistic rather than exaggerated.
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