Amplitude vs. level

In general acoustic terms, amplitude describes the extent of air pressure deviation from the normal atmospheric pressure. A microphone converts changes in air pressure to voltages. An A/D converter converts these voltages into discrete numbers. Changes in air pressure happen above and below the normal atmospheric pressure (the zero reference), resulting in sound amplitude that is bipolar—it has both positive and negative magnitudes. The voltages and numbers used to describe audio signals are also bipolar. An audio system has an equal positive and negative capacity. Professional audio gear, for example, uses –1.23 to +1.23 volts; an audio sequencer uses the –1 to +1 rational range.

Figure 9.1 Amplitude and level. The top graph shows the amplitude of the waveform, which has both positive and negative magnitudes. The level representation is simply the absolute magnitude of the signal.

One of our main interests is to make sure a signal will not exceed the limits of a system. For example, we do not want the samples within our audio sequencer to rise above +1 or drop below –1. But we do not really care whether the signal exceeds the positive or negative limits; we only care about its absolute magnitude. The term level in this book denotes the absolute magnitude of signals. Amplitude of +0.5 and –0.5 both denote a level of 0.5. Figure 9.1 demonstrates these differences.

Using numbers and voltages to express signal levels would be hugely inconvenient. A professional desk indicating that the signal level is 0.3070730560393407377579071411V, or an audio sequencer showing us that the sample value is 0.25, is rather overwhelming, especially considering the two denote exactly the same level, only in different units. The decibel system provides an elegant solution—it expresses levels with more friendly values that mean the same on all systems. The two numbers above are simply –12 dB.

The limit, or the standard operating level of a system, is denoted by 0 dBr (dB reference). On professional audio equipment, 0 dBr is a level of 1.23 V; within an audio sequencer, it is a sample level of 1. Since 0 dBr is the highest limit of the system, levels are mostly negative. But on analog equipment, signals can go above 0 dBr—they might clip, they might distort, but they can still go there—so we also have positive levels. We call the range above 0 dBr headroom. Even within an audio sequencer, signals can go above 0 dBr, but for a few good reasons we are made to believe that they cannot (more on this in Chapter 11). For now, we should regard 0 dBr as the absolute limit of a digital system.

Figure 9.2 Cubase track meter facility. In addition to the standard level meter, there is also a graphical and numeral peak hold indicator and a clip indicator.

Mechanical and bar meters

Mechanical meters involve a magnet and a coil that move a needle. They take up quite a lot of space, and usually only involve a scale of around 24 dB. Bar meters involve either a column of LEDs, a plasma screen or a control on a computer screen. Bar meters might provide some extra indicators in addition to the standard level gauge:

• Peak hold—a held line on the meter indicating the highest meter reading. Usually, the hold duration can be set to forever, a few seconds, or off. This feature tells us how far below 0 dB the highest peak of the signal is, which can be useful during recording or bouncing, where we use this information to push levels further up.
• Peak level—a numeral display that shows the level of the highest peak.
• Clip indicator—an indicator that lights when the signal exceeds the clipping level, which is normally set to 0 dB on a digital system. On analog equipment, the clipping level might be set above 0 dB, to the level where the signal is expected to distort.

Peak meters

Peak meters are straightforward—they display the instantaneous level of the signal. Their response to level changes is immediate. Peak meters are mandatory when the signal level must not exceed a predefined limit, such as 0 dB on a digital system. In essence, this is their main role.

On a digital system, the highest level of a peak meter scale is 0 dB. As the lowest level may vary with relation to the bit depth (being –96 dB for 16-bit audio, –144 dB for 24-bit audio, and so on), many digital meters have their bottom level set around –70 dB. If a peak meter is installed on analog equipment, the scale might extend above 0 dB.

Average meters

Our ears perceive loudness in relation to the average level of sounds, not their peak level (Figure 9.3 demonstrates the difference between the two). One disavantage of peak meters is that they teach us little about the loudness of signals. In order to bear any resemblance to loudness, a meter has to incorporate some averaging mechanism. There are various ways to achieve this. A mechanical meter might employ an RC circuit (resistor-capacitor) to slow down the rise and fall of the needle. In the digital domain, the mathematical root mean square (RMS) function might be used. Regardless, the movement of the meter should roughly reflect the loudness we perceive. One example where this can be useful is when compressing vocals—it is the perceived loudness of the vocals we want even.

Of all the averaging meters in mixing, the mechanical VU (volume unit) meter is the most popular. Figure 9.4 shows a plugin version. The scale spans –20 to +3 dB, and it is worth

Figure 9.3 Peak vs. average readings. A peak meter tracks the level of the signal instantaneously; therefore, its reading is identical to the signal level. An average reading takes time to rise and fall, but the resultant readout is reflective of our perception of loudness. We can say that an average readout seems lazy compared to peak readout.

Figure 9.4 The PSP VintageMeter. This free plugin offers averaging metering characteristics similar to those on the mechanical VU meters.

noting the distribution of different levels. For example, the right side of the scale covers nearly 6 dB, while the left covers the remaining 17 dB. Many studio engineers are accustomed to this type of meter, and some even have enough experience to set rough levels of various instruments just by looking at them.

VU readings can also be displayed on bar meters. This simply involves showing the RMS reading on the standard peak scale (–70 to 0 dB). However, since people are so used to the –20 to +3 range of the mechanical VU meters, a VU bar meter might have its scale covering these 23 dB only.

Figure 9.5 The Sonalksis FreeG plugin. This free plugin provides extended metering capabilities, among other features. The main meter characteristic is based on one of four ballistics settings (seen on the right panel); these include types of meters not discussed here such as the broadcast BBC type. The narrow bar within the main meter shows RMS levels. There are numeral and graphical (arrows) peak hold indicators for both peak and RMS.

Phase meters

VU or peak meters are often provided per channel and for the stereo mix. One more type of meter worth briefly mentioning is the phase meter. Phase meters are a common part of large-format consoles. They meter the phase relationship between the left and right channels of the mix. The meter scale ranges from –1 to +1. The +1 position denotes that both the channels are perfectly in phase; that is, that they output exactly the same signal (which is essentially mono). 0 denotes that each channel plays something completely different (essentially perfect stereo). –1 denotes that the two channels are perfectly phase-inverted. Generally speaking, positive readings tell us that our mix is phase-healthy, whereas negative readings suggest that there might be a problem. We want the meter to remain on the positive side of the scale unless we deliberately used an effect that involves phase inversion between the left and the right speakers (like the out-of-speakers effect described later in Chapter 12).

Figure 9.6 The Digidesign PhaseScope plugin. The phase meter can be seen at the bottom. Also note the –20 to +3 dB VU scale on the meters, and the number of possible meter characteristics. The psychedelic graph is actually a very useful tool called the Lissajous curve. It provides visual imaging of the stereo signal, from which we can learn about stereo image shifting, stereo width, phase problems, and more.