PROBLEM DESCRIPTION and DISCUSSION
Analog and digital are metered differently but TriCaster, incorporating a hybrid analog and digital audio mixer, had to conform to one manner of display for this information. Most hybrid audio mixers available on the market at the time of design represented digital audio levels using analog metering. Armed with this information, actual levels - analog and digital - were designed, tested, and reviewed in-depth by experts both in-house and representative of major US broadcast networks. Today’s audio mixer user interface benefits not only from this research and testing but subsequent industry feedback which has been integrated in the form of minor enhancements, but has been overall positive and indicative of TriCaster incorporating the correct interface.
TriCaster is a hybrid analog/digital device; its VU meters follow an analog scale, so it can be confusing, especially if moving from an older model TriCaster where only 6 dB of headroom was available (such as the TriCaster Studio). The first key thing to be sure of is that the terms being used are consistent. Crucial to understand is that “dB” is a measure of the difference between two levels, not an absolute sound level. To be technical, it’s a logarithmic-scaled ratio between levels, but the point is that a reference level is needed which the dB measurement compares with the one being measured. “dBu” and “dBV,” on the other hand, are absolute expressions of signal voltage.
Analog vs. Digital
In the analog audio world, the professional standard for audio levels is measured relative to 0 dBu, which is a 0.775Vrms signal, and the unity level (nominal or alignment signal level) is defined as +4 dBu (1.23Vrms). Altogether this standard is often referred to by the “+4dBu” moniker, as opposed to the consumer-oriented “-10dBV” standard which references 0 dBV (1.0Vrms).
The TriCaster rightly displays its audio levels in this fashion, relative to +4dBu. Thus, 0 dB (“unity”) on the VU meters is equivalent to +4 dBu, +20 dB is equivalent to +24 dBu, etc. This corresponds to the way that professional analog audio mixers display decibel levels.
Digital audio signals differ from analog in two significant ways: there is no physical "voltage" to measure and the signal exists within a numerically-defined amplitude range. The overwhelming majority of systems use “dBFS” to represent the signal where the reference, 0 dBFS, is the maximum defined level (“Full Scale”). So then typical implementation, such as within SDI, places the reference point at the maximum level allowable in the digital signal and thus all signal values are represented as negative decibel numbers.
The key question is how to relate an analog audio level, defined relative to a specific real-world voltage and set against a defined nominal signal level, to a digital one, defined based on the theoretical maximum quantification. Conveniently, the SMPTE RP155 standard specifies how this is to be done. It indicates +20 dB of headroom above the +4dBU (1.23V RMS) unity level, making the maximum value +24 dBu or, conversely, the unity level -20 dBFS.
Figure 1: Typical VU Meter
In other words, what this means is that a 0 dB unity level on the TriCaster audio mixer (+4 dBu) corresponds to an SDI signal level of -20 dBFS. Likewise, a +20 dB level on the TriCaster audio mixer corresponds to an SDI audio level of 0 dBFS.
Implications for Operators
This combination of analog and digital metering display creates two important considerations users must be cognizant of. First is that a digital output, although displayed on a meter which nominally implies a voltage, is not a physical voltage. This means that a downstream device employed to convert the digital signal to analog, if necessary, must conform to the same professional analog (+4dBu) standard as the TriCaster. Equipment conforming to the -10dBV standard will convert the signal to a lower level, and to an engineer watching the TriCaster metering this may seem almost 12 dB lower than it should be.
The second consideration is the meter bar color. Users moving from previous TriCasters or other mixing equipment may be conditioned to expect red will indicate signal clipping, as in the pink area in the diagram shown in Fig. 1. However, the red indicator begins at 0 dB, which in fact allows another 20 dB of headroom. Operators attempting to avoid this range will invariably introduce a 15 – 20 dB manual attenuation. On modern TriCasters, nominal audio levels that occasionally rise into the 'red' range (TriCaster Standard Edition) are not problematic provided peaks do reach the top of the meter. Information regarding compression and limiting to insure clipping does not occur can be found in the User Guide.
Figure 2: TriCaster Pro Line
In TriCaster Advanced Edition, metering has been re-designed to make things clearer and reduce concerns or quality issues rising from lack of familiarity. VU meter gradient colors have been adjusted to make it easier to discern ‘safe’ levels. The color red is reserved for clipping, indicated by flashing the channel label.
Figure 3: TriCaster Advanced Edition