MANAGING AUDIO LEVELS
Sep 1, 2005 12:00 PM, BY JIM STARZYNSKI
 Figure 1. The typical levels in analog voltage measurement. Click here to see an enlarged diagram. |
Maintaining the accuracy of the original acoustic energy is critical to the recording and transmitting of audio signals. As this energy is transformed electrically and sampled digitally, the goal is to maintain the integrity and exactness of the audio in all of the forms it may take as it moves within a modern TV infrastructure.
Passive or active, analog or digital, today's audio devices all have a finite capability to handle the maximum amplitude of the audio signal. If this capacity is exceeded, the resulting audio — due to a failure of the electronics, current limitations of the circuit or exhaustion of available bits — will not match the original audio and will be distorted.
Establishing and maintaining the proper relationship of the signal amplitude to the minimum and maximum capabilities of the equipment and storage medium in every step of the signal chain is essential for quality audio. Maintaining accurate levels during A-to-D conversions and always tracking perceived loudness (and how it is affected by level changes) must be done to safeguard signal accuracy. Identifying the critical steps in the signal chain and employing proper signal measurement practices to mix and distribute program audio is key. Problems can arise when analog signals are used in a digital plant if the differences in level characteristic and scaling are misunderstood and enough care isn't taken to the process of handling both types of audio signals.
To master the challenges of today's hybrid analog-digital world, let's review the terms and explore the practices that are fundamental to maintaining a modern TV facility's audio.
The reference level establishes a nominal amount of signal at an understood calibration point that is well above the noise floor of the signal and well below the overload point. This is the comfort zone. This level and the amplitude just below is where the majority of the audio signal should be mixed, transmitted or recorded.
The area above the reference level but below the maximum amplitude or overload level is referred to as headroom and is needed to maintain undistorted recording or transmission of the fast peaks of the audio material.
Measuring analog
The entire area from the noise floor to the recording or transmission overload level encompasses the dynamic range of the medium or electronics. Dynamic range is measured as a ratio in dB, and its span varies from system to system.
Contemporary analog voltage measurement levels are in dBu. dBu is used when an audio voltage is connected to another circuit that does not reduce the voltage of the original signal. These bridging circuits are the opposite of older matching circuits that required a signal to be loaded down by the destination devices for proper level. Matching circuitry was given up for the most part in the '70s and '80s, giving way to the simplicity of bridging circuits that have low source impedances around 60Ω or less and high load impedances that are typically 10,000Ω or more. Maintaining a ratio 10 times or higher of load impedance to source impedance bridges and does not load down the circuit.
dBu represents the level compared with 0.775 volt RMS with an unloaded, open circuit source. (The “u” in dBu stands for unloaded or unterminate. It is a voltage that is not related to power by an impedance.)
Analog operating levels and the the VU meter
dBm represents the power level compared with 1mW. This is a level compared to 0.775 volt RMS across a 600Ω load impedance.
A typical noise floor measurement for a studio analog mixing console with a single channel routed to program out is about -82dBu or 86dB below the +4dBu operating level. We frequently use 0VU on the meter scale to actually mean +4dBu. (See Figure 1.)
John Woram's “Recording Studio Handbook” explains this as “A carry-over from the earlier days when meter construction was difficult and additional resistance was necessary for accuracy. This resistance loaded down the meter by 4dBm and therefore 0VU was actually +4dBm. Rather than redefine the zero reference level, it was considered expedient to leave 0dBm at 1mw across 600Ω, with the understanding that 4dBm above this value would correspond to a zero meter reading.”
Many modern VU meters are adjustable but may read 0VU with a signal of +4dBu applied.
Broadcasters still use this convention today for analog circuits, including mixing consoles, distribution amps and many signals that interface with telephone company circuitry, even as carriers move from analog to digital. Analog inputs connecting to digital multiplexing devices frequently operate with a +4dBu reference level.
Headroom's at the top
This audio signal, combined with video, feeds an encoder with A-D conversion, establishing an audio-video digital bitstream for carriage to the destination. This type of signal path is frequently used for studio-to-transmitter links as land-based fiber continues to be chosen over microwave for getting the studio's signal to the transmitter site.
 Figure 2. Analog operating levels and their relationship to the VU meter and digital signal levels. Click here to see an enlarged diagram. |
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