More In-Depth Information:
Digital and Analog
The optimum audio levels for digital audio signals are different than those for analog signals.
Whereas the 0dB peak setting is the standard operating level (SOL) for analog systems, for digital equipment the maximum level (in North America) is ▲typically -20dB.
With both analog and digital signals it comes down to headroom.
Headroom is defined as the safe area beyond the SOL (standard operating level) point. With a SOL of -20dB, (which is typically the standard in North America) this leaves another 20dB for headroom. European countries tend to allow for -18dB of headroom.
Analog audio systems often use an analog meter, such as the one shown here. With analog systems a maximum level of 0dB is considered the standard.
With digital signals, however, a digital meter, such as the one shown on the right, or a PPM meter (to be discussed below), are used. In the case of the digital meter on the right, when the signal touches the red area, we're entered the "headroom area."
If a digital signal, were to go to the very top of the scale, clipping would occur. Unlike analog audio, where exceeding the maximum level (generally 0dB) will result in signal distortion, in digital audio you might not notice mild clipping (the elimination of occasional audio peaks).
Actually, an occasional full-scale digital sample (to the top of the red range above) is considered inevitable; but, a regular string of "top of the scale" occurrences means that the digital audio levels are too high, and you are losing audio information.
Different types of meters respond differently to audio peaks. In the case of the standard VU meter shown above, the needle tends to swing past peaks because of inertia. At the same time, the needle will not quickly respond to short bursts of audio. Thus, this type of meter tends to average out audio levels..
Because of the limited headroom with digital audio signals, a peak program meter (PPM) or the previously discussed digital meter is preferred over a meter than tends to average out levels. On the outside, a PPM looks like the animated meter above. Even so, it is designed to much more accurately track short bursts of loud audio (the peaks), which can result in clipping with digital systems.
Before you can really get serious about maintaining correct audio levels throughout a production facility, you must see that the audio meters in all equipment are accurately calibrated to a standard audio reference level.
Although, facilities can adopt their own in-house standards, typically, a 1,000Hz audio tone should register 0dB on analog equipment and -20dB on digital equipment. Thus, the appropriate headroom is maintained in each case.
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