Monitoring Levels |
Sound Forge's play meters display peak levels during playback. Use the meters to monitor levels and ensure no clipping occurs in your file.
The standard method for digital metering is to use the maximum possible sample value as a reference point. This value is referred to as 0 dB. Decibels are used to represent fractions logarithmically. In this case, the fraction is: sample amplitude divided by the maximum possible amplitude. The actual equation used to convert to decibels is: dB = 20 log (amplitude/32,768)
To illustrate this, consider a sine wave with a peak amplitude of 50% of full scale. Inserting the values in the appropriate places yields 20 log (0.50) = -6.0 dB. In fact, each time a signals amplitude is divided by two, its dB value is decreased by 6 dB. Likewise, doubling the amplitude of a signal increases its dB value by 6 dB. Dividing the sine wave until its peak amplitude is equal to 1 produces lowest peak dB possible, -90.3 dB.
Why are dBs used when talking about audio? Decibels are typically used when dealing with sound pressure levels because of the vast range of sound (about 120 dB) that the human ear can perceive. Its also easier to say -90 dB than 0.000030 (1/32,768).
When recording to an analog medium such as magnetic tape, recording engineers typically try to keep VU (volume unit) meters as close to zero as possible. This ensures a high signal-to-noise ratio while preserving adequate headroom as to keep the tape from saturating and distorting. In addition, occasional peaks above 0 do not cause problems because the tape saturation point is not an absolute.
However, this is not true in the digital realm, where amplitudes are stored as discrete numbers instead of continuous variables. The flexible recording ceiling of analog is replaced by the absolute maximum sample values of digital audio. Stored signals must never have a value above these maximums, as the wave peaks are literally clipped. This clipping adds audible distortion and though it can go unnoticed, it can also ruin an entire project. Therefore, sample with the understanding that digital audio has absolutely no headroom. The following image shows a simple sine wave that has been clipped to 0 dB:
Because digital audio has no headroom, setting the sampling level becomes critical. If the loudest section of the audio is identified in advance, the recording level should be set so that the peak is as close to 0 dB as possible to maximize the dynamic range of the digital medium. If the loudest section of audio is unknown, allow 3 to 6 dB of headroom for unexpected peaks.
From
the Tools menu, choose Find
and use the Find dialog to
identify the largest peak in your file.