Definitions

Dynamic range

Dynamic range

Dynamic range is a term used frequently in numerous fields to describe the ratio between the smallest and largest possible values of a changeable quantity, such as in sound and light.

Dynamic range and human perception

The human senses of sight and hearing have a very high dynamic range. A human is capable of hearing (and usefully discerning) anything from a quiet murmur in a soundproofed room to the sound of the loudest rock concert. A difference like this can be 100dB which represents a factor of 10,000,000,000 difference in energy. Equally, a human can see objects in starlight (although colour differentiation is reduced at low light levels) or in bright sunlight, even though on a moonless night objects receive 1/1,000,000,000 of the illumination they would on a bright sunny day: that is a dynamic range of 90 dB. A human cannot perform these feats of perception at both extremes of the scale at the same time. The eyes take time to adjust to different light levels and the dynamic range of the human eye without any adjustment of the pupil is only approximately 30 dB . The instantaneous dynamic range of human audio perception is similar, so that, for example, a whisper cannot be heard in loud surroundings. Nevertheless, a good quality audio reproduction system should be able to reproduce accurately both the quiet sounds and the loud; and a good quality visual display system should be able to show both shadow details in nighttime scenes and the full brightness of sunny scenes.

In practice, it is difficult to achieve the full dynamic range seen by human beings using electronic equipment, since most electronic reproduction equipment is essentially linear rather than logarithmic like human perception. Electronically reproduced audio and video often uses some trickery to fit original material with a wide dynamic range into a narrower recorded dynamic range that can more easily be reproduced: this is dynamic compression. For example, a good quality LCD display has a dynamic range of around 1,000, or 30 dB (commercially the dynamic range is often called the "contrast ratio" meaning the full on/full off contrast ratio), and some of the latest CMOS image sensors now have dynamic ranges of 110dB, compressing the range into a range, near 30dB, that the human eye may understand. When showing a movie or a game such a display is able to show both shadowy nighttime scenes and bright outdoor sunlit scenes, but in fact the level of light coming from the display is much the same for both types of scene (perhaps different by a factor of 10). Knowing that the display does not have a huge dynamic range, the program makers do not attempt to make the nighttime scenes millions of times less bright than the daytime scenes, but instead use other cues to suggest night or day. A nighttime scene will usually contain duller colours and will often be lit with blue lighting, which reflects the way that the human eye sees colours at low light levels.

Examples of usage

Audio

Audio engineers often use dynamic range to describe the ratio of the loudest possible undistorted sound to the quietest or to the noise level, say of a microphone or loudspeaker. In digital audio, the maximum possible dynamic range is given by the audio bit depth (see signal-to-noise ratio).

Dynamic range of an audio device is also sometimes referred to as the dynamic window. The dynamic range is defined as the difference between the minimum and maximum amplitude a given device can record. For example, if the ceiling of a device is 10 dBV and the noise floor is 3 dBV then the dynamic range is 4.85 dBV.

10dBV=10^{left(frac{10}{20} right)}=3.162V

3dBV=10^{left(frac{3}{20} right)} =1.416V

3.162V-1.416V=1.749V

20 times log_{10} left(frac{1.749V}{1V}right)=4.859,dBV

In digital audio systems the dynamic range is limited by quantization error. The maximum achievable dynamic range for a digital audio system is:

mathrm{SNR_{ADC}} = left (1.761 + 6.0206 cdot Q right ) mathrm{dB} ,!
The 16-bit Compact Disc has a dynamic range of 98dB.

In the analog domain multiple noise processes determine the noise floor of a system and its corresponding dynamic range. Magnetic tape has a dynamic range of approximately 55dB. A vinyl disk has a dynamic range of approximately 65dB.

Since the early 1990s, it has been recommended by several authorities, including the Audio Engineering Society, that measurements of dynamic range be made with an audio signal present. This avoids questionable measurements based on the use of blank media, or muting circuits.

Electronics

Electronics engineers apply the term to:

In audio and electronics applications, the ratio involved is often so huge that it is converted to a logarithm and specified in decibels.

Metrology

In metrology, such as when performed in support of science, engineering or manufacturing objectives, dynamic range refers to the range of values that can be measured by a sensor or metrology instrument. Often this dynamic range of measurement is limited at one end of the range by saturation of a sensing signal sensor or by physical limits that exist on the motion or other response capability of a mechanical indicator. The other end of the dynamic range of measurement is often limited by one or more sources of random noise or uncertainty in signal levels that may be described as the defining the sensitivity of the sensor or metrology device. When digital sensors or sensor signal converters are a component of the sensor or metrology device, the dynamic range of measurement will be also related to the number of binary digits (“bits”) into which any analog measurement quantities are converted to create digital numeric values. For example, a 12-bit digital sensor or converter can only provide a dynamic range in which the ratio of the maximum measured value to the minimum measured value is limited to 4096-to-1.

Metrology systems and devices may use several basic methods to increase their basic dynamic range. These methods include averaging and other forms of filtering, repetition of measurements, nonlinear transformations to avoid saturation, etc. In more advance forms of metrology, such as multiwavelength digital holography, interferometry measurements made at different scales (different wavelengths) can be combined to retain the same low-end resolution while extending the upper end of the dynamic range of measurement by orders of magnitude.

Music

In music, dynamic range is the difference between the quietest and loudest volume of an instrument, part or piece of music. In modern recording, this range is often limited through audio level compression, which allows for louder volume, but can make the recording sound less exciting or live. Popular music typically has a dynamic range of 6 to 10 dB, with some forms of music having as little as 1 dB or as much as 15 dB. See Loudness war for additional information.

Photography

Photographers use exposure range as a synonym for the luminosity range of a scene being photographed; the light sensitivity range of photographic film, paper and digital camera sensors; the opacity range of developed film images; the reflectance range of images on photographic papers.

During the film era of photography, Galen Rowell created a filter to increase apparent dynamic range. It consisted of a graduated gradient, neutral density filter positioned in front of the lens at the time the exposure was made. The top half was dark and the bottom half was clear. The dark area was placed over the scenes' high intensity region; usually the sky. The result was more evenly exposed film with increased detail in the shadows and low light areas. Though this didn't increase the fixed dynamic range available at the surface of the film's emulsion, it stretched usable dynamic range in practice. When digital photography became popular, Eugene F. Lally conceived an algorithm that mapped the image and adjusted the signal gain of the pixels in shadows and in highlights to better distribute the lighting range across the image. This technique eventually was adopted by Nikon and called D-Lighting. The technique became the standard in digital cameras and image editing software to expand dynamic range. Eugene F. Lally took the technique the next step into high-dynamic-range imaging (HDR). It represented a new and radical departure from traditional forms of photography. It solved the fundamental challenge of making photographs fit the dynamic range of how we see in real life into the dynamic range available in digital imaging sensors. This technique can be applied directly to in-camera processing and into image editing software. Finally the dynamic range we are capable of "seeing" with our eye/brain processing can be replicated in photography.

Others

  • High Dynamic Range Imaging - is an emerging field in computer graphics which seeks to represent light levels (either measured or synthesised) as an open-ended range of absolute values, rather than as a simple ratio of 'full' brightness. This allows more accurate and realistic renderings.
  • Standard Operating Level - A specified reference level. In recording applications, standard operating level is defined as:

O VU = + 4 dBm

See also

References

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