Dynamic range

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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.

Dynamic range and human perception

Dynamic range is an important indicator of the quality of a system intended either to record or to reproduce information for human perception. The human senses of sight and hearing have a very high dynamic range. A person is capable of hearing (and usefully discerning) anything from a quiet murmur in a soundproofed room to the melody in the loudest rock concert: the latter is 10,000,000,000 times louder than the former, that is a dynamic range of 100 dB. Equally a person 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 person 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 1000, or 30 dB (commercially the dynamic range is often called the "contrast ratio" meaning the full on/full off contrast ratio). 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 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 bit resolution (see signal-to-noise ratio). Dynamic range of an audio device is also sometimes referred to as the dynamic window.

To mathematically determine a dynamic range you must add the headroom to the signal to noise ratio OR take the difference between the ceiling and noise floor of an audio device. For example, if the ceiling of a device is 10 dB and the floor is 3 dB then the dynamic range is 7 dB, since 10-3 = 7.

Electronics

Electronics engineers apply the term to:

• the ratio of a specified maximum level of a parameter, such as power, current, voltage or frequency, to the minimum detectable value of that parameter. (See Audio system measurements.)
• In a transmission system, the ratio of the overload level (the maximum signal power that the system can tolerate without distortion of the signal) to the noise level of the system.
• In digital systems or devices, the ratio of maximum and minimum signal levels required to maintain a specified bit error ratio.

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

Music

In music, dynamic range is the difference between the quietest and loudest volume of an instrument, part or piece of music.

It is also the range of amplitudes an audio device can reproduce. Dynamic range is the headroom plus the signal to noise ratio ranges added together. It can be calculated by taking the difference between the ceiling and noise floor of an audio device. 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[1].

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. It can be controlled through the use of a graduated ND filter. More details about dynamic range and dynamic range optimization can be found here [2].

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.

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.