Tuesday, 25 September 2012

dB in relation to Digital Video

  I was confused by the Iris control scale on my HD Video camera as it mentioned dB. Gleaned this from WIKI:

Video and digital imaging In connection with video and digital image sensors, decibels generally represent ratios of video voltages or digitized light levels, using 20 log of the ratio, even when the represented optical power is directly proportional to the voltage or level, not to its square, as in a CCD imager where response voltage is linear in intensity.[20] Thus, a camera signal-to-noise ratio or dynamic range of 40 dB represents a power ratio of 100:1 between signal power and noise power, not 10,000:1.[21] Sometimes the 20 log ratio definition is applied to electron counts or photon counts directly, which are proportional to intensity without the need to consider whether the voltage response is linear.[22] However, as mentioned above, the 10 log intensity convention prevails more generally in physical optics, including fiber optics, so the terminology can become murky between the conventions of digital photographic technology and physics. Most commonly, quantities called "dynamic range" or "signal-to-noise" (of the camera) would be specified in 20 log dBs, but in related contexts (e.g. attenuation, gain, intensifier SNR, or rejection ratio) the term should be interpreted cautiously, as confusion of the two units can result in very large misunderstandings of the value. Photographers also often use an alternative base-2 log unit, the f-stop, and in software contexts these image level ratios, particularly dynamic range, are often loosely referred to by the number of bits needed to represent the quantity, such that 60 dB (digital photographic) is roughly equal to 10 f-stops or 10 bits, since 103 is nearly equal to 210.

And from my friendly electronics guru (I'm not sure what her source was):

The camera adjusts the brightness of the video normally by opening the iris to control the illumination of the CCD. However once the iris is fully open, it is possible to bring up the effective brightness by increasing the electrical amplification of the CCD signal. 0 dB is the normal setting for electrical gain. plus 6 dB is a gain increase of a factor of x2 and 18 dB is a gain increase of a factor of x6. The problem with electrical gain is it also amplifies the noise, especially fixed pattern noise, in the picture. A well made CCD can tolerate 6 dB gain without much noticeable grain. 18 dB is certain to give a grainy picture but sometimes a noisy picture is better than no picture. A camera that allows manual setting of the gain is preferable to one that adjusts automatically if you need to minimize picture noise. I think this explains the change from f stops to dB scale on the iris control. I may not fully understand the maths (my weak point) but now I know that dB apply not only to sound.

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