All codecs have a similar set of parameters. These include image resolution, image composition (single frame versus two fields), de-Bayered versus raw (progressive-only), image frame rate or field rate, color sampling (4:4:4, 4:2:2, 4:1:1 or 4:2:0), RGB versus YCrCb, compression ratio, and bit depth.
Traditional codecs employ bit depths of either 8 or 10 bits. The number of bits used for recording is independent of the number of bits output by the sensor’s analog-to-digital converter.
Nevertheless, a camera’s dynamic range is a function of sensor performance. To minimize noise, the analog signal may be clipped above the noise floor. To minimize highlight distortion, the analog signal may be clipped before full sensor saturation. Both enable a clean signal to be presented to the A/D.
Interestingly, the number of A/D bits has no relation to a camera's dynamic range. Rather, as a camera’s A/D bit depth increases, the tonal resolution of the camera’s gray scale increases. (See Figure 1.)
Both ProRes 4444 and AVC-Ultra can provide 12-bit sample depth. Alternately, data can be converted to log values. In this case, 16 bits can be represented by only 10 bits. Thus, when looking at bit depth specifications, it’s important to know whether it’s log data.
Consider an illumination range of 18 stops. Assuming older sensor technology, at best only 12 stops can be captured by the sensor. However, these 12 stops are not all usable. Low illumination causes several stops to be lost because of high levels of noise. Likewise, at high illumination, several stops are lost due to clipping under extreme light levels. The effective dynamic range is only about six stops. (See Figure 2.)
In Figure 2, the brown diagonal line shows a perfectly linear gamma. In order for a video signal to be displayed correctly on a monitor, a nonlinear gamma must be applied to the signal from the A/D. In the HD world, it’s called Rec. 709. (Red curve.) This curve provides the video image that we are used to looking at. When video will be transferred to film, a lower contrast video image is required. (Blue curve.) The “X” marks the point where the filmic curve yields a brighter mid-tone image that reduces apparent contrast.
Consider a contemporary sensor. (See Figure 3.) The illumination range remains the same at 18 stops. The potential sensor range, however, has increased to 15 stops. Because of improved technology, fewer stops are lost to noise and bright light clipping. Thus, the sensor is able to capture a usable 12-stop dynamic range.
Once again, the brown diagonal shows a linear gamma curve, and the red curve shows Rec. 709 gamma. To record a 12-stop signal, a 12-bit codec can be employed. Alternately, some cinema cameras utilize a logarithmic gamma (green curve) that is applied to sensor data. At point “Y,” the logarithmic curve yields a brighter picture that reduces apparent contrast. Likewise, at point “Z,” the logarithmic curve yields a darker picture that also reduces apparent contrast.
This explains why a logarithmic image looks so much “flatter” than a Rec. 709 image. After log conversion, only 10 bits are required to carry the 12-stop signal range.