Digital Video - November 2007 - (Page 48) MASTER&COMPRESSOR you may even need to adjust the ratio of bits allocated between I, P and B frames. Let’s review the HD encoding process step-by-step, then take a closer look at the adjustment parameters for segment reencoding in Sonic’s H.264/AVC and VC1 products. 1. CONVERT TO 8-BIT 4:2:0 YUV COLOR SPACE. Blu-ray, HD DVD and standard DVD all use this format exclusively. If you shot with The CineVision 2.0 Timeline View. In the area between the green and red vertical lines, you can see that the an HDV camera, you started in this peak signal-to-noise ratio is low and the quantization is high, indicating this area might need attention. space, but hopefully you did your color correction in a wider gamut continued from 45 with a higher bit depth. The key thing is that the conversion to this the most sophisticated apps let you do something called a segment limited space must be done right—some encoding applications reencode. Briefly defined, this is where the compressionist marks a introduce unwanted color banding at this step. section from an encoded clip, in order to reencode at higher quality. 2. PREPROCESSING. Before the compression system begins renThis becomes critical for really long files. Without segment reencode, dering, you can remove or add grain and noise, and, if necessary, you might have to reencode the entire feature just to fix one shot. restore lost sharpness with aperture correction. When you segment reencode, the finer and more obscure com3. SET UP YOUR BASE ENCODE. This involves setting parameters pression parameters are brought to bear. Foremost among these for the first and second passes. The first is an analysis or investigaadjustments is quantization, loosely defined as the allocation of bits tion pass, in which the encoder examines the content and makes within a shot, or within an defined area of the frame. Sometimes > DIGITAL VISION’S SOFTWARE PLUG-INS FOR HD DISC ENCODING Left: Original unprocessed grainy image. Right: Image after degrain filter applied. You may not really want to remove this much grain, if it was intentionally created as an in-camera effect. Sometimes a side effect of extreme grain removal is a change in the color balance of the image. Grain contains less red than blue and green, so removing most of the grain makes the image more red. PHOTOS: SPEED AND ANGELS, COURTESY SALIENT MEDIA nyone who’s ever authored more than a few DVD titles for commercial release has used or wanted to use noise and grain reduction. Outside of Hollywood title work (and even inside it) you count yourself lucky if the content was shot on low-grain 35mm film or well-lit digital video. Only then might you feel able to pass the video straight on to the encoder without any preprocessing. This year the Swedish company Digital Vision introduced a line of plug-ins for the Sonic Cinevision encoder, called DVO, or Digital Vision Optics. These are known as DVO Grain, DVO Regrain, DVO Aperture, DVO Brickwall, DVO Alias and DVO Dust. A What’s different about these plug-ins, other than their quality, is that they are integrated into the encoder at two different steps in the process. You can use them as normal preprocessing filters, before compression is applied, but you can also use them in segment reencoding. The latter is helpful when upon close inspection, artifacts are noticed after the first pass of compression. Sometimes increasing the bit rate or improving quantization doesn’t solve a problem, where filtering would. The following images give an example of the DVO Grain plug-in in action, removing unwanted film grain and video noise. —D.O.W. 48 dv november 2007 www.dv.com http://www.dv.com
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