It is possible to find video cards with
It is possible to find video cards with digital inputs (DV and 601) and these do offer the potential for higher quality encoding. The video cards also include a preview output so that the encoded files can be viewed on a conventional television monitor. Many video cards are designed as platforms to support nonlinear editors, so they will have many additional facilities that are not required for a straight encode. In an ideal world the streaming format would be encoded directly from a 4:2:2 source (at 270 Mbit/s); in fact, this is the way that DVDs of motion pictures are encoded. In this way the studios can ensure the best possible picture quality for the DVD release. To get the best possible encoding, a digital source should be used, with a clean picture and a low level of noise. If a 601 source is not available, DV can be used as an alternative. The mild DCT (discrete cosine transform) compression used by DV should not produce significant artifacts visible in the final stream. The second best is an analog source; here Y/C is preferable to composite. Analog sources exhibit more noise, which potentially can use up bandwidth at the compression stage. Composite signals are first decoded, and there will be visible artifacts that will produce a slight impairment of the final encoded quality (called cross-color and cross-luminance). There are different ways of compressing; the intermediate AVI may be at the same resolution as the original video, or it may be scaled to the target size. Each method has advantages and disadvantages. It is a trade-off between video quality and data rate. If many different output formats are required, an intermediate file may have to be stored for the duration of the encoding process. This file should be at least the size of the largest encoded picture, if not larger. So if you are encoding to 160 120 and 192 144, you may want the intermediate file at SIF resolution 352 240. The AVI file may even be archived. But the higher the quality, the higher the storage costs. The other consideration is the processing time, as a high-resolution file will take longer to process. The processing time can be important for two reasons. First, it could impact the throughput of work in a busy encoding center. Second, with live feeds the encoding has to take place in real-time, so the lower the resolution, the less CPU power required for the codecs. VTR control It is much simpler to encode a tape if the video capture application has a remote control facility for the VTR. This means that you do not have to start the capture on the fly, plus it allows batch encoding to a prepared schedule. Most professional tape decks can be controlled via an RS-422 port (DV decks also can be controlled through the IEEE-1394 interface). The port allows full transport 162 The Technology of Video and Audio Streaming
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