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Article 0013
Category:
WMV Export Component
Subject: Compression Recommendations
Date:

Constant Bit Rate. This is best used from a streaming server NOT to be confused with Progressive Download (http). On streaming servers one would need to have very tight control over the bandwidth in use at any moment.  CBR does that.

With CBR, the encoder decides what data to "throw out" in order to keep the constant bit rate. Complex video such as dissolves, camera motion, fast action suffers most as far as image quality, whereas "talking head" video would suffer less loss in quality. Complex video needs more data to look good.

2 Pass CBR means the encoder does an analytical pass so it can "see the future" so it can make more "intelligent" decisions. 2 Pass CBR generally can/will look better than 1 Pass CBR at the same data rate but it takes longer to encode.

VBR is best for progressive download where you have some flexibility over the data rate at any given moment. Variable Bit Rate means the encoder can allow for some increase and decrease in data rate as needed and dictated by the settings you chose.

With Windows Media, 1 Pass VBR, the Quality Slider is the target. The encoder targets the quality setting and does whatever it needs to the bandwidth to get there. You could get very different data rates with two different videos if one were talking heads and the other was fast action sports. The fast action would need a higher data rate to get the same quality setting.

2 Pass VBR, like 2 Pass CBR does an analytical pass so it can "know the future." This also means 2 Pass VBR will make "intelligent" decisions. It's decision making is guided by both the data rate you target and the Quality slider. The data rate you set is an AVERAGE and can vary from frame to frame. That 2nd pass really helps here because it has a better idea where it can toss data to keep the average data rate over time. The Quality slider helps in how it makes decisions when it is faced with one. It determines at what point the encoder should drop frames and keep a sharper imager OR keep smooth playback and loss a bit of image quality. Personally I always prefer smooth playback so I'll often set Q to 0 when doing 2 pass VBR. If I want sharper image I'll simply raise the data rate. It's my aesthetic as a compressionist.

Input Frame Type. Sources can be either progressive (all scan lines on) or interlace, two fields of every other scan line starting at either upper or the next (lower) scan line. Most (but not all) video is Interlace Lower Field first. It's quite possible to have progressive video 24p (24 or 23.98 progressive frames), 720p (such as DVCPro HD).

Output Frame type. Computer (and web) playback are progressive so in general you'd set this to progressive. If you desired to play back your video on a TV set you might chose interlace.

Frame Rate. PAL is 25 fps, NTSC is 29.97 fps (Not 30), film is 24 and there's also 23.98 (I won't get technical about that here) If you want to cut frame rate some people will cut it in half. Many newbie compressionists think cutting the frame rate in half cuts the file size in half. It will cut the size but not in half. Encoders make important decisions about throwing out data based on how much different one frame is compared to the next. It needs to keep more data to keep up a level of quality if the frames are very different. The more similarity from frame to frame the more it can throw out duplicate data. Well, lower frame rate means greater difference from frame to frame so it doesn't throw out as much data as some would think. In short, cutting the frame rate in half won't cut the file size in half.

Output frame size. Simply the size of the frame in square pixels. What you chose depends on factors ranging from readability, size of monitor used by the viewer and ability to decode the image. It will affect data rate and file size.

The ability to decode an image is an important thing many fledgling compressionists overlook. A 400mhz Pentium II with 64MB of ram running Windows 98 might have problems playing back a file that would look great on the latest greatest 3Ghz computer running Windows XP. Also the connection speed of the viewer is important. Is your target using a 56k dial up connection, DSL/Cable Modem, T1 or T3 or playing right from the hard drive?

The above is very simplified and a good compressionist will find many exceptions and variations. This is just to get you going.

BTW anything you encode with Windows Media Advanced won't play back on WM9 on the Mac.