The telephone book is 6 CDs. Encoders Part II:......................
See Chart here...............................
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MPEG 2 Die For, (continued) November 3, 1997 Contents
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PRODUCT
TABLE MPEG 2 Encoders
A full breakdown of product features, prices and contact information.
Comparing Encoder Features
What's the difference between high-end and inexpensive encoder systems? Ultimately it shows up in the quality of the encoded video -- how good it looks upon playback (see "MPEG 1 or MPEG 2?" January 6, for sample photos comparing MPEG 2 and MPEG 1 frames). The difference is also apparent in the efficiency with which the video data gets compressed. Most variable bit rate (VBR) encoding systems let you specify average and maximum bit rates, so quality comparisons involve looking at different systems using the same bit rate. Less expensive constant bit rate systems are fine for short clips.
ÿÿÿ<Picture>ÿÿ The algorithms needed to decode MPEG 2 are public information (although a consortium of manufacturers has recently created a licensing fee for MPEG 2 decoders, despite the fact that MPEG was previously considered to be in the public domain). But the algorithms for encoding MPEG 2 are not part of the standard and are closely held secrets of encoder manufacturers. The same is true for proprietary preprocessing and prefiltering systems that massage the video signal prior to encoding to minimize artifacts. (Sometimes these filters are sold separately, as in the case of Minerva's $5,995 Video Preprocessing Module for its $29,995 Minerva Publisher MPEG 2 encoder system.) The quality of encoded video thus varies with the equipment used.
ÿÿÿ<Picture>ÿÿ Most of the MPEG 2 encoders listed in the chart deliver 720-by-480 resolution (or 704-by-480), which yields four times the detail of MPEG 1. But an interesting in-between alternative, called half-resolution MPEG 2, is 352 by 480. This format offers better resolution than MPEG 1 and incorporates interlace (each frame consists of two 352-by-240 images running at 60 fields per second), so fast motion looks as good as it would with MPEG 2. It's offered, for example, on FutureTel's PrimeView Duet Pro encoder, in Optibase's MPEG Forge system, and as a midpriced alternative in Optivision's VSTOR line.
ÿÿÿ<Picture>ÿÿ Besides picture quality, there are several other important features to look for. First on the list is real-time encoding -- the ability actually to create the MPEG 2 file at the same rate at which the video is coming in, so the file is ready to use immediately after the video clip has finished playing.
ÿÿÿ<Picture>ÿÿ The ability to decode while encoding also is a big time-saver. This lets you see what the decoded image will look like as it is being encoded, so you can spot errors without waiting for the file to be finished. Sometimes this is accomplished through an optional add-on card.
ÿÿÿ<Picture>ÿÿ If you do see errors, you can make several kinds of corrections. Besides allowing you to change the bit rate, most MPEG 2 systems offer filters that you can invoke, as well as color correction adjustments. Some systems incorporate on-screen test equipment -- a video waveform monitor and/or vectorscope. Used in conjunction with test patterns such as color bars, these can help maintain maximum transfer quality.
ÿÿÿ<Picture>ÿÿ MPEG files consists of three types of frames, called I-frames (intra), P-frames (predicted), and B-frames (bidirectional). Most MPEG 2 encoders have a feature that automatically identifies the start of a new scene, for the purpose of placing an I-frame there in the encoded stream. The I-frames contain the complete information needed to create a picture, while the more efficient P-frames and B-frames contain only information that has changed since the previous I-frame (or the next, in the case of the B-frame). Typically, there's only one I-frame every half-second. (This is why it takes so long to channel surf with digital satellite TV systems -- viewers have to wait for the next I-frame to come around.)
ÿÿÿ<Picture>ÿÿ To edit video, you'll need access to each frame independently, which means ordinary IPB MPEG files are unworkable. For video editing applications, your encoder should have the ability to create I-frame-only files. Fortunately, even the less expensive encoders, such as Wired's ButaneII card, can finesse this task.
ÿÿÿ<Picture>ÿÿ If you're already editing video using a nonlinear system, such as those from Avid or Media 100, you might benefit from an MPEG 2 authoring system that converts those files directly (in the digital domain), rather than converting them back to analog video and then feeding them in through the jack. You might not even need a board -- Heuris' MPEG Power Professional 2, for example, is a Mac/PC software solution that converts Avid OMF, Media 100, QuickTime, and generic AVI files into MPEG 2 files.
ÿÿÿ<Picture>ÿÿ Most encoder boards offer video input jacks that accept standard composite and S-video signals. Higher-end systems, like Optivision's VSTOR line, have jacks for component video signals, such as those from Betacam VCRs.
ÿÿÿ<Picture>ÿÿ Professional video producers working in the D1 digital video format should look for an encoder that accepts this digital signal directly, rather than converting from digital to analog and then back to digital again. The MPEG Power Box from Heuris is one encoder that offers D1-to-MPEG 2 conversion.
ÿÿÿ<Picture>ÿÿ Some encoder systems, such as Optibase's MPEG Forge, offer VTR (videotape recorder) control and batch encoding. These can automatically locate a number of video clips on a videocassette (based on the time-code numbers you specify) and then create a separate MPEG file for each.
ÿÿÿ<Picture>ÿÿ You will also want to consider the type of audio input jacks the system has (XLR balanced connections are best; RCA phono jacks are second; minijacks are the weakest), as well as the audio file formats your MPEG 2 encoding solution offers. While all MPEG 2 systems can compress audio signals into MPEG 1-compatible audio formats (called Layer 1 and 2), the DVD format includes several other audio options that are of particular interest to those encoding feature films. These include AC-3 (Dolby Digital) surround sound and the ability to carry up to eight simultaneous foreign-language soundtracks. The high-end ($99,000) Minerva Compressionist 250 system incorporates AC-3 encoding, as does CagEnt's MPEGXpress encoder board.
ÿÿÿ<Picture>ÿÿ Another feature, called inverse telecine, is designed to improve performance when encoding movies that were originally shot at the standard 24fps film rate. This process, found on most units, removes frames that were duplicated when the film was transferred to video, thus reducing the amount of information that needs to be compressed by MPEG 2. When encoding lengthy material such as feature films, the ability to selectively re-encode just a segment of an MPEG 2 video bitstream makes revisions faster. Selective re-encoding is available with Sonic Solutions's DVD Creator system.
ÿÿÿ<Picture>ÿÿ Finally, note that most MPEG 2 encoders allow you to multiplex video -- to put several different video streams (as well as audio) into a single file.
ÿÿÿ<Picture>ÿÿ Start your search by looking at samples of video compressed with the different systems you're considering. At the very least, for multimedia production, look at the clips that manufacturers make available on promotional CD-ROMs or the Internet. For high-end systems, don't be shy to ask to have your own sample video clip encoded; this will give you a standard of comparison.
Cliff Roth is author of The Low Budget Video Bible, and Mobile Computing For Dummies.
<Picture: continue>ÿÿ MPEG 2 Die Forÿ November 3, 1997 Contents |
