Frequently Asked Questions

How do I encode MPEG files?

The Digitizer is the unit that accomplishes this function.  A different way to ask this same question might be:  "How do I digitize tapes?"  You will want to have a high-quality source deck(s) for digitizing, probably the highest-quality deck in your facility for each format you use.  Your Digicaster is very versatile; depending on your custom specifications, you may be able to control:  a Betacam SP deck, a 3/4" deck, an SVHS deck, a DVCPro deck, or others.  In addition, you may control many different decks of each type.

III.  Isn’t all digital video the same?

No, all digital video is not the same.  It is not possible to convert analog video to digital video (or analog sound to digital sound) directly; there must by definition be some form of compression.  What distinguishes digital video formats are the different compression algorithms used to create their files.  Nor is this simply a matter of different formats taking up more or less space depending on their compression ratios, although that is a necessary consequence of this phenomenon.  Some compression algorithms are proprietary:  Indeo and QuickTime, for example.  QuickTime is probably the most popular proprietary compression format, used in many applications for the Macintosh OS.  Others, such as MPEG video, were established for universal purposes and according to internationally agreed-upon standards. 

IV.       What is MPEG Video?

MPEG stands for the Moving Picture Experts Group, a working group comprised of members of both the Internal Standardization Organization (ISO) and the International Electrotechnical Commission (IEC).  MPEG video is not a style or type; rather, it is defined by a particular method of compressing analog data (the original data source being a microphone or camera lens) into digital format.  There are currently two forms of MPEG video widely used for broadcast purposes, MPEG1 and MPEG2.  For some time, MPEG worked on an even higher standard, MPEG3, designed specifically for the HDTV market; however, MPEG2 compression algorithms had gotten so effective at reproducing even HDTV-level quality, MPEG3 research was essentially halted.  There are other versions of MPEG video as well.  MPEG4 is a low-bitrate video algorithm designed primarily for video telephony, multimedia email, and other Internet applications.  MPEG7 has yet to be released to the public.  

V.        How much disk space does an MPEG file consume?  Do MPEG files take up as much space as my AVID QuickTime files?

Engineers familiar with non-linear editing systems are no doubt aware that the most common method of encoding and decoding used in that field is an older method, Motion JPEG (MJPEG) technology.  In this system, each video frame is encoded into a JPEG (Joint Photographic Experts Group) image, which is then stored on the local hard disk.  Even if each frame is exactly the same as the previous one, an entirely new image is created from frame to frame.  Since single JPEG images frequently require more than half a megabyte of storage space, even a few seconds of video might easily encompass 50 megabytes of disk space or more. MPEG video works entirely differently.  The Group realized that, for most video, certain aspects of the images conveyed do not change from frame to frame (excluding basic editing techniques such as jump-cuts).  A static image -- a 30-second frozen shot of a piece or artwork, for example -- does not need to be reconstructed frame by frame from scratch.  Rather, a single image can be continually rebroadcast for the duration of the shot, saving huge amounts of storage space and losing no quality in the process.  Fully static images are rare in normal broadcasting; static portions of images are quite common, however.  By breaking images into a grid system, portions of those images that do not change from frame to frame do not need to be re-encoded.  Under a grid system, the image becomes a series of packetized macroblocks (MBs) which are transmitted as a group.  In effect, the image becomes encoded as a series of pixel instructions, laid out in a grid.  Since the human eye is more sensitive to changes in brightness than changes in color, the grid for image luminance encoding is 16x16, while the grid for image chrominance encoding is smaller (meaning the MBs are larger) at 8x8.

VI.       What are I-frames?

Intraframes, or I-frames, are the independent frames of reference for an MPEG file.  These occur roughly twice per second.  I-frames require higher bitrates than the average for a given compression ratio.  However, they "anchor" the video, allowing such basic functionality as shuttling.  (By comparison, the JPEG images used in MJPEG encoding are allconsidered I-frames, which allows for frame-specific editing, but uses up much more disk space.) Surrounding I-frames in an MPEG video clip are P-frames (Predicted) and B-frames (Bi-Directional).  P-frames are compressed with reference to a previous frame and may themselves be used as references; B-frames are compressed with reference to both a previous and a subsequent frame and thus cannot themselves be references.  Since they refer to other frames, P- and B- frames do not need to be reconstructed from scratch, and they may be encoded using significantly higher rates of compression than I-frames.  B-frames are the most tightly compressed images in an MPEG file.

VII.     What's the difference between MPEG1 and MPEG2?

Standardized in 1992, MPEG1 was intended for VHS-quality signal transmission primarily for the then-nascent digital video market and is still considered an efficient use of bandwidth and storage space.  MPEG2 was created as the standard for digital broadcasting to provide higher levels of bandwidth transmission needed by, amongst others, direct satellite service (DSS) providers.  MPEG1 has an average compression rate of about 1.5 megabits per second (Mbps); the largest possible compression rate for MPEG1 is slightly more than 5 Mbps.  MPEG2 bitrates fall between about 3Mbps and about 15Mbps. Interestingly, at bitrates below 3 Mbps, MPEG1 actually performs better than MPEG2.  This is because the higher level of precision built into the MPEG2 algorithm requires more processing than MPEG1.  At lower bitrates, the percentage difference is great enough to cause significant digital artifacts to appear in MPEG2 encoding that do not appear in MPEG1.  MPEG2 should not be used at bitrates lower than 3 Mbps.

VI.       How does the Digitizer know which deck I want to control?

In many applications a single deck is normally used as a source deck.  If you have more than one source deck a few different strategies can be used for selecting the type of deck that will be used for a given encode session.  One possibility is to use a "black box" switcher, that is, one specifically designed to switch audio, video, control, and sync all at once.  A more complete solution for larger integration applications may require full integration with a router or patch bay combined with an upgraded V‑LAN system.  Of course, the most economical solution may be for a technician simply to change the connections manually.

VIII.    What is the RAID?

RAID stands for Redundant Array of Independent Disks.  It is a method of linking independent computer hard drives such that they act as one unit.  The advantage of acting as one unit is that data recording and retrieval becomes much faster and more efficient.  A RAID is also a cost-effective method of building redundancy into your hard drive system.  There are different levels of RAID configurations, each of which confers different advantages to the user.

Why do you recommend we have twice as many channels as we actually need?

For optimum performance, your system should be able to alternate MPEG sources for program output.  D.Co’s proprietary software, ShowMaker Pro, is designed to automate this process.  By integrating switcher control with MPEG playback control, ShowMaker Pro prepares clip files for air on separate channels, then switches input files on cue.  This allows for seamless clip playback without any possibility of transmission errors. Transmission errors may occur when MPEG clips are streamed continuously from one channel source.  Temporary modulations in CPU usage – normal in the operation of any digital system --may slow system function by a few thousandths of a second, enough to cause timing errors on a single channel.  While this cannot affect the playback of any one clip, it may impact the back-to-back compiling of an MPEG playlist in a negative manner.  Switching alternate channels while building lead and preroll times into a program schedule is the best way to provide ideal system performance.

I have a limited amount of storage space.  What is the lowest level of encoding I can use for broadcast quality video?

This really depends on two things:  the type of video you will be encoding, and what you believe is acceptable broadcast quality.  Static video is easily compressed, while dynamic video is less compressed and thus requires more space.  A client who needs a short clip of an unchanging image, such as might occur in security applications (CCTV, etc.), can use a lower compression rate than a client encoding a live sporting event, for instance, and achieve the same image quality. The level of acceptable quality is up to you.  For over-air broadcast, D.Co recommends a minimum level of 6 Mbps encoding; for high-intensity applications such as sporting events, we recommend 8 Mbps encoding or higher.  Certain low-intensity applications, such as local access programming, may be encoded at 4 Mbps.  It may be possible for MPEG1 encoding to satisfy your requirements if you require smaller file size.  However, there will be a substantial loss in quality from MPEG2.  Colors will be dimmer, and hard luminance boundaries will be blurred.  In addition, motion may well become pixelated.  At very low bitrates, however, in the range of 1.5 Mbps, MPEG1 is preferred to MPEG2.

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