File conversion
Feb 1, 2009 12:00 PM, By Stoyan Marinov
Avoid systems that promise a universal solution.
Today, the “tapeless” dream has already been realized. Tape is still used for acquisition and archiving but has been long ago forgotten in the post-production and playout departments. Although file-based technology is progressing, compatibility issues still remain. While using fast, nonlinear applications to broadcast media is a true time-saver, dealing with various file formats and compression types can be a real headache.
In the past, we had to deal with a number of mutually incompatible tape formats, such as BetaCam, DVCAM, DVCPRO, D1, D9, etc. Now we are drowning in a mixture of file formats and compression types, such as MJPEG, MPEG-1,- 2, -4, Windows Media, QuickTime, DV, JPEG2000, etc., as well HD and SD and the requirement to work with other non-TV image formats including those of mobile phones. In many cases, the speed and flexibility benefits are compromised by the hassle and quality degradation involved when converting from one format to another.
It seems the file-based approach is not a panacea for content workflow. While modern networks and storage backbones can transfer content blazingly fast, dozens of times faster than the real-time content duration itself, content conversion is still a tedious, semiautomated task. New file formats or compression types introduce even more challenges to content conversion engines. Not to mention that every manufacturer tends to create and fight for its “own” media implementation of the “same” format, sometimes within the standard specs, sometimes not — and this is when a nice standard emerges. A good example would be AVC-Intra, which is essentially AVC (H.264 compression) that doesn't take advantage of the interframe compression algorithm; therefore, every frame is independent from the others.
Why so many file formats and compression types?
This question may sound familiar to many because it reflects the not-so-old “tape format war” discussions. And the answer is still the same: No single existing format is perfect for all applications. As a rule of thumb, the larger the tape is, the better quality it provides (for its time). However, there are always a bunch of lightweight, portable formats that are not up to the large tape quality but are acceptable for newsgathering and other outdoor activities. It is the same situation today — MPEG-2 is perfect for transmission and storage purposes, but it is not so good for editing, while DV edits easily, but has a relatively huge file size. Economy versus flexibility, reliability versus mobility, quality versus speed — these are all trade-offs when dealing with formats to suit particular parts of the scene-to-screen chain.
Role of content conversion
Simply put, content conversion is necessary because of the compromises and limitations of the various individual file formats and compression systems. Many of the file-based products on the market (NLEs, playout servers, etc.) are designed to work with one or two media formats. This simplifies product development and significantly shortens the time to market. For example, a server that supports only DV can be launched in less than half a year; a server that supports only MPEG-2 can start shipping in about one year. Imagine how long it would take to launch a product that supports every existing compression system or format today; it is just not feasible.
Some manufacturers go in the opposite direction by using their own proprietary formats so they can lock customers to their product line only. Previously, content had to undergo at least three or four conversion stages in a file workflow from ingest to transmission and archiving. Today, many systems can consistently support the major compression types and file formats, so such conversion is not mandatory. (See Table 1, bottom of page 2.)
Figure 1. Tapeless broadcast workflows can include both file-based and baseband video supported by a series of encoders and decoders. With playout able to read the files from ingest and post production, a file-based area is formed around central storage with baseband used only in the real-time areas of live video input and transmission playout.
Click to enlarge
Content conversion is a complex process to consider. Do we need to convert from one compression type to another, e.g. MPEG-2 to DV? Do we need to convert from one resolution to another, e.g. SD to HD? Do we just need to move content from one file container to another, e.g. AVI to MXF? Or do we need to do all at the same time? The conversion can be lossless or lossy, quick or slow, simple or multistep, depending on exactly which processes are required.
Despite all efforts to avoid compression conversion, we usually end up with at least one conversion: the transmission encoding. Whatever the choice for the in-house file format, the transmission format is always different, or at least the bit rate is. Even if we choose to stick to MPEG-2 (studio quality) for good in-house storage ratio, for the MPEG-2 satellite transmission, we need to provide an even lower bit rate, somewhere in the 2.5Mb/s-4Mb/s range. And although the compression is essentially the same, recompression needs to be done. (See Figure 1.)
| Want to use this article? Click here for options! |  |
