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Every Journey Begins with a Single Step

So you are thinking about compressing some video. It might seem like a daunting subject, but it isn’t really. If you begin with small steps, soon you’ll be taking the whole thing in stride. In this article, we will examine the history and significance of video compression, along with its practical uses—some of the reasons you are on this journey today. There are quite a few products and services available today that just wouldn’t be possible without compression. Many more are being developed. Let’s see what’s out there.

Compression Is Necessary Because . . .

Delivering digital video and audio through the available networks is simply impossible without compressing the content first.

To give you some history, there has been a desire to deliver TV services through telephone networks for many years. Trials were carried out during the 1980s. Ultimately, they were all unsuccessful because they couldn’t get the information down the wire quickly enough.

Now we are on the threshold of being able to compress TV services enough that they can fit into the bandwidth being made available to broadband users. The crossing point of those two technologies is a very important threshold. Beyond it, even more sophisticated services become available as the broadcast on-air TV service comes to occupy a smaller percentage of the available bandwidth.

So, as bandwidth increases and compressors get better, all kinds of new ways to enjoy TV and Internet services come online. For example, a weather forecasting service could be packaged as an interactive presentation and downloaded in the background. If this is cached on a local hard disk, it will always be available on demand, at an instant’s notice. An updated copy can be delivered in the background as often as needed. Similar services can be developed around airline flight details, traffic conditions, and sports results.

Compression Is About Trade-Offs

Compressing video is all about making the best compromises possible without giving up too much quality. To that end, anything that reduces the amount of video to be encoded will help reduce the overall size of the finished output file or stream.
Compression is not only about keeping overall file size small. It also deals with optimizing data throughput—the amount of data that will steadily move through your playback pipeline and get onto the screen. If you don’t compress the video properly, it will not fit the pipe and therefore cannot be streamed in real time.

Reducing the number of frames to be delivered helps reduce the capacity required, but the motion becomes jerky and unrealistic. Keeping the frame count up may mean you
have to compromise on the amount of data per frame. That leads to loss of quality and a blocky appearance. Judging the right setting is difficult, because certain content compresses more easily, while other material creates a spike in the bit rate required. That spike can be allowed to momentarily absorb a higher bit rate, in which case the quality will stay the same. Alternatively, you can cap the bit rate that is available. If you cap the bit rate, the quality will momentarily decline and then recover after the spike has passed. A good example of this is a dissolve between two scenes when compressed using MPEG-2 for broadcast TV services operating within a fixed and capped bit rate.

First We Have to Digitize

Although some compression can take place while video is still in an analog form, we only get the large compression ratios by first converting the data to a digital representation and then reducing the redundancy. Converting from analog to digital form is popularly called digitizing. We now have techniques for digitally representing virtually every thing that we might consume. The whole world is being digitized, but we aren’t yet living in the world of The Matrix. Digitizing processes are normally only concerned with creating a representation of a view. Video structure allows us to isolate a view at a particular time, but unless we apply a lot more processing, we cannot easily isolate objects within a scene or reconstruct the 3D spatial model of a scene.

Software exists that can do that kind of analysis, but it is very difficult. It does lead to very efficient compression, though. So standards like MPEG-4 allow for 3D models of real-world objects to be used. That content would have the necessary structure to exploit this kind of compression because it was preserved during the creation process.
Movie special effects use 3D-model and 2D-view digitizing to combine artificially created scene components and characters with real-world pictures. Even so, many measurements must still be taken when the plates (footage) are shot.

Spatial Compression

Spatial compression squashes a single image. The encoder only considers that data, which is self-contained within a single picture and bears no relationship to other frames in a sequence. This process should already be familiar to you. We use it all the time when we take pictures with digital still cameras and upload them as a JPEG file. GIF and TIFF images are also examples of spatial compression. Simple video codecs just create a sequence of still frames that are coded in this way. Motion JPEG is an example in which
every frame is discrete from the others.

The process starts with uncompressed data that describes a color value at a Cartesian (or X-Y) point in the image.
The next stage is to apply some run-length encoding, which is a way of describing a range of pixels whose value is the same. Descriptions of the image, such as “pixels 0,0 to 100,100 are all black,” are recorded in the file.

This coding mechanism assumes that the coding operates on scan lines. Otherwise it would just describe a diagonal line.

The run-length encoding technique eliminates much redundant data without losing quality. A lossless compressor such as this reduces the data to about 50% of the original size, depending on the image complexity. This is particularly good for cell-animated footage.
The TIFF image format uses this technique and is sometimes called LZW compression after its inventors, Lempel, Ziv, and Welch. Use of LZW coding is subject to some royalty fees if you want to implement it, because the concepts embodied in it are patented. This should be included in the purchase price of any tools you buy.

The next level of spatial compression in terms of complexity is the JPEG technique, which breaks the image into macroblocks and applies the discrete cosine transform (DCT ).
This kind of compression starts to become lossy. Minimal losses are undetectable by the human eye, but as the compression ratio increases, the image visibly degrades. Compression using the JPEG technique reduces the data to about 10% of the original size.

Temporal Compression

Video presentation is concerned with time and the presentation of the images at regular intervals. The time axis gives us extra opportunities to save space by looking for redundancy across multiple images.

This kind of compression is always lossy. It is founded on the concept of looking for differences between successive images and describing those differences, without having to repeat the description of any part of the image that is unchanged.

Spatial compression is used to define a starting point or key frame. After that, only the differences are described. Reasonably good quality is achieved at a data rate of one tenth of the original data size of the original uncompressed format.
Research efforts are underway to investigate ever more complex ways to encode the video without requiring the decoder to work much harder. The innovation in encoders leads to significantly improved compression factors during the player deployment lifetime without needing to replace the player.

Ashortcut to temporal compression is to lose some frames, however it is not recommended. In any case, it is not a suitable option for TV transmission that must maintain the frame rate.

Why Do I Need Video Compression

Service providers and content owners are constantly looking for new avenues of profit from the material they own the rights to. For this reason, technology that provides a means to facilitate the delivery of that content to new markets is very attractive to them. Content owners require an efficient way to deliver content to their centralized repositories. Cheap and effective ways to provide that content to end users are needed, too. Video compression can be used at the point where video is imported into your workflow at the beginning of the content chain as well as at the delivery end. If you are using video compression at the input, you must be very careful not to introduce undesirable artifacts. For archival and transcoding reasons, you should store only uncompressed source video if you can afford sufficient storage capacity.

With the increasing trends toward technological convergence, devices that were inconceivable as potential targets for video content are now becoming viable. Science fiction writers have been extolling the virtues of portable handheld video devices for years, and now the technology is here to realize that capability. In fact, modern third-generation mobile phones are more functional and more compact than science fiction writers had envisaged being available hundreds of years into the future. Handheld video, and touch-screen, flat-screen, and large-screen video, are all available here and now. They are being rolled out in a front room near you right this minute. What we take for granted and routinely use every day is already way beyond the futuristic technologies of the Star Trek crew.

Portable Video Shoot and Edit

Portable cameras have been around for a long time. Amateur film formats were made available to the consumer as 8-mm home movie products; they replaced earlier and more unwieldy film gauges. The 8-mm formats became increasingly popular in the 1950s and ‘60s. The major shortcomings of these were that they held only enough footage to shoot 4 minutes, and most models required that the film be turned over halfway through, so your maximum shot length was only 2 minutes. At the time, battery technology was less sophisticated than what we take for granted now, and many cameras were driven by clockwork mechanisms.

These devices were displaced quite rapidly with the introduction of VHS homevideo systems in the late 1970s. Several formats were introduced to try and encourage mass appeal. But editing the content was cumbersome and required several expensive four-head video recorders.

Just after the start of the new millennium, digital cameras reached a price point that was affordable for the home-movie enthusiast. Now that the cameras can be fitted with Firewire interfaces (also called iLink and IEEE 1394), their connection to a computer has
revolutionized the video workflow. These cameras use the digital video (DV) format that is virtually identical to the DVCAM format used by professional videographers and TV companies. The DV format was originally conceived by Sony as digital 8-mm tape for use in Sony Handycam® recorders.

Of course, there are alternative software offerings, and other manufacturers’ laptops support the same functionality. Sony VAIO computers are very video capable because they are designed to complement Sony’s range of cameras, and the Adobe Premier and Avid DV editing systems are comparable to the Apple Final Cut Pro software if you want to use Windows-based machines.

This is all done more effectively on desktop machines with greater processing power. The laptop solution is part of an end-to-end process of workflow that allows a lot of work to be done in the field before content is shipped back to base.

Handheld video playback is becoming quite commonplace. There are several classes of device available depending on what you need. Obviously, the more sophisticated they are, the more expensive the hardware. There is a natural convergence here, so that ultimately all of these capabilities may be found in a single generic device. These fall into a family of mobile video devices that include

● Portable TV sets supporting terrestrial digital-TV reception

● Portable DVD viewers

● Diskless portable movie players

● PDA viewers

The new generation of mobile-phone devices is converging with the role of the handheld personal digital assistant (PDA). These mobile phones are widely available and have cameras and video playback built in. They also have address books and other PDA-like applications, although these may be less sophisticated than those found in a genuine PDA. Some services were being developed for so-called 2.5G mobile phones, but now that the genuine 3G phones are shipping, they will likely replace the 2.5G offerings.

H.264 on Mobile Devices

H.264 is designed to be useful for mobile devices and consumer playback of video. Rolling this standard out for some applications must take account of the installed base of players, and that will take some time. So it is likely that, initially, H.264 will be used primarily as a mobile format.

The Ultimate Handheld Device

Taking the capabilities of a portable TV, DVD player, PDA, and mobile phone and integrating them into a single device gets very close to the ultimate handheld portable media device. Well it might, if the capabilities of a sub-notebook computer are included.
Afair use policy is now required for consumer digital video that allows us to transfer our legitimately purchased DVD to a “memory card” or other local-storage medium. These memory cards are useful gadgets to take on a long journey to occupy us as we travel, but the content owners are not comfortable with us being able to make such copies.

There are still issues with the form factor for a handheld device like this. To create a viewing experience that is convenient for long journeys, we might end up with a device that is a little bulkier than a phone should be. Maybe a hands-free kit addresses that issue or possibly a Bluetooth headset. Usable keypads increase the size of these devices. Currently, it is quite expensive to provide sufficient storage capacity without resorting to an embedded hard disk. That tends to reduce the battery life, so we might look to the new storage technologies that are being developed. Terabyte memory chips based on holographic techniques may yield the power-size-weight combination that is required. Newer display technologies such as organic LED devices may offer brighter images with less power consumed. Cameras are already reduced to a tiny charged cathode device (CCD)
assembled on a chip, which is smaller than a cubic centimeter.

The key to this will be standardization. Common screen sizes, players, video codecs, and connection protocols could enable an entire industry to be built around these devices. Open standards facilitate this sort of thing, and there are high hopes that H.264 (AVC) and
the other parts of the MPEG-4 standard will play an important role here.

3GPP home page: http://www.3gpp.org/

Personal video recorders (PVRs) are often generically referred to as TiVo, although they are manufactured by a variety of different companies. Some of them do indeed license the TiVo software, but others do not. Another popular brand is DirecTV.

Analog Off-Air PVR Devices

A classic TiVo device works very hard to compress incoming analog video to store it effectively and provide trick-play features. The compression quality level can be set in the preferences. The compromise is space versus visible artifacts. At the lowest quality, the video is fairly noisy if the picture contains a lot of movement. This is okay if you are just recording a program that you don’t want to keep forever—for example, just a time shift to view the program at a different time. If you want to record a movie, you will probably choose a higher-quality recording format than you would for a news program.

The functionality is broadly divided into trick-play capabilities and a mechanism to ensure that you record all the programs you want to, even if you do not know when they were going to be aired.

In the longer term, these devices scale from a single-box solution up to a home media server with several connected clients. This would be attractive to schools for streaming TV services directly to the classroom. University campus TV, hospital TV services, and corporate video-distribution services are candidates. Standards-based solutions offer good economies of scale, low thresholds of lock-in to one supplier, and good commercial opportunities for independent content developers.

Digital Off-Air PVR Devices

When digital PVR devices are deployed, recording television programs off-air becomes far more efficient. In the digital domain, the incoming transport stream must be de-multiplexed, and packets belonging to the program stream we are interested in are stored on a hard disk. The broadcaster already optimally compresses these streams. Some storage benefits could be gained by transcoding them. Note that we certainly cannot add any data back to the video that has already been removed at source.

Future development work on PVR devices will focus on storing and managing content that has been delivered digitally. This is within the reach of software-based product designs and does not require massive amounts of expensive hardware.
There are complex rights issues attached to home digital-recording technology, and it is in constant evolution.

TiVo: http://www.tivo.com/ DirecTV: http://www.directv.com/

Mobile PVR Solutions

Another interesting product was demonstrated by Pace at the International Broadcasting Convention (IBC) in 2004. It was a handheld PVR designed to record material being broadcast using the DVB-H mobile-TV standard. Coupling this with the H.264 codec and a
working DRM solution brings us very close to a system that could be rolled out very soon. Provided rights issues and the content-delivery technology can be developed at the front end, products such as the PVR2GO could be very successful.

The technology that enables PVR devices is getting cheaper, and the coding techniques are pushing the storage capacity (measured in hours) ever upward. Nevertheless, not every household will want to own a PVR. In addition, the higher end of the functionality spectrum may only ever be available to users with a lot of disposable income. Some of the basic functionality may just be built into a TV set. As TV receivers are gradually replaced with the new technology, they ship with video compression and local storage already built in.

Pause and rewind of live video, for instance, is very likely to be built into TV sets, and for it to be manufactured cheaply enough, the functionality will be implemented in just a few integrated circuits and will then be as ubiquitous as the Teletext decoders found in European TV sets.

Broadband connectivity is penetrating the marketplace very rapidly—perhaps not quite as fast as DVD players did, but in quite large numbers all the same.
A critical threshold is reached when video codecs are good enough to deliver satisfactory video at a bit rate that is equal to or less than what is available on a Broadband connection. Indeed, H.264 encoding packed into MPEG-4 multimedia containers coupled with a PVR storage facility and a fast, low-contention broadband link is a potential fourth
TV platform that offers solutions to many of the problems that cannot be easily solved on the satellite-, terrestrial- and cable-based digital-TV platforms. MPEG-4 interactive multimedia packages could be delivered alongside the existing digital-TV content in an MPEG-2 transport stream. Indeed, the standards body has made special provision to allow this delivery mechanism, and MPEG-4 itself does not need to standardize a transport stream because there is already one available.

On with the Journey

So we have some ideas now about the kind of platform the video we compress might be played on. This helps us to assess the best way to encode it. We also know something about the target size and the delivery mechanism. That will help us choose players and codecs.

Now we can consider the scenario from the other end. The material we use will be sourced in a range of formats, and we need to know something about how each one works.