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It is possible to find video cards with digital inputs (DV and 601) and these do offer the potential for higher quality encoding. The video cards also include a preview output so that the encoded files can be viewed on a conventional television monitor. Many video cards are designed as platforms to support nonlinear editors, so they will have many additional facilities that are not required for a straight encode. In an ideal world the streaming format would be encoded directly from a 4:2:2 source (at 270 Mbit/s); in fact, this is the way that DVDs of motion pictures are encoded. In this way the studios can ensure the best possible picture quality for the DVD release. To get the best possible encoding, a digital source should be used, with a clean picture and a low level of noise. If a 601 source is not available, DV can be used as an alternative. The mild DCT (discrete cosine transform) compression used by DV should not produce significant artifacts visible in the final stream. The second best is an analog source; here Y/C is preferable to composite. Analog sources exhibit more noise, which potentially can use up bandwidth at the compression stage. Composite signals are first decoded, and there will be visible artifacts that will produce a slight impairment of the final encoded quality (called cross-color and cross-luminance). There are different ways of compressing; the intermediate AVI may be at the same resolution as the original video, or it may be scaled to the target size. Each method has advantages and disadvantages. It is a trade-off between video quality and data rate. If many different output formats are required, an intermediate file may have to be stored for the duration of the encoding process. This file should be at least the size of the largest encoded picture, if not larger. So if you are encoding to 160 120 and 192 144, you may want the intermediate file at SIF resolution 352 240. The AVI file may even be archived. But the higher the quality, the higher the storage costs. The other consideration is the processing time, as a high-resolution file will take longer to process. The processing time can be important for two reasons. First, it could impact the throughput of work in a busy encoding center. Second, with live feeds the encoding has to take place in real-time, so the lower the resolution, the less CPU power required for the codecs. VTR control It is much simpler to encode a tape if the video capture application has a remote control facility for the VTR. This means that you do not have to start the capture on the fly, plus it allows batch encoding to a prepared schedule. Most professional tape decks can be controlled via an RS-422 port (DV decks also can be controlled through the IEEE-1394 interface). The port allows full transport 162 The Technology of Video and Audio Streaming

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There are different interconnection formats for video to add to all these formats: Analog composite (NTSC, PAL) Y/C or S-video (analog) Digital component (601, 4:2:2) 270 Mbit/s (SDI) DV (FireWire, IEEE-1394) 25 Mbit/s, 5:1 compression As you can see, encoding is partly video format conversion. Interconnections Interconnection formats include analog composite and component, and digital component. All require processing to arrive at the final AVI file format. To this end the video capture card has several components: a composite decoder, analog-to-digital conversion, DV decompression, and possibly hardware scaling. If modern digital tape decks use component recording, why do most video cards have analog composite inputs? For historical reasons analog composite has become a standard for interconnections and is still in very common use. Video encoding 161 decode demod decode select ADC Y/C YUV NTSC /PAL S-VHS DV 601 ANALOG Analog Sources Digital Sources Digital File DIGITAL YUV YUV YUV YUV AVI Figure 8.4 Capture card blocks.

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160 The Technology of Video and Audio Streaming switcher workstation network option to streaming server video camera workstation network to content server VCR encode workstation network clips clips files to streaming server non-linear editing workstation editing file server file transfer from editing system encoding from tape live encoding Figure 8.3 Video capture systems.

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Most video content is edited into a finished program before conversion to a streaming format. The final cut can be dubbed onto videotape to transfer into the streaming encoder, but it is often simpler to transfer the program as a file directly from the editing system to the streaming media processor over a LAN. Video capture Most streaming media content is created in a conventional television format, either direct from a camera or as an edited program on videotape. Live streaming is a special case of encoding and places more exacting demands on the processing hardware. The first step is to convert the television signal as a computer file format, often Audio-Video Interleaved (AVI). In the case of a live event the capture will take place at the venue. With videotape the capture usually will be at the encoding center. It is also possible to ingest files directly from nonlinear editing systems. The first problem you will encounter with video is that there is a large number of different formats that you might want to encode: analog or digital, component or composite. Videotape also comes in many formats, as shown in Table 8.3. This list is by no means exhaustive; there are older analog composite formats (1-inch) and a number of other digital formats (D-1, D-3, D-5, D-9). The source material may even be supplied in a high-definition format like HD-CAM. I haven t mentioned audio-only sources. Again, these can be analog or digital, and delivered on tape, cassette, and mini-disc, or burnt onto a CD-ROM. Video encoding 159 Table 8.3 Tape Formats Tape width Name Color encoding Notes 3/4 inch U-matic Analog Y/C Archive material 1/2 inch VHS Analog Y/C Consumer format S-VHS Analog Y/C Betacam (SP) Analog Y/C Digital Betacam Digital YUV Beta SX Digital YUV 8 mm DV Digital YUV DVC-PRO Digital YUV DVCAM Digital YUV

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158 The Technology of Video and Audio Streaming VCR Videotape Streaming file Workstation PCI bus Video card AVI Video Figure 8.2 Encoder system. compact streaming format. Decompression then takes place in a compatible decoder, or media player, installed in the end-user s computer. The process can be likened to zipping and unzipping a file on a PC. Codecs can be symmetrical or asymmetrical. A symmetric codec usually requires less processing and is more efficient. Streaming media codecs are usually asymmetrical; this means the encoding process requires much more processing than the decoder. Since only one encoder is required, but all the viewers need a decoder, it makes sense to put the complexity in the encoding end. The players usually are downloaded over the Internet, so the executables must be kept to a reasonable size very large downloads are not popular with the public. Ingest station The video capture can be carried out on a separate ingest station ahead of the compression process and the intermediate AVI file stored on a server. These files can be very large so you will need a generous-sized array of disk drives. Also beware of the 2 GByte limit for the file size for the original AVI format.

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de-facto standard for computer audio and video files. From this point on, regular desktop computer technology now can be used for downstream processing. The AVI files themselves cannot be delivered over the Internet for two reasons: one, AVI files are usually very large, each second of video can be more than 1 Mbyte of data; two, AVI files can t be streamed in real-time over the Internet. You must download the entire file to a local drive before you can play it. Hence the need for compression and packetization. Once captured, the AVI file is routed to the encoding software. The encoder uses a software codec to convert it to a streaming format (typically MPEG-4, Windows Media, Real, or Apple QuickTime). Finally, the streaming file is uploaded to a server for delivery over the Internet. If you are making a live webcast, this process has to take place in real-time. The computer will need a minimum level of processing power; this varies from codec to codec. Codec Codec is short for compression/decompression. Codecs are software programs that employ a mathematical algorithm to squeeze the media files into a more Video encoding 157 270 Mbit/s uncompressed 4 Mbit/s MPEG-2, SD-TV 1.5 Mbit/s MPEG-1 500 kbit/s ADSL, cable modem 56 kbit/s analog modem Figure 8.1 Relative file sizes.

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156 The Technology of Video and Audio Streaming Table 8.2 Channel Data Parameters Uncompressed SD broadcast ADSL or Analog SD video source television cable modem modem Frame size 720 480 720 480 192 144 160 120 Frame rate 30 30 15 5 Color sampling 4:2:2 4:2:0 YUV12 YUV12 Video source rate 166 Mbit/s Uncompressed data rate 124 Mbit/s 5 Mbit/s 1.15 Mbit/s after scaling Target data rate 4 Mbit/s 500 kbit/s 35 kbit/s Total data reduction to 40:1 330:1 4700:1 meet target rate Scaled data rate 1:1.33 1:33 1:144 Compression from scaled 30:1 10:1 30:1 rate to target rate Note: SD is standard definition. The table shows how scaling down before encoding reduces the uncompressed data rate. A thumbnail video for a dial-up modem requires that the data rate is reduced to nearly 1/5000 of the original. This is accomplished by scaling down by a factor of 144:1, and then compressing by a further 30:1. Figure 8.1 shows the relative file sizes for encoded media. Data compression The raw data rate from a standard definition television camera is 166 Mbit/s. This has to be reduced to rates as low as 30 kbit/s for delivery to dial-up modems. How is a reduction in video data of almost 5000:1 achieved, while retaining a viewable picture? This reduction takes place in several steps. The scaling usually takes place on the video capture card. The compression is performed by the codec, a software application running on the main processors. The usual way to encode is first to capture the video with a dedicated PCI card installed in the encoding workstation. The card converts the video signal into an intermediate format that can be handled as a computer file, usually one of the Audio-Video Interleaved (AVI) formats. AVI has become the

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Distribution channels A broadcast teleproduction usually is created in a single format. If other formats are needed for distribution, then standards conversion will be used, but as a separate downstream process. In contrast, when encoding media for streaming it is common to produce multiple output formats. You may be called upon to encode up to 20 different versions of a video clip. These are files at different stream rates, each rate repeated for a number of different streaming codecs. Television has an allocated bandwidth, and the MPEG-2 compression is configured to fit. It may be variable bit rate, but within narrow limits; for standard definition it could be 2 to 6 Mbit/s. The channels used to deliver streams vary from 3G wireless, dial-up modems, to E-1/T-1 lines. Video encoding 155 Table 8.1 Typical Distribution Channels Channel data rate Guaranteed Notes bandwidth 601 270 Mbit/s Yes Uncompressed, standard definition video Digital television 4 6 Mbit/s Yes Variable bit rate T-1 (E-1) 1.5 Mbit/s (2 Mbit/s) Yes DSL 144 kbit/s 8 Mbit/s No Cable modem 128 kbit/s 1 Mbit/s No Dual ISDN 128 kbit/s Yes ISDN 64 kbit/s Yes Dial-up modem 56 kbit/s No Typical capacity 35 Kbit/s In Table 8.1, the digital television example is the standard definition television that is broadcast to the DVB standard, and uses MPEG-2 MP@ML coding. It is very similar in quality to DVD video. Some channels offer a guaranteed bandwidth; others are dependent upon the capacity of the local loop, plus the contention ratios within distribution nodes. Typically the local ends offered to the consumer do not have a guaranteed quality of service (albeit they are at a much reduced cost compared to the traditional telecoms circuits). To encode video for all these different applications, the video is delivered with a range of basic parameters. Table 8.2 shows some typical data rates for the video alone, excluding audio and control data.

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8 Video encoding Introduction Encoding is the conversion of a video signal into a streaming media file. The video signal first must be compressed, then packetized, and finally encapsulated within a streaming wrapper. This chapter covers the encoding stage from a video signal to the point where the streaming file is transferred to the streaming server. The encoding takes place in several stages. The first stage is to capture a conventional video or television signal and convert it to a file format that can be processed by computer software. The second stage is data rate reduction by scaling and compression to a bit rate that can be delivered over dial-up or broadband circuits. The third stage is to wrap the compressed video in a packetized format that can be streamed over an IP network. The majority of developments that have led to the rapid adoption of streaming media lie at the second stage, compression. There is no standard way of encoding; there are many different ways to get from video to a streaming format. The route that you choose will depend upon the hardware platform that you have selected, the required throughput of encoded material, and the final viewing quality that you require. This chapter describes some typical architectures, but is not intended to be prescriptive. To find the optimum solution for your application, you may want to undertake comparative trials between some different solutions. It is quite possible to assemble your own system from PCs, video capture cards, and suitable choice of software. Alternately a shrink-wrap solution can be purchased. Do you require the best quality at very low bit rates? Do you encode a few hours a week, or do you have a large archive to put online? All these factors have to be considered before deciding upon an encoding solution. Broadly speaking the price will rise with performance and with the convenience of operation. Some high-end applications allow automated batch encoding. If the encoder will be used in a integrated workflow with other systems, how easy is it to interface with the encoding process?

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tangular natural video picture with associated audio, its object-based approach has great potential for the creative content developer. You must never forget that content is for the users, not the creators. It is vital that the complex technology application is transparent to the potential consumer. The early viewers of streaming media may have overcome any technical obstacles, but the bulk of the public will not tolerate quirks or unpredictable behavior if streaming is to be a mainstream media. It must be as easy as watching television or a DVD. With careful HTML design, you can make smart decisions about which file format to direct a browser to by detecting the plug-ins installed in the browser. Standardization often is put forward as the answer to some of these issues. If there was one player that could render any media format, perhaps using proprietary plug-ins, all would be simple. After all, we can receive a television broadcast from any station without needing a special receiver. The flip side to this is that it is extremely unlikely that streaming could have developed from inception to be a mainstream communication medium in the period of only five years. The proprietary architectures can continuously improve without the constraints of standards, and are spurred on by intense competition. Whether this situation continues as streaming moves out of the PC environment into general consumer devices remains to be seen. There is far more pressure to standardize when streaming is deployed to set-top boxes or wireless devices. The life of a television receiver or set-top box can be 10 times that of a software release, so there will be a drive to standardize. Introduction to streaming media 153

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