19 ADVERTISEMENT FEATURE AWS Elemental
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which is more than 20 years old. The crucial goal in codec development is to achieve lower bitrates and smaller file sizes while maintaining similar quality to the source. Until fairly recently, most video encoding was performed using specially built hardware chips. AWS Elemental pioneered software-based encoding, which runs on off-the-shelf hardware. HOWVIDEO COMPRESSIONWORKS To reduce video size, encoders implement different types of compression schemes. Intra-frame compression algorithms reduce the file size of key frames (also called intra-coded frames or I-frames) using techniques similar to standard digital image compression. Other codec algorithms apply interframe compression schemes to process the frames between key frames. For standards-based codecs, AWS Elemental builds and maintains its own algorithms that it uses to analyse I-frames and the differences in subsequent frames, storing information on the changes between these frames in predicted frames (P-frames) and processing only the parts of the image that change. Bi-directional predicted frames (B-frames) can look backward and forward at other frames and store the changes. Once compression has been applied to the pixels in all the frames of a video, the next parameter in video delivery is the bitrate or data rate – the amount of data required to store all the compressed data for each frame over time, usually measured in kilobits or megabits per second (Kbps or Mbps). Generally speaking, the higher the bitrate, the higher the video quality – and therefore, the more storage required to hold the file and the greater the bandwidth required to deliver the stream. LIVE AND VOD ENCODING AWS Elemental compression products are divided into two different categories – live encoders and video-on-demand encoders. Live encoders take in a live video signal and
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encode it for immediate delivery, in cases such as a live concert or sporting event, or for traditional linear broadcast channels. While live encoders are limited to analysing and encoding content in real time, VOD encoders that use file-based media sources are not restricted to processing the content in real time. VOD encoders may take multiple passes at the video, analysing it for motion and complexity. For example, explosions in an action movie might require more effort to encode than a talking-head interview. Constant bitrate (CBR) encoding encodes the video at a more or less consistent bitrate for the duration of a live stream or entire video file. Variation occurs because different frame types will have different data rates, and because buffers on clients allow a small amount of variability in the bitrate during playback. Variable bitrate (VBR) encoding means the video bitrate can vary greatly over the video’s duration. A VBR encode spreads the bits around, using more bits to encode complex parts of content (like action or high-motion scenes) and fewer bits for simpler, more static shots. In either CBR or VBR encoding, the bitrate target is followed as closely as possible, resulting in outputs that are very close in size. Overall, especially at lower average bitrates, VBR-encoded video is going to look better than CBR-encoded video. Another factor with VBR is that the encoder needs to be configured so that the peak data rate isn’t set so high that the viewing device can’t process the stream or file.
Quality-defined variable bitrate encoding (QVBR) – a new encoding mode from AWS Elemental – takes VBR one step further. For live or VOD content, QVBR brings sophisticated algorithms to consistently deliver the highest-quality image and the lowest bitrate required for each video. Simply set a quality target and maximum bitrate, and the algorithm determines the optimal results no matter what type of source content you have (high action, talking heads, and everything in between). OPTIMISING QUALITY For premium content, such as a television series or movie, AWS Elemental Server on- premises or AWS Elemental MediaConvert on the AWS Cloud encodes video files to the highest quality. On-demand services may employ a fleet of AWS Elemental Servers, or just use the auto-scaling MediaConvert service to process VOD assets. The codec of choice for delivering video evolves every few years and each advance brings with it improved video quality at lower bitrates. At the same time, screen sizes and resolutions get bigger, so it takes considerable computing power to generate compressed files with high video quality. Until recently, H.264 (also known as AVC) was the best codec for optimising quality and reducing file sizes; many content publishers still use it today. But it’s quickly giving way to HEVC: High-Efficiency Video Coding. It’s also referred to as H.265; different standards groups employ different designations to refer to the same standard. HEVC requires more computing power than H.264, but it’s more efficient. As AWS Elemental writes its encoding software from scratch, rather than relying on off-the-shelf encoding software, it can take the efficiency gains in each new generation of codec and adjust the software accordingly to get the best quality and/or smallest video file sizes, as well as the shortest encoding times.
EVOLUTION OF CODECS
BIT BY BIT Despite the evolution of codecs over the last 20 years, some broadcasters are still using MPEG-2 with its high bitrate
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