Buyers' Guide to Transcoders 2017
When it comes to viewing content on any device, anywhere in the world, the marketing hype often outstrips the reality. Content publishers pick a particular compression technology (codec) based on its popularity or accessibility from a particular set of devices, but older devices—or even devices that are on a competing operating system (OS)—may be unable to stream the content.
That’s where transcoding comes in. At its basic level, a transcoder captures the stream of one type of codec and then converts it to another type of codec.
Transcoders sit squarely in the middle of the signal acquisition and viewing continuum. If an encoder captures the original signal and packetizes it, and a player receives the packets and converts them back to the original signal, then a transcoder acts as the translator between the acquisition codec and the list of codecs that the player has at its disposal for playback.
What About Converters?
We are sometimes asked about devices that will change a video signal from one format to another. These are more often referred to as converters— they’re tasked with changing DVI to HDMI, NTSC to PAL, interlaced content to progressive frames, or even analog VGA to HDMI—since they don’t actually packetize the video into segments or chunks that will be delivered via IP.
That said, there are some converters that have built-in encoding or transcoding, mainly for real-time delivery within a local area network (LAN) for image magnification, command and control, or digital signage. Even those products fall outside the realm of wide-area networks (WANs) or even internet-based video streaming.
Unified Communications and Real-Time Transcoding
One area we covered quite a bit in recent issues of Streaming Media magazine is the homogenization of enterprise video and its broader-scoped sibling: unified communications. In both of these areas, transcoding plays a central role.
Consider for a moment the need to live-stream a CEO all-hands meeting. For those unable to attend, the traditional H.264 encoding and streaming would suffice. Yet, for those who may not be able to view the content on a network, an audio-only call may be established. Most audio systems use either G.279 or G.711 audio compression, while most audio streaming systems use other audio codecs.
The same is true for playback of audio- or video-on-demand files. A quick glance at design and engineering questions on Stack Overflow shows that solutions built around audioconferencing codecs—even for fairly ubiquitous player options, like ffmpeg and its ffplay player component—require some level of transcoding, since ffplay recognizes G.711 but not G.729 streams.
Doing this level of real-time transcoding for audio codecs is much less processor-intensive than video transcoding, which means most audio transcoding is now handled in software on commodity processors (and even on some mobile phones or tablets, given the dual-core or quad-core processor offerings on today’s consumer smartphones).
Video transcoding, on the other hand, often still requires specialized hardware and dedicated software.
What About Media Servers?
Several transcoding features have moved toward the network core, though, in part due to the additional mission-critical role that media servers have been tasked with.
For several years now, media servers have been able to perform simple transcoding, such as decoding an MPEG-2 Transport Stream (M2TS) and converting the contained MPEG-2 primary video stream to a more modern streaming codec like Advanced Video Coding (AVC, aka H.264). But recent trends toward adaptive bitrate (ABR) video streaming have also put the burden of downscaling a single high-resolution stream into several lower bitrates and frame sizes.
An example of this, which is officially considered transrating, might be a live H.264 stream sent to the media server at 1080p30 (1920x1080 resolution at 30 frames per second). The transcoding, whether at a media server or via a standalone transcoder, would then convert the stream to at least one 720p30 (1280x720 resolution at 30 fps) and perhaps one standard-definition (SD) stream at 480p30 (854x480 at 30 fps).
In addition, either the transcoder or media server can perform packaging or segmentation steps, whereby the video files are broken down into very small files that can be delivered by a standard HTTP server— which scales much better than a traditional streaming protocol such as RTP or RTMP, which require a persistent session between the media server and the end-user’s player.
These software transcoding tools typically are either built into the operating system, are open source, or are bundled with a video editing tool.
Built-in transcoding options are often limited to a very specific set of options. Apple’s integrated QuickTime player, up until recently, had a wide set of options for saving content in various formats. Recent versions, though, only offer options for transcoding or resizing to meet the form factors and resolutions of other Apple devices such as the iPad or iPhone.
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