2017年转码器买家指南

当涉及到在任何设备上查看内容, 世界上任何地方, 市场宣传常常超出实际情况. 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.

这就是转码的由来. 在基本层面上, 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. 如果编码器捕获原始信号并对其进行分组, 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.

转换器怎么样??

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转PAL, 将内容穿插到渐进式框架, or even analog VGA to HDMI—since they don’t actually packetize the video into segments or chunks that will be delivered via IP.

也就是说, 有一些转换器具有内置编码或转码功能, mainly for real-time delivery within a local area network (LAN) for image magnification, 指挥与控制, 或者数字标牌. Even those products fall outside the realm of wide-area networks (WANs) or even internet-based video streaming.

统一通信和实时转码

有一个领域我们在最近几期的 流媒体 magazine is the homogenization of enterprise video and its broader-scoped sibling: unified communications. 在这两个领域中，转码都起着核心作用.

Consider for a moment the need to live-stream a CEO all-hands meeting. 对于那些无法参加的人，传统的H.264编码和流式传输就足够了. 然而,, 对于那些可能无法在网络上查看内容的人, 可以建立纯音频通话. 大多数音频系统使用G.279或G.711 audio compression, while most audio streaming systems use other audio codecs.

播放音频或视频点播文件也是如此. 快速浏览关于堆栈溢出的设计和工程问题 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, 因为ffplay认识G.711 . but not.729年流.

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 软件 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).

视频转码, 另一方面, 通常仍然需要专门的硬件和专用的软件.

媒体服务器怎么样??

一些转码功能已经转移到网络核心, 虽然, in part due to the additional mission-critical role that media servers have been tasked with.

几年来, 媒体服务器已经能够执行简单的转码, 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, 又名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). 的代码转换, 无论是在媒体服务器上还是通过独立的转码器, 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).

除了, 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.

桌面转换器已经

These 软件 transcoding tools typically are either built into the operating system, 都是开源的, 或者与视频编辑工具捆绑使用.

Built-in transcoding options are often limited to a very specific set of options. 苹果的集成QuickTime播放器, 直到最近, 是否有大量的选项来保存各种格式的内容. 最新版本, 虽然, only offer options for transcoding or resizing to meet the form factors and resolutions of other 苹果 devices such as the iPad or iPhone.

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