Transcoding 101 with Telestream, Wowza, 微软, and Rhozet

For those who have questions about how to effectively transcode their source video for online use, Telestream的代表, Wowza媒体系统, 微软, and Rhozet came together in an online roundtable hosted by StreamingMedia.com. The event provided a great deal of practical advice for both those just starting out and those with some transcoding experience.

圆桌会议由凯文·劳登开始, Telestream的项目经理, who advised that before people can transcode into a streaming format, they need to decide which format is the best choice. 目前的战斗是在H.264和WebM视频，他说.264享受更多使用. 至于决议, he suggested going with a smaller size since many devices offer good upscaling nowadays. 一个MP4 H.264 AAC file at 640x360 will play on almost anything, 他说.

For those who are looking to shave some time of their encoding sessions, 劳登建议尝试单次编码, 而不是双重传递. Using it takes about half time, so videos can be ready much faster.

查理很好, Wowza媒体系统的首席技术官, picked it up there and dug in a little deeper with encoding settings. 基线配置文件是最容易玩的, 他说, and many mobile devices only support Baseline video.

拥有正确的关键帧间隔非常重要, 说好的, suggesting an interval of between one and five seconds. A shorter interval leads to quicker startup times, 他说. Silverlight video works best with a two-second interval.

传送多比特率视频非常重要, 他说, because you don't know what network conditions your viewers will experience or what device they'll be using. 创建多比特率视频时, 关键帧对齐是很重要的, so that the player can switch between streams as needed.

亚历克斯Zambelli, a technical evangelist for the 微软 Media Platform, 对Silverlight的编码给出了建议. 用户可以编码到1080p. Smooth Streaming, Silverlight's version of adaptive bitrate delivery, is used by most people.

As for how many versions of a file a content owner should produce for Smooth Streaming, 赞贝利建议12个, 再多说就太过分了. Typically, 他说, create 4 for standard definition streaming, 6 for 720p, and 8 for 1080p. Your top quality level should be the original file's resolution. 对于最低的，不要低于288x160. For bitrates below 300kbps, 他说, it's better to half the frame rate than to lower the resolution.

最后出场的是乔恩·罗宾斯, 他是Rhozet的解决方案经理, who explained that it's important to monitor your transcoding system once you have one in place, to make sure that what you think is happening is really happening. 他还解释了转码的步骤, explaining how the audio and video portions of a file get transformed and the whole package is then reassembled.

When shopping for a transcoder, he advised, format is key. Make sure it can decode what you have and encode the format you need. 之后，看看视频的质量和速度.

When it came time for the question and answer section, viewers showed that they still had lots of basic questions about transcoding. One viewer wanted Louden to explain why he recommended single-pass encoding when double-pass produces better video. He explained that it's a good solution for organizations trying to squeeze every second out of their workflow and gain as much efficiency as possible.

Other viewers needed some transcoding terms explained. Robbins said that the mezzanine format is what the video archive is kept in, and is the master file that all other files should come from. Content owners will go back to that original when making new versions in the future.

Robbins also explained Baseline, Main, and High profiles. 它们是为硬件制造商而存在的, 他说, to give them and easy way to say what level of video they support. The Baseline profile is less complex and is typically supported by phones. Main更复杂，由机顶盒使用. The High profile uses the most processing power, so it's supported by desktop and notebook computers.

感兴趣的读者可以查看全文存档的圆桌会议在这里.