如何选择视频采集卡

本文首次发表于2007年 流媒体行业资料手册. 订阅 流媒体 杂志并免费获得电子版 原始资料,点击 在这里

Just when it seems safe to make a video capture card purchase, something new seems to come along. What’s a content creator to do, and what criteria should one assess to make the best purchase? 在计算机功能更加强大的今天，这种捕捉卡也是必不可少的? 本文将提供几个可供考虑的选项.

正确评估内容捕获, we’re going to break down the field into three areas: live capture and transmission; asynchronous capture and delivery (live capture and encoding for later playback); and edited on-demand content—in other words, 可以分段创建的内容, 编辑在一起, 然后是消费产出.

实时捕获和传输
This area is the most misunderstood and also the one with the least amount of innovation, 至少最近几年是这样. Perhaps the lack of innovation is due to financial difficulties facing particular industry leaders or the simple fact that we’re in a transition from standard-definition content to high-definition content as the primary medium. One thing is certain, though: Live streaming has never been easier or more cost-effective to do.

正确评估直播捕获卡, one must consider three areas: the types of inputs—both audio and video—that will be captured; the format or formats and bit rate or bit rates that must be generated; and the decision regarding whether all content will generate from a single server or be mirrored on multiple servers. 让我们来看看这三个方面.

输入
GIGO(垃圾进垃圾出). 即使有高质量的相机作为采集设备, too many content creators settle for the use of composite inputs and substandard cabling for use in their streaming media projects. The rule of thumb for analog capture is that every 3 decibels of noise (snow, 工件, etc) results in a doubling of the bit rate required to achieve equivalent quality. The converse is also true: eliminate noise or artifacting in the acquisition device, and the bit rate will fall while the compressed content quality remains high.

考虑使用带有3-CCD(或更新的CMOS)捕获芯片组的相机, and then use either S-video or component video outputs to connect the camera to the streaming media card’s input. Better yet, with the advent of digital video cameras—especially those that use USB 2 or FireWire/i.Link connectors—consider connecting directly to the capture card or the computer if it contains internal FireWire connectors in order to maintain a direct digital path from the camera to the streaming software. 在高端相机上, the option to use SDI (serial digital interface) should also be considered, 因为这是提供在几个流捕获卡. A note of clarification, though: with the exception of very new cameras that use the H.264编解码器, any content captured with a digital camera will still be re-encoded with a streaming codec. The reason for this re-encoding is strictly a matter of bandwidth: most digital video cameras capture at 25 megabits per second, or almost 50 times the average sustained bandwidth of a consumer’s cable modem or DSL.

On the audio side, if possible, don’t use the on-camera microphone for audio capture. Doing so adds additional extraneous audio noise that is very difficult for the audio codec to translate, and extremely distracting to consumers when they listen through low-end desktop or laptop speakers. 而不是, 放置一个外部麦克风，尽可能靠近视频的主题, 必要时使用无线麦克风.

输入 on streaming audio boards and analog-to-USB audio capture devices typically fall into two categories: balanced and unbalanced. Unbalanced connectors are typically RCA connectors or the noise-prone 1/8" (3.5mm) stereo jack—the same types of connectors used on VCRs or headphones, respectively. 平衡连接, 另一方面, 通常是3针XLR连接器, 专业麦克风上使用的连接器类型. The XLR connector is normally attached to a streaming media capture card by way of a breakout cable, 因为连接器本身太大，不能安装在标准PCI卡上. Some cards will also generate enough power via the XLR connector to power an external microphone through a process known as phantom powering.

格式和比特率
当正确的连接器已经确定, the next step in live capture is to determine the codec and bit rate of the content that will be streamed. The use of multiple 编解码器 and bit rates used to require a 1:1 ratio of inputs to capture cards, 以及大量的外部齿轮. 幸运的是, companies such as Viewcast have created software solutions like SimulStream, which allows multiple bit rates or even different 编解码器 to be simultaneously captured and streamed from a single video and audio input. 在写这篇文章的时候, 最流行的实时编解码器是WindowsMedia, QuickTime和Real, but the recent advent of a live streaming SDK for the On2 VP6 codec (better know as Flash Video 8) will probably propel that format to one of the top three 编解码器 required for live streaming, especially given the fact that the installed base of Flash players far exceeds the number of installed Real Players.

交付
一旦输入, 编解码器, 比特率已经确定, the last step in the live streaming scenario is to choose whether to deliver directly from the streaming capture device or to offload delivery to other servers that have more robust streaming and bandwidth capacity. This decision is typically based on three key criteria: number of simultaneous users, 所选编解码器的数量, 以及捕获时可用的处理器功率/带宽.