How to Choose a Video Capture Card

This article first appeared in the 2007 Streaming Media Industry Sourcebook. To subscribe to Streaming Media magazine and receive free access to the digital version of the Sourcebook, click here

就在购买视频采集卡看起来很安全的时候，一些新的东西似乎出现了. 内容创造者该怎么做，他们应该评估什么标准才能做出最好的购买? 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? This article will provide several options to consider.

To properly assess content capture, 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, content that may be created in pieces, edited together, and then output for consumption.

Live Capture and Transmission
这是最容易被误解的领域，也是最缺乏创新的领域, at least in recent years. 也许缺乏创新是由于某些行业领导者面临的财政困难，或者是我们正处于从标准清晰度内容向高清内容作为主要媒介的过渡这一简单事实. 但有一件事是肯定的:直播从来没有像现在这样简单，成本效益也从来没有这么高过.

To properly assess live streaming capture cards, 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. Let’s look at each of these three areas.

Inputs
GIGO (Garbage In, Garbage Out). Even with a high-quality camera as the acquisition device, 太多的内容创作者满足于在他们的流媒体项目中使用复合输入和不合格的电缆. 模拟捕获的经验法则是，每3分贝的噪音(雪), artifacts, 等)导致达到同等质量所需的比特率加倍. 反之亦然:消除采集设备中的噪声或伪影, 比特率会下降，但压缩后的内容质量仍然很高.

Consider using a camera with a 3-CCD (or newer CMOS) capture chipset, 然后使用S-video或分量视频输出将摄像机连接到流媒体卡的输入端. 更好的是，随着数码摄像机的出现，尤其是那些使用USB 2或FireWire/i的摄像机.链接连接器-考虑直接连接到捕获卡或计算机，如果它包含内部火线连接器，以保持从相机到流媒体软件的直接数字路径. On really high-end cameras, 还应该考虑使用SDI(串行数字接口)的选项, as this is offered on several streaming capture cards. 不过，这里需要澄清一下:除了一些非常新的相机使用H.264 codec, 用数码相机捕获的任何内容仍将使用流编解码器重新编码. 这种重新编码的原因是严格意义上的带宽问题:大多数数码摄像机的捕获速度为每秒25兆比特, 或者几乎是消费者的电缆调制解调器或DSL平均持续带宽的50倍.

在音频方面，如果可能的话，不要使用相机上的麦克风进行音频捕捉. 这样做会增加额外的无关音频噪声，这对音频编解码器来说很难翻译, 当消费者通过低端的台式机或笔记本电脑扬声器听音乐时，他们会非常分散注意力. Instead, place an external microphone as close to the subject of your video, and use wireless microphones if necessary.

流音频板和模拟到usb音频捕获设备的输入通常分为两类:平衡和不平衡. 不平衡连接器通常是RCA连接器或容易产生噪声的1/8”(3).5毫米)立体声插孔-分别用于录像机或耳机的相同类型的连接器. Balanced connections, on the other hand, are typically 3-pin XLR connectors, the type of connector used on a professional microphone. XLR连接器通常通过断线连接到流媒体捕获卡上, as the connector itself is too large to fit on a standard PCI card. 一些卡还将通过XLR连接器产生足够的电力，通过一个被称为幻象供电的过程为外部麦克风供电.

Formats and Bit Rates
When the proper connectors have been determined, 实时捕获的下一步是确定将流式传输的内容的编解码器和比特率. 使用多个编解码器和比特率，需要1:1的输入比例来捕获卡, along with a significant amount of external gear. Fortunately, 像Viewcast这样的公司已经创建了像SimulStream这样的软件解决方案, 它允许多个比特率甚至不同的编解码器同时捕获并从单个视频和音频输入流. At the time of this writing, the most popular live codecs were WindowsMedia, QuickTime and Real, 但最近出现的On2 VP6编解码器(更广为人知的是Flash Video 8)的直播SDK可能会推动该格式成为直播所需的三大编解码器之一, 特别是考虑到Flash播放器的安装基数远远超过了Real player的安装数量.

Delivery
Once the inputs, codecs, and bit rates have been determined, 实时流场景的最后一步是选择是直接从流捕获设备交付，还是将交付卸载到具有更强大的流和带宽容量的其他服务器. 这个决定通常基于三个关键标准:同时用户的数量, number of chosen codecs, and processor power/bandwidth available at the point of capture.