| Just when it seems safe to make a video capture card purchase, something new seems to come along. What criteria should one assess to make the best purchase? 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 this article we will discuss the issues with live capture and transmission. |
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Live Capture and Transmission
To properly assess live streaming capture cards, one must consider three areas:
1. The types of inputs—both audio and video—that will be captured;
2. The format or formats and bit rate or bit rates that must be generated;
3. The decision whether the content will generate from a single server or be mirrored on multiple servers.
Inputs
GIGO (Garbage In, Garbage Out). Even with a high-quality camera, too many users settle for 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, artifacts, 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 signal flow and the bit rate will fall while the compressed content quality remains high. |
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Consider using a camera with a 3-CCD (or newer CMOS) capture chipset, and then use either S-video or component video outputs to connect the camera to the streaming media card's input. High-end cameras come with the SDI (serial digital interface) signal flow. SDI is the new 'gold standard' for signal transmission but the higher quality is commanding a higher price on the cameras and the encoder cards.
On the audio side, if possible, don't use the on-camera microphone for audio capture. These can introduce distracting noise. Remember that most of the audience hears the program through low-end desktop or laptop speakers. Instead, place an external microphone as close to the subject of your video, and use wireless microphones if necessary.
Audio inputs on high quality streaming encoder cards 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. Balanced connections, on the other hand, are typically 3-pin XLR connectors, the type of connector used on a professional microphone. The XLR connector is normally attached to a streaming media capture card by way of a breakout cable, as the connector itself is too large to fit on a standard PCI card.
Formats and Bit Rates
When the proper connectors have been determined, the next step in live capture is to determine the codec and bit rate. The use of multiple codecs and bit rates used to require a 1:1 ratio of inputs to capture cards, along with a significant amount of external gear. Fortunately, companies such as Viewcast have created software solutions like SimulStream, which allows multiple bit rates or even different codecs to be simultaneously captured and streamed from a single video and audio input.
The most common live codecs are WindowsMedia, QuickTime and 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 codecs required for live streaming, especially given the fact that the installed base of Flash players far exceeds the number of installed Real Players.
Delivery
Once the inputs, codecs, and bit rates have been determined, 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 service is provided by a Content Delivery Network.
To deliver a video stream directly to a viewer on the other side of the internet requires two calculations: #1) The upload speed from the point of origin and #2) The download speed at the viewer's computer.
For example, Chris wants to do a video presentation to four prospects. At Chris's office, he has a cable modem service with a 2 megabit per second upload speed. Using a high quality card like the ViewCast Osprey 240, he can output 400Kbps x 4 = 1600Kbps (1.6 megabits per second). This is within the upload speed of the cable modem, leaving room for other traffic.
The second calculation is that each viewer has broadband internet access with a minimum download speed of 400Kbps. (Hence the upload calculation 4x400Kbps).
This 'streaming math' points out one of the advantages of a Content Delivery Network (CDN) provider. An organization that want to serve streaming video to many viewers probably does not have the bandwidth on hand. CDN's are built to support these large, occasional bandwidth requirements. (There will be a detailed discussion of the CDN choice process in later articles.)
The other delivery issue goes back to viewer's computer. With a high quality encoder, like the ViewCast Osprey-240, Chris is not limiting his viewers to one codec. The Osprey is capable of creating multiple codec streams at the same time.
Conclusion
In conclusion, web-casters have a wide range of encoding choices from computer-based software solutions to dedicated appliances like the ViewCast GoStream. Software-based solutions can perform well for hobby-grade web-casting but they are dependant on the power and resources of the computer. Software-based solutions also condense the signal flow to either FireWire or USB input.
High quality capture cards, like those made by ViewCast, provide multiple benefits. They are created to provide the most broadcast-like quality. The pre-processing of video and audio signals is cleaner and is not dependent on the computer's CPU. The connectors and signal flow are native to video camera output. And ViewCast gives the web-caster the flexibility to stream multiple codecs simultaneously.
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