Buyer's Guide: Live Transcoders
This article appears in the February/March issue of Streaming Media magazine, the annual Streaming Media Industry Sourcebook. In these Buyer's Guide articles, we don't claim to cover every product or vendor in a particular category, but rather provide our readers with the information they need to make smart purchasing decisions, sometimes using specific vendors or products as exemplars of those features and services.
We covered live encoders in another Buyers' Guide but only touched briefly on live transcoding. In this Buyers' Guide, we'll delve into the options for hardware- and server-based live transcoders.
In the broadcast world, where live transcoding has been a staple for almost forty years, the process of converting from one format or signal type to another is often referred to a format conversion. In the analog transmission era, a PAL signal might have been converted to NTSC or SECAM for broadcast across borders or geographies. For live events, this format conversion meant converting both the frequency and resolution of the signal.
Not much has changed in the world of digital over-the-air (OTA) transmission. Before the widespread adoption of HDTVs that had a 16:9 aspect ratio, format conversion included both frequency conversion and aspect ratio changes, to accommodate the large number of legacy televisions in the 4:3 aspect ratio. These digital format convertors are still a large part of OTA delivery, and play a part in the world of live online streaming video.
For streaming video, format conversion is coupled with packaging and segmentation, conversion of one codec type to another and potentially even from one frame rate to another.
What are some of the key features to look for?
1. How many codecs, formats, resolutions and/or bitrates can be transcoded in real time? While many hardware-based transcoding solutions are capable of real-time conversion from one format or codec to another, there are a limited number that can do multiple simultaneous transcodes.
The rule of thumb seems to be that most dedicated format convertors can do two conversions, choosing from a combination of frame rate, output resolution, aspect ratio, and color-space shifting. These dedicated convertors can also handle comb filters, and some will also handle the pre-processing requirements for accompanying audio signals.
Conversely, as we see in the buyer's guide on page NN, there are some hardware-software solutions that are much lower in price but work equally well as long as the production workflow doesn't require real-time conversion.
2. How critical is real-time conversion? Live hardware transcoders, especially those that work in format conversion, are known for latencies measured in the milliseconds. For streaming, the option for server-based live transcoding also exists, where significant additional firepower can be thrown at the transcoding step. The few available server-based solutions are able to handle everything-including frame rate, output resolution, aspect ratio, and segmentation-but require the additional latencies that streaming solutions have grown accustomed to.
3. Can more than one input be transcoded in real time? A format convertor, in order to deal with the computationally heavy pre-processing, will convert one signal type to another signal type. On the other hand, those live transcoding solutions that have IP inputs often have enough horsepower to handle multiple transcodes of multiple signals. One example of this would be a live transcoding box that receives in MPEG-2 Transport Streams, demuxes the content (e.g., MPEG-2 codec content), transcodes the content into H.264, and then re-packetizes it into another MPEG-2 Transport Stream for re-transmission.
4. How important is a single-purpose device? In terms of server-based or appliance-based transcoding solutions, the question of single-purpose device is also an essential consideration. For those involved in broadcast or high-profile events, a single-purpose device that can be dedicated to a task is probably the most appealing. There's just something about an on-off switch and the instant response of a dedicated appliance that warms the heart of a broadcast engineer in the way that a server-based solution never can.
On the flip side, a single-purpose device is often locked into a point in time, with enough horsepower to do the task at hand but limited ability to be upgraded to handle additional formats, codecs, or adaptive bitrate scenarios, as we'll see in the next point. Resellers of broadcast equipment can show you warehouses full of outdated equipment that, once serving their intended single purpose, were jettisoned to make way for the next shiny black box capable of providing an incrementally upgraded capability.
5. Is adaptive bitrate a required part of the production workflow? The last example brings up another key part of live transcoding: the potential need for adaptive bitrate delivery. While this is traditionally thought of for live encoding, adaptive bitrate in its various competing formats is fast becoming a necessity for any type of content, including on-demand content that has been stored in a mezzanine file format. Server-based live transcoding solutions will receive a request from the end user's player, determine the necessary resolution and bandwidth requirements, and then transcode/segment the mezzanine file to the appropriate output for the client device.
You might ask why this isn't done beforehand, saving the media server from the additional processing at the time of content request. While it is often done this way for current content, many long-tail videos have not been converted over for adaptive bitrate delivery. Rather than telling the end user that a piece of content is not available, the real-time transcoding allows that long-tail video content to be served up right away. Additional business rules at the media server level can be enforced, keeping a copy of the transcoded content for a set period of time, to avoid the need to transcode for every single request.
Another reason to stay with a high-quality mezzanine file and then transcode on the fly has to do with the competing adaptive bitrate solutions from Adobe, Apple, Microsoft and the newly ratified DASH standard. Until the adaptive bitrate landscape stabilizes, real-time transcoding offers a way to maintain a single asset that is then turned into various delivery versions at the time of request.
To wrap up this buyer's guide, let's consider one final factor: cost. The cost of a dedicated, single-purpose format convertor often is higher than a server and software solution, although the price of appliance format convertors has dropped in recent years. The initial cost alone, though, can't be the sole decision point, especially if a software-based solution gives way during a major event. That forty-second lag while a software solution is being rebooted might just be the difference between a job well done and a job long gone.
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