A Buyer's Guide to Format Conversion Tools

When it comes to the conversion of streaming content from one format to another, there's a fair amount of confusion, plus a decent number of terms: encoding, transcoding, transrating, transmuxing, and several others. We've covered encoding, both in hardware and software, in two separate Buyer's Guides in the Sourcebook, but let's take a look at the other three terms -- transcoding, transrating, and transmuxing -- in this Buyer's Guide.

Just like there are significant differences between hardware and software encoders, transcoding can be broken down into two groups: real-time and non-real-time transcoding. We'll look at live transcoders here -- also known as format conversion -- and then look at non-real-time transcoding in a separate Buyer's Guide.

Four Questions to Ask When Purchasing Format Conversion Tools

• Is real-time conversion necessary?
• Do you need transcoding or just transmuxing?
• Do you need to change aspect ratio?
• How many inputs must be transcoded?

Real-Time Conversion

The first question to ask when it comes to transcoding is whether live format conversion is necessary. 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 users of streaming solutions have grown accustomed to.

In addition, one must consider both the opportunity cost of live and the cost of a dedicated, single-purpose format convertor, as the latter expense is often 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 40-second lag while a software solution is being rebooted might just be the difference between a job well done and a job long gone.

Transcoding vs. Transmuxing

This is the second question around content delivery format conversion; after all, the vast majority of content created today is encoded using the H.264 codec, and much of it meets the primary H.264 profiles: Base, Main, and High Profiles, respectively, in terms of bitrate and perceived quality.

Given the use of H.264 profiles for live encoding, many uses of the term "transcoding" actually refer to "transmuxing." Transmuxing -- the multiplexing conversion of two or more discrete elementary streams -- is the process of combining the discrete streams into a single stream, segmented into manageable packet sizes, each of which is transmitted independently across the data network.

Even for newer adaptive bitrate (ABR) content formats, such as Adobe HTTP Dynamic Streaming (HDS) or Microsoft Smooth Streaming, the ability to convert from one proprietary set of fragmented MP4 (fMP4) segments to another proprietary or standards-based set of ABR segments is only a transmux, not a transcode.