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September 19, 2011

Codecs: A Brief History

Written by Jeff Sobel

 

If you’ve worked in video you’ve certainly been in contact with codecs.  Probably many codecs, in fact.  So what is a codec?  Codec is an abbreviation for “Compressor-Decompresor” or “Coder-Decoder” (technically speaking, it’s a portmanteau, an amalgamation of two words (a favorite tool of tabloid sites who live to gossip about ScarJo or Brangelina), but i digress).

The vast majority of digital video is processed by a codec, each of which has its strengths and weaknesses.  In the broadest sense, there are two types of codecs:  Lossy and Lossless.  Almost all commonly used video codecs today are lossy, but many “lose” so little information in the encoding process that the resulting video appears identical to the original video source under normal viewing conditions, yet achieves much lower bandwidth than uncompressed video.  Another upper-level classification of codecs is “Intraframe” or “Interframe.”  Intraframe codecs compress each frame of video discreetly, while interframe codecs look at sequences of frames and compress them in groups by looking for the elements in the group that change from frame to frame (anything in motion, for example) and applying more bandwidth to encoding those elements while omitting the repetitive data inherent in the static elements (backgrounds, for example) of the group (groups of frames are referred to as “GOPs” (Group of Pictures).  There are pros and cons to both types of encoding and this frequently results in choosing different codecs for capturing, processing and delivering video content (more on that in a later blog).

The first popular consumer digital video cameras used the DV codec, which worked in standard definition and lossily compressed video at a ratio of about 5:1, resulting in a 25mbps bandwidth that could be written to inexpensive tapes.  The DV codec was spawned into more advanced flavors including DVCAM, DVCPRO, DVCPRO50 (which doubled the bitrate to 50mbps for high quality SD video), and ultimately DVCPRO HD, which supported 1080i and 720p HD formats and is found only in professional (typically shoulder-mount, ENG-style) cameras.  All codecs in the DV family are intraframe, allowing for quick editing on the relatively low-powered computer systems available at the time these codecs were popular (the dawn of consumer video production, roughly 2000-2006).

As prosumer and consumer HD cameras hit the market, HDV became the most common recording format.  HDV is a flavor of MPEG-2 compression, the same type of compression used to encode video for DVDs and digital cable television signals.  Because of HDV’s high compression ratio it is able to pack 4.5X the number of pixels (1440x1080 vs 720x480) into the same 25mbps bitrate that DV used, allowing HD quality video to be recorded to the same inexpensive DV tapes.  MPEG-2 is an interframe codec, not ideal for editing, but highly-useable for most consumer & prosumer producers who aren’t doing heavy post-production processing.  HDV was at the forefront of the tsunami of popularity for handheld HD cameras.  Many modern prosumer cameras still use MPEG-2 codecs today that are very similar to HDV, such as Sony’s XDCAM EX series which record in a codec closely related to HDV, but in a higher 35mbps bitrate.

The final codec to be mentioned in this installment of the blog is h.264.  h.264 was designed to be a highly-versatile codec, capable of encoding very low-bitrate video for mobile devices and internet applications that could support limited bandwidth, but also efficiently encode very high quality, high-bitrate HD video.  As such, h.264 is used in recording and sending video in mobile phones (often at a bitrate of just 300kbps), and is also the codec used in the very high-quality Blu-Ray Disc format (typically a 40mbps video stream), and everything in between. h.264 simplified the delivery of video by providing one codec that worked very well in an extremely wide range of bitrates (previously, most codecs worked most efficiently at a small range of bitrates so were designated for specific applications).  Prosumer cameras, especially DSLRs such as the Canon 5D/7D, are also using h.264 to record video.  h.264 produces much higher quality video than MPEG-2 at the same bitrate, but due to this very high compression ratio is an undesirable codec for editing and post-production.  For this reason, most producers transcode from the source h.264 codec to a codec designed for post-production, then either transcode back to h.264 or to a 3rd codec for delivery.  We’ll talk about post-production codecs (such as Avid DNxHD and Apple ProRes) and delivery codecs in the next installments of this blog.

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Next entry: Photography in My Eye: Part 1: Glass

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