DNxHD codec

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Avid DNxHD is a high-definition video post-production codec engineered for multi-generation compositing with reduced storage and bandwidth requirements. It was developed by Avid Technology, Inc.

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Uncompressed high definition digital video has a substantially higher bit-rate than standard definition and can require powerful computers to process and edit. Other codecs such as HDV, DVCPRO HD, AVC-Intra, AVCHD, and HDCAM use compression techniques that limit the spatial and temporal resolution of the image. While suitable for acquisition, these codecs will tend to degrade the image over the multiple encode-decode cycles that are typically required during the post-production of complex layered imagery.

DNxHD offers a choice of 8- or 10-bit depth at three user-selectable bit rates. DNxHD data is typically stored in an MXF wrapper, although it can also be stored in Quicktime or AVI wrappers. A standalone Quicktime codec for both Windows XP and Mac OS X is available to create and play Quicktime files containing DNxHD material.

DNxHD is intended to be an open standard, but as of March 2008, has remained effectively a proprietary Avid format. Ikegami's Editcam camera system is unique in its support for for DNxHD, and records directly to DNxHD encoded video. Such material is immediately accessible by editing platforms that support the DNxHD codec. The source code for the Avid DNxHD codec is licensable free of charge, though few (if any) companies have done so as of March 2008.

DNxHD was first supported in Avid DS Nitris (Sept 2004), then Avid Media Composer Adrenaline with the DNxcel option (Dec 2004) and finally by Avid Symphony Nitris (Dec 2005). Xpress Pro is limited to using DNxHD 8-bit compression, which is either imported from file or captured using a Media Composer with Adrenaline hardware. Media Composer 2.5 also allows editing of fully uncompressed HD material that was either imported or captured on a Symphony Nitris or DS Nitris system.

Avid reports that DNxHD was approved as compliant with the SMPTE VC3 standard.^[1]

In 2007, Apple unveiled ProRes 422, a codec matching many of the features of DNxHD. ProRes lacks a low bandwidth offline resolution like DNxHD 36. ProRes is only supported for playback on Apple Macintosh computers, and is only supplied and licensed for use when purchased as part of Apple's professional video editing software package, Final Cut Studio, (version 2 or later). In contrast, DNxHD is available in 8 and 10 bit formats on any system which supports Quicktime. Unlike DNxHD, ProRes 422 provides full functionality at advanced resolutions (2K and 4K cinema) and SD.

Since September 2007 FFmpeg is providing VC-3/DNxHD encoding and decoding features thanks to BBC Research who sponsored the project and Baptiste Coudurier who implemented it. (ffmpeg -i <input_file> -vcodec dnxhd -b <bitrate> -an output.mov)

[edit] Technical Details

DNxHD is very similar to JPEG. Every frame is independent and consists of VLC-coded DCT coefficients.

Header consists of many parts and may include quantization tables and 2048 bits of user data. Also each frame has two GUIDs and timestamp. The frame header is packed into big-endian dwords.

Actual frame data consists of packed macroblocks using a technique almost identical to JPEG: DC prediction and variable-length codes with run length encoding for other 63 coefficients.

DCT coefficient is not quantized.