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Avid Codecs : DNxHD and DNxHR

Avid had to create a set of high-quality High Definition codecs back in the mid-2000's to allow them to move High Definition video content over their I/O connection pipe of choice for the time, a domestic 400Mbs Firewire cable! This was the birth of DNxHD codecs.

DNxHD became a very popular finishing and editing codec as it was a Media Composer friendly format and the compression also allowed users to store HD content on drives which, at the time, had limited performance and capacity.
With recent UHD formats becoming more widely used in cameras, users now need to edit in 2K, 4K and even 8K. As a result, Avid revised its codecs in line with this demand and released an updated DNxHR codec set. While this new codec has been released this does not mean the end of DNxHD.

Let's look at them both and what they are suitable for.

Avid DNxHD

As we have mentioned already DNxHD is widely used by video editors but it is also used by other manufacturers and is supported within the Quicktime wrapper. Some camera manufacturers can create DNxHD proxy files allowing edit systems quick and easy access to footage (the Arri Alexa, for example, offers this functionality, as well as QT ProRes)

But what are the codecs that are available and how do you interpret their figures and names? The list reads straightforward enough, but when mixed in with varying frame rates (60/50/30/25/24 fps etc) and raster sizes (1080 or 720) and the figures may change a little. Let's keep things simple at the moment and compare the US standard format 1080p30 with PAL 1080p25 format. The complete white paper for Avid is available here. We'll start with the highest quality and work our way down to the lowest...

Avid DNxHD 444: For 1920x1080 progressive projects. Full resolution, 10-bit 4:4:4 RGB video sampling and high bit-rate is a visually loss less compression. This codec is only seen in projects made in the RGB colour space.

Avid DNxHD 220x: For a high-quality image in YCbCr-colour space for 10-bit sources. The data rate is dependent on frame rate. For example, 220Mbps is the data rate for 1920 x 1080 30fps interlaced sources (60 fields) while progressive sources at 24fps will be 175Mbps and at 25fps will be 185Mbps.

Avid DNxHD 220: For high-quality images when using 8-bit colour sources, the data rate remains the same is the 10-bit version detailed above.

Avid DNxHD 145: For high-quality mastering when using 8-bit lower data rate sources such as HDCAM and DVCPRO. 145Mbps is the data rate for 1920 x 1080 30fps interlaced sources (60 fields). Progressive sources at 24fps will be 115Mbps and at 25fps will be 120Mbps.

Avid DNxHD 100: For optimal visual impact where workflow speed and storage capacity are important factors. Suitable replacement for DV100 compression and offers lower processing overhead than AVC-Intra 50/100. Sub-samples the video raster from 1920 to 1440 or from 1280 to 960 to reduce compression artifacts, providing a balance of reduced compressed bandwidth and visual quality. Progressive sources at 24fps will be 80Mbps and at 25fps will be 85Mbps.

Avid DNxHD 36: High-quality offline editing of HD progressive sources only. Designed for projects using an offline/online workflow due to large quantities of source media and/or needing more real-time preview streams for craft editorial or multicamera editing.

"A controlled test was performed to compare uncompressed HD to DNxHD 220 for a film out. To get uncompressed HD; the film was scanned at 2K then rendered as uncompressed HD, and then DNxHD 220. The two HD elements were edited together with a diagonal wipe between the two formats. The resulting sequence was exported as DPX files for a film out. The 35mm film was projected, as well as the digital files. In a calibrated screening room, experienced engineers were unable to pick where the split was done, nor which part of the image was Avid DNxHD." -PostWorks, New York

With the exception of DNxHD 444, the number at the end of the DNX codec is the data rate required for a single stream of video. DNxHD 120 requiring 120Mbs for example.

Avid DNxHR

As we have seen already, Avids DNxHD codecs were designed for HD workflows up to 1920x1080. The later DNxHR codecs take the leap into 2K, UHD, 4K and 8K formats. So what codecs are available?

DNxHR 444—4:4:4 RGB colour space at 12 bit, ideal for high-quality color correction and finishing
DNxHR HQX—High quality extended; 12 bit for colour correction and mastering
DNxHR HQ—High quality 8 bit; offers a great balance of beauty at a smaller bandwidth for editorial
DNxHR SQ—Standard quality 8 bit; ideal for editorial
DNxHR LB—Low bandwidth 8 bit; ideal for remote workflows and saving storage; LB 1/4 resolution and LB 1/16 resolution also available for the most bandwidth/space-constrained workflows.

As can be seen, the codecs are named and don't allude to their required respective data rates which makes them a little harder to account for in terms of disk performance and file size on disk. The table below shows examples of the data rates you should expect from the DNxHR codec family. The last 2 columns are datarates in MBs.

It's also worth knowing that the DNxHR codecs in a High Definition project are backward compatible with DNxHD. But as the DNxHD codecs are still available in HD projects I would recommend sticking with these where possible and use DNxHR in UHD and above codecs.

If you want to dive deeper into these codecs take a look at the Avid article here which also offers additional resources.


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