Topics covered in this article:
- Delivering Dolby Vision with only L1 metadata
- The Content Mapping Unit (CMU)
- Monitoring the CMU output
To deliver a Dolby Vision Master, the following steps will have to be completed:
- Preparation for Dolby Vision Analysis
- Create L1 metadata using Dolby Vision Analysis
- Check the L1 metadata using Dolby Content Mapping (CMU)
- Performing the necessary modifications to the metadata through the Trim Pass thereby creating L2/L3/L8 metadata (optional)
Dolby Vision Delivery with Level 1 (L1) Metadata
It is possible to deliver Dolby Vision content with only Level 1 or L1 metadata. Dolby Vision content delivered with only L1 can be encoded and sent to playback devices like consumer televisions, tablets, or phones, and these devices in turn will decode the content correctly with Dolby Vision mapping. While L1 metadata is enough to enable Dolby Vision, it is recommended to always check the mapping resulting from the generated L1 metadata before delivering Dolby Vision content for distribution.
The Content Mapping Unit (CMU)
The mapping resulting from L1 metadata can be checked using an application called the CMU. In the early days of Dolby Vision, the CMU used to run on an external workstation that had to be physically connected to the color correction system over a video and data network.
More recently, many of Dolby’s technology partners have integrated the CMU application into their systems and now offer an integrated solution to create Dolby Vision content. Customers today have a choice between using an external/hardware CMU (eCMU) or the integrated/software CMU (iCMU) in their color correction or mastering systems.
The CMU takes the HDR (master grade) images along with the (generated) L1 metadata and, using Dolby Vision content mapping, generates a mapped result based on the selected target.
Most studios require the colorist to check the mapping at an SDR target of 100 nits, Rec709, Gamma 2.4. The colorist therefore will switch the CMU output to SDR (100 nits, Rec709, Gamma 2.4) and check the mapping for the content that they are working on. If the mapping is acceptable, the content can be delivered along with the metadata.
The output of the iCMU can be monitored in one of two ways on Resolve. The user can use either of the following:
- The Enable Tone Mapping Preview feature with the output of the CMU displayed on the primary display.
- The Use dual outputs on SDI option in the Video Monitoring settings to obtain two separate outputs that can be displayed on two separate monitors, one HDR and the other SDR or HDR (depending on the target selected on the CMU). Using Resolve, both SDI outputs would output the same resolution. A suitable video output card is required on the Resolve system in order to output two UHD signals simultaneously.
With the Use dual outputs on SDI option enabled, the colorist may choose to monitor the HDR image and the mapped output of the iCMU/eCMU simultaneously on two monitors placed side by side.
Alternatively, they can also use a single monitor and switch between HDR/SDR as required.
Since the iCMU can be set to map to both SDR and HDR targets, it is important to ensure that the target output monitor settings and calibration match the target selected on the iCMU. Hence when the CMU is set to map to SDR, the target display must also be set to SDR. If the CMU is set to map to a 600nit/PQ target, the target display must be set accordingly.
If the colorist finds the mapping to be acceptable, the content can be rendered out in the requested format and delivered along with the metadata which can exported as an XML file. If the colorist is not satisfied with the mapping, they can use the Dolby Vision trim controls to adjust and modify the mapping to suit their taste and match their creative intent.