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Hybrid Log Gamma for HDR TV broadcasts: What is it?

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Hybrid Log Gamma for HDR


HDR (high dynamic range) is a term that we see almost every day and what it exactly means we have already described extensively . However, a new standard for the av world wouldn’t be a new standard if we didn’t have a number of different sub-standards. For example, we have already seen Dolby Vision pass by and many TVs today have support for HDR10. However, there is another standard that is being worked on; HLG (Hybrid Log Gamma). What exactly is this?

What is Hybrid Log Gamma?

To view Dolby Vision and HDR 10 content, you always need a TV that can display the wide range of HDR. However, HLG is an HDR standard that is interesting for everyone, including those without a TV that can display high dynamic range material. The data can be understood by both HDR and SDR televisions. This must ensure that the contrast and color reproduction of a standard dynamic range TV are also improved. HLG is especially interesting for broadcasters, which in this way can relatively easily send HDR signals to consumers.

HLG was developed by the British BBC and the Japanese NHK. These companies aimed to develop an HDR standard that is compatible with the current 10-bit infrastructure and that only requires modifications to cameras and key monitor displays. In particular, it is a standard that makes it relatively simple and cheap for broadcasters to broadcast HDR content.

The HLG signal represents relative light levels in the original scene, allowing the images of a broadcast to be adjusted for the same artistic effect on a very bright (HDR) or darker (SDR) display. Only the display itself needs information about its capabilities and the environment to display the signal credibly and accurately. As a result, the sending of metadata, which is done with other HDR standards, is not necessary. The same infrastructure can be used for HLG as for SDR TV signals. Standards such as Dolby Vision and HDR10 place a much higher burden on the broadcasters as they require extra bandwidth and are not compatible with SDR TVs, so two signals have to be sent.

Hybrid Log Gamma is therefore a ‘simplified’ way to give an HDR look to TV broadcasts. A look that should have added value on both HDR and SDR TVs. Of course, the quality will not match an HDR10 or Dolby Vision signal with specific metadata that is optimized for high dynamic range TVs, but it does make it relatively easy to display TV broadcasts with a larger dynamic range on any TV .

When can we expect HLG?

During IFA 2016, various manufacturers, including Sony, Panasonic and LG, showed the possibilities of HLG on their TVs with high dynamic range. Our impression was very positive; Hybrid Log Gamma provides a wider dynamic range and is quite close to the other HDR standards. It is not yet clear when broadcasters will actually start using the standard.

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