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Wowza

BT.709 Wowza

Problem

Noticed that our lowest transcoded video streams had a different colors to the rest of the streams. We do multiple transcoding’s, both 720p, 480p, 360p, 270p and it was only 360p and 270p that had this disparity from the original. The difference is slight but annoying enough that you notice it if you are constantly switching between qualities.

  • Original
  • 360p

Solution

Setting the three values colour_primaries, transfer_characteristics, matrix_coefficients to be BT.709 for 360p and 270p.

You do this by setting the following encoder properties

NameValue
mainconcept.colour_primaries1
mainconcept.transfer_characteristics1
mainconcept.matrix_coefficients1

Background

There is multiple values that says what color space that is being used. using MPC-HC we can se that the color space we are using for most of the files is BT.709, as it should be for HD broadcasts.

Color primariesBT.709
Transfer characteristicsBT.709
Matrix coefficientsBT.709
For 1080p, 720p and 480p, MPC-HC reported the following color information

however then dropping the resolution to below 360p Wowza and Mainconcept switches the color space.

Color primariesBT.601 PAL
Transfer characteristicsBT.470 System B, BT.470 System G
Matrix coefficientsBT.601
For 360p and 270p different color information was reported compared to the original 1080p

Tests

Wowza allows you to set advanced settings for the Mainconcept encoder, however you must configure Wowza to output what those settings are. you can look at the post Wowza Transcoder options to se which settings that exists.

ParameterPossible Values
mainconcept.colour_primaries1: ITU-R Rec. BT.709-5,
4: ITU-R Rec. BT.470-6 System M,
5: ITU-R Rec. BT.601-6 625,
6: ITU-R Rec. BT.601-6 525,
7: SMPTE 240M, 8: Generic film (colour filters using Illuminant C),
9: Rec. ITU-R BT.2020
mainconcept.transfer_characteristics0:auto (default),
1: BT.709-5,
4: BT.470-6 System M,
5: BT.470-6 System B G,
6: BT.709-5,
7: SMPTE 240M,
8: Linear,
14: Rec. ITU-R BT.2020
mainconcept.matrix_coefficients0:auto (default),
1: BT.709-5 System 1125,
4..6: BT.709-5 System 1250,
7: SMPTE 240M,
9: Rec. ITU-R BT.2020 non-constant luminance system
Difference settings for colors

My educated guess would be that I should set the values for BT.709-5 System 1250 since I am in a pal country running 50fps. But doing so resultat in an outputted file that had Matrix Coeffcient as BT.601

The only combination of the above settings that resultat in BT.709 for all settings was the value 1 for all three properties.