Should You Use Internal or External Patterns with HCFR

The below article contains excerpts from The Display Calibration Guide – Volume 1 SDR.

HCFR is able to generate patterns from a PC or laptop either through the primary screen connection (e.g. when connecting a desktop PC) or through a secondary screen connection (e.g. external monitor when connecting a laptop).

Internal Patterns

Running automated patterns through HCFR is more convenient and more versatile as the patterns generated will follow the workflow and settings selected. However, it isn’t guaranteed that you will get accurate patterns if the output isn’t set up correctly.

Black Level Mismatch

The biggest issue you will encounter when running patterns from HCFR’s GDI option is something called level mismatch. This is because there are three places where the limited or expanded range is set:

1. In Windows on your graphics card
2. In HCFR when configuring the output
3. On the display

Since video is recorded and played back in the limited range (16-235), it should be obvious that setting all outputs to be in this range should produce the correct image.

This isn’t so obvious when setting this up however. When a display is plugged into Windows, it can either be recognised as a computer monitor and Windows will display enhanced levels (0 – 255) or as a TV and Windows will force limited levels (16 – 235) even IF it is saying it has passed full-range.

As an added complication, HCFR’s configuration is a bit backwards: it needs to be set up for full-range patterns when the display is in limited range. So the most fool-proof setup is the following:

1. Windows graphics card outputting limited range (to a TV device – unless the TV is reporting a “PC input”, it is in limited range already.)
2. HCFR pattern generator configured for FULL RANGE (0 -255) output.
3. Display configured for limited range input (although this should be automatic)

Once set up correctly, black will be black when getting HCFR to display a 0% stimulus pattern using its pattern generator. If not set up correctly, the pattern will display as dark grey. You can switch HCFR pattern generator between 0 – 255 and 16 – 235 output to double check you have the correct option selected, in case something in the chain isn’t behaving.

All the valid options are detailed in the below table for easy reference.

Please note that if you are going to use more than 20 greyscale / gamma steps in HCFR, or for absolute accuracy, you should be using option 2, which allows HCFR to scale the output, as opposed to Windows. This will result in more accurate levels.*

* Thank you Dominic Chan of AVSForum for pointing this out.

External Patterns

As great as HCFR’s internal pattern generator is, running patterns using an external pattern generator might be more accurate.

If you watch disc-based media, then running patterns from a pattern-generator disc is the most accurate as it takes into consideration the end to end playback system. For example, Blu Ray players from different brands and using different chipsets will behave slightly differently when translating between colour spaces (YUV —> RGB —> YUV, etc) and playing the patterns from disc will take this into consideration.

However, if the external pattern generator cannot be automatically controlled from the software – in this case HCFR – then it might be a lot slower to complete the calibration. You also run the risk of calling up the wrong pattern or patch. This can be slightly frustrating especially while you are learning.

The next most accurate pattern generator that can be controlled from HCFR is a Raspberry Pi pattern generator called PGenerator. You can read more about it here. While it is a bit more complicated to set up, it provides the best of both worlds: the convenience of HCFR’s internal pattern generator with – almost – the accuracy of a disc.

The Display Calibration Guide

If you would like to learn more about displays, and display calibration, you can get The Display calibration Guide here.