BLACK IS BLACK .... NOT

It seems like a simple question. ‘What is black?’ Back in the first grade, the teacher told us that black is the absence of color. I was happy with this answer for many years. Then came television and computers.

In the early days of television, American engineers developed a broadcast standard. This was named after the National Television Standards Committee (NTSC). As it evolved, there were minor problems to correct. One issue was interference between the audio portion of the broadcast signal and the video color subcarrier. The correction for this was to change the frame rate from 30 frames per second to 29.97 frames per second. But that’s a different story.

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IRE units are a linear scale from +120 to -50.
The zero reference is at the blanking level. Component systems normally operate with 100 IRE units equal to 0.7 V p-p
Composite video normally operates with 140 IRE units equal to 1.0V p-p
SMPTE
Society of Motion Picture and Television Engineers
Standard 170M -
Composite Analog Video Signal

Another issue was the transmitters of the time. They adjusted power by monitoring the signal level. Engineers were concerned that if there were low video (0 IRE) in the active area the transmitters would add too much gain to the signal. In addition, engineers feared that if the chroma portion of the signal reached too low a level (minus 20 IRE), it would interfere with the sub-carrier. They decided to make a minimum level for active video and added what is known as ‘set-up’. This required a minimum signal of 7.5 IRE (Institute of Radio Engineers) units in the active portion of the waveform. This minimal signal was added to SMPTE s170M as a base line so that the early transmitters would have a positive IRE level in the active video to set gain. It also raised the chroma portion of the signal by the same amount.
By the time PAL (Phase Alternate Line) was developed in Europe, other provisions were in place. Therefore no set-up was required. PAL uses the full 700 mV amplitude in the active area. NTSC uses 92.5 IRE of the possible 100 IRE available.

Analog NTSC video still adheres to this requirement.

ColorBars

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SMPTE Standard 259M
10-Bit 4:2:2 Component and 4fsc Composite Digital Signals -
Serial Digital Interface
SMPTE Standard 125M
Component Video Signal 4:2:2 -
Bit-Parallel Digital Interface
Digital values describe the correspondence between video signals and quantization levels.
In a 10 bit video system there are 877 quantization levels between black (64) and white (940) In an 8 bit video system there are 220 quantization levels between black (16) and white (235)

As we step forward about 50 years, digital systems were being developed. Digital signals (any system based on SMPTE 259M) include the SDI, MPEG, DV and DVCAM standards. It is also the basis for the HDTV systems.
Video levels are measured in digital units. SMPTE s125M describes the range for television as 16 to 235. This allows for sub-black (0-15) and above normal (236-255) levels. In digital television signals there was no need for set-up. Without set-up there is more room for signal information and the full 16-235 levels can be fully utilized.

Other digital products such as graphics programs use the full 0-255 range (RGB). That is why files created in graphics programs must be limited to 16-235 when being imported to video programs.

This makes the situation seemingly straightforward. Analog NTSC signals use set-up in the active video region. Digital signals complying with SMPTE s259M do not include set-up.

Most of the confusion occurs when analog to digital and digital to analog conversions take place. In NTSC, a composite analog signal enters a decoder and the 7.5 IRE black levels are mapped to a digital value of 16. In PAL systems, 0 millivolt (black) is also mapped to a digital level of 16. NTSC-J systems are similar to NTSC except that there is no set-up. Black (0 IRE) levels are also mapped to 16.

A-D Conversions

D-A Conversions

During processing in non-linear-editing, compositing or other digital video systems, more color space conversions may be used to convert from video levels to RGB levels. Before outputting the finished video, it must be converted back to SMPTE s125M levels.

Set-up is only added at the digital to analog encoder. When you set the project to 'NTSC has set-up' this only affects the decoder (digitize) and encoder (playback) analog functions. When it decodes an incoming analog level of 7.5 IRE it maps it to a digital value of 16. When it encodes a digital value of 16 it maps it to an analog value of 7.5 IRE in the active video section of the signal. That's why the optional video sections (lines 10-20) in the vertical interval area will normally have a level of 0 IRE. In PAL the digital level 16 is mapped to 0 millivolts. In NTSC-J (no set-up), digital 16 is mapped to 0 IRE.

All of this becomes important during transitions in a mixed format facility. When the decoders are properly calibrated they expect that incoming NTSC analog signals will have proper set-up. A properly calibrated NTSC encoder will expect that a digital 16 level represents black. They will take that digital 16 and map it to 7.5 IRE. This makes signal monitoring extremely important at every transition.

Monitoring of analog and digital signal flows have two different criteria.

Analog signals will have set-up at all times in the active area.

White Black levels

Digital signals will have black set to a value of 16.

Digital Monitoring

Dig levels

 

Welcome to the real world

Monitoring is required because not all signals are created properly.

Some cameras have a set-up switch. This should only affect the composite output. There are some models that add set-up to both the analog and digital outputs. A waveform monitor is the only method of checking for proper levels.
Due to misunderstanding the standards involved, some facilities will digitally process an SDI signal to include set-up in the active area.
Some sites will remove set-up in the decoder/encoder and raise the SDI level to compensate. These two wrongs can create a composite signal that appears correct. The problem is that this method only allows 203 levels in SDI instead of the proper 219 (32-235 instead of 16-235)
Certain editing and DVD production software and hardware do not manage black levels properly. They process signals using RGB levels. This causes analog black levels to vary from 0 IRE to 15 IRE. There is usually no method of determining how black is treated except direct measurement.

These and other problems demonstrate why constant monitoring should be used in every step of a process. If signal integrity is correct throughout the path, there should be no reason to alter the signal. Any extraneous signal manipulation will degrade the video. In this case less is better.

In theory all of this should be transparent to the user. In the real world there is confusion in this area. Some products produce improper levels and some people adjust a signal to improper levels through misunderstanding the concept.

Checking parameters with a scope is the only way to insure proper levels in your productions. Never take any incoming material for granted.

Jim Alfonse, owner of Tri-Sys Designs, is a Systems Integrator with twenty-five years experience in the Broadcast Industry. He's designed, built and commissioned installations from Satellite News Vehicles to Production Suites to OB vans. Jim has been involved with several equipment manufacturers performing video standards compliance and signal integrity testing.
For more articles on various subjects visit www.tri-sysdesigns.com