Differential gain and phase: Why measure it, if we can’t see it?
Many say that video Differential Gain and Differential Phase (DG and DP) are not visible to the human eye. So why would anyone want to measure something that is invisible? (The reason that DG and DP are said to be "invisible" is because the magnitude is usually small and the change in scene brightness often masks the errors.)
The DG and DP tests were designed to detect very small errors before they could bother a human. This ensures good video quality when the video passes through hundreds of amplifiers in succession as it goes from source to destination. For amplifiers, analog-to-digital converters (ADCs) and digital-to analog-converters (DACs), there are some simple ways to evaluate their DG and DP performance and verify the performance at both the sweet spot and near the power rails.
Again, this detects very small errors, ensuring signal integrity with multiple stages. To better understand the impact of DG and DP errors, let’s examine how DG and DP are applied to amplifiers, ADCs and DACs.
Simply, the impact of a DG or DP video error could translate into a person’s flesh tone changing as they move from a brightly lit area to a dimly lit area. First, for subcarrier type television systems like NTSC, (North America and Japan), DG change directly changes the saturation or how vivid the color is, much like the chroma control on your TV. The DP error will change the hue of the color (toward Green or Purple) like the TV’s tint control.
Second, in subcarrier TV systems such as PAL (Europe and China) DG directly applies where DP results in a second-order saturation change. Third, for High Definition (HD) and component systems, DG and channel gain differences result in colorimetry changes. Though NTSC is not broadcast widely over-air in the USA, industrial and security video systems are still dominated by the legacy technology.
Why do we need DG and DP Specifications?
Think of how a TV program is created. Multiple camera signals are switched and sent through special effects equipment, recorded, played back and edited, all on their way to become a program. The program may be distributed over long distances by microwave, fiber optics or satellite systems, and eventually be broadcast over-the-air. A cable, DVD or satellite system then brings the program into our homes so we can enjoy it.
In this process, the video may pass through hundreds of amplifiers. Each amplifier contributes a small amount of DG and DP to the video signal. To be sure that the video signal is preserved, engineers designed a very sensitive test signal.
All amplifiers have some amount of non-linear amplitude response. Negative feedback helps to reduce this non-linearity. DG and DP are really specialized linearity measurements that take frequency response into account. NTSC and PAL television systems send the color information on a subcarrier (3.58 MHz and 4.43MHz respectively). Differential Gain is defined as the change in amplitude of the high-frequency subcarrier when there is a change in the low-frequency video level or brightness.
In the NTSC video waveform, Figure 1, the 3.58MHz subcarrier is superimposed on a lower-frequency luminance signal with five brightness steps. The subcarrier is drawn as single, large sine waves for clarity. In fact, there are more than two hundred subcarrier cycles across one horizontal line.
Figure 1: Video differential gain (DG) and differential phase (DP)
Differential Phase is defined as the change in phase of the high-frequency subcarrier when the lower-frequency video level or brightness changes in NTSC and PAL. The Hue or Tint of the color displayed is controlled by the phase relationship between the reference burst located in the black region of the video and the subcarrier present in the active video picture time. If the proper color is to be displayed, the phase must be accurately controlled.
Amplifiers, ADCs and DACs have a "sweet spot" where they are most linear and meet the highest specifications or standards. This sweet spot is usually located midway between the two power rails (Figure 2) although the IC designer could place it at another point if necessary. In this sweet spot, the amplifier is operating with the best control over feedback and is naturally most linear. That means that as a signal approaches the power-supply rails, the linearity decreases.
Figure 2: Amplifier DG and DP near the power rails
Superimposing a high-frequency sine wave on a low-frequency signal allows the amplifier’s entire operating range to be explored. For example, an amplifier such as the MAX4389 is specified with a low typical DG of 0.015% and DP of 0.015° for subcarrier television systems. However, DG is also used in wider-bandwidth television and non-video applications. If we need a 10MHz signal, we can exercise an amplifier like the MAX4389 by applying a 7MHz sine wave and then changing the DC bias. If the required bandwidth is 30MHz, we can apply a 22MHz sine wave.
Typically, the high-frequency sine wave is selected to be about two-thirds to three-quarters of the system bandwidth. The sine wave signal biased to mid range DC (Figure 2) will have the best response. As the DC is changed to move the sine wave toward either power supply rail, the sine wave amplitude will change.
Usually, the high-frequency response is reduced as the power rail is approached and the transistor operating current is reduced. At the extreme condition, the amplifier simply runs out of current, stops operating, and clipping occurs. ADCs and DACs can be checked in a similar manner.
As the integrated circuit becomes more complex, with more than just a single amplifier, larger DG and DP errors are expected. Such higher-complexity circuits might typically contain multiplex switches, a six-pole Sallen-Key filter (which has three or more amplifier stages of its own), and a video buffer amplifier. One such circuit, the MAX7428 developed by Maxim, keeps the typical DG error to just 0.2% and the DP error to only 0.2°.
DG started as a test method for amplifiers, and video requirements added DP to create a very sensitive test that allows characterization of many cascaded stages to insure signal integrity. This has resulted in the use of DG for other products such as ADCs and DACs. Applications requiring wider bandwidth are enabled by varying the DG test signal to match the bandwidth.
DG is a versatile test methodology to thoroughly explore the linearity of various components over the full range of the available power supplies. Measuring the invisible DG is a very useful tool. It acts like a microscope for closely inspecting the signal integrity thus insuring quality through long chains of analog circuits.
About the author
Bill Laumeister is Principal Member of the Technical Staff with the Advanced Video Strategic Applications Group at Maxim Integrated Products, and works with companies selling consumer products. He has thirty-eight years experience and holds several patents in the video field. He is the inventor of a video communications method called VEIL (Video Encoded Invisible Light). It is being considered by the U.S. Congress in the "Digital Transition Content Security Act" as a possible patch for the "analog hole". He can be reached at Bill.laumeister@maxim-ic.com.
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