Measuring composite-video signal performance requires understanding differential gain and phase, Part 1 of 2

Cliff Win, Jr., Senior Applications Engineer, National Semiconductor Corporation - August 21, 2007

The composite video signal of the 1950s still has a large presence in today's world of high-definition analog and digital video. Therefore, most parameters associated with composite video remain major considerations in system design. Two key parameters that are unique to composite video performance are differential gain (DG) and differential phase (DP), which are such critical factors in the composite video signal path that it has become standard practice to include them in nearly all high-speed amplifier datasheets.

However, measurement and verification of these parameters may not be simple. This article will provide an overview of composite NSTC/PAL video and the importance of DP and DG. It will also focus on the interpretation of these specifications in datasheets and, most importantly, the various measurement methodologies for DG and DP.

The composite video signal
In the 1940s, the National Television System Committee (NTSC) introduced a composite monochrome video standard, which encoded timing (horizontal and vertical synchronization) and brightness (luminance) information into one channel, Figure 1.

Figure 1: NTSC monochrome video signal
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In the following decade, the committee reconvened to develop a color television standard, Figure 2,

Figure 2: NTSC color video signal
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which is now known as "NTSC."

This color standard retained the same video signal and bandwidth for full backward compatibility with previous black-and-white television sets.

To achieve this, a color (chrominance) "subcarrier" and reference "burst" (also called a. "color burst") were embedded onto the original monochrome video signal. First, the chrominance and luminance frequencies are interleaved so that the color and brightness information can occupy the same available bandwidth. This interleaved spectrum is centered at approximately 3.58 MHz, which is the chrominance subcarrier frequency. To reduce visibility of the subcarrier, the frequency was purposely chosen to be an odd multiple of one-half the horizontal synchronization rate (15.75 kHz/2).

Secondly, the color burst, which consists of nine cycles of the subcarrier signal, is located on the "back porch" of the horizontal blanking interval. The purpose of the burst is to synchronize the 3.58 MHz color oscillator in the television set or receiver with the transmitted signal's color reference.

NTSC's European counterpart is the Phase Alternating Line (PAL) video standard. The primary distinction between the two standards is that PAL's subcarrier frequency is 4.43 MHz and its sync-to-video amplitude ratio is slightly different.

The chrominance information can be divided into two elements: color saturation and color hue. Saturation is determined by the amplitude of the subcarrier, and hue is determined by the phase of the subcarrier relative to the color reference burst. Saturation can be described as the intensity of the color, while hue is the accuracy of the base color, Table 1.

(Click to enlarge image)

For example, a red subcarrier that has lower amplitude but is still in proper phase with the color burst will appear pink, which is a less saturated version of red. On the other hand, if a subcarrier is ∼283° out of phase with the color burst rather than ∼241°, then the color is cyan instead of green.

Note that video signals use the "IRE" unit instead of DC voltages to describe levels and amplitudes. Based on a standard 1 Vpp NTSC composite-video signal that swings from -286 mV (sync tip) to +714 mV (peak video), a 140 IRE peak-to-peak convention has been established. One NTSC IRE unit is 7.14mV, where -40 IRE is equivalent to -286 mV, and +100 IRE is equivalent to +714 mV. Note that 0 IRE is equivalent to 0 V.

The PAL video signal is slightly different in that it swings from -300 mV to +700 mV, instead. Thus, one PAL IRE unit is 7mV, where -43 IRE is equivalent to -300 mV at the sync tip, and +100 IRE is equivalent to +700 mV at the peak video level. Throughout this discussion IRE units will be used mostly, instead of volts or millivolts.

Differential gain and differential phase
As illustrated in Figure 2, the average value or midpoint of the chrominance subcarrier is the luminance level. The chrominance can be thought of as the amplitude of a sinusoidal signal and the luminance can be thought of as this signal's DC offset level. Differential gain error is the change in the amplitude of the chrominance subcarrier due to a change in the luminance level.

Therefore, differential gain gauges how the color saturation changes when picture brightness changes. Consider the sinusoidal amplitude output of a video amplifier which varies when the dc offset level varies. This would result in a red car under daylight that turns pink as evening approaches.

Differential phase error is the change in the phase of the chrominance subcarrier due to a change in the luminance level. Differential phase indicates how the color hue changes when picture brightness changes. Consider the sinusoidal phase output of a video amplifier which varies when the DC offset level varies. This would result in a blue shirt indoors that turns purple when it's outdoors. Ideally, color saturation (amplitude) and hue (phase) should remain constant regardless of the luminance or DC offset level. A red car should appear red under any lighting, and a blue shirt should always remain blue whether it's indoors or outdoors.

Generally, DG and DP errors of up to 1% and 1°, respectively, are unnoticeable to the human eye. Thus, one should consider these numbers as the total error budget for the entire video-signal chain.

There can be several error-contributing blocks before the signal is finally viewed by the human eye. A typical path may include the video camera, the recording unit, the transmitter, the receiver, and the display monitor. The individual DG/DP requirement for each processing stage or amplifier within a signal chain is much lower than 1%/1°, depending on how many successive stages there are.

Consider, for example, the design of a video-distribution system with five op amps between the input and output. Assuming that each op amp contributes 0.01% of DG and 0.01° DP maximum, these errors will result in a cumulative DG/DP error of 0.05%/0.05° maximum, respectively. If the designer desires the overall system specification of his video distribution board to be 0.05% DG and 0.05° DP, then these particular op amps would just barely meet his requirements. Studio-broadcast quality ICs and amplifiers require far lower DG/DP specifications than their consumer-grade counterparts, because there are many more successive stages in their signal chain. Some of these video systems may demand even lower than 0.01%/0.01° of DG/DP for each of their op amps.

DG/DP measurement basics
Measuring DG and DP is essentially measuring the error in amplitude and phase of a sinusoidal signal as its dc-offset level is varied. Therefore, the test signal is typically a 3.58 MHz (NTSC) or 4.43 MHz (PAL) sine wave with a dc offset that varies from the 0 IRE up to 100 IRE. One way to generate this type of signal is to use a video test pattern generator to output a "color bars" waveform like the one in Figure 2. The DG error can then be calculated as the change in percentage between the measured peak-to-peak amplitude of the subcarrier at the lowest step on the staircase, relative to that of the highest step.

DP can be measured and calculated similarly with respect to the phase of the subcarrier. However, this end-points approach can be misleading since DG and DP may not always behave linearly with luminance or dc level. Besides, for merely measuring two end points, manually shifting the offset of a sine wave with a dc voltage would have been simpler to do.

Thus, a better approach would be to measure the amplitudes of each subcarrier at every step of the staircase and determine the greatest delta between the amplitudes. Using a color bars or modulated staircase waveform with a greater number of steps will increase the number of measurement points to provide further resolution in determining the maximum DG/DP error. Therefore, a modulated saw tooth ramp may be more desirable than a staircase limited to a relatively small number of steps.

Manually measuring multiple gain and phase error points over the entire luminance range can become quite tedious. One alternative way of evaluating DG/DP is with the use of specialized video test equipment such as vectorscopes and waveform monitors. These types of instruments have built-in DG/DP test functions that can report the largest peak-to-peak delta over the entire, continuous luminance range. However, such equipment does not have the resolution to measure the DG/DP of individual op amps down in the 0.01%/0.01° to 0.001%/0.001° range, as they are better suited for evaluating complete video systems. Therefore, one of the most effective and preferred methods of measuring DG/DP today is through the use of a network analyzer.

DG/DP measurements with network analyzers
Network analyzers are capable of performing gain and phase error measurements simultaneously over a DC bias range, and have very high resolutions to resolve DG/DP errors less than 0.01%/0.01°. An example of a DG/DP test setup with the Agilent HP4195 network analyzer is shown in Figure 3.

Figure 3: DG/DP test setup with network analyzer
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The HP4195's resolution is 0.001 dB/division and 0.01°/division, and also has a built-in dc source, which is used to provide the dc offset or luminance level. This source can be programmed to sweep the dc level across the luminance range in increments as fine as 10 mV per step. It is also filtered with a 500 kHz low-pass filter to remove unwanted noise that may affect the measurements, and is then combined with the HP4195's oscillator output (3.58 MHz or 4.43 MHz).

This is done passively with a delta configuration circuit to maintain line impedance and prevent any additional DG or DP from being introduced into the test signal. At the output of the device under test, an 8 dB attenuator is placed in series with the output resistor. The attenuator ensures that the signal amplitude is low enough to prevent any overloading or distortion at the network analyzer's front end. The analyzer interface and test setup components are 50 Ω impedance, including the splitter, combiner, and attenuator.

The appropriate oscillator amplitude and dc-sweep range is set up and measured with an oscilloscope at the input of the device under test. The oscillator amplitude is adjusted to yield 286 mVpp (40 IRE), which is the standard test signal amplitude for NTSC. The network analyzer is calibrated and configured to perform the "S21" S-parameter measurement. An average of a number of measurement sweeps should be taken for the required resolution. For measuring ultra-low DG/DP devices, 50 or more measurements should be averaged. Figure 4 illustrates an op amp's typical gain error and phase plot from a network analyzer.

Figure 4: Gain and phase error (-100 to +100 IRE)
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The gain error, which is typically in dB is expressed as a percentage with the calculation of Equation 1:

This is just one way of performing the measurement with a network analyzer. There are, of course, other slightly different methods and network analyzer models employed in DG/DP tests. Some analyzers may not have the built-in dc source and an external dc-sweep source may be necessary. Other analyzers may be equipped with a dc-bias input, in which case a sawtooth ramp from a function generator could be fed into this input. This would eliminate the need for the combiner circuit at the input of the device under test, since the DC bias would be combined with the sinusoid inside the network analyzer.

(Part 2 of this article will examine measurement conditions for DG/DP in datasheets; you can go to Part 2 by clicking here.)

About the author
Cliff Win is a Senior Applications Engineer for National Semiconductor's Amplifier Product Line. Previously, he has worked extensively with analog and mixed signal video display ICs in National's CRT and Digital Television group. Cliff holds a Bachelor of Science degree in Electrical Engineering from the University of California at Davis.