Part 4 : Definition of DG (Differential Gain) / DP (Differential Phase)
References :
1. Walter Fischer Presentation, Analog Television,
2. ITU-T Recommendation J.61 (TRANSMISSION PERFORMANCE) and J.64 (VITS)
3. PAL Systems Television Measurements, Tektronix®, February 2000
4. VM700T PAL Datasheet
Definition
Differential Gain / Differential Phase are interferences on a video signal caused by Non-Linear
Distortion. (1)
Differential Gain:
Measured as peak-to-peak differential gain. The 5-riser staircase chrominance packet with the greatest peak-to-peak amplitude is found and the ratio of that amplitude to the peak-to-peak amplitude of the blanking level chrominance packet is determined and subtracted from unity. A similar ratio is determined using the packet with the least peak-to-peak amplitude and that ratio is subtracted from unity. The measurement result is the sum of the two differences.
See element D2 and CCIR Recommendation 569. (4)
Differential Phase:
Measured as peak-to-peak differential phase. The maximum phase difference (absolute value) between a 5-riser staircase chrominance packet and the blanking-level chrominance packet is determined. Likewise, the minimum phase difference (absolute value) is determined. The measurement result is the sum of these two phase differences and is expressed in degrees. See element D2 and CCIR Recommendation 569.(4)
Differential Gain
If a constant small amplitude of chrominance sub-carrier, su perimposed on a luminance signal, is applied to the input of the circuit, the differential gain is defined as the change in the amplitude of the sub-carrier at the output as the luminance varies from blanking level to white level, the average picture level being maintained at a particular value.(2)
Differential Phase
If a constant small amplitude of chrominance sub-carrier without phase modulation, superimposed on a luminance signal, is applied to the input of the circuit, the differential phase is defined as the change in the phase of the sub-carrier at the output as the luminance. (2)
Intermodulation from the luminance signal into the chrominance signal also known as, Differential gain, differential phase Intermodulation is measured with the test signal element D2 shown in Figs. 1 consisting of a 5- riser staircase with superimposed sub-carrier.
Figure 1 : Signal D2 for 625-lines system
At the receiving end, the sub-carrier is filtered from the rest of the test signal and its six sections are compared in amplitude and phase.
Differential gain
Differential gain is expressed by two values, +x % and –y %, which represent the maximum (peak) differences in amplitude between the sub-carrier on the treads of the received test signal and the sub-carrier on its blanking level, expressed as a percentage of the latter. In the case of a monotonic characteristic either x or y will be zero.
Differential phase
Differential phase is expressed by two values, +x and –y, in degrees, which represent the maximum (peak) differences in phase between the sub-carrier on the treads of the received test signal, and the sub-carrier on its blanking level expressed in degrees difference from the latter. In the case of a monotonic characteristic either x or y will be zero.
Side notes 1. Source from PAL Systems Television Measurements, Tektronix®, February 2000
Non-Linear Distortion are Amplitude dependent waveform distortions. This classification includes distortions that are dependent on APL (Average Picture Level) changes and/or instantaneous signal level changes. Since amplifiers and other electronic circuits are linear over only a limited range, they may tend to compress or clip large signals. The result is nonlinear distortion of one type or another. Nonlinear distortions may also manifest themselves as crosstalk and intermodulation effects between the luminance and chrominance portions of the signal. The first three distortions discussed in this section are differential phase, differential gain, and luminance nonlinearity. These are by far the most familiar and most frequently measured nonlinear distortions. These parameters are included in the performance specifications of most video equipment and are regularly evaluated in television facilities. The other distortions are not as routinely tested, however, most measurement standards and performance checks include them. It is generally recommended that nonlinear distortions be measured at different average picture levels. Some test signal generators provide variable APL signals by combining the test signal with a variable level pedestal. Since inservice measurements cannot be made with these test signals, measurements requiring control of APL are often eliminated from routine testing.
Side notes 2. Source from PAL Systems Television Measurements, Tektronix®, February 2000
VM700T Automatic Measurement.
To make an automatic measurement of differential phase with the VM700T, select DGDP in the
MEASURE mode. Both differential phase and differential gain are shown on the same display (the lower graph is differential phase). Measurement results are also available in the AUTO mode.
Figure 5 : The VM700T Differential Gain and Phase Measurement Display
Differential gain should not exceed the following figures at high or low average picture level:
For 4.43 MHz: x or y : 10%; x + y : 12%,
Differential phase should not exceed the following figures at high or low average picture level:
For 4.43 MHz: x or y : 5°; x + y : 6°.
Test setup for Differential Phase measurements
Test setup for Differential Gain measurements
No comments:
Post a Comment