Common Mistakes when Running Color Tests
There are some details in the rules that are easily missed. However, its important to understand what's actually required. Let's take a look.
What channels, when, and where?Technically, the color tests only need be conducted on "a minimum of four (4) channels plus one additional channel for every 100 MHz, or fraction thereof, of cable distribution system upper frequency limit (e.g., 5 channels for cable television systems with a cable distribution systems upper frequency limit of 101 to 216 MHz; 6 channels for cable television systems with a cable distribution system upper frequency limit of 217-300 MHz; 7 channels for ..... 300 to 400 MHz, etc.)".
In practice, as long as you're running the tests, its a very good idea to run them on all the channels.
The color tests are required to be conducted on at least a triennial (once every three years) basis. They only need to be conducted "at the output of the modulating or processing equipment (generally the headend of the system)."
Chrominance to Luminance Delay Inequality (CLDI): "which is the change in delay time of the chrominance component of the signal relative to the luminance component, shall be within 170 nS." This is reasonably clear. Another way of stating it is that timing of the chrominance (color) signal needs to be within +/- 170 nS of the luminance (black-and-white) signal.This is the color parameter that most commonly fails to meet the FCC's requirements. Failure to meet the spec can be caused by a wide variety of problems. In nearly all cases, the problem is caused by the phase response of filters somewhere in the system. Examples include single channel bandpass filters on antenna feeds, video/audio separation filters (used to separate the video and audio signals of "composite" video signals), and lack of CLDI precorrection filters in modulators. By the way, CLDI precorrection filters in modulators are necessary to correct for CLDI problems that are inherent in television sets.
Measurement practices have been discussed in many publications. In practice, it can be very difficult to measure CLDI on signals that have substantial CLDI and noise. Instruments that can average signals over many samples, such as the TEK VM700A provide this feature.
«Click for an example of CLDI that would be a little tough to measure.
Differential Gain: "which is measured as the difference in amplitude between the largest and smallest segments of the chrominance signal (divided by the largest and expressed in percent), shall not exceed ±20%."Based on the equation provided in the rules: (100% * (MAX-MIN)/MAX), the actual requirement is 20%, NOT ±20%. That equation can never yield a negative number!
Here are displays from two common measurement devices. The Tek VM700A, and a vectorscope.
«click here if you use a VM700A for diff gain tests
«click here if you use a vectorscope for diff gain tests
In the case of the VM700A, read the p-p/max number. For a vectorscope, adjust the gain so that the test signal just touches the compass row. Then read the differential gain at the point furthest from the compass row.
Differential Phase: "which is measured as the largest phase difference in degrees between each segment of the chrominance signal and reference segment (the segment at the blanking level of 0 IRE), shall not exceed ±10 degrees."Several things can cause errors when running diff phase tests.
First, the requirements are specified differently than for the diff gain tests. The rules use a definition (there's more than one) of diff phase that specifies maximum deviation from a reference point (the lowest packet on a linearity staircase or ramp signal). Diff gain was specified differently. Its a peak to peak specification.
