20-06-2019, 11:59 AM
Some other components of the ITS waveform are a composite pulse and a staircase.
The composite pulse is a 10 or 20T raised cosine luminance pulse combined with a burst of subcarrier shaped using an identical filter. When these are combined, the positive peaks follow the raised cosine outline and the negative peaks produce a flat line across the base. It is used to check for and measure group delay - the time difference between delays to the luminance and chrominance components of the signal.
Viewed on a scope, movement of the chrominance component will cause the flat baseline to become a horizontal 'S', the height of which increases as the delay increases. A multiburst signal can be used to measure group delay across the entire spectrum by using pulses containing, say, 1, 2, 3, 4 & 4.43MHz signals (for 625 lines).
The staircase performs a similar function to the ramp and may or may not have subcarrier superimposed on it.
At first sight of a 5-step staircase, the obvious conclusion would be that you would just measure the relative heights of each step using a scope. Modern scopes are capable of very accurate waveform measurements but that wasn't the case in the heyday of ITS - such methods would not not give the accuracy demanded in a broadcast environment!
Instead, the waveform was differentiated - basically passed through a low value capacitor to remove the low frequency content and retain only the vertical transitions. These could then be amplified and the peaks viewed on the scope. In an ideal world they would all be the same height but it was now much easier to measure any differences.
The composite pulse is a 10 or 20T raised cosine luminance pulse combined with a burst of subcarrier shaped using an identical filter. When these are combined, the positive peaks follow the raised cosine outline and the negative peaks produce a flat line across the base. It is used to check for and measure group delay - the time difference between delays to the luminance and chrominance components of the signal.
Viewed on a scope, movement of the chrominance component will cause the flat baseline to become a horizontal 'S', the height of which increases as the delay increases. A multiburst signal can be used to measure group delay across the entire spectrum by using pulses containing, say, 1, 2, 3, 4 & 4.43MHz signals (for 625 lines).
The staircase performs a similar function to the ramp and may or may not have subcarrier superimposed on it.
At first sight of a 5-step staircase, the obvious conclusion would be that you would just measure the relative heights of each step using a scope. Modern scopes are capable of very accurate waveform measurements but that wasn't the case in the heyday of ITS - such methods would not not give the accuracy demanded in a broadcast environment!
Instead, the waveform was differentiated - basically passed through a low value capacitor to remove the low frequency content and retain only the vertical transitions. These could then be amplified and the peaks viewed on the scope. In an ideal world they would all be the same height but it was now much easier to measure any differences.
