scopes [was: how to troubleshoot? ...]

[Goddard, Duncan]

>>>The XYZ unit sounds like a vectorscope. I'm not sure what the
> professional use of these are, I think some kind of video
> or communication use...
> 
> 19" is HUGE for a scope.  Maybe its some kind of TV monitor or
> test gear ???<<<
> 
I'd go with that- 19" rack mount 'scopes may crop up in other areas, but in tv they are all over the shop. the z-input allows the use of an "ordinary" 'scope to display tv images; it was quite common on early tek stuff (when they were still a "british" company, based on guernsey).

vectorscopes are used to display information from the two amplitude-modulated and 90 degrees apart components of the tv colour signal; tv colour information for pal or ntsc transmission is created by matrixing the red, green and blue signals from a three tube/chip camera to produce a luminance signal (y) and the two colour-difference signals, red-minus-luminance and blue-minus-luminance. this choice ensures as full a recovery of the data at the other end as possible. the r-y and b-y are then used to modulate the two different phases of the 3.58 or 4.43MHz subcarrier; these are then added together and stuck on top of the luminance signal for transmission. I'm missing out tons of detail here but pretty soon it'll be obsolete anyway........ 
(pal stands for "phase alternate line"; one of the two subcarriers changes phase by 180 degrees every other line of the tv picture, an attempt to overcome the phase shift and subsequent hue errors afflicting ntsc, which we english video engineers call "Never Twice the Same Colour")
 
the vector display on such a device is useful not only for alignment of coding/decoding equipment but for matching and colour-balancing cameras in a studio or an OB truck. other tv-specialised devices derived from oscilloscopes display luminance or composite waveforms line-by-line or field-by-field.

d.


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