Technical Question about Video Signal

Discussion in 'Archived Threads 2001-2004' started by Vince Maskeeper, Mar 4, 2003.

  1. Vince Maskeeper

    Vince Maskeeper Producer

    Jan 18, 1999
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    Currently taking a broacast engineering class, and loving FINALLY getting into the more tech side of my chosen major.

    Unfortunately, my teacher is an engineer- and not much of a teacher (or communicator for that matter)- and I'm getting confused a little on the nature of IRE scale.

    In class we went over the IRE scale- and he showed the Sync at -20ire and the peak white at 100ire- and said the scale was "1 volt peak to peak".

    That seemed to make sense, but I wondered if they was a specified output voltage of 100ire (similar to audio scale, where a +4dbu is equal to a specified output voltage of 1.23v)-- the teacher said no- but I thought this seemed silly.

    So, a little side research showed 100ire (white) as 2.0 volts and 7.5ire (black) as .5 volts. This would seem to indicate that the scale between 7.5 and 100 would be 1.5 volts in scale--- which seems in direct opposition to the "1 volt peak to peak" phrase he repeated a hundred times in ref to the -20 ire Sync to the 100ire white range.

    Now I'll be the first to admit I'm often confused on electrical theory in terms of practical measure of voltage and current--- but it seemed impossible that the entire scale would equal 1 volt, when it seems that the scale of the visual stuff (7.5 to 100 ire) was 1.5 volts itself.

    Anyone have some good "dummies" websites that takes you through this stuff a bit more step by step?

  2. Allan Jayne

    Allan Jayne Cinematographer

    Nov 1, 1998
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    One source I found awhile back (I forget the URL) had 100 IRE as 0.714 volt peak while the sync. was equivalent to -40 IRE at -0.286 volt. This makes the total signal with sync included as 1.000 volt.

    This is consistent with one spec. sheet for an IScan doubler where Y/Pb/Pr and RGB outputs were given as 0.7 volt except that with sync. on green the voltage was 1.0 volt.

    Within a piece of equipment, the video signal could be amplified or attenuated to any peak to peak voltage so long as the proportions for 0 IRE, 7.5 IRE, 100 IRE, etc. were correct. The 1.0 pp voltage looks more correct to me for the signal as presented at an output or expected by an input.

    I don't totally trust that web site because it gave the NTSC chrominance subcarrier as 3.597 something which did not even round to 3.58 that everybody else talks about.

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