[Tig] Combined measurement of Luminance plus Chrominance

Dave Best daveb
Wed Oct 23 14:40:52 BST 2002


Combined measurement of Luminance plus ChrominanceBill, this is like deja vu all over again.
  ----- Original Message ----- 
  From: Bill Topazio 
  To: 'tig at tig.alegria.com' 
  Sent: Wednesday, October 23, 2002 8:10 AM
  Subject: RE: [Tig] Combined measurement of Luminance plus Chrominance


  The answer to this has its roots in the BROADCASTABILITY of the signal.  What you DON'T want is a tape to air that has the potential of overmodulating the transmitter.  That is why you are looking at Vp-p, or Y+C, which in my opinion is the correct way to do that.  Some people get hung up on "vectors being out of the boxes" but it's the Y+C that concerns the transmission folks.

  In general, we use an OUTSIDE limit of +120IRE and -20 IRE for signal components (naturally sync extends to -40).  Signals in NTSC can be "legal" up to 133IRE but that will certainly overmodulate a transmitter.  Remember, 100% saturated bars produce that 133IRE (perfectly legal according the NTSC math), so they went to 75% for broadcasters so they could actually transmit the darn thing.

  In practice, as Bob Lovejoy mentioned, we do give ourselves a lower value (115-118) so that we have a margin of error as well as room to soft-clip gracefully.  You also want to look at the Y-only component to ensure white levels are in check (typically hard clip at 100-102IRE).  It is not uncommon for flashes or specularities to exceed these amounts, but often they will not be noticed if they are BRIEF enough.

  Keep in mind that all of this relates to ANALOGUE BROADCASTING (OK, and perhaps to less robust tape formats like VHS and 3/4") and that in today's world you should also be looking at component YPrPb as well to ensure they stay between 16 and 235 (8-bit Y) and 16-240 (8-bit Prb).

  Lastly, it's always good to have a file of delivery specs from your clients.  In the US for example, PBS is stricter than any of the networks, especially in the Y area (titles, etc.), and will happily reject programs wth titles over 90 IRE!
    -----Original Message-----
    From: Broadcast [mailto:broadcast at video-8.com]
    Sent: Wednesday, October 23, 2002 12:32 AM
    To: 'tig at tig.alegria.com'
    Subject: [Tig] Combined measurement of Luminance plus Chrominance


            
    Hi Guys,would anybody be able to answer the following question? 
    When viewing a waveform monitor in FLAT,what is considered  as being the maximum combined level of both luminance plus chrominance ? i.e. when measured in mV....IRE

    At Video-8 we supply to around the world (Broadcasters) and constantly get conflicting answers. 

    Here's Hoping 

    Vince Brant 

    Video-8 Broadcast 
    Australia 

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