Latest post Sat, Jun 16 2018 11:40 AM by George Angeludis. 3 replies.
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  • Fri, Jun 15 2018 4:36 AM

    • dreid
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    Problem with black levels (frequency) of DNxHR HQX when exporting UHD timeline

    Hello all, I was wondering if you could help me with a vexing problem that perhaps one or more of you has encountered in the past.

    I am exporting my timeline: 2160p, 29.97 UHD. Codec, as per the AMA File Export tool is DNxHR HQX in an MXF OP1a wrapper.

    Problem is, when I send this file to the QC facility, they are reporting “frequency issues” (not level issues per se or how it looks (it looks great) but frequency issues in their terminology.) First of all, what does that mean?

    Well, they say that, when looking at my file using a Tektronix scope they see “black levels to around minus 30mV and video levels to about plus 730mV. Mostly from ringing around the fonts.”

    Well, I say that is impossible because I have applied the Avid Safe Color Limiter (rendered before export of course) which clips the levels at 0mV and 700mV. My scopes certainly reflect this: there is no excursions past 700 or below 0 when viewed with my built in Avid scopes and my external Blackmagic scopes. I realize the Blackmagic SmartScope Duo is no Tektronix, but a clip is a clip, it is either there or it is not. Blackmagic or Tektronix it shouldn’t matter: if I have applied a limiter, and my scopes show the limiter is working, then why is it not that way after I transfer the file to them?

    The QC facility says that, although the illegal excursions are completely invisible to the naked eye and are only visible on a high end scope, these -30 or +30 excursions could be enough for a broadcaster to reject my show.

    I say that I have rendered the limiter or clip before my export and that, on my scopes, all looks well, so how could the levels be out of whack when they reach their facility? The problem must be on their end, no?

    I am using Avid 8.5.3 on a PC. They are looking at my file (linking via AMA import) using Avid 8.4.2 on a Mac. Could that have anything to do with it?

    I know there are a lot of brilliant online editors in this forum who are very smart when it comes to file sharing and levels issues/incompatibilities. You have no idea how much this would mean to me to get this sorted out as we are at a standstill with my project right now.

    Essentially it seems to me to be a problem with Mac-Avid ability to understand the black levels of DNxHR HQX files exported by PC-Avid.

    Any advice at all would be EXTREMELY appreciated and rewarded.

    Thanks in advance everyone who reads this post!

    MC 8.5.3, DNxIO w/ 10.5.4, Win 7 Pro 64 bit SP1, Dell 7910, Dual 10 core Xeon 3.10, 128 GB DDR4 RAM, Quadro K5200, 1 TB SSD system drive, 1 TB SSD cache... [view my complete system specs]

    And back to a windowless room full of Avids he goes.

  • Fri, Jun 15 2018 10:50 PM In reply to

    • Lukas Boeck
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    Re: Problem with black levels (frequency) of DNxHR HQX when exporting UHD timeline

    hmm,

     

    any chance the "ringing around the fonts" correlates with external graphics that have been imported incorrectly with "do not modify" color levels?

    i'd get another opinion on the file in say after effects. measure with e.g. color picker.

     

     

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  • Sat, Jun 16 2018 9:45 AM In reply to

    • Bruno M
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    Re: Problem with black levels (frequency) of DNxHR HQX when exporting UHD timeline

    Although I'm by no means a modern-day TV engineer, I did study television engineering in the late 1980's, and transient under/overshoots (or 'ringing') is a well known problem which is difficult to completely eradicate.

    The first thing you should do is bring the offending export back into MC and see if you can see any difference between it and your final sequence. If all looks OK to you using whatever internal /external scopes you have, we must assume that unless the the QC facility have some fault with their equipment, you are effectively blind to this problem.

    dreid:
    Blackmagic or Tektronix it shouldn’t matter

    Not all scopes are created equal. Proper signal measuring equipment that costs many thousands of dollars (like the Tektronic scopes) are designed to measure very high frequencies - after all, they're designed for accurate analysis of signals to identify the very problem we're talking about. Scopes costing a few hundred dollars are not going to have the circuitry to do this. In fact, if these cheaper scopes were showing any of these artifacts, it's likely they'd employ filtering circuits to 'clean up' the display.

    Knowing these limitations, I have a HD Leader scope which cost me several thousand dollars, and (with all the filters switched off) that shows various transient 'spikes' with SDI signals coming from my video hardware (blackmagic and Avid Mojo DX). These spikes are not visible in the software scopes in apps such as MC or Resolve.

    I'm not familiar with the technical specs in th US or Canada, but here in the UK this is a known problem that is mentioned in broadcaster's technical specifications.

    The BBC document specifically mentions the tolerances allowed - I've copeid and pasted them below...

    ---------------------------------------------------------------------------------

    1.2.1. Video Level Tolerance
    In practice, it is difficult to avoid generating signals slightly out of range, and it is considered reasonable to allow a small tolerance:
    • the RGB components and the corresponding Luminance (Y) signal, should not normally exceed the “Preferred Minimum/Maximum” range of digital sample levels in the table below,
    • measuring equipment should indicate an “Out-of-Gamut” occurrence only after the error exceeds 1% of an integrated area of the active image.
    For further details see the EBU Recommendation, EBU R103.
    Any signals outside the “Preferred Minimum/Maximum” range are described as having a gamut error (or as being out of gamut). Signals cannot exceed the “Total Video Signal Range” and will therefore be clipped.
    Full range video levels must not be used for delivered television programmes.
    Colour gamut "legalisers" should be used with caution as they may create artefacts in the picture that are more disturbing than the gamut errors they are attempting to correct. It is advisable not to “legalise” video signals before all signal processing has been carried out.

    -----------------------------------------------------------

    I can't seem to paste the chart referred to, but here's link the EBU document...

    https://tech.ebu.ch/docs/r/r103.pdf

    In respect of the 1% mentioned, I know there are Tektronic scopes that have this 1% 'button' specifically for the purpose of assisting QC sessions.

    So the question is, how bad is your out-of-gamut problem and do they fall within the technical specs demanded?

    It's difficult to know where your problem lies without looking at the material using a proper scope - certainly graphics and titles are obvious candidates.  In fact, Avid have for years used filtering on titles created with their title tool to help with this problem and when we all went HD, people noticed this softness compared to (say) titles brought in from other applications. I can't quite find the forum topics that covered this, but there was a console command to switch this off, with the possible problems this might cause.

    As far as the Avid legaliser is concerned, I certainly remember I never relied on this to corerectly legalise my exports as I discovered it wasn't properly catching everything - according to my separate hardware legaliser. This was in the days of outputting to tape. Today, with file based delivery, some facilities are using software specifically designed to look at all sorts of errors can manifest themselves in digital deliveries.

    At the end of the day, I guess you are going to have to comply with the conditions set by the specs and the QC facility, and you'll need to firstly be able to identify the problem, and secondly find ways to correct it, possibly with other software-based solutions or legalisers. You could of course just get a QC facility to do the legalising for you, and build this into the program costs.

     

     

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  • Sat, Jun 16 2018 11:40 AM In reply to

    Re: Problem with black levels (frequency) of DNxHR HQX when exporting UHD timeline

    I always apply the Safe Color at 4:2:2 but only above video tracks and below title track (s).

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