Analog Domain Hardware Proc Amp adjustments (VHS) and Tektronix WFM601A

[Angies_Husband]

Ok, so not sure if this is a philosophical question or not, but I have a few questions around best practices for analog domain proc amp adjustments (i.e., before the content hits the video capture card).

My workflow is:
Mitsubishi HD-HS2000U ----(svideo)---> Leitch X75 TBC ----(SDI1)----> Aja Kona LHi
and
Leitch X75 TBC ----(SDI2)----> Tek WFM 601A

So I am able to watch the capture in real-time on the scope.
The Leitch X75 also has several options for proc amp adjustment:

• Black Level
• Luma Gain
• Chroma Gain
• Hue Phase
• Cb Gain
• Cr Gain
• Cb Adjust
• Cr Adjust

Which seems like a lot of flexibility, so my question is around tradeoffs while capturing.

Content is home movie VHS from 1985. This is raw capture footage and I know it's got a ton of issues. Right now, just trying the get the best capture possible so I don't have to go back to the analog domain (again).

Given a scene like this one:
VHS_Scene_Screenshot.jpg


You can see that the Diamond Display is showing out of range with the Cb channel (top right).

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However, the YPbPr Parade waveform is all legal, and then the RGB Parade waveform confirms the issue where the Blue channel would exceed legal values (and clip). Note: I have the Waveform cursors to show the 7.5 IRE to 100 IRE range (e.g., about 53mV to 714mV)
VHS_Diamond_scope.jpg

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Now... If you refer back to the scene, the clipping area appear to be on the left shoulder of the light blue dress when it's getting sun highlights?


So, my question is around the tradeoffs:
1. How much effort should be put into pulling all the values into allowable spec?
2. Do folks have a "best practice" for iterating on proc amp adjustments?
3. What approach would you take, given the proc amp adjustments available on the Leitch?
4. Are software limiters with soft-clip "knees" a useful post-processing tool? (e.g., in Adobe Media Encoder) If so what sort of "knee" is a good standard (3%, 5%, 10%?)

I am assuming the tradeoff is around compressing the dynamic range of the majority of the content signal (i.e., if like 80-90% of the tape content was say in a rage of 500mV p-p, and by getting all the content into spec you end up compressing that to 250mV p-p, won't that degrade more quality in the capture and make software correction more difficult)?

I am pretty new to all of this, and actually just learned all of the above after spending a week doing captures wrong and realizing it too late... So trying to get it less wrong on take 2!

Looking forward to thoughts on the topic, and hopefully learn more from the depth of professional experience here in the forums!

Cheers,
Angies_Husband


P.S. I know there is another good thread recently here:
http://www.digitalfaq.com/forum/vide...s-capture.html
But this seemed to be more focused on SOFTWARE adjustments...

[msgohan]

Quick thoughts; don't have time at the moment to read the thread.

• Attach a sample. It's tough to say much based on one photo of a monitor showing the scene.
• If you're feeding SDI into the WFM, it's not going to tell you much that software scopes can't, since it's already digital at that point.
• Given that you're asking about the proc amp controls of a TBC, they may not be analog.

[Angies_Husband]

Quote:

Originally Posted by msgohan

• If you're feeding SDI into the WFM, it's not going to tell you much that software scopes can't, since it's already digital at that point.
• Given that you're asking about the proc amp controls of a TBC, they may not be analog.

Sorry, I should have articulated the question a bit differently. You are correct that the signal is in the digital domain once entering the external TBC. I was referring to the "analog domain" as "everything prior to the file produced by the capture card." Is there a better way to phrase that?

Anyway, I might imagine that if the capture card has a sw proc amp then it might be doing the same thing... But once the file is saved, post-processing in software isn't going to be able to bring back details that were clipped at capture (thus my question). Also, if you are using a software scope on the file that was captured, I am pretty sure that's NOT the same thing.

For fun, here is the flow diagram of the X75, and you can see that the A/D conversion is the first step before Frame Sync and the Proc Amp.
X75_InternalSignalFlow.jpg



The X75 does 8-bit quantization of the 1v p-p analog signal (in TBC mode)...

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...and the SD-SDI output is at the SMPTE 259M-C standard (270Mbps, 10-bit quantization).
X75_AnalogInputSpec.jpg


Now, I actually had to look this part up, but here is what wikipedia had to say about SDI

Quote:

"...For all serial digital interfaces (excluding the obsolete composite encodings), the native color encoding is 4:2:2 YCbCr format. The luminance channel (Y) is encoded at full bandwidth (13.5 MHz in 270 Mbit/s SD, ~75 MHz in HD), and the two chrominance channels (Cb and Cr) are subsampled horizontally, and encoded at half bandwidth (6.75 MHz or 37.5 MHz). The Y, Cr, and Cb samples are co-sited (acquired at the same instance in time), and the Y' sample is acquired at the time halfway between two adjacent Y samples..."

So not meaning to make this a discussion about Nyquist/Shannon sampling theorem, and the fact that decades of broadcast engineers can't be wrong, the 13.5MHz SDI is more than sufficient to reconstruct a continuous time-domain signal of <5MHz (e.g., Luma).

Quote:

Originally Posted by msgohan

• Attach a sample. It's tough to say much based on one photo of a monitor showing the scene.

I could attach a sample, but my question is not about a particular scene. It's more about philosophy and best practice methodology.

Hope that helps!

[msgohan]

Quote:

Originally Posted by Angies_Husband

I could attach a sample, but my question is not about a particular scene. It's more about philosophy and best practice methodology.

It's so much easier to answer with examples that can then be extrapolated.

Quote:

Originally Posted by Angies_Husband

if you are using a software scope on the file that was captured, I am pretty sure that's NOT the same thing.

Besides the digital blanking areas, what information does your SDI scope have available to it that isn't contained in the file dumped from the same SDI stream onto your hard drive?

[Angies_Husband]

Quote:

Originally Posted by msgohan

Besides the digital blanking areas, what information does your SDI scope have available to it that isn't contained in the file dumped from the same SDI stream onto your hard drive?

Maybe I'm confused, I thought the "digital blanking areas" were the whole point of doing proc amp adjustments prior to capture? Meaning, getting the analog signal into an acceptable range so it's not clipped?

Is that not true?

[msgohan]

Quote:

Originally Posted by Angies_Husband

getting the analog signal into an acceptable range so it's not clipped

This is the goal.

But that has nothing to do with digital (or analog) blanking. That's the area outside the active video.

SDI blanking.PNG

[Angies_Husband]

Ah, got it. Thanks for the figure, that helps explain it. "blanking" != "clipping"

But so back to the original question, and how much effort should go into proc amp adjustments?

And for clarity maybe there are three different places you could "scope?" [a,b,c]?
where:
VHS --[a]--> Capture Card --[b]--> "Captured Clip" --> Some other Software (e.g., Avisynth) --[c]--> "Edited Clip"

Are you saying the the video content is the same at all three points [a], [b], [c]?
I thought that at [b] and [c] is where things get "clipped"?


EDIT:
I did just manage to find an older post from 2016 that I think talks about my original concern:
http://www.digitalfaq.com/forum/vide...html#post45895

Sanlyn points out that the RGB content clipped that original YUV content.

So... is that to say that as long as you keep your capture in YUV, the full range of the 1v p-p analog signal is in there, and thus you can adjust it appropriately later in the workflow?