Quantcast Is VCR blue screen a reference point? - digitalFAQ Forum
  #1  
02-22-2019, 05:27 PM
Angies_Husband Angies_Husband is offline
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Just curious if anyone knows if the default "blue screen" that a VCR outputs supposed to be a true analog "blue" at some reference level.

When I look at my VCRs blue screen on my Tektronix WDM601, the Vector display (100% bars) has a single dot that is only a small chroma gain and phase adjustment from the "box"... feels like I should just make the proc amp adjustments in the TBC to get it in there... But that ended up being too hot on the blue, so I readjusted the gain for 75% bars.

This now seems to also correctly match the upper limit of about 0.6v (i.e 85% of the 0.7v luma signal) when I look on the waveform display.

So really two questions:
1. Does the VCR think it's displaying a pure blue signal, and if so, is it valid to calibrate to? (either chroma phase or chroma gain)
2. Is it best practice to ensure the content you are playing is within NTSC spec (i.e., "legal") and then use post processing after capture to improve color, contrast, etc...?

Thanks!
(I am not a broadcast engineer, but rather an EE that likes old scopes, so this is kinda fun! Still learning!)
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  #2  
02-24-2019, 07:28 AM
Eric-Jan Eric-Jan is offline
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I guess it's more of a "default" and sometimes also without sync signal, or nothing is connected..(if there's no OSD text/menu in it) so you might want to avoid it.
By default it is mostly something the electronics go default on... no signal input > blue screen.
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02-24-2019, 08:52 AM
dpalomaki dpalomaki is offline
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Quote:
1. Does the VCR think it's displaying a pure blue signal, and if so, is it valid to calibrate to? (either chroma phase or chroma gain)
2. Is it best practice to ensure the content you are playing is within NTSC spec (i.e., "legal") and then use post processing after capture to improve color, contrast, etc...?

Read more: http://www.digitalfaq.com/forum/newreply.php?do=newreply&noquote=1&p=59623#ixzz5gS YriH95
1. One would have to dig though the design information for the specific VCR to sort out what it is supposed to be outputting as the blue screen, and then measure it with a calibrated waveform monitor to see what it actually is, and whether on not is is consistent from day to day. I would not count on it as a calibration source. It is an internally generated signal and not related to what may be recorded on a tape you are playing.

2. A good practice, since you have the 'scope is to check the output of the VCR as you play tapes and using a proc amp adjust the analog signal to be within the applicable standards for the format signal you are capturing. Also make sure your capture system is set for input at the corresponding standards; e.g., the the pedestal (black level) for USA NTSC should be 7.5 IRE all the way. Highlights you want to preserve should be no more than 100 IRE.
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02-24-2019, 08:33 PM
Angies_Husband Angies_Husband is offline
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Quote:
Originally Posted by dpalomaki View Post
1. One would have to dig though the design information for the specific VCR to sort out what it is supposed to be outputting as the blue screen, and then measure it with a calibrated waveform monitor to see what it actually is, and whether on not is is consistent from day to day. I would not count on it as a calibration source. It is an internally generated signal and not related to what may be recorded on a tape you are playing.
Makes sense -thanks!

Quote:
Originally Posted by dpalomaki View Post
2. A good practice, since you have the 'scope is to check the output of the VCR as you play tapes and using a proc amp adjust the analog signal to be within the applicable standards for the format signal you are capturing. Also make sure your capture system is set for input at the corresponding standards; e.g., the the pedestal (black level) for USA NTSC should be 7.5 IRE all the way. Highlights you want to preserve should be no more than 100 IRE.
One thing I found confusing is the 'scoping of analog vs SDI signals... I understand that the NTSC analog should be 7.5 IRE for black. However - after doing some research it appears that when you look at the SDI "luma" waveform (i.e., the analog signal has been brought into the digital domain for SDI transmission), the black is set to 0 IRE (e.g., 0 volts). This would be consistent with what I am seeing on my WFM601A (i.e., black at 0.0V and white at 0.7V), but just want to be sure...

Is that understanding correct?
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  #5  
02-24-2019, 09:17 PM
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lordsmurf lordsmurf is offline
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Having used many JVC models, the answer is a definite "no". The blues are all sorts of tints, levels and saturations.

It's only useful if you measure the blue from that deck in advance. You can always calibrate it against a Pantone swatch, but then everything must be calibrated (monitor, swatch source).

I use JVC blue screens for testing, and calibration of TBCs (ie output matches expected blue for that VCR), but nothing more.

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  #6  
02-25-2019, 06:20 AM
dpalomaki dpalomaki is offline
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IRE and volts apply to the analog signal and image. They have digital domain equivalents, but problems arise when you start talking real world implementation, especially with consumer gear and formats. USA used 7.5 IRE as the black level (setup/pedestal) for SD NTSC video, Japan's NTAS and the PAL system used 0 IRE as black.

http://www.glennchan.info/articles/technical/setup/75IREsetup.html
provides an interesting take on the issue.

Bottom line is the playback device should output a black level that corresponds to what the display device expects to see. And the intermediate steps along the way to the output should preserve the signal including its dynamic range.
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02-26-2019, 08:22 PM
jwillis84 jwillis84 is offline
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I have a "vague" recollection from the 70's when I was growing up that GE televisions first introduced the "blue background" instead of "static" upon signal loss.

I think Popular Electronics even published a renegade "blue circuit" to the do the same in TV's without the feature. You couldn't buy it in other brands for a time because of patent or trademark issues.. but how you patent or trademark a color was beyond me at ten years old.

If that recollection has even a slim bit of truth to it.. then "no".. I don't think it was regulated or sanctioned as any kind of "standard" by any body other than the lawyers that might pursue you for infringing on their shade of blue. Reminds me of the "blue men from Intel" or the "blue meanies from Firefly".

By the time VCRs came about I also sort of remember this getting sorted out by Panasonic in their Omnivision line? Again.. I have a faulty memory.. that was the 80's.. but by the 90's I do recall "blue screen" started showing up in lots of VHS players.

When Gemstar and Tivo/Macrovision (Rovi'sion) started sueing people for electronic program guides.. I seem to recall someone also citing the "blue wars" of old.. but no one thought it was funny. Everyone was tired of the "format wars" by then.
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  #8  
02-26-2019, 09:37 PM
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Quote:
Originally Posted by jwillis84 View Post
blue .. but how you patent or trademark a color was beyond me
Papa Smurf would be pissed.

I remember snow and black screen until the mid 90s.

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  #9  
02-27-2019, 08:14 AM
dpalomaki dpalomaki is offline
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The patent might have covered the muting (where in the noise display associated with loss of signal was "silenced" to a solid color or some other pattern) aspect of the circuit, not the specific color. A bit like squelch in radios. Copyright or trademark protections might have applied to the description or name of the process.

IBM was knows and "big blue" and the "black boards" in their buildings were white with blue chalk back in the 1960s. Over the air broadcast TV was great for ghosts, and in fringe locations, snowy images and maybe one or two channels.

Windows gave us the "blue screen of death."

And who says blue is a cold color? consider Smurfette - was she hot or what ?
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  #10  
02-27-2019, 06:19 PM
Eric-Jan Eric-Jan is offline
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What i know of the blue screen, is that it prevented a television to automaticly switch off, the normal noise/snow screen caused a tv to go to auto shutdown/standby mode
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