Revised NTSC Video to Analog Oscilloscope Converter circuit - YouTube Video

[w2aew]

I revisited the NTSC to Analog Scope converter circuit that made a video on last year, and made some improvements to it  .  I added an improved z-axis drive amplifier which gives better contrast and brightness controls for a larger variety of analog scopes.  Here is the video describing the circuit:


I have also attached a PDF scan of the schematic.

Enjoy!

[BiOzZ]

very nice! cant wait to see how this is going to turn out!

[GK]

This might be an effective way to test out the capabilities of a DSO with the "intensity grading" feature?

[w2aew]

This might be an effective way to test out the capabilities of a DSO with the "intensity grading" feature?

Hmm, not sure.  The intensity grading feature on the DSOs have to do with helping you see which portion of the waveform(s) are being written more often, or most recently - simulating the intensity of the old glowing phosphor trace.  However, I don't think many (any?) of the DSOs give you a way of *controlling* the intensity based on a voltage value (video luminance).  It is this Z-axis modulation that is lacking in most (all?) DSOs. 

I'd love to hear which DSOs, if any, have Z-axis input for externally modulating intensity.

[BravoV]

Alan, how about PAL signal ?

[GK]

I'd love to hear which DSOs, if any, have Z-axis input for externally modulating intensity.

I really don't know; the last time I turned on a DSO with the intensity grading feature, it was one of the first Tektronix DPO models provided for evaluation. After barfing at the terrible display it was put straight back into the box!

However that was ~8 years ago or so and things have moved on since then. I see no technical reason for a "DPO" not to have a Z input. In fact, since they're generally marketed as being capable of emulating the display of an analogue oscilloscope, they should damn well have one!

[w2aew]

I'd love to hear which DSOs, if any, have Z-axis input for externally modulating intensity.

I really don't know; the last time I turned on a DSO with the intensity grading feature, it was one of the first Tektronix DPO models provided for evaluation. After barfing at the terrible display it was put straight back into the box!

However that was ~8 years ago or so and things have moved on since then. I see no technical reason for a "DPO" not to have a Z input. In fact, since they're generally marketed as being capable of emulating the display of an analogue oscilloscope, they should damn well have one!

They "emulate" the phosphor intensity grading and persistence/decay based on writing rate and density, but do not (as far as I know) emulate any kind of external intensity modulation (z-axis).

[notsob]

LM1881 also does PAL - R-ext is used for a timing change, or have a look at LMH1980 does all 1881 does plus HD 1080p etc.

http://www.ti.com/lit/ds/symlink/lm1881.pdf
http://www.ti.com/lit/ds/symlink/lmh1980.pdf

[GK]

They "emulate" the phosphor intensity grading and persistence/decay based on writing rate and density, but do not (as far as I know) emulate any kind of external intensity modulation (z-axis).

Yes, but there surely isn't any technical reason why the "intensity" could not be modulated by a sampled analogue z-axis input rather than computed from the writing rate and density.

 
From page 26

http://docs-europe.electrocomponents.com/webdocs/0da8/0900766b80da861a.pdf

"Z Axis

A digital phosphor oscilloscope (DPO) has a high display
sample density and an innate ability to capture intensity
information. With its intensity axis (Z axis), the DPO is able
to provide a three-dimensional, real-time display similar to
that of an analog oscilloscope. As you look at the waveform
trace on a DPO, you can see brightened areas – the areas
where a signal occurs most often. This display makes it easy
to distinguish the basic signal shape from a transient that
occurs only once in a while – the basic signal would appear
much brighter. One application of the Z axis is to feed special
timed signals into the separate Z input to create highlighted
“marker” dots at known intervals in the waveform.

XYZ Mode with DPO and XYZ Record Display

Some DPOs can use the Z input to create an XY display
with intensity grading. In this case, the DPO samples the
instantaneous data value at the Z input and uses that value
to qualify a specific part of the waveform. Once you have
qualified samples, these samples can accumulate, resulting
in an intensity-graded XYZ display. XYZ mode is especially
useful for displaying the polar patterns commonly used in
testing wireless communication devices – a constellation
diagram, for example. Another method of displaying XYZ
data is XYZ record display. This mode the data from the
acquisition memory is used rather than the DPO database."

Pity they don't give a list of model numbers with such capability. Don't have the time right now to trawl through DPO/DSO specification sheets but perhaps someone else out there could chime in.

[w2aew]

Yes, the current crop of Tek DPO/DSA/MSO scopes (5000, 7000 and 70000 series) do have an XYZ mode.  I haven't tried this out yet to see how well it works in this application - but I will!

BTW - for anyone following this thread, I corrected a small mistake in the component values in the schematic attached to the original message.  Please re-download it if you were intending to replicate this circuit.

[MMR]

Hello sir,
           I found your circuit and which i seen it ll be useful to my project , because  i am working in 1980's model instrument it has a CRT output. To provide the video signal MC6845 crt controller is been used on that board. so to the crt there is only five cable is connected  from the main board(6845). Three of it for power supply to the crt and two of it for (shield cable) video i/p and video ground. I got the problem with the crt. so i thought by using your circuit can i receive the video signal from the main board to oscilloscope. I had a dual channel oscilloscope. Please guide me for this issue.
_ Is there any other method to confirm the video signal production from MC6845 using oscilloscope.

[David Hess]

They "emulate" the phosphor intensity grading and persistence/decay based on writing rate and density, but do not (as far as I know) emulate any kind of external intensity modulation (z-axis).

Yes, but there surely isn't any technical reason why the "intensity" could not be modulated by a sampled analogue z-axis input rather than computed from the writing rate and density.

I would amount to adding another high speed digitizer albeit with simplified signal conditioning.  That would not make economic sense for an inexpensive DSO.  Better would be a 4 channel DSO which used a 3rd channel as a Z-axis input for X-Y displays but I am not aware of anything like that.

Some old DSOs can use their z-axis input to gate sampling for blanking purposes but that does not support intensity modulation.