Author Topic: Smallest colour CRT viewfinder?  (Read 618 times)

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Offline MindBender

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Smallest colour CRT viewfinder?
« on: June 11, 2019, 07:47:34 pm »
Back in the day, camcorders had black & white miniature CRT based viewfinders at first. At the end of this era, colour LCD viewfinders became available too, but because their resolution was very poor in the beginning, many high-end camera’s stuck with black & white CRTs. Many wannabe-hobby-electronics-engineers harvest these tiny black & white CRTs from old camcorders and show them off on youtube and hack-a-day as if they have just invented warm water.

This particularly small sized CRTs were never made in colour versions, probably due to size constraints of the electron gun. The smallest colour CRT I am aware of is the one in the Panasonic CT-101 portable TV, but that tube is already significantly larger, and still they have to resort to all kinds of tricks to make it display colour.

Around this era I was still in school, studying electronic engineering, and I remember there briefly was a camcorder with an extraordinary viewfinder on the market, combining best of both worlds: CRT resolution, but with colours. The way they did this was by using the same black & white miniature CRT everybody else did, but they combined it with a colour wheel. Colours weren’t brilliant to modern standards, but they were a lot better than the colours and the resolution on miniature LCDs at that time.

After many nights of researching (there isn't much information on vintage 90's camcorders on the Internet) I found out that this camera in my memories must be a Mitsubishi HS-CX6. And with some luck I was able to purchase a working version off of eBay. I cracked it open and indeed, it has a colour wheel. Or better; Colour cone, so save space.

Now I wanted to convert this viewfinder in a little gadget. 3D print an open housing for it, connect a raspberry pi to it and show people how great it is. But unfortunately this viewfinder isn't as self-contained as I hoped it would be. I expected it to take a black & white video signal, containing field data of red, green and blue time-domain multiplexed. Still 50 fields per second, consisting of 16.7 red, 16.7 green and 16.7 blue fields. Simple technologie, no need for memory.

But unfortunately it works differently. De viewfinder runs on 5 volts and received a 4.5 volt DC signal of which I do not know the function. Further more it receives a 50Hz block wave (95% high) which seems to be field sync, and a 15.625kHz block wave (don't recall the duty cycle) which appears to be the line sync. So far so good, but the last signal it receives is really unexpected.

The last signal resembles a video signal, but contains 3 fields between field sync pulses, and 3 lines between line sync pulses. That suggests some memory is involved in time-domain multiplexing three separate colour fields into the time slot of a full-colour field.

Does anybody know a cheap way of doing that? I will post photo's of this viewfinder later.
 

Offline dzseki

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Re: Smallest colour CRT viewfinder?
« Reply #1 on: June 12, 2019, 06:58:26 am »
Very interesting project.
I don't know for sure, but the trick must lie in that VHS has lower resolution (both luma and chroma) than standard PAL, and they may take advantage of that during the display.
 

Online TurboTom

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Re: Smallest colour CRT viewfinder?
« Reply #2 on: June 12, 2019, 08:32:42 am »
Have a look at the RCA CKC-021 viewfinder utilizing the 1.5" Hitachi beam index color CRT. That's probably as small as it gets color-CRT-wise. Here's a very nice web site on the beam index technology. Once in a while, one of these cameras with color CRT viewfinders can be found on certain well-known online auctions...
« Last Edit: June 12, 2019, 11:59:36 pm by TurboTom »
 
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Online james_s

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Re: Smallest colour CRT viewfinder?
« Reply #3 on: June 12, 2019, 04:24:23 pm »
The color wheel viewfinder sounds fascinating, can you post some pictures of the inside?

I remember those beam index color CRT viewfinders, I saw a few of them in store demos back when they were cutting edge. Always wanted one to play with.
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #4 on: June 12, 2019, 07:38:20 pm »
Mitsubishi coined this technology 'TrueFinder'. Attached a couple of photos of the exterior, in case others want to hunt down a piece of this technology too. Note the bulge on the right-hand side of the unit, and the 'tint' and 'color' adjustments on the left-hand side.
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #5 on: June 12, 2019, 07:39:58 pm »
And, to the best tradition of this forum, a couple of x-rays of the unit.
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #6 on: June 12, 2019, 07:54:04 pm »
And here are the vintage tech pr0n money shots. Note that the colour wheel is more like a color cone, to save space. It has RGBRGB filters, where the blue is more violet than blue.
« Last Edit: June 12, 2019, 07:56:31 pm by MindBender »
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #7 on: June 12, 2019, 08:19:44 pm »
And a couple of pictures with the top PCB removed.
A service manual for this camera, schematics or a repair guide for this camera would be really helpful (https://www.eevblog.com/forum/repair/mitsubishi-hs-cx6-camcorder-schematicsservice-manual-wanted/).
« Last Edit: June 12, 2019, 08:24:03 pm by MindBender »
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #8 on: June 12, 2019, 09:04:36 pm »
Ok, this is not simple timed analog demultiplexing. There's some serious electronics on the top PCB:
Top side
74HC4046 (High Speed CMOS Logic Phase-Locked-Loop with VCO)
Sony CDX1172AM
3x Mitsubishi M5M4C500AVP (Field memory -> Thanks Berni)
Mitsubishi M51289FP (PAL video chroma signal processor-> Thanks Berni)
Motorola C28W (MC68HC05P9)
BA6458FP (3-phase motor driver)
Bottom side
NEC 65025 (?)
Sony CXD1170M (6-bit 40MSPS High Speed D/A Converter)
« Last Edit: June 13, 2019, 09:15:30 am by MindBender »
 

Online james_s

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Re: Smallest colour CRT viewfinder?
« Reply #9 on: June 13, 2019, 02:39:08 am »
Hey that's neat, thanks!

RAM? Is it enough that they could be digitizing a whole frame at a time and then scanning it out 3 times to get the color? You might learn something by measuring the speed of the color wheel and the vertical scan.

I bet that thing makes a better image than the little beam index tubes. Something I really like about the NuColor display on my Tek scope is that it's a B&W CRT so there's no shadow mask screendoor look.
 

Offline Berni

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Re: Smallest colour CRT viewfinder?
« Reply #10 on: June 13, 2019, 07:28:53 am »
Yep those three identical chips are RAM

To be more precise its Mitsubishi 81920x6 RAM. They call it being "field memory" because it has 320x256x6bit of space in it. So by the looks of it they are likely storing the red green and blue each in its own RAM chip to form RGB666 format giving you 18bit color.

So this thing must store one entire field of video into RAM. Then as soon as there is enough data the CRT must start drawing from RAM very quickly to make room for the next frame. Some of this could possibly overlap since the data is moving out of RAM faster than the video is filling into it.

This being DRAM its also structured to be 320x256 so that row select always happens in the horizontal blanking period.

The other Mitsubishi chip is just analog PAL video processor that turns the composite signal into red, green and blue signals.
 
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Online TurboTom

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Re: Smallest colour CRT viewfinder?
« Reply #11 on: June 13, 2019, 07:55:17 am »
Since the TV video signal consists of two half frames that are interlaced, and the tiny CRT for sure doesn't need (or is capable of) a higher full-frame resolution than 320x256, even with the limited amount of memory in the "field memory" chips, a full double buffering is possible. I cannot imagine how the engineers would have gotten along with contemporary (parallel) storing the RGB signal and sequential (color-wise) scanning it to the screen on a full-frame base.

I assume the color wheel will be rotating at 3000RPM (PAL) or 3600RPM (NTSC), the tube thus having to scan vertically at 150Hz (PAL) or 180Hz (NTSC). Anyway, very peculiar design and quite an effort for a view finder.

It's amazing how things have changed with the invention of the tiny TFT panels. But these "curiosities" besides the main-stream are the spice that make this hobby so fascinating...
 

Offline Berni

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Re: Smallest colour CRT viewfinder?
« Reply #12 on: June 13, 2019, 08:13:36 am »
Well this RAM has separate data input and data output pins unlike most RAM where the data bus is bidirectional. Also it appears that this RAM is capable of reading an entire 256 word row at once and then slowly shifting it out while other operations are going on. So by the looks of it it appears like this RAM chip was designed to be easily written and read from simultaneously and being operated on entire lines at a time.

I do find it amazing to what effort all of the engineers working with analog video had to go trough. So many products did hugely convoluted and elaborate things in order to provide extra functionality. Just fitting color into a composite signal is rather complicated, let alone the design challenges involved with later on storing the signal on magnetic tape or LaserDisc. Modern digital video is boring.
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #13 on: June 13, 2019, 05:25:58 pm »
The amount of collective knowledge just never ceases to amaze me. One of the Mitsubishi chips being a common PAL decoder gives me hope that I can offer it a regular PAL signal, even though I'm pretty sure I saw 3 fields between field sync pulses. I guess I have to make a decent break-out cable and measure again. I will post my oscillographs here.

Anyway, this viewfinder turns out to be much more complex than anticipated. Perhaps even over engineered, given the simple problem is solves. Exactly the way I like it! Perhaps Mitsubishi borrowed this technology from their projection TV division, or perhaps it is the other way around. In hindsight not a bad investment of engineering, I suppose.

Attached is an x-ray of the PCB, for the curious among us.
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #14 on: June 13, 2019, 07:00:55 pm »
I don't know what it is I measured before, but currently I'm measuring a solid colour composite (CVBS) signal op pin 3, colour burst and all. It seems like we're getting to repurpose this technology gem without too much effort after all!
 

Offline MindBender

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Re: Smallest colour CRT viewfinder?
« Reply #15 on: June 13, 2019, 08:17:19 pm »
And here's the pinout:
1. Field sync (50Hz)
2. Power supply (5V)
3. Composite video (colour, PAL)
4. Ground
5. Line sync (15.625kHz)
6. !Reset? (4.55V, rises 5ms after power, lowers simultaneously)

In the oscillographs attached, pin 3 is shown on channel 1 (yellow trace), pin 1 in shown on channel 2 (green) and pin 5 is shown op channel 3 (blue trace).
Note the weather large DC offset on the composite video signal, and the difference in the line sync between both fields.
« Last Edit: June 13, 2019, 08:23:11 pm by MindBender »
 

Offline Berni

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Re: Smallest colour CRT viewfinder?
« Reply #16 on: June 14, 2019, 05:50:08 am »
That's pretty easy then.

The camera likely has these sync signals available in its internal circuitry so they saved themselves some work (And PCB space, this thing is pretty compact) by simply running the signals over to the viewfinder rather than recovering them from the video signal.

But this is actually a easy thing to do. There are chips that do exactly this such as the LMH1981:
http://www.ti.com/product/LMH1981
 


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