HD NBTV?

Forum for discussion of narrow-bandwidth mechanical television

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Postby Steve Anderson » Sun Jun 01, 2008 12:20 pm

Well, I'm surprised by all the positive remarks, thanks all.

Steve, I know what you mean about 'fudging' to get mechanical (non-moving) parts to align, what you can't see in the photo is the power supply which is on the underside of the chassis, that really is ugly!

Chris, the tube I used is a 3" DG7-32 which may not be easy to get in Australia, but common in Europe. It has a maximum voltage rating of 800V and the datasheet shows typical operation at 500V. It was made by Philips and Mullard in both the UK and France. It has a P1 phosphor, the 'DG' being the European designation for that.

In this display I'm running it at around 650V but as I mentioned elsewhere I'm going to mothball this display and start afresh with a 5" 5ADP1 which requires around 3000V, but has the advantage of not only a larger display, but a flat faceplate and also no internal graticule. I also have a couple of 5ADP7s with the long persistence phosphor, so I'm tempted to make the display dual-standard between NBTV and SSTV, swapping the CRT depending on what use I wish to put it to.

Now those of you who remember the 60s in the UK will also recall those horrendous dual-standard TVs, 405/625. Huge great switches that changed almost everything in the electronics between standards. They were cheaply made and perhaps one of the most unreliable electronic components ever produced. TV repair guys must have loved them!

The reason these dual-standard TVs were made was that the new BBC2 channel was 625 and for some time both BBC1 and ATV/ITV stayed on 405 lines, this changed with the introduction of colour.

Footnote: Something I recently found out is that 405 lines was not unique to the UK, it was also used in various places for a while, Hong Kong and New Zealand amongst them.

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Postby DrZarkov » Sun Jun 01, 2008 5:21 pm

There was even a short history of 405 lines in Germany: According to "Practical Television" the british army in occupied Germany wanted to watch the coronation. Of course it was transmitted by the German TV (in 625 lines) in that time, but Bielefeld/Paderborn area (many thousand british soldiers were in that area in that time) was not covered by any German transmitters, so the army got a licence to make their own transmitter in Bielefeld. One day with 405 lines in Germany!

(about 15 to 20 years later BFBS-television was started in all towns with British Forces in Germany. Of course in 625 lines, norm "I". Some of those transmitters are still running, I saw "Dr. Who" the first time on "BFBS colour" as this station was called until the late 80s. I was the only one in our village who could receive BFBS TV, most people did not even knew that it existed! But the programme was without sound, I learned about different systems an I wanted a decoder, which costed me 80 DM. One of the things that got me interested in television technics as a boy of twelve...)
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Postby Steve Anderson » Sun Jun 01, 2008 5:42 pm

Well Volker, that is an interesting piece of history. I had completely forgotten about BFBS, yet I now remember them from my schooldays. I recall listening to "Family Favourites" on the BBC on Sundays, BFBS carried this around the world, not live, but on tape. In fact to keep the costs down the chit-chat between the two presenters (say in London and Cyprus) was pre-recorded and the music edited in later or perhaps played live.

So now we have yet another location where 405 was used! I'm sure Steve O will be interested!

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Postby Steve Anderson » Sun Jun 01, 2008 6:07 pm

DrZarkov wrote:One of the things that got me interested in television technics as a boy of twelve...)


This should go onto the 'Off-Topic' section, but I'll leave it as it is. At the age of eight or nine someone gave me a chemistry set as a present. I loved it, bad smells, carpet stains, all those boyish type of things.

One of the experiments was about electrolysis, this used a 4.5V battery to plate a key (or whatever) in a solution of Copper Sulphate (I think). Well, it was a slow process so to speed things up I connected it to the 240V AC mains.......the aftermath got me hooked...."I like this electrical stuff!" How I never killed myself in the ensuing years is beyond me, the things I did would now petrify me! Youth.

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Postby Panrock » Sun Jun 01, 2008 7:34 pm

Steve Anderson wrote:One of the experiments was about electrolysis, this used a 4.5V battery to plate a key (or whatever) in a solution of Copper Sulphate (I think). Well, it was a slow process so to speed things up I connected it to the 240V AC mains.......the aftermath got me hooked...."I like this electrical stuff!" How I never killed myself in the ensuing years is beyond me, the things I did would now petrify me! Youth.

Steve A.


Just as OT... I was about 13 when I made an 'arc lamp' from two pencils joined across the 240v mains (having calculated the likely current I had even connected two 13 amp sockets in parallel for this, not then understanding how a ring main was wired...).

I put my sunglasses on to view, and struck the arc. The arc lamp worked! But the pencils caught fire and I had to drop them. They left a nasty mark on the drawing room carpet. My parents were not best pleased. :cry:

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Postby Steve Anderson » Sun Jun 01, 2008 7:53 pm

Panrock wrote:I was about 13 when I made an 'arc lamp' from two pencils joined across the 240v mains......My parents were not best pleased. :cry: SteveO


That gave me a darn good chuckle! And neither were my patents, they insisted that I repaint the ceiling on my own and withheld my pocket money to pay for the paint. Now the tables are turned, and I have to instill the same senses into the girls here. Boy, it never gets any easier!

Steve A.

Anyway, let's return to the main theme of this thread.....
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Higher definition.

Postby Steve Anderson » Sun Jun 01, 2008 8:36 pm

Now those that have been following what I have been posting in this thread will understand I have a bandwidth limitation, namely 24kHz as it stands. At 48 lines and 12.5Hz frame rate in the aspect ratio I have chosen, that's the limit.

So if I wish to increase definition the frame rate has to suffer although I do have a small margin to use. If one ignores the flicker (or can eliminate it), quoting the late Doug Pitt (Vol. 29 No. 3) in the article 'Super NBTV-An Epilogue', "Perhaps complaints about "narrow band flicker" will soon be a thing of the past and amateur band transmissions at 6.25 frames per sec (just adequate for lip movement) will take off in the near future." End of quote.

So halving the frame rate to 6.25fps allows me to go to the the square-root of two increase in lines, plus my small margin, say 72 lines? Is this worth investigating gents?

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Re: Higher definition.

Postby Steve Anderson » Mon Jun 02, 2008 1:38 pm

Steve Anderson wrote:...Is this worth investigating gents? Steve A.


No massive response, but I'll plough on anyway. Looking closely at the 3" display it is feasible that it might handle 72 lines, and for the change of only four resistors and a slight rearrangement of the frequency dividers, it's worth a try.

Results in a few days...

Steve A.

Added later....As it turns out the frame rate won't be 6.25Hz but 6.667Hz, it's just the way the numbers work out:-

72 lines at an aspect ratio of 4:3 requires 96 active pixels, using 100 pixels/line allowing for retrace time and a sample rate of 48kHz results in a 6.667Hz frame rate, the line frequency being 480Hz.

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Re: Higher definition.

Postby Viewmaster » Mon Jun 02, 2008 7:00 pm

Steve Anderson wrote:Added later....As it turns out the frame rate won't be 6.25Hz but 6.667Hz, it's just the way the numbers work out:-

72 lines at an aspect ratio of 4:3 requires 96 active pixels, using 100 pixels/line allowing for retrace time and a sample rate of 48kHz results in a 6.667Hz frame rate, the line frequency being 480Hz.

Steve A.


I'm glad that you are going for 4:3, which in my little book, is the best aspect ratio of all over the years.
Nowadays everyone goes potty over 16:9 which is a compositional nightmare being too wide for its height. OK for filming letterboxes but no good for chimneys!

I am reminded of the story of the three bears....
"My aspect ratio of 3:2 is too high." :cry:
"My aspect ratio of 16:9 is too wide." :cry:
"My ratio of 4:3 is just right." :)
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Postby chris_vk3aml » Mon Jun 02, 2008 9:50 pm

Just a few figures and a warning about some (perhaps) unexpected problems:

You propose 72 lines = 5 angular degrees per Nipkow disc sector.
With 3:4 aspect ratio = 72 x 96 pixels = 6912 pixels x 6.667 = 46082.3 pixels per second. Top frequency required = 46082 / 2 = 23041 Hz.

This will probably be OK with CD recording, even better at 48 KHz CD-ROM sampling BUT problems are likely to occur with low frequency phase shifts as the frame repetition rate is so incredibly low. To get a system PHASE FLAT to 6 Hz requires a frequency response extending well BELOW even that frequency. We are already stretching this to the limit of most audio A/D cards at the 12.5 Hz rate.

There is also NO WAY that you can legally put a 23 KHz wide uncompressed video signal on hf ham radio below 29 MHz. Here in telecommunication region 3 (Australia) the legal bandwidth limit for the whole signal is 8 KHz on those hf bands. In your part of the world I believe the bandwidth limits are even more stringent. Even on 10 metres (above 29 MHz) you are limited to 16 KHz, which means 8 KHz of legal modulation on a.m. or about 15 KHz on vestigial sideband. NBFM will be a washout, owing to its higher order sidebands demanding even greater bandwidth, and due to most commercial NBFM transceivers having pre-emph/de-emph curves causing wild video phase shifts.

Are you aware of the difficulty of watching a 6.7 Hz frame repetition picture with no storage or interlace? Interlace, perhaps more than 2:1 interlace might be worthwhile, but the electronics to generate this may be a bit fearsome. The Sanabria concept of 3:1 interlace at 45 lines was a good one.

There are also good reasons for NOT using horizontal scanning OR the 3:4 ratio on a picture of less than 100 lines where head-and-shoulders are the most suitable subject - as Baird empirically determined all those years ago. Facial images simply suit vertical scanning and narrow vertical aspect ratios better, by disposing the limited number of pixels in a more economical way around the face. Indeed, another good reason for using a tall narrow picture was to decrease the number of lines while retaining a fair number of image pixels. Why should we want to do this? Well, the sector angle for a 30 line scanning disc is 12 degrees - and that gives a fair leeway for angular errors in laying out the scanning disc. The usage of a narrow picture also avoids the bogey of low frequency phase shift, as a tight close-up of a face covering the full width of the vertically scanned picture will have minimal transverse shading - shading that would be resolved into low frequency video components down near the frame rate. It is actually my opinion that our 2:3 ratio is not narrow enough, and that Baird hit the desirable target somewhat better at 3:7. However in setting a standard I bowed to Doug Pitt's and Stan Kujawinski's wishes in the early 1970s.

Let's say you increase the number of lines to 72, and decrease the sector angle from Baird's 12 degrees (with 30 line) to your 5 degrees (with 72). The accuracy you're going to need in positioning the holes in that 72-hole disc to maintain 1/4 pixel positioning accuracy will be, as we discussed in another posting, incredibly stringent. The angles decrease by a factor of 11.25/5 = 2.25, the picture area goes down by a factor of about 6, the hole positioning accuracy required goes up by a factor of something like 12.

To cut the disc to anything like this degree of accuracy you will definitely need:

(1) An accurate and unworn turntable dividing head from a metal-worker's lathe, suitable for high-accuracy gear-cutting.

(2) A rigid and accurately centered boss to hold the disc axially in the dividing head while you punch the holes.

(3) A suitable and rigidly mounted jig for the hole punching to one side of the dividing head, with micrometer adjustment for the radial measure.

Unconvinced? Consider the scanning disc:

With a 32 line disc, one pixel = 1/48 of 11.25 degree = 0.23 degree = 14 arc minutes.
One quarter pixel positioning accuracy (to avoid image streaks and jagged edges on reproduced lines) = approx 3.5 minutes of arc.

With your 72 line disc, one pixel = 1/96 of 5 degrees = 0.05 degrees = 0.3 minutes of arc.
One quarter pixel positioning accuracy (to avoid image streaks and jagged edges on reproduced lines) = 0.75 minutes of arc.
Angular layout of the 72-line scanning disc MUST be 3.5/0.3 greater accuracy = about TWELVE (12) TIMES MORE ACCURATE!

I wish you luck but you'll need VERY accurate tools - and even if you find those tools and manage to make a disc, who else will be able to make one? There are very good reasons for a group keeping to 32 lines as a general standard where mechanical scanners are concerned.

When one gets up to those line numbers, something like the Mihaly-Traub stationary mirror drum - adjustable while the scanner is in operation - becomes a much more attractive constructional proposition - or maybe even keeping the system wholly electronic by using CRT's.

I would LOVE to see such a scanner, but I just felt that I should emphasize the problems and perhaps indicate a few potential solutions. I'd be seeing a firm of engineers with an ACCURATE metalworker's lathe as a first priority. Then I'd be building a custom vestigial sideband transmitter for use on the broadband section of the two metre amateur band before I proceeded.

Or - look at the alternative - use digital systems, a webcam and software compress to .mpeg in real time, and transmit those compressed images in by DRM system on hf, which I suspect would be more practical - and may even be EASIER to achieve.

I honestly wish you luck...

And if you find this type of engineering facility PLEASE let me know, please. I wouldn't mind getting some discs of that accuracy myself.

Best wishes,

Chris Long VK3AML.
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72 Lines.

Postby Steve Anderson » Mon Jun 02, 2008 10:43 pm

Chris,

Yes, I take board all of your points and agree with them. However the display and source are not mechanical. I'm not a mechanical sort of guy, nor do I use any spectrum space below far IR. I started to build a mechanical drum display comprising of 32 lines and frankly got bored and never completed it.

I know that goes against the grain of this forum which is headed "Forum for discussion of Mechanical Television", but as I have pointed out elsewhere it's the signal's format that interests me. The concept of squeezing a video signal down an audio path has fascinated me since sometime in the early 70s.

So one could say our objectives are similar, yet different. Hence my interest in SSTV too, and I agree, it's a sequence of stills, rather than the common perception of 'television' which like cinema is a portrayal of moving images recreating life on a screen.

As for the flicker problem, this could be solved by a converter that refreshed the display at say 50Hz. In essence it becomes no different to a web-cam. It begs the question, is it worth the effort?

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Postby chris_vk3aml » Mon Jun 02, 2008 11:13 pm

I take your point and I think you're very wise to go for that 72-line project using the software and CRT approach.

Incidentally, I made a slip of a decimal point in my last posting - just as I did last time I discussed this problem (yes, I'm a twit!).

The requisite 1/4 pixel positioning accuracy on the 72 line disc would be not be 0.75 minutes of arc as stated, it would be 0.075 minutes of arc, meaning that this order of increased accuracy over the 32 line disc would be required:

3.5/0.075 = FORTY SEVEN (47) TIMES MORE ACCURACY needed for the 72 line disc than the 32, for positioning the holes correctly.

If people are trying to fabricate scanning discs for this number of lines, beware, you WILL need access to engineering and measuring equipment of clinical accuracy! At 32 lines, scanning discs are an inexpensive and elegant answer to the problem of basic television. Above 50 lines, the mechanical engineering difficulties become overwhelming.

Best wishes,

Chris Long VK3AML.
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Errata

Postby Steve Anderson » Mon Jun 02, 2008 11:27 pm

chris_vk3aml wrote:Incidentally, I made a slip of a decimal point in my last posting - just as I did last time I discussed this problem (yes, I'm a twit!). Chris Long VK3AML.


Never mind, it's so easy to push that 'submit' button not realizing your posting is flawed. It's happened to me many times, and to others I'm sure. The word 'errata' comes in useful sometimes.

Steve A.

P.S. In some other thread, possibly a year ago I moaned that there was no spell-checker on this board. Someone suggested composing in Word, spell-checking, then a cut-and-paste into the forum. (I'm a crappy speller). I've recently dumped Microsoft and now run Linux, and as I type this very text it's spell checking all the time. OK, it doesn't like colour as opposed to color, but it can be added to the dictionary. If you have access to a second machine it's worth having a play with Linux, believe me.....wanna good deal on a used car?
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Re: 72 Lines.

Postby Viewmaster » Tue Jun 03, 2008 12:11 am

Steve Anderson wrote:Chris,
Yes, I take board all of your points and agree with them. However the display and source are not mechanical. I'm not a mechanical sort of guy, nor do I use any spectrum space below far IR. I started to build a mechanical drum display comprising of 32 lines and frankly got bored and never completed it.
Steve A.


I've always known that you were not mechanically inclined, Steve, but you really must build a simple 32 line Nipkow disc mech NBTV one day (if you haven't already done so in the past?).

Watching other folks' machines is nowhere near as exciting as switching on your own first home built NBTV and watching those images take form.

Even more exciting than building your own 405 line TV and watching the CRT light up for the first time. ....if you are that old? :)
I've been fortunate in doing both and the first NBTV experience beat 405 albeit much poorer quality.

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Re: 72 Lines.

Postby Steve Anderson » Tue Jun 03, 2008 1:27 am

Viewmaster wrote:I've always known that you were not mechanically inclined, Steve, but you really must build a simple 32 line Nipkow disc mech NBTV one day (if you haven't already done so in the past?). Watching other folks' machines is nowhere near as exciting as switching on your own first home built NBTV and watching those images take form. Albert.


I'm sure you're quite right Albert, and yes, I should get a mechanical display operational, if nothing else than to gain some kudos here. (That's not serious). Given the time I will, but I have a few projects in the pipeline, most unrelated to NBTV in a direct sense.

Sir Frank Whittle, "I didn't hesitate, on the first run of the RB211 I slammed open the throttle, 55,000 lbs of thrust in less than ten seconds." Now that takes b**ls! From his biography, "Not much of an Engineer."

My point? You're right Albert. You never know until you give it a go.

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