NBTV Frame Rate Converter

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

NBTV Frame Rate Converter

Postby Steve Anderson » Sat Jul 17, 2021 4:44 pm

Due to the stoppages caused by the chip shortage I'm looking for something that I can design and build with the parts I already have. One thing I've had my eye on for a few years is Graham Lewis's "NBTV Frame Rate Converter"[1], in this case it would be an update resulting in far fewer components, though the principle would be basically identical, so no originality claimed...

If anyone's interested I'll make a start. A small CRT display will be a companion unit for this too. I think I can reduce the chip-count from 13 or so to maybe 4-5.

{1} Newsletter Vol.29 No.3 (March 2004).

Scans of Grahams original follow...

Postscript: If you've seen Chris Long's 'Slow NTBV' at half frame rate (6.25Hz frame, 200Hz line) that may be a worthwhile addition to this...I was quite impressed with it. It also means the video bandwidth requirement is halved to around 5kHz, perhaps more so. I wish I could find the link...

I've sent Chris an e-mail, updates later...

Steve A.

Mar 2004_04.gif

Mar 2004_05.gif

Mar 2004_06.gif
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Re: NBTV Frame Rate Converter

Postby Harry Dalek » Sat Jul 17, 2021 6:16 pm

I had not heard of this one be bit of wiring for sure so i can see why you want to reduce update the circuit idea .
Keen to see your CRT come together too ! nearly finished mine depending on any thing else comes to mind i have one control hole to fill ; )
Any way watch with interest see how it go's.
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Sat Jul 17, 2021 6:40 pm

Thanks Harry, the CRT bit will be much as per the DG7-32 stuff I've done before. Nothing really new, why re-invent the wheel? I'll probably use the same circuits from years back...though the timebase speeds will need altering (easy). I'd have to check the grid/cathode arrangements which will now require 40-60kHz of bandwidth.

The largest chip will be a 28-pin DIL/DIP 0.3" device, no more difficult than wiring up two 7400s, the rest should be the more familiar 8/14/16/18-pin DIL/DIP parts. I will avoid SMD parts too.

Steve A.
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Re: NBTV Frame Rate Converter

Postby Harry Dalek » Sun Jul 18, 2021 10:38 am

Steve Anderson wrote:Thanks Harry, the CRT bit will be much as per the DG7-32 stuff I've done before. Nothing really new, why re-invent the wheel? I'll probably use the same circuits from years back...though the timebase speeds will need altering (easy). I'd have to check the grid/cathode arrangements which will now require 40-60kHz of bandwidth.

Thanks to Andrew i have a DG7 32 as well ,I sort of do the same most of the time as in using what i know works which is all your stuff but some times you have to change things to adjust to what you have handy in my case this was power supply and crt modulation for this project CRT.
Yes comes in handy when you have circuits already built .
40 to 60 khz video in this bandwidth the converter changing the frame rate from 12.5 to 25 yes

The largest chip will be a 28-pin DIL/DIP 0.3" device, no more difficult than wiring up two 7400s, the rest should be the more familiar 8/14/16/18-pin DIL/DIP parts. I will avoid SMD parts too.

Steve A.



Yes watch with interest see how it go's the CRT part will be very enjoyable to watch come together
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Sun Jul 18, 2021 12:11 pm

Harry Dalek wrote:40 to 60 khz video in this bandwidth the converter changing the frame rate from 12.5 to 25 yes

Actually 12.5Hz to 50Hz, even at 25Hz flicker can still be seen, though far less than at 12.5Hz. Don't get confused with 25Hz interlaced TV (analogue 625). Although 25 frames/sec, it comprised of two fields at 50 per second.

The first field is all the odd line numbers (1,3,5,7 etc.) followed by all the even line numbers (2,4,6,8 etc.). This has two benefits, though the frame rate is 25Hz (all 625 lines) the flicker rate is 50Hz. It also cuts the bandwidth in half from a pure non-interlaced 625/50Hz display.

It was derived from film projectors in cinemas. Although 35mm movie film is usually 24 frames per second, each frame is shown twice raising the flicker rate to 48Hz, sometimes three times to 72Hz. This is done with a shutter in the light-path synchronised to the film advance mechanism.

https://hackaday.com/2015/07/26/sheddin ... %20problem.

Here our signal is non-interlaced, so although 25Hz is a good improvement, 50Hz is better. It doesn't require any more electronics to convert to 50Hz rather than 25Hz. You could go all-out and convert to 100Hz, but the rewards really aren't worth it.

Also consider mechanical displays, the usual 12.5Hz/750rpm becomes 50Hz/3000rpm, I think that's enough!

As for bandwidth, nominally 12.5Hz NBTV has a bandwidth of 10kHz, though often it can be a bit more, hence 40-60kHz, more won't hurt...

Steve A.

You may think, "Why not interlace NBTV?" First it requires an odd number of lines, hence 405, 525, 625, 819 in the old major analogue standards. Second I did try it with 33 lines interlaced many years ago. With so few lines it really doesn't work...at least that's my experience...
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Sun Jul 18, 2021 3:59 pm

One of the other issues is dealing with off-frequency and/or erratic/wobbly line rates. It calls for a timebase corrector, no mean feat...

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Re: NBTV Frame Rate Converter

Postby smeezekitty » Mon Jul 19, 2021 1:01 pm

Steve Anderson wrote:One of the other issues is dealing with off-frequency and/or erratic/wobbly line rates. It calls for a timebase corrector, no mean feat...

Steve A.

To be quite honest, at this point it is probably better to turn to software solutions at that point.
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Mon Jul 19, 2021 1:29 pm

smeezekitty wrote:To be quite honest, at this point it is probably better to turn to software solutions at that point.

This 12.5Hz to 50Hz converter is already done in software, but not on a PC, it's a stand-alone unit not requiring a PC at all, a PC perhaps could be simply used as an audio recorder replacing a tape machine.

Using a PC is a perfectly valid way of doing it, but simply loading someone else's software doesn't give me a sense of achievement, but that's me, others may feel different...and quite honestly my way doesn't make logical sense, again that's me... When have I been known to be logical?

I could be viewed as daft, stupid or non-sensical, guilty on all counts! Here we have the luxury of being such, we're not under any commercial pressure or constraints, whims and fancies are allowed...

Steve A.
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Mon Jul 19, 2021 5:02 pm

Here's my initial go at a concept...I did say 4-5 chips, it's come down to three! Though I may have missed something obvious - it wouldn't have been the first time!

I could have used a smaller package micro device (e.g. 20-pin DIL) except none have enough RAM... Your thoughts?

I've got two gain controls...no idea why...a silly error...ignore one or the other...scatterbrain me...

Graham Lewis's original version used 128 samples per line, due to the restricted amount of RAM in these devices I've chosen to use just 96, we'll see how that works out. I was reluctant to add an additional RAM device.

I'm also trying to make this a single +5V device, which is generally unlike me...hence the choice of op-amps...

...I might add, there's no TBC implemented...yet...should one happen it'll not be as simple as this...something to consider...

Steve A.
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Mon Jul 19, 2021 6:09 pm

I can only get the MCP604s in SOIC packages here, but in the UK Farnell DIL/DIP...1852055. Others similar too probably...

At first the LM358 looks suitable, but below 0.7V output it can only 'pull-down' 50uA...notice all Graham's voltages from the LM385s are at least 1V. Here I need a good firm 'pull-down' to 0V. Though once the concept is proven I may try using LM358s...

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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Sun Jul 25, 2021 3:27 pm

After some re-arrangement within the available RAM of the micro it should be possible to increase the samples to 112 per line, which is 87% of Grahams without an external RAM chip.

I'm still looking into the op-amp situation, I'd rather try and find something more widely available...and non-SMD.

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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Tue Jul 27, 2021 2:00 pm

Harry Dalek wrote:Thanks to Andrew i have a DG7 32 as well...

Here's a datasheet for the DG7-32 that's new to me, though I have others, this seems more recent...but quite a large file!

Steve A.

DG7-32.pdf
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Re: NBTV Frame Rate Converter

Postby Harry Dalek » Tue Jul 27, 2021 6:49 pm

Steve Anderson wrote:
Harry Dalek wrote:Thanks to Andrew i have a DG7 32 as well...

Here's a datasheet for the DG7-32 that's new to me, though I have others, this seems more recent...but quite a large file!

Steve A.

DG7-32.pdf


Its a nice Tube not to long i only have one so the PDF will come in useful for me too .
I recall reading you mentioned once its a dimmer CRT for NBTV ? , would like to do an all Valve monitor one day .
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Re: NBTV Frame Rate Converter

Postby Steve Anderson » Tue Jul 27, 2021 7:08 pm

Harry Dalek wrote:I recall reading you mentioned once its a dimmer CRT for NBTV ? , would like to do an all Valve monitor one day .

Really it's quite bright enough to be viewed in normal room lighting, though turning down the room light and the CRT intensity does improve matters, but that true of most NBTV displays...see my photos of my first NBTV CRT display, 32/48/64/72 lines...96 lines I never quite got to...perhaps because of the limitations of a 3" CRT.

All valve/tube, no that's a non-starter, some valves/tubes OK. But an all valve/tube device is a headache at a minimum. A hybrid of sand-state and vacuum-state makes sense... it also depends on what you have and what you can get at a reasonable price...some valves/tubes go for stupid prices now...

Steve A

Harry, unless you have some valves/tubes now, look up at what an ECC83/12AX7 is gonna cost you now...the most basic, yet good audio amp, then the sockets for the same...sure you want to go all tube/valve? Somehow I doubt it...

Then there's power transformers...good luck there too...plunder some old radiograms or similar for those valuable parts, most were 'disposed of'' (junked) in the 70s or 80s...
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Re: NBTV Frame Rate Converter

Postby smeezekitty » Wed Jul 28, 2021 8:30 am

Harry, unless you have some valves/tubes now, look up at what an ECC83/12AX7 is gonna cost you now...the most basic, yet good audio amp, then the sockets for the same...sure you want to go all tube/valve? Somehow I doubt it...

Then don't use 12AX7s ;)
There are plenty of capable triodes that can be had for dirt cheap on eBay. You might need to look around tube references.
12AX7s are only expensive because the audiophools use them but there is nothing particularly special about their characteristics
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