NBTV-scope project

[aussie_bloke]

G'day all.

I recently been wanting to display 32 line NBTV images on an oscilloscope tube and have found a brilliant circuit on GW3JGA John Lawrence website http://www.gw3jga.net/ , his NBTV-scope circuit page can be viewed here http://www.gw3jga.net/jl-notebook-082 and the schematic and screen display can be viewed in the following links:
http://www.gw3jga.net/jl-images/082fig1.jpg (schematic)
http://www.gw3jga.net/jl-images/082fig2.jpg (NBTV displayed on scope)

Anyhow I have constructed the circuit from John's schematic and have been testing it out on my Tektronix scope and am getting line scan and Y input, video signal to Z input but am not getting field scan to X input. So hence I am getting no picture and when expanding the X input I am getting fuzz as seen in the pics below. Testing the circuit out I can see the problem lies in the flyback output from pin 7 of the 4528 chip to pin 6 of the 4066 chip, I am not getting a flyback pulse but 5VDC instead with only slight appearance of what looks to be pulsed luminance variations in the 5VDC line but no pulse spikes whatsoever. I have traced over the circuit a few times to make sure everything is wired up correctly and have fixed a couple of my mistakes I made but everything is now all correctly wired up and am getting correct voltages to the chips. I have uploaded a video of this circuit to YouTube for you all to see http://www.youtube.com/watch?v=vdf4N3ENQ2c (am yet to add description).

Anyhow am in need of help to get this circuit working so would much appreciate any assistance from you guys in solving this field scan/flyback pulse issue. John Lawrence doesn't have an email address on his site so I have been unable to contact him, does he post in these forums?

Lastly for the +/-5V power supply source I used a centre-tap transformer to +/-5V regulators circuit which can be found here http://3.bp.blogspot.com/-JIv5UDwKylQ/T ... supply.bmp , I used a transformer I pulled out of (I think) an old amplifier and have used the two 10V windings with centre-tap for the earth and have wired it up according to that schematic and the NBTV-scope circuit's earth is connected to that centre-tap. As I'm using a variac and a two prong lead connected to the transformer I wonder if the centre-tap to NBTV-scope circuit's earth is good enough as an earth or should I connect a big metallic object to the earth line??? Anyhow the voltages relative to earth are correct so I guess it's probably fine.

Anyhow here below are pics of the NBTV-scope circuit:

[Steve Anderson]

There's an error in the schematic...pin 4 of IC2 should go to -5v NOT ground/0v. If that doesn't fix things replace the chips, your previous mistakes (or his) could have zapped them.

Good luck,

Steve A.

I don't know why he made it split-supply, it doesn't need to be.

In addition I would also connect pins 12 & 13 of IC3 to -5v. They're not used, but it's not a good idea to leave CMOS inputs 'floating'.

[aussie_bloke]

Thanks for that, will connect it to -5V and see how that goes, I had a feeling pin 4 connected to earth looked a bit out of place! Anyhow hope that will fix the problem, have spare chips to replace in case the chips are zapped.

[aussie_bloke]

Okay I have made those fixes to the NBTV-scope circuit and unfortunately it's done jack diddly squat change to the flyback pulse output at pin 7 except shifting the DC voltage from +5V to -5.3V and I even changed the chips too with no results. So the problem with this circuit is still eluding me big time.

I am now wondering if the sync pulse is right with this circuit. The sync pulse when correctly tuned with minimal noise is at a pulse width of 2.5ms with an on pulse of about 150-200us and amplitude level of around 8Vp-p (see picture below), is that the correct sync pulse?

Also here's picture of the X and Y output together on screen, line output (Y-axis) looks like a steep sawtooth but with a long off time between each sawtooth and also unstable and the field output looks to be pretty much nothing at all.

Lastly I have just made a video demonstrating what I am now getting on scope display after making those fixes and I also show the signals I am getting at key areas of the circuit http://www.youtube.com/watch?v=YnSa8RsrtwA .

[Steve Anderson]

Troy, I assume that the pic that ends in...265.jpg is the output of IC1 (pin 6). If so that looks about right for a crappy 741, but it should do. The other pic (...264.jpg) makes no sense at all, can you say at what point this waveform is from?

Also, what is the source of your NBTV signals?

Steve A.

I've worked out the timing formula from the Philips datasheet and I get the feeling that the R5/C2 combination is wrong. At 10V supply the formula is Tw = 0.32.R.C which comes out to 5.76ms. Throw in resistor/capacitor tolerances and there's every chance that this thing will never trigger. I suggest changing R5 to 100k which yields a nominal timing of 3.2ms.

The fact that IC2 pin 7 is now sitting at -5V is a good sign.

[aussie_bloke]

Pic 264 is showing both output signals for the X and Y axis from the X and Y output sockets connected to pin 6 of their associate 741 opamps, X being the field scan and Y being the line scan, the fuzzy waveform at the top is the field can for the X axis and the pulsed waveform for the line scan for the Y axis. The source of the signal is from my MP3 player which the signal is compressed to 320kbps mono MP3 (with vision only channel) which should be not much degraded from the standard WAV file, it worked fine with my two televisors so I assume it should work okay with this circuit.

[Steve Anderson]

Troy, when I asked,"What is the source of the NBTV video?" I meant who made them? From club CDs? Or somewhere else.

I wouldn't even consider using MP3 for this, even in mono and especially at such a low sampling rate. Others here (I hope) will back me up on that. You'll notice all the video on this forum is not in MP3. The fact it worked on your other displays...you were lucky.

Steve A.

[aussie_bloke]

Troy, when I asked,"What is the source of the NBTV video?" I meant who made them? From club CDs? Or somewhere else.

I wouldn't even consider using MP3 for this, even in mono and especially at such a low sampling rate. Others here (I hope) will back me up on that. You'll notice all the video on this forum is not in MP3. The fact it worked on your other displays...you were lucky.

Steve A.

I don't have the club CDs as I haven't yet became a member, I simply make the NBTV videos myself using Gary's Video2NBTV software http://users.tpg.com.au/users/gmillard/nbtv/nbtv.htm which I convert pictures and video to NBTV standard and other standard presets in his software.

In regards of MP3s, comparing standard WAV to 320kbps MP3 on the spectrogram in Goldwave I find there's not a great deal of deterioration and 320kbps is on the high end of the sampling scale for MP3 opposed ot lower rates like 128kbps and 192kbps at least in terms of music. Anyhow the reason I put them to MP3 is for convenience as I don't have a portable CD player handy and a lot of the testing occurs in the garage and my MP3 player won't recognize WAV files.

I have decided to do a test to see if there's any noticeable difference between a NBTV video WAV file and NBTV MP3 file and to unlike with music, the difference in quality between the two is more outstanding according to the spectrogram in Goldwave and the NBTV The Big Picture program when playing back the files, (had to convert the MP3 back to WAV for playback of course). I tested with a NBTV converted picture of Felix The Cat and I can see converting to 320kbps MP3 created noticeable noise in the picture and a bit of stability problems which would not be real noticeable on a televisor but very evident on NBTV The Big Picture. Anyhow I've made a graphical illustration of my tests so yep, you're right MP3 is crapola big time even at its highest sampling rate of 320kbps!!! I still wonder though if it would have any bearing on the performance of the NBTV-scope circuit other than a displaying a bit less stable noisy picture should the circuit be 100% functional?

[Steve Anderson]

There you go. MP3 and other audio compression systems are geared around disposing of supposedly redundant information that the ear-brain combination doesn't notice is absent. Notice the word 'supposedly'.

A video compression algorithm works in a very different manner, although many of the principles are the same, here we're dealing with the eye-brain connection.

Now, how this applies to your current situation I don't know. But there is still something wrong at the hardware level. The scanning waveforms should be there irrespective of the MP3 corruption. In time you do meed to sort out an AVI system for this.

Gary's software is spot-on so we know that is not the source of any errors.

Steve A.

[Steve Anderson]

Troy, any update on this?

Steve A.

[aussie_bloke]

I'm afraid none as of yet, have not touched on the circuit as I am completely lost as to why the circuit is not working how it should despite being wired correctly and have no idea what I could try to get it to work .

[Harry Dalek]

Hi troy i know i have tried the monostable circuit and it worked the values are a bit different to your circuit..i think from a quick look the pin connections are different as in the one i tried one monostable feed the other a neater way to do it really
So am wondering about hes version of the circuit .
you have nothing to loose do a little rewiring and see .

[Steve Anderson]

Troy, have you tried changing R5 to 100k yet? You'll notice that's the same value the 'Club' circuit uses as posted by Harry uses.

Now, careful here, the club circuit is designed to trigger on positive-going edges, this one (GW3JGA's version) on negative.

Steve A.

[aussie_bloke]

Troy, have you tried changing R5 to 100k yet? You'll notice that's the same value the 'Club' circuit uses as posted by Harry uses.

Now, careful here, the club circuit is designed to trigger on positive-going edges, this one (GW3JGA's version) on negative.

Steve A.

Oh I totally forgot about that, will change the resistor and see how it goes and get back to you on the results.