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TX9 scart interface.

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Rebel Rafter
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Hi folks, RR here. Let's see if anyone can figure this out, the answer's probably obvious but unfortunately not to me. After a lot of struggling and aggro I've finally managed to get a scart interface to work on my 20" stereo Thorn TX9 set. It has both composite and RGB inputs and works quite well in both modes but there's a strange fault which in all my years of playing about with TV's I've never encountered, but then I've never tried fitting such an interface on a TX9. Whenever I switch back to the "off air" mode, i.e. through the aerial socket and the tuner and IF module the picture has continuously varying coloured horizontal bars coming and going and I've traced this and managed to find out it's being caused by the RGB input signal, even though in the off air mode the RGB is turned off by removing the blanking voltage at pin 9 of the TDA3560 pal decoder chip. I've used near enough the same interface as used with the old teletext, i.e. an 82 ohm resistor to earth followed by a 0.1 uF polyester capacitor in series with each of the three inputs, and about 2.5 volts at the blanking pin of the chip when in RGB mode and zero volts when turned off. And this effect stops if I turn off RGB on the freeview box or if I unplug the RGB input, but strangely enough it doesn't happen when in composite mode, even though the RGB signal is still present at the decoder chip's inputs, albeit with the input turned off, for some strange reason it only happens in the off air mode. I've done voltage measurements all over the place and it all seems to be working fine apart from this strange fault. The blanking voltage is definitely not present in the off air mode because I've checked it. Does anyone know if this is caused by a faulty TDA3560? is this a known fault? Any ideas anyone? RR.

 
Posted : 04/09/2015 12:08 am
Cathovisor
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I get this with my Panasonic set fed RGB from my HDD recorder.

If the recorder is on when the telly is first switched on it defaults to the aerial input, and I see the unlocked pattern you describe. It's only when I turn the set to the AV input the recorder is connected to that I get a locked picture because it's only then that the sync input (read: composite video) on the SCART gets routed to the sync. separator and therefore locks the timebases to the incoming RGB video.

The same happens with any other AV input (the set has three) whilst the recorder is on, and similarly if I set the recorder to composite only I don't see this problem.

I'm not sure if it's a feature or a fault: I know my Panasonic doesn't implement pin 8 input switching which of course would stop this happening. I didn't see this on my mother's Salora, for example but that only had one SCART: my Panasonic has one SCART, one BNC/S-Video connector and one front-panel BNC connector.

 
Posted : 04/09/2015 5:16 pm
Jayceebee
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Hi RR,

As Cathovisor states this is not a fault but normal operation of the TDA3560. The fast blanking pin9 is not to switch the chip over to RGB mode but to "Blank" the video on pin8 of the IC, if it is taken permanently high then it produces a completely black background and you only see the RGB input signal. You can also switch the blanking on and off at very high speed to produce multiple black boxes for subtitles, time, newsflashes etc. So to prevent the effect you are seeing the source of the RGB signal must be turned off when not required.

Hope this helps.

John.

John.

 
Posted : 04/09/2015 10:53 pm
Jayceebee
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Sorry, the video input (luminance) pin of the IC is actually pin10 :ccb

John.

 
Posted : 05/09/2015 12:25 am
Rebel Rafter
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Hi folks, RR here. Thanks everyone for the advice. I forgot that when the set is in "off air" mode the RGB which is still present up to the inputs of the TDA3560 is then unsynchronised but it still baffles me a bit why this would cause it to interfere with the off air picture, this would also explain why it doesn't happen in scart composite mode because the RGB signal is then synchronised. So it looks like I might have to install a little relay or two to disconnect the RGB signal from the decoder chip inputs when in off air mode and control it with the RGB blanking control line. I've already installed one little relay to disconnect the output from the home made video preamp when in off air mode because I thought the composite signal was causing the effect, but the relay can stay in as it stops unwanted voltage from the IF module finding it's way back up the line to the video preamp. RR.

 
Posted : 05/09/2015 6:42 pm
Mark Hennessy
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Pin 9 activates a switch between internally generated RGB signals from the decoder, and the external RGB inputs. One of the selling points of the TDA3560 was the presence of this fast-acting switch, so that clean "holes" in the composite signal could be punched - far better than previous arrangements. So with pin 9 low, I agree that any signal present on the RGB external inputs should not be able to interfere with the on-screen picture.

However, I think that what is happening here related to the fact that the signals present on the RGB inputs are non-sync with the composite signal that the IC is decoding. In that case, the presence of the RGB signals is somehow upsetting the black level clamps, causing the symptoms described.

In every external AV circuit that I've seen for these ICs, external RGB signals aren't permitted to reach the TDA3560 unless the set is in external mode (hence synchronised to the external input). Given that you need an analogue switch of some sort to select between internal teletext and external RGB, this is normally easy to arrange.

Last time I added RGB inputs to a TX10 was in 1991, so these are old memories, but the more I think about it, the stronger the memory becomes. I wonder if I can find the documentation somewhere? At the time, an uncle worked for them in Enfield, and gave me a bare PCB to assemble. Learnt a lot doing that...

 
Posted : 05/09/2015 7:37 pm
Rebel Rafter
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Hi, enveryone, RR here again. I think I understand this situation a bit better now, I thought the answer should be obvious. I thought that with pin 9 of the TDA3560 at zero volts the RGB input was turned off, but it now looks like I was wrong. I think what is happening is that with the composite scart input the signal being processed is the same as that on the RGB inputs and therefore in sync. with it, hence no unwanted effects, whereas in off air mode with a different signal which is obviously not in sync. with the RGB signal and can't be the incoming RGB signal is trying to mix with the off air signal and modulating it, hence the unwanted effects. So now I've gone into my freeview box's settings and turned off the RGB leaving just composite which gives a good enough picture. I only use the aerial input for a security camera so I can see whoever comes to my front door. The main reason I want RGB input is so I can watch my dvd's of the absolutely stunning Catherine Jenkins in colour as the discs I've got are unfortunately ntsc so there's no colour with these in composite mode but they look glorious in RGB! I only got these ntsc discs because when I bought them the proper UK ones were sold out all over the UK and I had to get Woolworths to import some from Jersey and for some reason they were ntsc. I think they must've been export stock for the states or Japan. RR.

 
Posted : 05/09/2015 9:12 pm
Jayceebee
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Thomson who made the Ferguson SRB1 BSB set top boxes which had RGB output were aware of these foibles. The standby function had a dual action, the first press would disable RGB but leave composite output active. This is so you could still feed your VCR to record BSB channels and watch a terrestrial programme without the RGB output floating across the picture. The second press put the box fully into standby.

John.

John.

 
Posted : 05/09/2015 9:54 pm
Doz
 Doz
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Thomson who made the Ferguson SRB1 BSB set top boxes which had RGB output were aware of these foibles. The standby function had a dual action, the first press would disable RGB but leave composite output active. This is so you could still feed your VCR to record BSB channels and watch a terrestrial programme without the RGB output floating across the picture. The second press put the box fully into standby.

John.

I always wondered why they did that!

 
Posted : 06/09/2015 10:58 pm
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