Featured
Latest
Scart Video Output
 
Share:
Notifications
Clear all

Forum 141

Scart Video Output

12 Posts
4 Users
0 Reactions
1,603 Views
valvekits
(@valvekits)
Posts: 780
Honorable Member Registered
Topic starter
 

I needed a composite video signal for a bit of UHF modulator testing today. No problem I just made up a little cable to "borrow" the signal off a scart socket pin 19 of a freeview box.
When I connected up, I knew immediately that the lack of sync and negative picture was due to the polarity of the waveform being inverted and my scope confirmed this. I was surprised because I was expecting the video to be like textbook video.

I'm unfamiliar with scart other than just connecting things up as it came after my time in the TV trade. So my question is why is the signal inverted - I would expect that the scart output would get fed to a buffer on anything it is connected to so why have another stage of inversion?

Eddie

 
Posted : 02/08/2014 10:04 pm
Refugee
(@refugee)
Posts: 4055
Noble Member Registered
 

I have chopped up a pound shop scart lead for the connectors and used them to drive normal circuits without issue using RCA and even BNC with proper screened cable.
Duff set top box??

 
Posted : 03/08/2014 3:12 am
sideband
(@sideband)
Posts: 4222
Famed Member Moderator
 

Nope. I've never seen inverted video at the Scart lead either. Should be just plain straightforward composite video. The only thing between the video output stage of the freeview box and Scart video output is likely to be a buffer stage but certainly not an inverter. Can't see any point as it wouldn't be compatible with anything else....

 
Posted : 03/08/2014 10:11 am
Refugee
(@refugee)
Posts: 4055
Noble Member Registered
 

It would be nice to know what the scope settings are on that waveform.
Some pound shop cables have real solder tags in them and then it is of great value to go back and buy a whole basket full of the things.

 
Posted : 03/08/2014 11:01 am
valvekits
(@valvekits)
Posts: 780
Honorable Member Registered
Topic starter
 

Thanks for your inputs but it only raises more questions than answers as I should of added that the first thing I did was to check the scart output of a second different brand freeview box and this was the same - inverted.
I do have an invert button on the scope but it wasn't that.
So then I connected the scart from the freeview into my old VHS recorder scart and took the RF output to a TV and all was working okay, so obviously the video recorder didn't seem to mind getting inverted video.
Anyway I just lashed up a transistor circuit to invert my scart output and I was then able to check my modulator and all was okay.
My assumption was the same as Rich (sideband) so it's time for someone else to take a look at a scart output video signal, something strange going off here. :ccf
Finally if you looked at my oscillogram and noticed a slight ghost sync pulse like the trigger needs setting properly, it is actually is rock solid to look at, so the camera can see something I can't.

Eddie

 
Posted : 03/08/2014 11:08 am
Refugee
(@refugee)
Posts: 4055
Noble Member Registered
 

Did you fit a 75 ohm resistor or terminator on the scope when you checked the set top box?
Lack of this might stop the circuit in the box being biased correctly.

 
Posted : 03/08/2014 11:43 am
sideband
(@sideband)
Posts: 4222
Famed Member Moderator
 

Did you fit a 75 ohm resistor or terminator on the scope when you checked the set top box?

That could be the reason but most boxes/scarts/etc are terminated internally, at least the ones I've played with have been.

 
Posted : 03/08/2014 11:58 am
Alastair
(@alastair)
Posts: 294
Reputable Member Registered
 

Recently, I got a 'new' scope, for practically nothing. Its a Tek 2445. After some renovation I checked it out on video waveforms from a Humax set-top box on SCART.

All the waveforms were the right-way-up, as it were....

I was looking also to see if there was any trace of subcarrier on the RGB signals--There wasnt. Something to keep in mind, if you fancy running monochrome sets, I found they don't much like the burst and subcarrier present, causes all sorts of weirdness on low contrast scenes!

But--All the signals from that box--were definitely the right way up!

 
Posted : 03/08/2014 3:46 pm
valvekits
(@valvekits)
Posts: 780
Honorable Member Registered
Topic starter
 

Thanks for your input Alastair that all sounds what I would have expected.
So to summarise I have got a seven year old Acoustic Solutions freeview box and one that I bought from Tesco last week. They both give inverted composite video out on pin 19 but despite this they work perfectly when connected to a video recorder or TV.
On the old box, the composite video is sitting on a slight negative dc pedestal of 250mV whereas on the Tesco (dion) box the composite video is sitting on a negative pedestal of 50mV.
The RGB signals on both boxes are also inverted and with similar dc pedestals.
Ref - out of curiosity I did put a 75Ω load resistor on the signal but it made no difference other than to lower the amplitude very slightly.
We might have to relegate this one to one of life's mysteries for now.

Eddie

 
Posted : 03/08/2014 5:44 pm
Refugee
(@refugee)
Posts: 4055
Noble Member Registered
 

You can test the scope by setting it to DC coupling and touching the probe on a battery. This will establish that the invert switch is correctly set on the scope.
You can also put a T piece on the lead and scope the signal there while showing the video on a monitor or through a VCR.
I have seen negative video on a monitor with the internal terminator switched off. On a VCR the terminator is on all the time and has no switch.

 
Posted : 03/08/2014 6:06 pm
valvekits
(@valvekits)
Posts: 780
Honorable Member Registered
Topic starter
 

I found the problem - Alastair had mentioned that he checked RGB to be free of subcarrier but checking my RGB signals, they were inverted and still contained burst so that couldn't be correct. Then I realised what was wrong because when I was checking RGB I was using the video "ground" which in my case wasn't actually ground.
If I checked RGB with the correct grounds the signal polarity was okay.
Here's what happened.
I had originally connected between 19 & 21(common ground) but didn't get an output and when I looked at the scart pin out schematic off the web, I noticed that there was a video out pin 19 and a video ground pin 17.
Now when I re-made the ground connection, I also removed pin 19 to make a better soldered job of it but alas re-connected to pin 17. My ground(common) then ended up connected to pin 19 and hey presto the connections were reversed! I re-checked my wiring several times but only ever saw my leads connected to pin 17 & 19 without noticing the polarity change. :ccg

Eddie

 
Posted : 04/08/2014 5:18 pm
Alastair
(@alastair)
Posts: 294
Reputable Member Registered
 

Call me old-fashioned, but I tend always to 'earth' to the metal-box/chassis of the device or any tuner type item/earthed heatsink on the board.....

 
Posted : 04/08/2014 6:00 pm
Share: