Flickering RGB -> SCART output.

Korinel · November 2014

Hello all,

For my toast rack I have bought a RGB -> Scart cable however when I connect the toast rack to the TV (Plasma), whilst I admit I get a glorious sharp picture, the picture constantly jumps to the right and then back again which makes things a tad annoying. The TV seems to have issues trying to work out which mode to display the output. I tried on a second TV (CRT) and whilst less frequently it still jumps to the right and back.

Is this a known issue with the Spectrum+ 128, or with RGB/Scart cables or with modern TVs?

Has anyone used Component or RGB to HDMI cables/Devices, if so are they any good?

Is the Component / RGB quality the same?

1024MAK · November 2014

Korinel wrote: »

For my toast rack I have bought a RGB -> Scart cable however when I connect the toast rack to the TV (Plasma), whilst I admit I get a glorious sharp picture, the picture constantly jumps to the right and then back again which makes things a tad annoying. The TV seems to have issues trying to work out which mode to display the output. I tried on a second TV (CRT) and whilst less frequently it still jumps to the right and back.

Is this a known issue with the Spectrum+ 128, or with RGB/Scart cables or with modern TVs?

I have the same type of cable from the same supplier. I normally use it for my Grey +2, but I know it works with 128k+ toast rack's as I tested a toast rack that I repaired & serviced for someone and got a nice sharp steady picture :smile:. No jumping pictures. This was with a Sharp Aquos LCD TV with manual (via remote control) SCART RGB and 4:3 switching (so it does not care about the SCART control voltages).

As this happens with another TV (different make I presume) I would say either there is a problem with the toast rack, or you have discovered another problem. It may just be that the TV's don't like the signal that is being used to feed the sync. signal or the SCART switching signals.

Does the computer produce an okay UHF output?

Do you have any other TV's to try it on?

Do you have any other ZX Spectrums? How well do they work with these TV's?

Korinel wrote: »

Has anyone used Component or RGB to HDMI cables/Devices, if so are they any good?

Is the Component / RGB quality the same?

HDMI is a digital standard and there are no such things as RGB to HDMI cables, as RGB is analogue video signals. SCART RGB is a type of component video.

I have a SCART RGB to HDMI converter box, it works great with my QL and my Acorn BBC machines, but when I tested it with my +2B and +3A I got an double image. I do intend to investigate this further, but have not yet made the time (I normally use RGB SCART for my +2, +2B and +3, I got the HDMI converter for use with the QL). I suspect it is the converter wanting a better quality sync. signal. However another poster has had good results with a SCART RGB to HDMI converter from a well known on-line shop, and this SCART RGB to HDMI converter looks just like mine.

Of the analogue video signals, the highest to lowest in terms of quality are: highest = RGB, S-Video, Composite and worst = UHF. If you can, I strongly recommend that SCART RGB be used ;).

Mark

mcleod_ideafix · November 2014

Korinel wrote: »

Is this a known issue with the Spectrum+ 128, or with RGB/Scart cables or with modern TVs?

It's a known issue with the Spectrum 128K and later models. These machines use a different timebase for generating syncs: 17,7345 MHz / 2.5 = 7.0938 MHz. he horizontal counter counts up to 456, so a scanline lasts 456/7.0938 = 64,28us instead of 64us.

Although 280ns of extra time per scan may sound as a tiny assumible error, it seems that the picture processor of some modern TV's doesn't like this at all. Even more: as these processors must cope with PAL and NTSC signals, they'd prefer a scan line with a duration of slighly less than 64us (to accomodate for NTSC signals) than slightly more than 64us (which don't fit in any video standard)

Sometimes I've thought about changing the crystal oscillator with a 18MHz one and see what would happen. The frequency change is not very noticeable, but it would allow for a scanline time of 63.333us, which is very close to the duration of a NTSC scanline, so issues with the RGB output would likely dissapear. Take into account that any change of this crystal will render the TEA2000 useless, as the colour clock will no longer be 4.33MHz.

ingo · November 2014

Make sure that the voltage at pin 8 of SCART is between 9,5V and 12V! This voltage tells the TV if the source (RGB or Composite) is 4:3 (9,5V ... 12V) or 16:9 (4,5V ... 7V).
If you have there about 8V the TV may constantly switch between 4:3 and 16:9. I had this at some TVs.

Greets Ingo.

Korinel · November 2014

Thanks all!

>Does the computer produce an okay UHF output?
Yes

> Do you have any other TV's to try it on?
No, just the Panasonic Plasma and the Sony CRT

> Do you have any other ZX Spectrums? How well do they work with these TV's?
Many :) They all work well both TVs however they all either use UHF or Composite, this is my only RGB one (I have no Amstrad models as I'm a Sinclair Snob :) )

It just appears not to be able to "lock" the picture, and instead seems to be switching from one format to another, so that could be what mcleod_ideafix and ingo said :( I'll check the voltage in the morning.

1024MAK · November 2014

Korinel wrote: »

> Do you have any other TV's to try it on?
No, just the Panasonic Plasma and the Sony CRT

Err, these makes are some of the most troublesome :x.

Think cheap when thinking retro :-P.

Sony sets don't like a low, or even a slightly low switching control voltage on the control pins of the SCART connector.

Pin 8 must be at least 9.5V DC (better if it is >=11V).
Pin 16 must be between 1.1V and 3V DC.

Also see if there are any on-screen menus that can override the auto mode selection (sadly these are being left out or hidden more and more :x).

Do you have a spare (good) 1.5V battery and a suitable battery box (suggest AAA or AA size, but C or D will do)?

Mark

Korinel · November 2014

Having read Paul's SCART page I know have some more background knowledge so I now understand... http://www.fruitcake.plus.com/Sinclair/Spectrum128/SCARTCable/Spectrum128SCARTCable.htm

I guess that the TVs are switching between RGB and Component due to pin 8/16 not having the correct values.

If I switch the Spectrum on using the SCART cable I pop the black probe on blanking ground ( 18 ) and red probe on blanking (16) and I get 4v (DC)

If I pop the black probe on common ground (21) and the red probe on function switching ( 8 ) I get 0v (DC).

These don't match what anyone is saying so I assume I've missed a point somewhere... could someone enlighten me please? :D

I also went through all the settings but they are all fairly non advanced so no RGB/COMP settings :(

I have the battery but not the box but I live in walking distance from MAPLIN :)

Korinel · November 2014

(sorry double post)

ingo · November 2014

Korinel wrote: »

If I switch the Spectrum on using the SCART cable I pop the black probe on blanking ground ( 18 ) and red probe on blanking (16) and I get 4v (DC)

If I pop the black probe on common ground (21) and the red probe on function switching ( 8 ) I get 0v (DC).

These don't match what anyone is saying so I assume I've missed a point somewhere... could someone enlighten me please? :D

The 4V at pin 16 are ok because this pin is inside the TV loaded with 75 Ohm to ground.
The voltage on pin 8 is not needed for all TVs because it is a voltage to AUTOMATICALLY switch the TV to SCART input. In most modern TVs this can also be done from the remote control. But in that case the user has to define the ratio 4:3 or 16:9 manually or the TV is trying to determine the right ratio from the input signal.
My experience especially with my Panasonoc Plasma TV is that it is best to input 12V at pin 8 to force the TV to 4:3 otherwise the TV is not right switched to 4:3.

So in your case it might be good to connect pin 8 of SCART via a resistor of about 150 Ohm to a 12V source (for example from the Spectrum itself).
For test you could try to use a new 9V battery between ground (-) and pin 8 (+) and see if this helps.

Greets
Ingo

1024MAK · November 2014

Hey, beaten to it :p

Yes, do as Ingo suggests. Note that Spectrums than do have "+12V" on their monitor sockets use an resistor to limit the current. When connected to a TV this causes the resistor to drop too much voltage which makes this "+12V" not very useful.

You can however get +12V from the serial port or the aux/keypad port. And before you ask, these connectors are very hard to find :x

For testing, use a suitable 12V (or at a push a brand new 9V battery).

Mark

ingo · November 2014

1024MAK wrote: »

Note that Spectrums than do have "+12V" on their monitor sockets use an resistor to limit the current. When connected to a TV this causes the resistor to drop too much voltage which makes this "+12V" not very useful.

If this is the case the TV is a bad one as the input resistance at pin 8 normally is high. A resistor in series to 12V makes sense to safe the 12V source in case of (undesired) shortage. Therefore in every case I add a resistor of about 150 Ohm in series to the 12V!

Greets Ingo.

1024MAK · November 2014

ingo wrote: »

If this is the case the TV is a bad one as the input resistance at pin 8 normally is high. A resistor in series to 12V makes sense to safe the 12V source in case of (undesired) shortage. Therefore in every case I add a resistor of about 150 Ohm in series to the 12V!

Yes, but often the Spectrums "+12V" is also being used for the SCART pin 16. As the resistor in the Spectrum is a 1k ohm, the volt drop can drop the voltage too low for some TV's when it is also used to feed pin 8.

Mark

Korinel · November 2014

Thanks, you chaps are awesome!

I took the SCART apart and compared it to Fruitcakes spec...

8 (Functional switching) is not connected at all, hence the 0v.

Most of the grounds are not connected either (e.g. Audio, Blue, Red, Green).

Brown and black cables from the monitor cable side are cut and open at the SCART end.

Comp video ground/Blanking ground is not correctly insulated and is touching the two cut wires, the heat insulation has not been heated and has slipped down so is now touching the two cut wires.

(The 150 ohm resistors that connect R,G,B are not all facing the same way... sigh, I'm no sparky but blooming eck I can put resistors so they look neat...)...

So I know what I need to do now, I don't see 12v battery packs (plenty of 9v) for sale, do they not exist? Would it be better to use mains dc instead? I don't really want to use my toast rack, this one is precious to me :)

*Edit*

Well there is this... http://i01.i.aliimg.com/wsphoto/v0/946455647/Portable-font-b-12V-b-font-font-b-18650-b-font-Li-ion-Super-Rechargeable-font.jpg

and http://www.maplin.co.uk/p/8-aa-battery-box-rk44x

How long would the batteries last?

ingo · November 2014

You can use any 12V DC adapter that gives a regulated output of 12V. If you use it only for pin 8 you should take a DC adapter made for low current. But before buying anything try to use a new 9V battery. Its voltage should be enough for a short test.

Ingo.

Spirantho · November 2014

The +2 has wire links next to the monitor port.... you can remove a link from the pin used as sync, and then connect it via a resistor to the 12V you get from the MIDI or serial ports, to give a proper voltage level that the TV can use.
I know this fixes the problem because I've done it several times!

Korinel · November 2014

I got myself a 9v battery box, hooked the red to pin 8 (Function Switching)and the black to 14 (Communication data ground), I tested a brand new battery with the multimeter and got 9.65v.

I plugged it into the back of the TV and then switched on the Spectrum and it still flickered :(

Any ideas?

Spirantho · November 2014

In my experience, pin 8 isn't necessary. Pin 16, on the other hand, must be between 1V and 3V and most be connected via a resistor to a voltage rail. Some SCART leads use the sync output (!) for this as it's roughly the right value, but this is a sync signal and not a constant voltage, so can send the TV haywire. Make sure pin 16 is a constant DC value, not an internal signal.

1024MAK · November 2014

1024MAK wrote: »

Pin 16 must be between 1.1V and 3V DC.

1024MAK wrote: »

Do you have a spare (good) 1.5V battery and a suitable battery box (suggest AAA or AA size, but C or D will do)?

So I suggest you try this next: disconnect and insulate the existing wire going to pin 16 of the SCART plug.
Then connect a new wire between pin 16 and the positive connection of a 1.5V battery. Connect the negative side of the battery to a 0V / ground connection on the SCART plug via another wire.
Then try you Spectrum again. Good luck :smile:.

Mark

1024MAK · November 2014

Also can you work out which pin on the Spectrums DIN monitor connection is wired to SCART pin 20?

Mark

Korinel · November 2014

Thanks again!
I just bought the bits to the pin 16 mod.

This is the pin layout:

Compared to Paul's it doesn't look right:

1024MAK · November 2014

There is no simple right way or wrong way, just different ways, as different televisions react differently. What works on one may or may not work on another.

The "Bright" output from pin 3 of the Spectrum should go not go directly to the reg, green and blue pins of the SCART plug but via some components. Do they?

You should also find some resistors in line with the red, green and blue video signals before reaching the relevant SCART plug pins.

The composite video feed is being used to try to "fool" the TV into switching to the correct mode via SCART plug pin 16. But it looks like your TV's don't like this. So try the 1.5V battery.

This "trick" is also used on SCART leads for other retro computers, and is common practice where there is a lack of purpose designed switching outputs, or no suitable source of power to use for this function.

Also don't worry about not all the grounds being connected. As far as I am aware, all the ground lines on the TV side of the SCART socket are connected together anyway. The only exception is sometimes pin 21.

Mark

Korinel · November 2014

1024MAK wrote: »

The "Bright" output from pin 3 of the Spectrum should go not go directly to the red, green and blue pins of the SCART plug but via some components. Do they?

No. The "bright" goes to a separate diode on each of the RGB wires, same as in Paul's diagram, they are marked "PH" so I assume they are 1N4148s.

1024MAK wrote: »

You should also find some resistors in line with the red, green and blue video signals before reaching the relevant SCART plug pins.

Yep, like Paul's diagram each RGB line has a 150 ohm (+-5) resistor.

1024MAK wrote: »

The composite video feed is being used to try to "fool" the TV into switching to the correct mode via SCART plug pin 16. But it looks like your TV's don't like this. So try the 1.5V battery.

This "trick" is also used on SCART leads for other retro computers, and is common practice where there is a lack of purpose designed switching outputs, or no suitable source of power to use for this function.

Also don't worry about not all the grounds being connected. As far as I am aware, all the ground lines on the TV side of the SCART socket are connected together anyway. The only exception is sometimes pin 21.

Mark

Superb thanks, that clarifies things immensely!
I have the AA battery box now so I'll try the 1.5v trick tomorrow.

Spirantho · November 2014

Yes, the composite video being used as a voltage rail is a common trick and it's a bit bonkers, to be honest. Whenever the composite video hits a sync, it'll go below 1V and tell the telly it's not RGB any more. Just because a multimeter can't det detect this dip doesn't mean the television can't.
I have yet to find a television that's completely happy with this kludge.

You need to modify the Spectrum (that's what I do) so that pin 20 becomes a 2V power rail, using a resistor to a 12V usually. And remember to disconnect the composite video from that pin too. :) Alternatively, wire up an active adapter using the power input like the Oric RGB leads do.

Korinel · November 2014

And (Drum roll.....) it worked! Thank-you!!!! Woo hoo :)

The picture is rock solid and very clear!

It is shifted to the left so I have a black band down the right. I'm assuming if I connect the 9v battery back to pin 8 that will disappear?

In the cable I bought the 16 and 20 SCART pins also had 150 ohm (+-5) resistors attached to them, they were underneath the sheath so had missed them.

I shall start a new thread :)

Flickering RGB -> SCART output.

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