andrum99
Posts: 910
Joined: Fri Jul 20, 2012 2:41 pm

Composite display tweaks

Mon Jul 30, 2012 1:41 pm

Is it possible to force the Raspberry Pi to enable display output on the composite video port? It seems that it is only enabled if the Pi detects something connected to the port on startup. I would like to be able to use the Pi headless but have the option of plugging in a TV to the composite port without having to reboot to enable display output.

Also, for some reason my TV produces a better display with the Pi in NTSC mode - in PAL mode the output goes beyond the left edge of the screen and there are lines across the middle of each line of text on the console. Is there a way to adjust the output to get full PAL resolution but keep the output where I can see it on the screen?

Thanks

Andrew.

JeremyF
Posts: 515
Joined: Wed Jan 25, 2012 9:06 pm
Contact: Website

Re: Composite display tweaks

Mon Jul 30, 2012 1:47 pm

andrum99 wrote:Is it possible to force the Raspberry Pi to enable display output on the composite video port? It seems that it is only enabled if the Pi detects something connected to the port on startup. I would like to be able to use the Pi headless but have the option of plugging in a TV to the composite port without having to reboot to enable display output.
If nothing is plugged into hdmi, then the Pi is constantly outputting to the composite port whether or not a display is attached to the other end. So if you attach a TV mid session, it'll output to it no problem.
{sig} Setup: Original version Raspberry Pi (B, rev1, 256MB), Dell 2001FP monitor (1600x1200), 8GB Class 4 SD Card with Raspbian and XBMC, DD-WRT wireless bridge

andrum99
Posts: 910
Joined: Fri Jul 20, 2012 2:41 pm

Re: Composite display tweaks

Mon Jul 30, 2012 4:01 pm

If nothing is plugged into hdmi, then the Pi is constantly outputting to the composite port whether or not a display is attached to the other end. So if you attach a TV mid session, it'll output to it no problem.
That is not what I have observed. I get no output on the composite port unless my TV is plugged into the composite port on bootup.

Andrew.

dom
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 5369
Joined: Wed Aug 17, 2011 7:41 pm
Location: Cambridge

Re: Composite display tweaks

Mon Jul 30, 2012 4:28 pm

andrum99 wrote: That is not what I have observed. I get no output on the composite port unless my TV is plugged into the composite port on bootup.
Andrew.
Not possible. There is no way of detecting whether the composite port is plugged in.
What you have said does apply to HDMI port.

User avatar
redhawk
Posts: 3465
Joined: Sun Mar 04, 2012 2:13 pm
Location: ::1

Re: Composite display tweaks

Mon Jul 30, 2012 9:16 pm

The way the PI console appears via composite pretty much depends on how the TV picture is configured most are set slightly zoomed in to avoid black areas from typical video content.
My 24" Panasonic TV also crops the PI image too so I lose a bit of the left side this isn't really a problem though because I'm using a TV capture card, Putty and Xming instead.
I tried to tweak the video settings to move the image but nothing happens I guess they only apply to the HDMI output.

If you still want PI on an old TV with Scart then this little device may help - http://www.ebay.co.uk/itm/320914582490? ... 1423.l2649

Richard S.

andrum99
Posts: 910
Joined: Fri Jul 20, 2012 2:41 pm

Re: Composite display tweaks

Mon Jul 30, 2012 10:44 pm

The way the PI console appears via composite pretty much depends on how the TV picture is configured most are set slightly zoomed in to avoid black areas from typical video content.
Good point - I had forgotten about overscan. I did have a TV that had been tweaked so it had minimal overscan but it went in the bin a few years ago. :-(

I do like the shimmering output I get from the Pi's composite port on my TV - reminds me of my old Sinclair Spectrum+. ;-) Wonder if there is a speccy emulator ported to ARM....

Andrew.

portets
Posts: 186
Joined: Sat Oct 29, 2011 6:24 am

Re: Composite display tweaks

Tue Jul 31, 2012 1:01 am

redhawk wrote:I tried to tweak the video settings to move the image but nothing happens I guess they only apply to the HDMI output.
The overscan settings effect both composite and hdmi. Did you make sure to remove the "#" at the beginning of each overscan line? Also, positive numbers are used to correct the image going offscreen. Negative for black bars.

andyroyal
Posts: 32
Joined: Fri Jul 20, 2012 7:41 pm
Location: Berkshire, UK
Contact: Website

Re: Composite display tweaks

Thu Aug 02, 2012 2:15 pm

andrum99 wrote:
Wonder if there is a speccy emulator ported to ARM....
There are Java-based Speccy emulators which theoretically are platform independent, so if you've got Java set up on your RPi that'd work. Probably.

User avatar
ukscone
Forum Moderator
Forum Moderator
Posts: 4182
Joined: Fri Jul 29, 2011 2:51 pm
Contact: Website

Re: Composite display tweaks

Thu Aug 02, 2012 2:26 pm

andrum99 wrote:.

I do like the shimmering output I get from the Pi's composite port on my TV - reminds me of my old Sinclair Spectrum+. ;-) Wonder if there is a speccy emulator ported to ARM....

Andrew.
see this thread http://www.raspberrypi.org/phpBB3/viewt ... 865#p65865

User avatar
alexeames
Forum Moderator
Forum Moderator
Posts: 2869
Joined: Sat Mar 03, 2012 11:57 am
Location: UK
Contact: Website

Re: Composite display tweaks

Thu Aug 02, 2012 6:46 pm

andrum99 wrote:Wonder if there is a speccy emulator ported to ARM....
There certainly is... and it's a very simple install as well. :)

http://raspi.tv/2012/how-to-install-fus ... spberry-pi
Alex Eames RasPi.TV, RasP.iO

User avatar
jbeale
Posts: 3511
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: Composite display tweaks

Sat Oct 26, 2013 3:21 pm

dom wrote:
andrum99 wrote: That is not what I have observed. I get no output on the composite port unless my TV is plugged into the composite port on bootup.
Not possible. There is no way of detecting whether the composite port is plugged in.
What you have said does apply to HDMI port.
I know this is an old thread, but I would like to support andrum99's statement, as I have observed it too (on recent system: Linux rp7 3.6.11+ #538 PREEMPT Fri Aug 30 20:42:08 BST 2013 armv6l GNU/Linux). There is no output on composite video unless it is connected at bootup.

Physically, it is easy to detect the presence of a connection: the TV presents a 75 ohm load on the video output, which would reduce the voltage amplitude of the video signal by a factor of 2 relative to an open connector in a standard video system. So a simple voltage comparator on the SoC wold give you that information, and I assume that has been implemented in the silicon.

karlkiste
Posts: 189
Joined: Tue Jan 22, 2013 8:50 am
Location: berlin, germany

Re: Composite display tweaks

Sat Oct 26, 2013 4:26 pm

I had the setup handy, and can say, my model b with latest raspbian image activates cvbs output while nothing is connected.

If this is not the case with all pis, a 78 ohm resistor can act as an impostor ;-)

User avatar
jbeale
Posts: 3511
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: Composite display tweaks

Sun Oct 27, 2013 12:55 am

actually, I did not look at the output with a scope so I have not ruled out the possibility that there was some screen-saver, that just made the screen look blank.

Siamak
Posts: 70
Joined: Sat May 31, 2014 1:48 pm

Re: Composite display tweaks

Tue Aug 04, 2015 1:16 pm

Hi every one,

For educational purposes I would like to use the Composite out put of the Raspi.

I have a digital modulator that works fine and I have been able to do the recording via my TV when it is configured as a DTV (it has a USB connection enabling me to record on a USB key).
The composite out-put of the RasPi works fine when connected directly to the TV, but cannot record, when I connect the Digital modulator there is nothing it like as if it is scrambles.

Is there any way that I can use a video amplifier to remedy the problem, in order to do so I need to know what the strength of the RasPi composite video signal is, can any body help at all, I don't have any oscillator to find out the signal strength.

Many thanks in advance

Siamak

drgeoff
Posts: 9912
Joined: Wed Jan 25, 2012 6:39 pm

Re: Composite display tweaks

Tue Aug 04, 2015 3:14 pm

The amplitide of the composite output from a RPi is close enough to the standard "1 volt into 75 ohms". Whatever your problem is I highly doubt that a video amplifier will fix it.

What digital modulator do you have? Can you give a link to information about it?

Your question is not at all related to this old thread. You should have started a new one.

BTW you need an oscilloscope, not an oscillator, to look at signals.

allanonmage
Posts: 9
Joined: Wed Jun 12, 2013 2:40 pm

Re: Composite display tweaks

Fri Nov 13, 2015 1:37 am

This may be a 3 year old thread, but it comes up highly in search results and does not answer the OPs request, so I will post what I found:

This raspberry pi stack exchange article addresses this problem and offers 2 step-by-step guides to address the OPs concerns:
http://raspberrypi.stackexchange.com/qu ... nt-working

This page goes over all the video parameters, and is linked to within the /boot/config.txt file:
http://elinux.org/RPi_config.txt

Apparently, there are different "standards" for the type of 3.5mm AV cable too:
http://www.raspberrypi-spy.co.uk/2014/0 ... ideo-jack/

Return to “Beginners”