Composite output quality

For the first time yesterday I tried out the composite video output to two different small lcd tvs that I have.
I have to say that the quality is very poor, it's like watching underwater ( well not quite) but it is bad, how is anyone supposed to use this with anything other than an expensive hdmi tv? Makes the Pi an expensive cheap computer.
Is there a way to "up" the output resolution via composite?

[rurwin]

The maximum resolution of a BBC Micro was 640x256, and it was unusable on a TV; anything more than 40 characters wide (320 pixels) was too high a resolution. (The VIC-20 used a 20 character-wide screen resolution.)

It's a tribute the TV technology that the RaspPi composite output works at all.

But what is the native resolution of those LCD TVs? If they only have 320x256 pixels, they are not going to be able to display 656x416 or whatever the effective resolution of the RaspPi's composite output is.

Not sure what the tv resolution is, I've had them a few years, I have lost the manuals by now. Even on my Sony Bravia, the composite picture is woeful.
As an aside, can the Pi output HDMI and composite simultaneously, or does it only do one at a time?

You should be able to configure the framebuffer dimensions to match the physical resolution of your LCD displays, which will avoid any poor scaling. The composite output works fine on my analogue CRT television. Some of the finer details are a bit flickery, because of the interlacing, but watching videos full screen is no problem at all.

I've always loved that ATV logo by the way!

Being born and reared in Staffordshire, ATV was at it's peak when I was growing up.
How do I configure the framebuffer settings, caveat, you are speaking to a relative linux newbie, with failing braincells ( too many get-offs from my motorbikes over the years )

Using config.txt in your /boot partition, see http://elinux.org/RPi_config.txt#Video_mode_options

If you scroll down, you should fine frambuffer width and height settings. Sorry I can't be more specific, am at work right now!

Thanks, I will take a look.

Being born and reared in Staffordshire, ATV was at it's peak when I was growing up.
How do I configure the framebuffer settings, caveat, you are speaking to a relative linux newbie, with failing braincells ( too many get-offs from my motorbikes over the years )

With a handle like that, I'd wager you're one of Maggie's victims in the eighties.....

Spot on.

Spot on. A victim of her policy, lasted until 1992.

Not much help on composite I'm afraid, never bothered with it. To the extent I butchered the connection off one of my Pi's so it would fit in a box I had.

It was a fair guess since you're from Staffordshire, it was a big mining area.

small lcd tvs

Casio TVs??

Composite video is crap no matter how you look at it the fact you mix chroma luma signals (Y/C) down one cable and then have to split it back into Y and C results in nasty side effects.
Ideally it would have been better if the Raspberry Pi provided an S-Video output but the Broadcom SoC doesn't support this kind of video output only composite.
Anyway the nasty artifacts can usually be well hidden providing your video capture device handles both odd and even fields at the same time like a proper TV.
If your capture device discards certain lines or only captures one field (like some crappy USB video grabbers) you basically you end up with rubbish video quality.

Richard S.

Composite is easy to 'produce' in software from a purely digital starting point, its an ancient system that looks a bit out of place in this day and age. It's only its cost (next to nothing) that keeps it going. Back in the day (early eighties) having a composite input monitor was a huge improvement over a UHF modulator system used on most home computers of the time (You could read an 80 column text display), but things have moved on a bit in the thirty years since.

Producing composite video is no more difficult than S-Video in fact many of the old computers and game consoles were already capable of Y/C and could be modded for it.
I guess with regards to Broadcom the probably didn't bother supporting Y/C because they expected everyone to be using HDMI instead.

In case anyone was wondering about composite video artifacts here are a few I made from my Pi:

Single field odd or even (commonly used by cheap and nasty USB video grabbers)


Fields mixed / blended


Interlaced video


Interlaced video software enhanced


Richard S.

Mine looks like #1 on both tv sets. Awful.

The composite quality depends on the TV/monitor you are using. I get similar results with one of my TVs (and an identical model TV I tried in my office break room)- looks like the first image. Both are a small 19" Panasonic CRT.

My other TV looks much better, also a CRT, a larger 32" Sony. Haven't tried it on my LCD screen as of yet, but that one has an HDMI input, so I wouldn't need to use the composite input (not sure if that TV even has one)

I think the problem with LCD TVs they don't actually take the video signal and interlace this like a regular CRT TV.
So either they're showing odd field and then even field (progressive scan) or they're mixing both fields together into one frame.
As you can see from the screen shots interlaced video has considerably less colour artifacts than blended fields.

Richard S.

I think you didn't set the output to PAL.
By default, it is set to American NTSC and it gets converted to PAL by your TV.
The conversion produces such artifacts which I also got until I uncommented:

Code: Select all

sdtv_mode=2

in /boot/config.txt.
The picture is much better now and I can't find any difference in picture quality between raspi and my dvd player and satellite receiver.

Thanks, and pardon my ignorance, but when I have uncommented that line, if I find it, how do I save it before exiting? Cant figure that bit out.

[rurwin]

You edit it as superuser, because otherwise you wont have permission to write it.

So
pi@raspberryPi ~/ $ sudo nano /boot/config.txt

Make the change and then CTRL-X to exit. (That's hold down Ctrl key and hit the X key.) It will ask if you want to save your changes, hit Y. Then it will ask where to save them, hit Enter.

Thanks, I'll try it tomorrow when I have time.

Ok, I'm using PiBang and that line is not in /boot/config.txt. Do I just add it at the bottom of the list? Or at the top,I have no idea.
Here is the entire contents of /boot/config.txt on my Pi with PiBang

core_freq=250
sdram_freq=400
over_voltage=0
disable_overscan=1
arm_freq=800

Apparently it doesn't matter

If you're going to add more things, it's probably good to keep similar things together, and so I would move the line

Code: Select all

disable_overscan=1

to the bottom, and add

Code: Select all

sdtv_mode=2

below or above that - that keeps the frequency stuff together, with the video stuff below

Thank you, I will try it now to see what happens