TweakVec

This is a utility for reconfiguring VEC, the composite video encoder on the Raspberry Pi.

ATTENTION! You probably don't need this!

Since 2021, the Raspberry Pi Foundation's fork of the Linux kernel includes robust support for the VEC features, including all the supported color encoding standards. Since early 2023, this is also included in the upstream kernel, along with kernel-wide overhaul of support for composite video.

TweakVec might still be useful if you want to play with advanced settings, custom subcarrier frequencies etc. But if you only want to enable one of the "less standard" modes like NTSC-J, NTSC-443, PAL-M, PAL-N, PAL60 or SECAM (as well as regular PAL and NTSC), you can use the kernel features instead and don't need TweakVec!

⚠️ IF YOU'RE A MAINTAINER OF A SOFTWARE PACKAGE THAT SWITCHES THE RASPBERRY PI'S COMPOSITE OUTPUT TO ONE OF THE AFOREMENTIONED MODES, YOU ARE STRONGLY ENCOURAGED TO MOVE AWAY FROM TWEAKVEC AND USE KERNEL'S NATIVE SUPPORT INSTEAD ⚠️

Switching composite color standards in the kernel

The following instructions are valid for Raspberry Pi 4 and older; see below if you're using a Raspberry Pi 5.

If you're using the Raspberry Pi Foundation's kernel, branch 5.10 or newer, you can select your preferred composite color standard:

• At boot, by adding vc4.tv_norm={mode} to the kernel command line (configured by the cmdline.txt file on the boot partition for most distributions)
• At runtime, by setting the mode DRM property; for example under Xorg, you can use xrandr --output Composite-1 --set mode {mode}

Supported values you can substitute for {mode} are: NTSC, NTSC-J, NTSC-443, PAL, PAL-M, PAL-N, PAL60 (since kernel 6.3, replaced with PAL-60 with a dash - but read on) and SECAM.

If you're using Linux kernel version 6.3 or newer (either upstream, Raspberry Pi Foundation, or any other release), you can use the native kernel-wide support for composite video encodings. You can select your preferred standard:

• At boot, by adding video=Composite-1:tv_mode={mode} (or together with the screen resolution, e.g. video=Composite-1:720x480i,tv_mode={mode}) to the kernel command line (configured by the cmdline.txt file on the boot partition for most distributions targeting the Raspberry Pi)
• At runtime, by setting the TV mode DRM property; for example under Xorg, you can use xrandr --output Composite-1 --set "TV mode" {mode}

Supported values you can substitute for {mode} in this case are: NTSC, NTSC-J, NTSC-443, PAL, PAL-M, PAL-N, PAL60 and SECAM. Note that there is no explicit PAL60 option - to achieve that mode, just use PAL together with a 480i/240p resolution, e.g. video=Composite-1:720x480i,tv_mode=PAL on the kernel command line or xrandr --output Composite-1 --mode 720x480i --set "TV mode" PAL under Xorg.

NOTE: If you set the PAL60 mode using the video= kernel command line option, it will set it for the text console, but it will NOT be set as the preferred mode for other programs, like Xorg. You will need to configure those separately.

NOTE: The early boot (the rainbow square and early kernel messages, if any) will be displayed in the mode set using sdtv_mode in config.txt, which is NTSC by default. The Pi will switch to your preferred mode after a couple of seconds, when the kernel-mode driver is initialized.

Note about Raspberry Pi 5

The BCM2712 chip present in the Pi 5 no longer contains a composite video encoder - a completely different one has been added as part of the RP1 southbridge chipset, designed in-house by the Raspberry Pi Foundation. This encoder is completely incompatible with the old VEC, uses a different kernel driver, and will never be supported by TweakVec.

There might one day be a similar but separate program for the Pi 5 but I have no immediate plans to make one. I don't have a Pi 5 at the moment so I wouldn't have means to test it anyway.

Thankfully, the new encoder supports most of the same color encoding standards - SECAM is unfortunately not supported, but all the rest should work.

• If you're using Linux kernel version 6.1 through 6.5, you can set the preferred standard:

  • At boot, by adding drm-rp1-vec.tv_norm={mode} to the kernel command line (configured by the cmdline.txt file on the boot partition for most distributions)

  • At runtime, by setting the mode DRM property; for example under Xorg, you can use xrandr --output Composite-1 --set mode {mode}

  • Supported values for {mode} are: NTSC, NTSC-J, NTSC-443, PAL, PAL-M, PAL-N and PAL60.

• If you're using Linux kernel version 6.6 or higher, released after February 2nd, 2024, you can set the preferred standard:

  • At boot, by adding video=Composite-1:tv_mode={mode} (or together with the screen resolution, e.g. video=Composite-1:720x480i,tv_mode={mode}) to the kernel command line (configured by the cmdline.txt file on the boot partition for most distributions targeting the Raspberry Pi)

  • At runtime, by setting the TV mode DRM property; for example under Xorg, you can use xrandr --output Composite-1 --set "TV mode" {mode}

  • Supported values for {mode} are: NTSC, NTSC-J, NTSC-443, PAL, PAL-M and PAL-N.

  • To get the PAL60 mode, just use PAL together with the 480i/240p mode, e.g. by setting video=Composite-1:720x480i,tv_mode=PAL on the kernel command line - but see the notes above.

  • NOTE: Unlike the old vc4 driver used on Pi 4 and older, the drm-rp1-vec driver did NOT retain compatibility with the old style settings. So depending on your kernel version, only one or the other style of configuration will work.

Why TweakVec was created?

The default NTSC and PAL modes are enough for most cases where you might want to output composite video, but the Pi's video encoder is actually capable of much more than that.

PAL-M was always broken in the official firmware, and things like PAL60 were never implemented properly, either. The Pi can do both of these, and even SECAM turned out to be implemented in hardware.

The factory NTSC and PAL modes cover most typical cases for a composite output. So why you might want to use this tool?

• On a display that supports it, PAL60 and/or NTSC 4.43 may produce sharper image than regular NTSC, while retaining the 60 Hz 480i or 240p raster favoured by retro enthusiasts.
• You may some old sets from e.g France or Russia that only supports SECAM, or from Brazil that only supports PAL-M. tweakvec lets you revive such sets without hardware mods - although the video quality will generally be worse in those modes compared to regular PAL or NTSC.
• Even if you don't need any of these special modes, tweakvec gives you access to some additional settings, such as setting horizontal position of the image, and relative horizontal position of luminance and chrominance signals.
• Playing with obsolete tech is fun ;) I personally find SECAM artifacts charming, even if they're objectively awful ;)

TweakVec was also used as a playground to research the VEC's capabilities when developing proper support for its features in the kernel.

Usage

TweakVec requires Python 3.7 and access to sysfs and /dev/mem (normally only available to root).

TweakVec will also not switch line standards (525-line vs. 625-line, a.k.a. 480i60 vs. 576i50), so please use config.txt, tvservice or the KMS driver to configure NTSC (for 525-line / 480i60) or PAL (for 625-line / 576i50) standard first as a baseline. You can also use the progressive modes for 240p60 or 288p50, respectively - TweakVec will also not touch this setting.

Examples:

Display current configuration

sudo python3 tweakvec.py

Configuring most common video modes

60 Hz / 525-line / 480i / 240p modes

You need to first configure NTSC mode (even for PAL-M and PAL60) using standard OS facilities (e.g. sdtv_mode=0 in config.txt or tvservice --sdtvon "NTSC 4:3") before running any of these. You can use either interlaced or progressive mode and this setting will be kept by tweakvec.

NOTE: These modes (including PAL-M and PAL60) have identical sync specification as the factory NTSC mode. If your display doesn't produce an image that is readable at least in black and white in NTSC mode, it is unlikely that any of these will work for you.

sudo python3 tweakvec.py --preset NTSC     # same as sdtv_mode=0
sudo python3 tweakvec.py --preset NTSC-J   # same as sdtv_mode=1
sudo python3 tweakvec.py --preset NTSC361  # same as sdtv_mode=3
sudo python3 tweakvec.py --preset NTSC443
sudo python3 tweakvec.py --preset PAL-M
sudo python3 tweakvec.py --preset PAL60
sudo python3 tweakvec.py --preset MONO525  # equivalent to sdtv_mode=8

50 Hz / 625-line / 576i / 288p modes

You need to first configure PAL mode using standard OS facilities (e.g. sdtv_mode=2 in config.txt or tvservice --sdtvon "PAL 4:3") before running any of these. You can configure either interlaced or progressive mode.

NOTE: These modes (including SECAM) have identical sync specification as the factory PAL mode. If your display doesn't produce an image that is readable at least in black and white in PAL mode, it is unlikely that any of these will work for you.

sudo python3 tweakvec.py --preset PAL      # same as sdtv_mode=2
sudo python3 tweakvec.py --preset PAL-N    # same as sdtv_mode=0x42
sudo python3 tweakvec.py --preset SECAM
sudo python3 tweakvec.py --preset MONO625  # equivalent to sdtv_mode=10

Setting individual options

# move image 7 pixels right - centers PAL/SECAM image
sudo python3 tweakvec.py --hshift 7

# move chroma channel only, relative to luma
sudo python3 tweakvec.py --chroma-shift -1

# enable/disable sync pulses
sudo python3 tweakvec.py --enable-sync 0
sudo python3 tweakvec.py --enable-sync 1

# custom chroma subcarrier frequency
sudo python3 tweakvec.py --fsc 4429687.5

Combining it all

# SECAM with full settings reset, full image shift and chroma shift
sudo python3 tweakvec.py --reset --preset SECAM --hshift 7 --chroma-shift -2

Use sudo python3 tweakvec.py --help for a full list of possible options.

PAL60 on RetroPie (interlaced and progressive) - quick setup guide

This seems to be the most common use case for this tool, so I'm attaching a quick setup guide for it.

NOTE: I'll stress this again - your TV/monitor has to be able to display NTSC image at least in usable black&white (don't worry about checkerboard patterns where colour is supposed to be if you get these). If that's not the case, PAL60 most likely won't work for you, either.

NOTE: You may substitute PAL60 with PAL-M, NTSC443 or MONO525 in the commands below if you're interested in some of those instead.

Before starting, ensure that git and Python 3.7 or newer are installed on the machine. Current RetroPie images include these by default.

1. Configure your installation to generally use NTSC for everything - use sdtv_mode=0 or sdtv_mode=0x10 in config.txt, and always use NTSC (never PAL) in things like videomodes.cfg or tvservice invocations in runcommand scripts.

   Note that RetroPie is configured for NTSC by default.

2. Download tweakvec onto your machine. The recommended steps are:

   1. Switch to terminal by pressing Alt+F4 in Emulation Station, or ssh onto your Pi. Verify that you're in the home directory (on a default installation, the prompt should read pi@retropie:~ $).

   2. Type git clone https://github.com/kFYatek/tweakvec.

      This will download tweakvec to /home/pi/tweakvec

3. Edit /opt/retropie/configs/all/autostart.sh. You can either use: Configuration Editor -> Advanced Configuration -> Manually edit global configs -> all/autostart.sh, or any editor of your choice.

   • Add sudo python3 /home/pi/tweakvec/tweakvec.py --preset PAL60 at the beginning of this file.

4. If you are using custom scripts (e.g. runcommand-onstart.sh) that invoke tvservice, you will need to repeat the above command after each such call.

   • Example set of commands for switching to progressive PAL60:

     tvservice -c "NTSC 4:3 P"; sudo python3 /home/pi/tweakvec/tweakvec.py --preset PAL60; fbset -depth 8; fbset -depth 32; tvservice -s

     To return to interlaced mode, use the same command, but with "NTSC 4:3" instead of "NTSC 4:3 P".

Enjoy! ;)

What can't it do?

It seems that the VEC is hard-wired to only support the 525-line and 625-line systems and will not deviate too much from their sync patterns. So sadly, no reviving of British 405-line or French 819-line sets :(

It is also not possible to configure NTSC-style color for 625-line mode or SECAM in the 525-line one. You can use PAL for both, but no NTSC50 or SECAM60 for you :(

I also did not find a way to generate a continuous signal without the blanking periods (you can disable sync pulses, but blanking will still occur).