OSSC

Introduction

Open Source Scan Converter is a low-latency video digitizer and scanconverter board designed mainly for connecting retro video game consoles and home computers into modern displays. It converts analog RGB/component video into digital format, and doubles (or triples) scanlines of a single frame if necessary to generate a valid mode for digital TVs or monitors.

The board and firmware was designed as a homebrew project mainly during 2015. The primary goal has been to make a scan converter with minimal latency and reasonable cost, which also can be built/assembled manually. Output image quality and features have also been important targets as long as they didn't contradict with primary goals.

Due to interest in the retrogaming community, both DIY kits and pre-assembled boards are released to public during early 2016. Firmware of the system is open source and new features can be expected after initial release.

Features

• Detection and digitization of various analog SDTV/EDTV/HDTV/PC modes
• Linedouble-support for 240p, 480i, 288p, 576i, 384p
• Linetriple-support for 240p with 4 different sampling modes
• Very low latency (less than 2 input scanlines)
• Fast "deinterlace" for 480i/576i
• Fast recover from input video mode change (e.g. 240p<->480i)
• All video processing done in RGB domain - no conversion to YCbCr
• Video and sync LPF for less-than optimal input signals
• Multiple inputs supporting various formats (see below)
• Full-range 24-bit RGB output through DVI/HDMI
• Emulated scanlines with configurable strength and position
• Configurable mask for overscan area
• Selectable sampling configuration for 480p input: DTV-480p or VGA 640x480
• Selectable CSC configuration for YPbPr source: Rec. 601 or Rec. 709

More informarion & discussion

• Original forum thread
• Console compatibility spreadsheet

Inputs

AV1 (RGB-SCART)

This input supports RGB video in RGBS and RGsB (sync on green) formats. Composite video, luma or c-sync can be used as sync source with RGBS mode. External sync splitters / boosters are generally not required or recommended as there is built-in sync filter & separator in the ADC frontend. The sync input has 75ohm termination, so TTL-level c-sync should not be connected directly there in order to avoid unnecessarily stressing source console and/or scanconverter. A 470ohm series resistor on the console side of the cable generally is a good solution when using cables which are wired for TTL-level c-sync output of a console. Video inputs also have standard 75ohm termination, so arcade boards may need extra resistors on the cable when connected directly without using a SuperGUN.

Analog audio is bypassed to a 3.5mm stereo output jack next to video output connector.

AV2 (Component)

3xRCA input supports for component video (YPbPr) and RGB (RGsB format).

AV3 (VGA)

VGA/HD-15 input supports RGB video in RGBHV, RGBS (pin 13) and RGsB formats. RGBHV and RGBS modes require clean TTL-level sync signals and cannot extract sync from composite/luma. VGA input is best suited for high-quality input sources as video LPF functionality is limited (SCART and Component inputs are routed through a dedicated LPF chip). Therefore, it is generally recommended to connect older consoles and arcade boards to other inputs.

Settings

Scanlines

Controls whether emulated scanlines are drawn on top of the picture

• Off: No scanlines drawn [default]
• Horizontal: Scanlines drawn on every other (digitize & linedouble modes) or every third (linetriple) output line
• Vertical: Scanlines drawn on every other output column

Scanline strength

• 12-100%: Strength of the emulated scanlines [default=12%]

Horizontal mask

• 0-60 pixels: Controls the size of a mask (black border) generated around the picture in horizontal direction (4-pixel steps). Can be used to mask areas which would get hidden in the overscan region of CRT TVs. [default=0]

Vertical mask

• 0-60 pixels: Controls the size of a mask (black border) generated around the picture in vertical direction (4-pixel steps). Can be used to mask areas which would get hidden in the overscan region of CRT TVs. [default=0]

480p in sampler

Controls the sampling mode when 525-line progressive signal (“480p”) is detected at input

• Auto: “VGA 640x480”-mode is selected when the signal comes from RGBHV input. “DTV 480p”-mode is selected with all other inputs [default]
• DTV 480p: Input is sampled at 858 samples per line, typically associated with 720x480 mode (CEA-861 spec.) used by DTV/DVD equipment and newer game consoles. This option forces the sampling mode for all inputs, which may be required for optimal image quality when e.g. Dreamcast with a VGA module is connected to RGBHV input.
• VGA 640x480: Input is sampled at 800 samples per line, typically associated with 640x480 mode used by PCs. This option forces the sampling mode for all inputs.

Sampling phase

• 0-347 deg: Selects the phase of regenerated pixel clock (=position where each sample is taken). When output rate matches the input DAC rate (PC graphics modes, newer consoles, 2 last linetriple modes), it is important to adjust sampling phase for optimal quality. This setting should be adjusted only after adjusting sync and video LPF since they can alter the relative position of video and sync signals. [default=180deg]

YPbPr input Color Space

Controls YPbPr->RGB colorspace conversion coefficients.

• Rec. 601: Input is assumed to be in Rec. 601 format, which is generally true for SD video [default]
• Rec. 709: Input is assumed to be in Rec. 709 format, which is generally true for HD video

Analog sync LPF

Low-pass filter selection for analog sync signals (SCART and component inputs). Required if there are noise or glitches on the sync line.

• Off: Sync is processed unfiltered [default]
• 33MHz: Lowest filtering
• 10MHz: Medium filtering
• 2.5MHz: Heavy filtering – recommended only if the above settings are not sufficient for stable sync, since excessive LPF can cause image jitter.

Video LPF

Video low-pass filter. Filters out high-frequency noise on video, and can reduce jitter when sampling clock does not match input video dot clock rate (older consoles in linedouble mode). NOTE: The last 3 settings are only effective with SCART and 3xRCA inputs.

• Auto: Suitable LPF is automatically selected based on input source and video mode [default]
• Off: LPF is disabled.
• 95MHz (HDTV II): 95MHz bandwidth – suitable for 1080p
• 35MHz (HDTV I): 35MHz bandwidth – suitable for 720p
• 16MHz (EDTV): 16MHz bandwidth – suitable for 480p etc. EDTV formats
• 9MHz (SDTV): 9MHz bandwidth – suitable for 240p, 480i etc. SDTV formats

240p linetriple

Controls whether 240p is linetripled instead of standard linedouble. NOTE: resulting modes do not have the same parameters (total lines, pixels per line) as the standard CEA 720p mode, so they are generally accepted only by monitors and not by many consumer TVs.

• Off: 240p is linedoubled, resulting to 480p output. [default]
• On: 240p is linetripled, which results to 720p output.

Linetriple mode

Controls the sampling and pixel clock multiplication mode for linetriple.

• Generic 16:9: Uses full horizontal sample rate without pixel multiplication, resulting to fully utilized 1280x720 output (16:9 aspect). [default]
• Generic 4:3: Uses 3/4 of full horizontal sample rate without pixel multiplication, resulting to 960x720 effective area of 1280x720 output (4:3 aspect).
• 320x240 optim.: Uses a sampling rate which matches the DAC rate of 426 dots per line used by various classic consoles (e.g. PSX) in 320x240 mode, resulting to pixel-perfect digitization. Output is pixel-multiplied by 3 in horizontal direction, resulting to 960x720 effective area of 1280x720 output. Note: If picture jitters when this mode is selected, adjust sampling phase until sweet spot is achieved.
• 256x240 optim.: Uses a sampling rate which matches the DAC rate of 341 dots per line used by various classic consoles (e.g. NES, SNES, MD) in 256x240 mode, resulting to pixel-perfect digitization. Output is pixel-multiplied by 4 in horizontal direction, resulting to 1024x720 effective area of 1280x720 output. Note: If picture jitters when this mode is selected, adjust sampling phase until sweet spot is achieved.

TX mode

Sets the output TX mode.

• HDMI: auxiliary HDMI packets are sent along with video data, such as Infoframes which indicate color settings (RGB, full-range). Only selectable in DIY boards.
• DVI: Only video data is sent to the display. Required if target display does not support HDMI.

Firmware update

See section below.

Save settings

Saves settings (all previous and remote control config) to flash.