Linux-Based 8mm Telecine

Media Conversions, my business, converts videotape and slides to DVD. My customers often ask if I also can convert 8mm film. This is the story of my adventure into converting film to DVD. There are a number of ways to make a conversion. You can run the film through a projector and use a video camera to capture the images. Although, finding a working projector is difficult. Belts and rubber drive components dry up. Worse, 30-year-old rolls of film, some with splices, may no longer stand up to the stress of being projected at 18 frames/second (f/s). Plus, most video cameras run at 30f/s and will not synchronize with the projector.

Telecines have been used since the early days of broadcast TV to convert film to video. A number of Web sites describe DIY Telecine projects (see Resources). Generally, they either rebuild a projector and use a still camera, or they utilize a flatbed scanner and a custom film transport. Based on my research, I decided to build a Telecine using a flatbed scanner. The cost of entry is low, and scanners running at 3,000dpi or above are a commodity item. You can get started on the conversion software without the film transport, and you don't need custom optics. The downside, if you're not a programmer, is that you have to write all of your own software.

I decided early in the project that I wanted to use only open-source software tools. I hosted it on an Ubuntu Linux desktop system. I knew I would need a programming language with support for scanning, serial (or parallel) port communication, a math library and an image library. A plotting and drawing library also would be helpful during program development. I also wanted a language that offered ease of programming in higher-level constructs. I was familiar with C, but did not want to use it for this project, so instead, I decided to use Python. Python is easy to learn, it's well supported by the Linux community in both on-line forums and with numerous examples of code, and error handling and type checking/conversion are part of the language. Plus, the Python Imaging Library includes an interface to SANE for scanner support.

I acquired an Epson Perfection 3490 photo scanner for the project. It has SANE drivers, a built-in backlight for film scanning and offers 3,200dpi resolution.

There are four steps to converting a roll of film: scan the film in segments, find the image frames in the segments, remove duplicate frames where the segments overlap and make a movie from the frames. I wrote three separate Python programs for the first three steps and used FFmpeg for the fourth. The software relies on cheap disk space. Frame files are copied from segment scans. Overlap removal makes a second, renumbered, copy of all of the frame files. This strategy allows each of those programs to be rerun with the same segment scans for debugging and program development.

The cost, for a 50-foot roll of film, is approximately 8GB of space for the segment scans and similar amounts of space for the log file (if debug is turned on) and each of the frame file sets. Files are written into subdirectories of the current directory and numbered sequentially. A root filename, given as a command-line argument, is used as a prefix. Scan data is written into the scans directory, and frame files are written to the frames directory. If logging is turned on, log files are written to the logs. If debug is left on (default setting), marked up copies of the scan files also are written to the logs. The markings show where the edges of the sprocket holes were found and the outline of the frame extracted. Finally, overlap-removed, renumbered frames are written to the movie directory.

The program for scanning film simply calls the SANE scanner interface, saves the scan data, advances the film and repeats for a count given as an argument on the command line. See the Film Transport sidebar for a description. You can do a project like this without a film transport, but it's tedious. Each scan takes about 80 seconds. Limits on the size of the backlight meant that I could use only about 7.7 inches out of the approximately 8.5 inches of scanner width. Allowing for overlap between the scans, a 50-foot roll of film will have about 90 scan segments and takes roughly two hours to scan.

Film Transport

There are two parts to this project. One involves the software that processes the scanned film and makes a movie. The other part is the design of a film transport. The film transport is the harder part of the project, because it involves creating one-of-a-kind hardware. My transport design is based on reel-to-reel tape recorders popular in the 1960s (Figure a). It feeds film from a supply reel, across the scanner and winds it up on a take-up reel. A pair of spring-loaded idlers maintains film tension. A sprocket drive advances the film.

Figure a. 8mm Film Transport—Photo by Frank Pirz

The film transport is controlled by an embedded microprocessor. It takes commands from the Linux system over a serial port, and controls supply and take-up reel rotation and a sprocket motor for advancing the film. I was able to find both a program development and device programming environment as well as a C compiler for the Microchip PIC series of microprocessors all running under Linux. See Resources for the list of software tools used in this project.

To simplify the software, I made a film guide out of 10mm thick clear plastic film. I first aligned a steel ruler with the scanner axis, and I used GIMP to examine scans of the ruler edge. I moved it between scans until it was aligned to within approximately 50 pixels with the grid in GIMP. At 3,200dpi, 50 pixels is about 0.015 inches and more than adequate for this application. Then, I placed a piece of plastic against the ruler and glued it down with CyanoAcrylate glue. Once the glue was dry, I removed the ruler and used a piece of 8mm leader as a spacer to glue down a second guide. A sheet of glass placed over the guides keeps the film being scanned in alignment. With the film aligned with the scanner, no corrections for skewed images are necessary.

The program for finding frames actually is looking for sprocket holes. It's substituting software registration for mechanical registration of the film. Figure 1 shows a short piece of scanned film. The left-hand side is the original scan, and the right-hand side is the same scan converted to black and white (B&W).

Before we look for sprocket holes, we first find the top edge of the film. Given the alignment of the film in the guides, we could skip this step, but at this point, I'd rather not. The location of the top edge and knowing whether it's Regular8 or Super8 film (see the A Short History of 8mm Film sidebar), tells us approximately where the centerline of the sprocket holes will be.