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Converting Prelinger Archive films to SVCD 08/06/01

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 editorial_icon.gif Converting Prelinger Archive films to SVCD
08/06/01
Alan Blount

While the Prelinger Archive has been available for several months, it has been difficult to watch the films on anything but a fast computer. The TMPEG encoder makes it possible to convert the files to the SVCD format, which retains most of the image quality of the original files, and can be played on most DVD players. SVCDs make it easy to throw an "Ephemeral Film Festival," where you can display the films on a conventional television or video projector.

Prelinger Archive films are coded as MPEG-2 files at one of three resolutions: 480x480, 368x480, or 352x480, at between 2.75 and 3.5 mbit/second. The desired playback aspect ration does not vary: all are meant to be played back at the Academy aspect ratio, 4:3, the standard for non-widescreen television sets.

There are three choices for playback of Prelinger material on a DVD player. The video can be coded as:

Encoding Format Resolution Bitrate
MPEG-1 VCD 352x240 (typical) 1.5 mbit
MPEG-2 SVCD 480x480 (NTSC) 2.6 mbit (typical)
MPEG-2 DVD 720x480 (typical) 2 - 8 mbit (typical)

This article is concerned with the middle choice--MPEG-2 encoding formatted for Super VCD (SVCD) playback.

SVCD Briefly

The first widely popular standard compact disc digital video format was Video-CD, or VCD, which uses MPEG-1 encoding for a 352x240 pixel image. Many (most) DVD players can play VCD discs, though some have problems playing CDROM VCDs (www.vcdhelp.com has a helpful reference that will help you determine if you player is capable of SVCD playback). VCDs run at a 1.5 mbit/second rate, and achieve a picture quality roughly on a par with VHS, though the character and quality of picture noise is different than (many would say inferior to) VHS.

VCD would be a likely format for viewing Prelinger archive material on a DVD player if something better wasn't available. As VCD is only capable of displaying 240 horizontal lines, half the vertical resolution is lost, as is the smoothness of motion, given NTSC's interlace.

SVCD improves on the VCD spec by allowing MPEG-2 coded video, at 480x480 pixels. The allowable bitrate is variable, and tops out at 2x regular CD speed, at around 2.6 mbit/sec.

Prelinger Films on SVCD

The SVCD spec is very close to the encoding found on the Prelinger films. Unfortunately, it is not exactly what SVCD requires. Even though the Prelinger films are encoded with MPEG-2, they do not match the encoding needed to be playable off SVCD.

With proper transcoding, Prelinger films can look very good when played as SVCD files. The object of the transcoding is to preserve as much image quality as possible while producing a disc playable on any SVCD-compliant player.

These are the differences between Prelinger's encoding and the encoding that an SVCD player is capable of playing:
  • Image size NTSC SVCDs must be encoded at 480x480 pixels.
  • Bit rate SVCD video must not run higher than around 2.4 mbit/second (allowing for audio). Prelinger files are coded at 2.75-3.5 mbit/sec
  • Progressive vs. Interlaced SVCD is capable of encoding films at either 30 frames-per-second interlaced or 24 frames-per-second. This capability can be used to your advantage when encoding Prelinger films.
  • Program Length This is not a problem. All of the Prelinger films are short enough (or pre-split) to fit on an SVCD.
Deinterlacing Films

Most of the transcoding parameters are intuitive--resize to 480x480 (while ensuring that the video plays back fullscreen), reduce the bitrate to less than 2.4mbit, etc. The trickiest part involves an optional framerate conversion.

Prelinger converted the films from their original celluloid source by telecine to analog videotape and then digitizing the tape. The hardware used for digitizing is a very good card, but does not allow for 24fps capture. Instead, the video was captured and MPEG-2 encoded at 30 interlaced frames per second. Since the original source was 24 frames per second film, approximately 1/5 of the available space is wasted by coding repeated fields twice.

A straightforward conversion to SVCD would involve simply encoding the video as is, at 30fps. The SVCD player will play back the video at the same rate, and the television will display 30 interlaced frames per second. SVCD players, however, have the capability of performing 3:2 pulldown on 24 fps progressive-source video, so that a film that is 24fps on the disc will be played back on the television at 30 interlaced fps.

Since the image quality is limited by the maximum SVCD bitrate (around 2.4mbit/second), it makes sense to spend those bits where they are needed most. By recoding Prelinger films to 24fps on the SVCD, you get an extra 20% of bits to work with, which makes for much improved quality at a given bitrate.

TMPEG

TMPEG (formerly Tsunami MPEG) is an excellent freely-available MPEG-2 encoder. It runs on Windows machines (sorry, no Linux). You will find it at http://www.jamsoft.com/tmpegenc. It provides an all-in-one package for converting Prelinger films to an SVCD-compliant file. The only other software required is an SVCD-burning program (Toast, Adaptec, cdrdao, etc).

You'll need a recent computer to run TMPEG. I'm using a 800mhz PIII, and it takes a couple of hours to convert each film.

  1. Install the TMPEG software.
  2. After starting up TMPEG, specify the Video Source file, which is the .mpg file you want to transcode.
  3. For Stream type (lower right corner of the main screen), select System (Video + Audio).
  4. Click the Setting button on the lower left. This will take you to the MPEG setup.
  5. In the Video tab, make the Stream type MPEG-2 video. Size is 480x480 pixels, aspect ratio is 1:1 (VGA), and frame rate is 23.976 fps.
  6. Set the Rate control mode to Constant quality (CQ). Click the Setting button.
  7. Set the Rate control parameters. I'm using a quality of 60, a maximum bitrate of 2200 (though you can probably go up to 2400), a minimum bitrate of 200 (voodoo--you probably don't need to set this), P picture spoilage at 0, and B picture spoilage at 10. You can play with these parameters to see what works best for you.
  8. Back to the Video tab, set Profile & Level to "Main Profile & Main Level (MP@ML), Video format to NTSC, Encode mode to 3:2 pulldown when playback, DC component precision to 10 bits, and Motion search precision to High quality (slow). If you know what you are doing, feel free to tweak these parameters.
Now you should set up the Inverse Telecine process, which will convert the 30fps source file to 24fps on the disc. Go to the Advanced tab, Select Video source type "Interlace," and set the Field order to "Top field first (field A)". Set Video arrange Method to "Full screen." Double click on (but don't check) Deinterlace.

You'll use the Deinterlace setup screen to determine if you have the field order set right. For Method, select "Even-Odd field (field)". Click and hold down the right arrow. As the film progresses, look at the motion. If it seems smooth, everything is good. If it's jerky, cancel out of the deinterlace screen and set the Field Order setting in the Advanced tab to "Bottom field first (field B)" and try again. This time the motion should be smooth.

Again, don't check deinterlace, but instead check and double click "Inverse telecine". Click Auto-setting, and using the "24 fps (Flicker priority)" method click "Start." TMPEG will deinterlace the movie (this takes a while). Click OK when done.

Back to setting the rest of the parameters. Go to the GOP Structure tab and set the following:
    Number of I picture in GOP: 1
    Number of P picture in GOP: 4
    Number of B picture in GOP: 2
    Output interval of sequence header: 2

    Output bitstream for edit (Closed GOP)
  x Detect Scene change
    Force picture type setting
Under the Audio tab, set the following:
    Stream type:  MPEG-1 Audio Layer II
    Sampling frequency: 44100
    Channel mode: Mono
    Bit rate: 112kbits/sec
    Emphasis: none
Finally, under Stream type, set "MPEG-2 Super VideoCD (VBR).

Back to the main screen. Be sure the output file name on the main screen is something different than your Video source. Now you're ready to encode! Click encode, go have a beer (or maybe six, depending on how slow your machine is), and you'll soon have an MPEG file ready to burn out as an SVCD using your favorite burn program. Note that it must be burned as an SVCD--a generic data CDROM won't play on most DVD players.

End Result

I've done tests with 14449.mpg, 05630.mpg, 07136b.mpg, and several others, and the results have been excellent. Picture quality seems gated mainly by the quality of the source material. Few encoding artifacts are noticible, or at least few are bothersome. The quality is far better than a VHS tape would offer.

Converting to DVD

While SVCD is high quality and compatible with most players, the DVD-R format seems to offer the least hassle and highest compatibility amongst players. The new Pioneer A03 DVD-R/RW drive looks like a great device to burn Prelinger films to DVD-R.

It's still an expensive choice, at somewhat less than $800, with blanks costing around $10. I believe that the MPEG-2 files may be writable as they are, without the somewhat complicated re-coding process described above. I intend to test this out as soon as I pick up a burner.

Helpful Links

As with anything, Google is your best bet for finding further info. Useful sites I've found through Google include:

http://sod.hgo.se/svcd
http://www.geocities.com/bug2kbug/svcd/svcd.htm

Final

I'd love to hear from you about your experiences in converting these files. Let me know if you find any improvments on these parameters--I can be reached at blount@alum.mit.edu.


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