Difference between revisions of "Sega FILM"

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Revision as of 09:34, 18 April 2006

This page is based on the document 'Description of the Sega FILM/CPK File Format' by Mike Melanson found at http://multimedia.cx/film-format.txt.

FILM is a multimedia container file format developed by Sega for use on its early CD-ROM home video game consoles. Based on analysis of a number of Sega CD and Saturn games, it appears that the format was first developed sometime during the era of the Sega CD, Sega's first CD-based video game console. There are many early variations of the FILM format observed on a number of Sega CD titles. Further, early FILM files have been observed on at least one 3DO game (Lemmings).

The Sega Saturn video game console, released in 1994, was also CD-ROM-based and apparently offered developers a standardized SDK for full motion video (FMV) playback using a modified variant of the well-known Cinepak video codec.

It is possible to store both audio and video in a FILM file. Alternatively, the format is able to handle either video without audio, or vice versa.

Sega Saturn CPK File Format

All multi-byte numbers are stored in big endian format.

Many Sega Saturn CD-ROM games use this file format to store frames of a Cinepak-compressed video interleaved with uncompressed PCM audio. When this is the case, the files will typically have the extension .CPK (which is why FILM files are usually known as CPK files). Sometimes, audio-only FILM files will have a .SND extension.

The general format is laid out as follows:

| +-------------------+ |
| | FILM header       | |
| |                   | |
| | +---------------+ | |
| | | FDSC chunk    | | |
| | +---------------+ | |
| |                   | |
| | +---------------+ | |
| | | STAB chunk    | | |
| | +---------------+ | |
| |                   | |
| +-------------------+ |
|                       |
| Audio or Video Data   |
|  ..                   |
|  ..                   |

A FILM file will usually consist of a header followed by interleaved audio and video chunks. The FILM header has the following structure:

 bytes 0-3    'FILM' signature
 bytes 4-7    length of FILM header (including signature and length
 bytes 8-11   FILM format version in ASCII (ex: '1.01' or '1.07')
 bytes 12-15  unknown (may be reserved and set to 0)
 bytes 16-n   remainder of FILM header

Different Sega Saturn games include FILM files with a variety of FILM version numbers. Sometimes, the same game will include FILM files with a variety of format versions. A version number implies that there has been some change to the file format, but I have yet to observe any differences between different FILM file versions (except possibly for version '1.09'; see STAB chunk for more details).

The remainder of the FILM header contains a number of chunks describing the media data in the file. Usually, the number of chunks is 2: A FDSC chunk and a STAB chunk.

A FDSC chunk contains file description information. The chunk format is as follows:

 bytes 0-3    'FDSC' chunk signature
 bytes 4-7    length of FDSC chunk (including signature and length
              fields); an FDSC chunk should be 32 (0x20) bytes long
 bytes 8-11   FOURCC of video codec (usually 'cvid' for Cinepak or null
              for no video)
 bytes 12-15  height of video frames
 bytes 16-19  width of video frames
 byte 20      unknown, but always seems to be 0x18 (24)
 byte 21      number of audio channels
 byte 22      audio sampling resolution in bits (either 8 or 16)
 byte 23      unknown
 bytes 24-25  audio sampling frequency in Hz
 bytes 26-31  unknown (may be reserved and set to 0)

Note that the height field precedes the width field, which is unusual since width usually precedes height when expressing video resolution.

The fields pertaining to audio (channels, bits, and sample rate) will all be 0 if there is no audio present in the file.

A STAB chunk contains a table of media sample information. The chunk format is as follows:

 bytes 0-3    'STAB' chunk signature
 bytes 4-7    length of STAB chunk (including signature and length
 bytes 8-11   framerate base frequency in Hz
 bytes 12-15  number of entries in the sample table
 bytes 16-n   sample table

Note that the length field ought to take the first 16 chunk bytes into account. However, it has been observed from some games (notably Burning Rangers using '1.09' version FILM files) that the length field sometimes does not account for these 16 bytes.

The sample table consists of a series of 16-byte sample records. Each record is laid out as follows:

 bytes 0-3    offset of sample chunk from beginning of sample data
 bytes 4-7    length of sample chunk
 bytes 8-11   sample info 1
 bytes 12-15  sample info 2

The offset is the offset from the start of the sample data, not the absolute offset in the file. The length of the FILM header implicitly serves as a pointer to the beginning of the sample data in the file.

If the sample info 1 field is set to all ones, the sample is an audio chunk. Otherwise, it is a video chunk. If it is a video chunk, the top bit of the 32-bit number specifies whether the chunk is an inter-coded or an intra-coded frame. 0=intra-coded (a.k.a. keyframe), 1=inter-coded. This is useful for seeking since it is a good idea to only jump to key frames when seeking through a file.

The rest of the first sample info field and the second sample info field pertain to framerate calculation. Refer to the section on FILM framerate calculation for more information on proper FILM playback using these sample info fields.

FILM Framerate Calculation

The STAB chunk contains a framerate base frequency in bytes 8-11. This frequency is expressed in ticks/second (Hz). Every video chunk has a frame tick count. For example, movie files in Panzer Dragoon I & II (FILM format versions 1.04 and 1.07, respectively) have a base frequency of 600 Hz. The frames have tick counts of 0, 20, 40, 60, 80, etc. Thus, there are 20 ticks between each successive frame.

 (600 ticks/sec) / (20 ticks/frame) = 30 frames/second

However, not all FILM files exhibit such a nice, even framerate. Myst, for example, contains FILM files (version 1.01) with a base frequency of 30 Hz. Some of the frames skip 2, then 3 ticks. Here's a sample tick count progression: 0, 2, 5, 7, 10, 12, 15, 17...

Each STAB record has 2 32-bit sample info fields. For an audio chunk, sample info 1 is all ones and sample info 2 is always 1. For a video chunk, sample info 1 contains the keyframe bit (as described in the STAB section) and the absolute timestamp in clock ticks of the video frame with respect to the file's base frequency clock. The sample info 2 field contains the number of clock ticks until the next frame is rendered. This type of information would be particularly useful in an interrupt-driven, real-time multimedia system like, for instance, a video game console (such as the Sega Saturn).

It is important to note that an application that knows how to play FILM files can not assume a constant framerate. The files will not play correctly with such a method. It is also important to note that converting FILM files to file formats that only support constant framerates (such as AVI) is not a winning strategy. The converted file will not play correctly.

FILM Deviant Cinepak (CVID) Video Data

The Cinepak data inside of a FILM file can not be decoded with a general purpose Cinepak decoding algorithm. It is reasonable to think that this was no accident. If FILM files contained standard CVID data, the files could be played on any computer that had some kind of Cinepak decoder implementation.

This document assumes familiarity with the Cinepak algorithm. For information on the entire decoding algorithm and data format, see the references at the end of this document.

A CVID chunk is supposed to start with a 10-byte chunk header:

 byte 0     flags
 bytes 1-3  length of CVID data
 bytes 4-5  width of coded frame
 bytes 6-7  height of coded frame
 bytes 8-9  number of coded strips

Following the chunk header ought to be the first byte of the first strip header. In the deviant CVID data stored in a FILM file, there appear to be an extra 2 bytes at the end of the chunk header, bringing the total header size to 12 bytes.

The length of the chunk as reported by the deviant CVID data chunk is also incorrect. The length field is supposed to report the length of the entire chunk including the header. The deviant CVID data reports a length that is 8 bytes too short. That is, the length reported in the CVID chunk is 8 bytes shorter than the length reported in the corresponding record of the FILM file's STAB chunk. The STAB chunk length takes into account the proper length, including the extra 2 bytes at the end of the chunk header.

Finally, the deviant CVID data appears to throw a potential decoder one more curveball with what appears to be data chunks that don't have, for lack of a better term, properly divisible chunk lengths. For example, a 0x2000 chunk commonly has a length of 0x604 bytes, which provides 2 bytes for the chunk ID, 2 bytes for the chunk length, and 0x600 bytes for 256 6-byte vectors. However, the deviant CVID data might have a 0x2000 chunk with a length of 0x600 bytes, which provides for the 4 chunk preamble bytes and 0x5FC bytes for the 6-byte vectors which is not evenly divisible by 6. This apparently confuses Cinepak decoders. The solution in this case is to unpack 255 6-byte vectors and then skip 2 bytes before attempting to decode the next chunk in the strip.

FILM Audio Data

Audio data in a FILM file can be stored in a variety of formats which are mostly linear PCM variants.

CPK files from Sega Saturn games almost always contain linear PCM audio. The one known exception is Burning Rangers. This game apparently uses CRI ADX ADPCM audio coding.

Saturn CPK audio data is always stored as signed data. This is important to remember when playing 8-bit CPK audio data on a PC which generally expects 8-bit data to be unsigned.

When a CPK file contains 16-bit audio data, the individual PCM samples are stored in big endian format. Again, this is important to remember when playing on a PC since a PC will generally expect little endian 16-bit data.

If the CPK audio data is stereo (8- or 16-bit), the channel data is non-interleaved. Usually, stereo data is stored interleaved as a left channel sample followed by a right channel sample as follows:

 L R L R L R L R ...

In a stereo CPK file, for each audio data chunk, the first half of the chunk contains left channel samples and the second half contains right channel samples. The likely reason for scheme is that console audio hardware such as that found on the Sega Saturn usually features a number of audio channels which can each play a mono PCM stream at a particular pan position (e.g., from 0-15 where 0 is extreme left, 7 and 8 are center, and 15 is extreme right). In order to play a stereo PCM stream, one physical audio channel must be assigned to extreme left while a second channel is assigned to extreme right. Then the appropriate channel samples are sent to the correct channel. Since these files are optimized for playback on the Sega Saturn it makes sense to arrange the audio samples like this.

The Sega CD audio playback hardware ostensibly supports native sign/magnitude audio samples. All Sega CD games studied use this format to store audio data. Sign/magnitude number coding for 8-bit numbers means that for each 8-bit byte:

 bit 7      sign
 bits 6-0   magnitude (value)

This is slightly different from twos complement encoding which is how computers typically store signed numbers. For example, in the sign/magnitude scheme, 0x81 represents -1 and 0xFF represents -127. 0x01 and 0x7F still represent 1 and 127, respectively, just as in unsigned, and signed twos complement coding.

Early Sega CD FILM Files

Some Sega CD games as well as at least one 3DO game (Lemmings) use an early version of the FILM format. Unfortunately, there are a lot of variations and a general-purpose player will probably require a lot of special-case logic in order to deal with every known circumstance.

The first difference in these early files is the version number. These early files do not have ASCII-readable version fields. Some have NULL or other non-ASCII versions. This helps automatic detection.

The second major difference that all of these early files appear to exhibit is an abbreviated FDSC chunk which omits any audio information. Thus, the FDSC chunk is laid out as follows:

 bytes 0-3    'FDSC' chunk signature
 bytes 4-7    length of FDSC chunk (including signature and length
              fields); this FDSC chunk is 20 (0x14) bytes long
 bytes 8-11   FOURCC of video codec
 bytes 12-15  height of video frames
 bytes 16-19  width of video frames

The lack of audio information leaves room for experimentation. The following is a list of known games which have early FILM files along with any special quirks observed.

On the Lemmings 3DO game, there are 2 files with the .film extension (the 3DO game console uses a custom filesystem which allows for file extensions longer than 3 characters). The files have a NULL version field. The files use CVID data which is deviant in the same manner as typical FILM CVID data with an added quirk: There appear to be 6 extra bytes between the end of the CVID chunk header and the start of the first strip header, rather than the usual 2 extra bytes. The Lemmings FILM files also contain 8-bit signed, monaural PCM data with a sampling frequency of 22050 Hz.

The Batman and Robin Sega CD title has a series of files with the extension .s. They are early FILM files with a version field of 0x00020000. Further, bytes 12-15 of the main FILM header are not set to 0 as is usually seen in FILM files. The files have a video fourcc of 'Seg4' which is probably Cinepak for Sega or a variant thereof. The files have STAB chunks which are a constant 0x20 (32) bytes long. This consists of the 'STAB' signature, a 4-byte chunk length, the base playback frequency, a sample count, and one 16-byte sample record. These files do not store the sample table in one place. Instead, they store one sample record followed immediately by the data corresponding to that sample record. Repeat until the end of the file.

The Dracula Unleashed Sega CD title contains a series of files beginning with video?? with no file extension. They are early FILM files with a NULL version field, abbreviated FDSC chunk, and 'sega' as a video codec (Cinepak for Sega). The audio is 8-bit sign/magnitude audio with a sampling frequency of 16000 Hz.

Jurassic Park for the Sega CD has a large number of files with the file extension .MVD. They are early FILM files with the same characteristics as the FILM files in Dracula Unleashed.

The Tomcat Alley title for the Sega CD has a massive data file called tca.bin. Inside this data file are a lot of FILM files. These files have NULL version fields. The files have abbreviated FDSC chunks and the 'sega' video fourcc. They also have short STAB chunks with an Apple Quicktime 'mdia' atom appended at the end.

Strategy For Detecting FILM File Types

There are quite a few variations and deviations in the FILM file format. This section presents logic for detecting and dealing with the variations.

File extension detection is not especially useful since many games will masquerade their files with another extension. The best method to determine whether a file is a FILM file is to read the first 4 bytes and check for the signature 'FILM'.

Upon validating the signature:

  • read the file version in the FILM header
  • if the file version consists of ASCII characters
    • if the file version is '1.09', watch out for bad length of STAB chunk as well as ADPCM audio
    • load as standard FILM file, be sure to feed CVID data through Cinepak decoder that can handle it
  • if the file version is 0x00020000
    • read the video fourcc
    • if the video fourcc is 'Seg4'
      • assume the file comes from Batman and Robin
  • if the file version is NULL
    • read the video fourcc
    • if the video fourcc is 'cvid'
      • assume the file comes from the Lemmings 3DO game
      • be prepared to decode special variant of modified CVID data
      • assume 8-bit, mono, 22050 Hz PCM audio
    • if the video fourcc is 'sega'
      • assume the file comes from one of several Sega CD games that agreed on this format
      • assume 8-bit, mono, 16000 Hz sign/magnitude audio

Games That Use FILM Files And Their FILM File Versions

These are some of the games that use the FILM format:

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