third optional chunk is identified by the "idx1" tag, which indexes the offsets of the data chunks within the file. By way of the RIFF format, the audio and visual data contained in the "movi" chunk can be encoded or decoded by software called a codec, which is an abbreviation for (en)coder/decoder. Upon creation of the file, the codec translates between raw data and the (compressed) data format used inside the chunk. An AVI file may carry audio and visual data inside the chunks in virtually any compression scheme, including Full Frame (Uncompressed), Indeo, run-length encoding, and Microsoft Video 1. Some programs, like VLC, complain when the "idx1" index sub-chunk is not found, as it is required for efficient moving among timestamps. They offer to "fix" the file by building an index temporarily or permanently. Metadata As a derivative of the Resource Interchange File Format (RIFF), AVI files are commonly tagged with metadata in the INFO chunk. In addition, AVI files can embed Extensible Metadata Platform. By design, any RIFF file can legally include additional chunks of data, each identified by a four-character code; software which does not understand that particular code should skip the chunk. As such, it is theoretically possible to expand any RIFF file format, including AVI, to support almost any conceivable metadata. Some of the limitations of AVI in modern use relate to a lack of standardization in this metadata (see Limitations below). Limitations Since its introduction in the early 90s, new computer video techniques have been introduced which the original AVI specification did not anticipate. The original AVI specification does not provide a standardized way to encode aspect ratio information, although the later OpenDML (AVI 2.0) specification does. Consequently, older players may not select the right aspect ratio automatically (though it may be possible to do so manually). There are several competing approaches to including a time code in AVI files, which affects usability of the format in film and television post-production, although it is widely used. For WAV audio files, Broadcast Wave (BWF) extensions were designed to standardize post-production metadata, but an equivalent for AVI files has not emerged. Some parties are known to write BWF chunks into AVI for metadata. AVI was not intended to contain video using any compression technique that requires access to future video frame data beyond the current frame (B-frame). Approaches exist to support modern video compression techniques (such