CD Ripping

CD 抓轨 (CD Ripping), also known technically as digital audio extraction, is the process of reading linear PCM data from the CD-DA sectors of an audio CD and saving it as WAV, AIFF, FLAC, or other computer files. This differs from recording the player’s analog output using a sound card: ripping directly captures the digital samples returned by the optical drive, while subsequent compression and file format selection are separate steps.

CD-DA organizes audio into consecutive sectors and track indices, rather than storing pre-made WAV files on the disc. Ripping software uses the directory table to locate track ranges, requests data at a rate of 75 audio sectors per second, and concatenates the 44.1 kHz, 16-bit stereo PCM data from each sector. Whether track gaps, hidden pre-track audio, index points, pre-emphasis flags, and end sectors are preserved depends on the software’s capabilities, the read range, and the output method; files split by track may also differ from the boundary definitions in a full-disc image with a CUE sheet. When reading audio, the optical drive may encounter issues such as scratches, focus failure, tracking errors, buffering, and position repeats. The interleaved Reed–Solomon code on CDs can correct a certain range of physical read errors, but CD-DA lacks the additional error-checking layers found in certain sector patterns on data CDs; drives may also mute or interpolate unrecoverable samples. Ripping software therefore uses methods such as repeated reads, sector overlap, C2 error information, CRC comparison, or AccurateRip to determine whether the results are consistent.

Different CD drives may read from the same physical location starting at a fixed number of audio samples apart, commonly referred to as a read offset. Correcting the offset aligns the ripped results with a unified reference boundary, but if the optical drive cannot read beyond the lead-in or lead-out zones, a small number of samples at the very beginning or end of the disc may still be lost. Offset differences typically manifest as an overall shift of the samples forward or backward and are not equivalent to audio being resampled.

If a track rip yields a checksum that matches that from a database or an independent re-read, this indicates with a high degree of probability that the extracted samples do not contain random read errors; however, it does not prove what type of master was used for the record, nor does it allow for an assessment of the quality of the recording, mixing, or mastering. Converting an error-free WAV file to FLAC preserves the PCM data without loss; converting it to lossy formats such as MP3 or AAC, however, alters the signal.