Compact Cassette

盒式录音带 (Compact Cassette, also known as Compact Audio Cassette) is a magnetic tape recording format developed by Philips and unveiled at the Berlin Radio Exhibition in 1963. It enclosed the tape, the feed reel, and the take-up reel within a portable plastic case, simplifying the process of threading tape into the player. Philips subsequently promoted standardization through a more open licensing approach, making 盒式录音带 a universal medium for home recording, portable playback, car audio, and commercial music distribution.

The tape is approximately 3.81 millimeters wide and runs at a nominal speed of 4.76 centimeters per second. Standard stereo recording divides the tape width into four magnetic tracks: two tracks are used on one side to record the left and right channels, and the other two tracks are used after the cassette is flipped over. Mono equipment may use wider magnetic tracks. Since Sides A and B are actually located on the same continuous piece of tape, the designation “C60” refers to a total of approximately 60 minutes for both sides combined, rather than 60 minutes per side. Longer C90 and C120 tapes typically use thinner tape and are more sensitive to stretching, creasing, and the condition of the tape mechanism. The recording head uses a varying magnetic field to align the magnetic particles according to the audio signal, and the playback head then detects changes in the residual magnetization. A high-frequency bias is superimposed on the program during recording to improve the linearity of the magnetic material; different tape formulations require corresponding bias and playback equalization. IEC Type I is typically iron oxide-based; Type II often uses high-bias materials; Type III once combined both types of coatings but did not gain widespread adoption; and Type IV uses metal particles. The identification notches on the cassette allow compatible devices to automatically select certain settings, but many recorders still require manual matching.

Noise-reduction systems such as Dolby B, C, S, and dbx reduce tape noise through preprocessing during recording and complementary decoding during playback. Using the wrong decoding method can alter the frequency balance and dynamic range; therefore, the “Dolby” logo does not indicate that every player should use the same mode. Commercially pre-recorded tapes may also use recording-side headroom expansion technologies such as HX Pro; this type of processing does not require additional decoding by the player.

The speed stability of the cassette mechanism and the position of the magnetic heads directly affect sound quality. The drive spindle and tape-pressing roller control the tape speed, while the reel is responsible for winding and unwinding; mechanical deviations can cause jitter, slow drift, or unstable tape movement. If the azimuth of the magnetic head does not match that used during recording, high frequencies will attenuate due to the phase difference between the two magnetic gaps. The tape itself is also subject to background noise, copy effects, powder loss, stretching, and demagnetization, the severity of which varies depending on the material, manufacturing, storage, and usage history.

盒式录音带 stores continuous analog magnetization changes and does not include sampling rate, bit depth, or file format. When digital recordings are copied to cassette tape, the final medium remains analog tape; conversely, digitization requires playback through a tape deck, analog-to-digital conversion, and subsequent file encapsulation. In the late 1980s, formats such as DCC—which used the same physical form factor but recorded digital data—emerged; these employed different recording and playback systems than traditional analog cassettes.