If you’re familiar with Dolby Laboratories it may be through their surround-sound encoding for movie theatres and home cinema systems, but it was in analogue recording noise reduction systems that they started. Magnetic tape is an inherently noisy medium, to the extent that the whooshing sound of background noise is clearly audible in a tape recording. The noise reductions chips with the Dolby logo in a Yamaha cassette deck. Different magnetic materials have been used in tape media including iron oxide, chromium dioxide, and finely divided iron particles, and each of those has different magnetic properties and requires a different bias. The AC bias is derived from an oscillator that also provides the drive for the tape erase head, and whose frequency and level are set to different values depending on the type of tape medium in use. Originally they used a DC bias with a fixed DC current flowing through the recording head, but later designs used an AC bias in which a high-frequency AC signal several tens of kHz higher than the audio signal is recorded alongside it, the AC being inaudible on playback but having the effect of providing a constant recording level to keep within the linear region of the tape’s magnetic properties. But it does have a range where it’s approximately linear, leading engineers to seek ways to keep the audio recording into the linear region and vastly lower the distortion. Magnetic tape is not a linear medium, meaning that the degree to which it can be magnetised is not linearly proportional to the magnetic field applied. The erase head is driven by a high-frequency AC signal when recording, with the intent of removing and overwriting any previous recordings. ![]() In some inexpensive machines these two amplifiers are the same physical circuit reversed by means of the record/playback switch, but in high quality recorders they will be separate. The recording and playback heads are both connected to audio amplifiers. Many tape recorders such as this Sanyo cassette deck had significant switches to repurpose parts of their circuit between play and record. The heads have adjustable azimuth, which is set such that the gap between the head’s poles is perpendicular to the edge of the tape. In some machines the record and playback are performed by the same head. First contact is with an erase head, followed by the record and playback heads. Before the tape passes through the pinch roller, it passes by two or three magnetic read and write heads. This pinch roller regulates the speed, with tape tension maintained on both sides of it by the reel drives. It’s important that the tape speed be kept constant, and this is achieved by clamping it between a constant-speed rotating metal roller and a rubber one. If the tape is simply pulled past the head by the force on the take-up reel, it will run at a variable speed dictated by the power on the reel and the radius of the tape spooled upon it. The pinch roller and tape head of a Sony cassette mechanism. The take-up reel is lightly driven to run faster than the tape speed, and the playback reel has a slight braking force to keep the tape under tension at all times. There are two reels that hold the tape: the playback reel that houses the recording, and the take-up reel that stores the tape as it plays in the machine. Whether it is a humble cassette recorder or a top-end studio multitrack, all tape recorders are very similar. But the key to the format’s runaway success is the technical advancements that differentiate those 1950s machines from their wire recorder ancestors. By coating a flexible plastic tape in ferrous particles, the same simple technique of laying down an audio signal as variations in the magnetic field could be made smaller, lighter, and more robust. The first magnetic recordings were made directly on metal wires, but metal fatigues as it bends. The Device That Made The Sound Of The Latter Half Of The 20th Century “Like a travelling razor blade”, a Blattnerphone steel-strip tape recorder at the BBC in 1937. ![]() Unless you’re an enthusiast or collector, it’s likely you won’t have a tape deck in your audio setup here in 2021 and you’ll probably be loading your 8-bit games from SD card rather than cassette, but surprisingly there are still plenty of audio cassettes released as novelties or ephemeral collectables. What’s missing? Magnetic tape, the once-ubiquitous recording medium that first revolutionised the broadcast and recording industries in the mid-20th-century, and went on to be a mainstay of home audio before spawning the entire field of personal audio. In our no-nonsense journey through the world of audio technology we’ve so far have looked at digital audio and the vinyl disk recording.
0 Comments
Leave a Reply. |