how does the erase head work on tape decks? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: April 23, 2021, 06:49:19 pm
content: I've seen 1/8 inch cassette decks just wire up the erase head to the power supply, but all 1/4 inch reel to reel decks all use a ... bias oscillator if I'm not mistaken?...I'm trying to put together a reel to reel deck for some experiments, but do I really need that oscillator thing? can't I just connect the erase head to a DC supply?

author: tooki
author_rank: Super Contributor
timestamp: April 23, 2021, 08:03:48 pm
content: The cheapest recorders use a permanent magnet as the erase head. The optimal method, however, is a high frequency alternating magnetic field. Good cassette decks use a proper erase head. (Not just open reel machines.) Given how easy it is to make a simple oscillator, I don’t know why you’d forgo their superior erase performance to save so little effort. And since you need an oscillator for the AC bias anyway...

author: MIS42N
author_rank: Frequent Contributor
timestamp: April 23, 2021, 10:02:11 pm
content: The recording process is not 100% efficient in removing previous content. You can record without erasing but for audio work it is unsatisfactory because the previous content can be heard softly in the background. If your experiment is digital, it may ignore low level unwanted signal.The reel to reel oscillator is a high frequency that effectively scrambles the tiny magnets on the tape. The particles have to have some hysteresis or the tape would be erased by any minor magnetic field. A DC field aligns them. The headroom on an aligned tape is smaller than a scrambled tape, so high frequency erase is preferred.The recording process can be considered as a form of pulse width modulation, the bias system providing the pulses and the audio providing the width changes. The magnetic material on tapes is quite similar to what is used to coat hard disk platters. The recording on disks doesn't use erase because the read back is digital so ignores low level previous data. But old data can be recovered by using analog methods. If you have really sensitive material to erase, it requires many full formats of the disk to reduce residuals to an unrecoverable level. Or crush the disk.

author: TimFox
author_rank: Super Contributor
timestamp: April 23, 2021, 10:10:47 pm
content: The alternating magnetic field H is maximal over a relatively short length of tape corresponding to the gap in the erase head.As the element of tape goes away from the gap, the oscillating field falls off.This is a standard way of "degaussing" where the magnetic material is first subjected to an H field strong enough to run the hysteresis loop's flux density B from +saturation to -saturation, and then slowly reduce the field magnitude so that the hysteresis loops shrink, eventually down to zero. The initial value of B is then "forgotten", having been mixed up by the initial saturated hysteresis loops.The tools sold to degauss screwdrivers do the same thing when you slowly pull the blade away from the AC-driven coil.

author: Gyro
author_rank: Super Contributor
timestamp: April 24, 2021, 05:30:05 pm
content: Only Mods can delete a thread. If you ask a question here, it stays.

author: RJSV
author_rank: Super Contributor
timestamp: April 25, 2021, 09:19:27 am
content: Nice replies!I would add, that the AC bias was usually approx. 44 khz, for old school reel to reel decks, and, I've noticed, the Schober Organ Co. Tape Reverb (as well as any modern 1/8 inch deck).