Talk:Tape bias

Theory
The diagram given in this section is useless for understanding the topic. A better diagram would be the one in the article on Hysteresis. This is how it was presented to me in school back in the days of sticks & rocks, when dirt was new. Back then it was called a B-H loop as that's how the axis were marked. One is the coersive force and the other the retained magnetism (retentivity?).

Using the hysteresis loop makes it so much easier to see how very low amplitude audio causes very little or no change in the magnetic domains. At the other end, too much saturates the domains and accomplishes little. That's why adding a bias to get up into the linear region works so well. No fancy Greek letters needed. — Preceding unsigned comment added by Dbryant 94560 (talk • contribs) 06:51, 12 November 2023 (UTC)

I'm sorry, but I find this a very poor entry. I've just been reading the book 'the science of sound recording' which touched on the subject of the use of bias but did not explain it in detail. I was hoping that the wikipedia article would explain the theory. I'm starting out understanding there is a problem with distortion due to hysteresis and I understand that tape bias overcomes this distortion, but I'm not sure why or how it works.


 * It does´t work at all. It is a ghost of old technology, that has nothing to do with the later tape medium. It simply intermodulates the audio with a high frequency beep, causing more distortion, on the already low headroom tape format. Similar to how some thought the RIAA filter was something.. And Dolby. And probably why some hesitate to do 24bit streaming..

The theory section on this article is woefully inadequate. More time is spent on the history of tape bias than on the theory, with only three sentences in the whole article attempting to explain how it works. Given that most people visiting this article will be looking for an explanation and not a potted history I feel more space needs to be dedicated to describing how and why it works as it does.

″A constantly, rapidly varying magnetic field also helps overcome magnetic hysteresis (the tendency of paramagnetic materials to resist either being magnetized or releasing its magnetism) of both the recording head and the tape medium.″ Does this mean that the very high frequency bias overcomes hysteresis because it causes a lot of flux? How does that work exactly? I was unaware that hysteresis changed its properties at different frequencies - the m-h graphs seem to show hysteresis as being predictable and linear. I wasn't aware hysteresis 'disappears' at high frequencies. That seems to be the key to understanding how bias works, if its true. That seems fundamentally at odds with everything I'd studied on hysteresis, so please explain why a high frequency signal should make hysteresis suddenly disappear.

The other sentence: "As the tape leaves the tape head, the applied bias partially offsets the tape's field and the remaining net induction is essentially the difference between the positive and negative half-cycles of the previously recorded signal. This differencing operation further cancels some of the nonlinearity." It doesn't matter how often I read this, it makes no sense to me. If I was being generous, I would say you assumed the reader knows too much about the topic (which is a mistake given the whole reason for them visiting wikipedia is because they don't know about the topic.) Break it down into simple stages. Don't just explain what happens, explain why it happens. That's what this article is all about, surely? The expression "applied bias partially offsets the tape's field" needs more explaining. What is 'net induction?' What is a 'differencing operation?'

I understand its a technical subject, but two or three sentences to describe what is happening is just not good enough. Whenever possible, use plain, simple English. There can never be too much plain language in the world when describing complex operations. I know jargon may make things easier to explain, but this is an encyclopaedia not a technical manual so you have to make allowances for the possibility that the reader is unfamiliar with jargon. Nor is this a dictionary entry - you won't be penalised for using more than once sentence to describe something! If you can use clear and simple text it'd be so much easier to read and understand.

This is not a criticism of the accuracy of the article, only a request to improve the presentation and fullness in its description. Hope this helps improve the article. — Preceding unsigned comment added by 86.161.173.239 (talk) 10:09, 5 April 2013 (UTC)


 * The major ptroblem with explaining how AC bias works is that no one really fully understands how (or even why) it works. There are at least three competing and mutually exclusive theories and the explanation in the article isn't one of them.  I B Wright (talk) 16:44, 18 April 2013 (UTC)


 * IBWright: "at least three competing and mutually exclusive theories". Really? Don't spew nonsense... Magnetic media aka Spin glass is a heavily researched and well-understood area, and is routinely modeled at large scales (Jiles-Atherton model) down to sub-micrometer length scales (Micromagnetics). Modeling the effect of AC bias on magnetic tape transfer function doesn't even require micrometer-scale modeling and is considered an undergraduate-level exercise. Magnetics 101. 93.172.159.180 (talk) 19:50, 15 April 2015 (UTC)


 * There are indeed multiple "explanations" which one finds when researching this topic. I suspect some are speculation or from those who know just enough to be dangerous.  I certainly agree that the explanation given here is inadequate.  If the theory is so well known, surely someone could do a better, more complete, and well-cited explanation for the theory. 141.219.101.151 (talk) 16:59, 10 August 2015 (UTC).
 * See also "Some Popular Misconceptions About Magnetic Recording History and Theory," By McKnight and McKnight (presented at the 2012 SF AES Convention), available at http://www.aes.org/aeshc/pdf/mcknight_some-popular-misconceptions.pdf . In particular, misconception 2: "We Understand How AC Bias Works."  While it has been heavily researched, maybe it is not so "well-understood" as you might think.  There have been many models and theories. Just because there's a model, doesn't mean the model is correct. 141.219.101.151 (talk) 17:33, 11 August 2015 (UTC)
 * I agree. I'm an electrical engineer and I can't figure out what in the hell bias is after reading this article.  Either I'm incredibly dense or this article is incredibly poorly written. 208.126.79.154 (talk) 18:58, 6 April 2018 (UTC)

FM is more obvious; why not?
I'm guessing it requires a faster tape, allowing for the high frequency signal, but that would be partly mitigated by the lack of a need to care about non-linear response. Add in a subsonic signal to control motor speed and I think you'd get amazing audio quality. 50.89.71.42 (talk) 06:48, 29 August 2014 (UTC)
 * Wouldn't be practical with cassette tapes but lots of people did just that with the audio portion of VHS "Hi-FI" enabled video recorders. 94.0.215.193 (talk) 12:05, 31 October 2015 (UTC)

Article needs table
Standards for cassette tapes defined by the International Electrotechnical Commission 94.0.215.193 (talk) 01:08, 31 October 2015 (UTC)


 * Here's a suggested citation for this. Not sure if this is best included in this article, Compact Cassette or both. ~Kvng (talk) 15:38, 9 August 2017 (UTC)

Uncited material in need of citations
I am moving the following uncited material here until it can be properly supported with inline citations of reliable, secondary sources, per WP:V, WP:NOR, WP:CS, WP:NOR, WP:IRS, WP:PSTS, et al. This diff shows where it was in the article. Nightscream (talk) 18:27, 29 August 2022 (UTC)


 * I have reverted this. Additional context on this editing pattern can be found at Talk:Radio, Talk:Phonograph_record and User_talk:Nightscream. ~Kvng (talk) 03:29, 30 August 2022 (UTC)

AC bias
Although the improvements with DC bias were significant, an even better recording is possible if an AC (alternating current) bias is used instead. While several people around the world rediscovered AC bias, it was the German developments that were widely used in practice and served as the model for future work

The German pair received several related patents, including for "high-frequency treatment of the sound carrier".

Possibly independently of Weber and Braunmühl, the UK company Boosey & Hawkes produced a steel-wire recorder under government contract during the Second World War that was equipped with AC bias. Examples still surface from time to time, many having been disposed of as government surplus stock. After the war, Boosey and Hawkes also produced a "Reporter" tape recorder in the early 1950s using magnetic tape, rather than wire, which was based on German wartime technology.

Theory
As the tape leaves the trailing edge of the gap in the tape head, the oscillating magnetic field due to the applied AC bias is rapidly reduced to the average magnetic field of the much slower-changing audio signal, and the tape particles are therefore left in this magnetic condition. The non-linearity of the magnetic particles in the tape coating is overcome by having the AC bias field greater by at least an order of magnitude (10× the maximal audio field), which saturates these particles in both magnetic directions while they pass the gap in the recording head. The AC bias level is quite critical and, after being adjusted for a particular tape formulation with a specific recording machine, is usually left unchanged.

The mechanism is similar to the demagnetizing signal, which is used to erase the tape except that the desired audio signal is superimposed and retained on the tape during the recording process. The large AC bias acts as a demagnetizing signal which decays exponentially as the tape moves beyond the head, while the audio signal is the residual field that remains imprinted on the magnetic medium.

Practice
The characteristics of the recording system change quite markedly as the level of the bias current is changed. There is a level at which the system gives the minimal distortion (which is the highest bias). There is also a level at which the high-frequency response is at maximum (lowest bias). These conditions, unfortunately, do not occur at the same bias level. Professional reel-to-reel and cassette recorders are always set up for minimal distortion. Consumer equipment, particularly Compact Cassette recorders, have the bias set at a compromise level (usually a little higher) to give good frequency response and acceptably low distortion.

Bang & Olufsen invented and patented the so-called Dolby HX PRO (Headroom eXtension) principle for combining bias control with the Dolby system for better high-frequency response in cassette recorders. Tandberg invented the cross-field recording system for tape recorders where a separate head with the bias was used. Adding bias to the recorded signal in the one head had tended to limit the system's high-frequency response due to interaction between bias and signal. The cross-field system produced less interference from the bias signal. This allowed extended high-frequency performance compared to mixing the two signals in the recording head, but mechanical tolerances for cross-field are tight. The system required frequent readjustment and was largely abandoned. Japanese manufacturer Akai, however, persisted with cross-field bias and successfully marketed portable and mains-operated machines featuring the cross-field system.

Different amplitudes of bias field are optimal for different types of tape, so most recorders offer a bias setting switch on the control panel, or, in the case of the compact audio cassette, may switch automatically according to cutouts on the cassette shell. Ferric-based tapes require the lowest bias field, while chrome-based tapes (including the pseudo-chromes) requiring a higher level, and metal-particle tapes requires even more. Metal-evaporated tape accepts the highest level of bias, but it is mostly used for digital recording (which does not use bias, as the non-linearity is not a major problem). The same is valid for a combination cassette tape, the FeCr variant, on which a thinner chrome layer covered a thicker ferric layer. The idea behind this was that at lower frequencies and higher head currents, the ferric layer would be more deeply magnetized, while at higher frequencies only the top Cr layer was active. In practice, this didn't work well, and some claimed that this thin chrome layer was quickly polished off in heavy use.