![]() | This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | Archive 2 | Archive 3 | → | Archive 5 |
Data rates for cassettes as data storage are from Slashdot. Not verified. Hotlorp 05:09, 10 Jan 2004 (UTC) http://slashdot.org/articles/03/09/18/1857204.shtml?tid=126
The name "musicassette" or MC refers ONLY to pre-recorded cassette albums, not recordable cassettes. Lee M 01:57, 16 Apr 2004 (UTC)
Is it just me, or do the metric measurements in the article seem overly precise? Is the width of a single track really specified down to tenths of a micrometre? To help visualise it, a single human hair is about 100 micrometres thick. This means that the track width is specified to thousandths of a hair's width.
Conversions from imperial to metric shouldn't just be mechanic calculations based on conversion factors taught by rote in school. They should actually have some perspective to the real-life things that are actually being measured.
Or to put it another way, how would you feel if I (being approximately 186 cm tall) expressed my height as 6 feet, 1.228346457 inches? — JIP | Talk 10:08, 3 Apr 2005 (UTC)
The dimensions currently given in the article look not very plausible:
If someone has access to the standards that I added to the reference section, could you please look up the real dimensions and replace the current inch mess? Markus Kuhn 21:04, 18 December 2005 (UTC)
Here is an online example of cassette tape specifications. -- Blainster 10:22, 19 December 2005 (UTC)
It's very notable that 1/8 inch is far closer to 3.18cm than 3.81cm. Is the latter really correct? Perhaps it was typed in wrongly. Multiple sources don't necessarily mean much if they all got their wrong information from the same place. Fourohfour 10:46, 19 December 2005 (UTC)
The article's second sentence reads: "It consists of a length of magnetic tape from BASF inside a protective plastic shell." This could be interpreted to mean that all cassetes - sold by all companies - contain tape made by BASF. I'm pretty sure that's not the case, and the intended meaning may have been simply that BASF invented the type of tape used in cassetes, but it needs to be clarified.
-- 4.245.5.118 23:46, 25 Apr 2005 (UTC)
I removed a link that was essentially an ad. I saw the person who added it said it was a good example of the cassette in today's market, but it seems irrelevant. Anyone who feels different, feel free to explain why... methelfilms 03:00, 14 July 2005 (UTC)
Can somebody with more technical expertise that what I have write a section that talks about endless-loop cassettes? Although I think they've fallen out of favor, they were popular in answering machines and in music-on-hold systems. Some had a metalic piece of tape that served as a "marker" for the equipment to know when the tape had made a complete cycle. Any takers to write such a section? Joe 19:49, 3 October 2005 (UTC)
Am I right in thinking C15 tapes were widely available for a period? I seem to remember using them for ZX Spectrum data.-- bodnotbod 12:39, 26 October 2005 (UTC)
The Coleco_Adam computer used two high-speed drives based on compact cassette technology.
"the two stereo tracks lie adjacent to each other rather than a 1/3 and 2/4 arrangement. " -- could someone explain this for the layperson? -- Tarquin
I think it means that the tape is laid out like this:
1111111 2222222 3333333 4444444 <---> direction of tape
rather than:
1111111 3333333 2222222 4444444
Where 1 and 2 are the stereo channels of side one, and 3 and 4 are the stereo channels of side two. How to put that into simple language without taking up half a page is a different matter, however... -- Camembert
I read somewhere that a Stereo compact audio cassette, actually has five tracks... Tracks 1, 2, 4, and 5 have audio on them. Track 3 is silent... Side one is made up of Tracks 1, 2, and half of Track 3. Side Two consists of The Second Half of Track 3, plus Tracks 4 and 5. The Silent Track in the Center is there to help prevent crosstalk between the two sides.§ —The preceding unsigned comment was added by Garr1984 ( talk • contribs).
It was in some Handyman's encyclopedia, in a section discussing how to repair 8 Track and Cassette Tapes. Garr1984 04:09, 22 March 2007 (UTC)§
Discussion continued at New info on track & tape width dated 22 March, 2007 -- Blainster 20:48, 26 March 2007 (UTC)
Is 'compact audio cassette' a meaningful term? The official name is "Compact Cassette", and it's usually referred to as an "audio cassette" or simply "cassette" nowadays... so where did this hybrid come from? Has it ever been used except as a result of this article?
I'm not convinced that it's worth changing the title of the article now (too much hassle with links), but if this is simply a neologism, wouldn't it be preferable to discourage its use elsewhere?
Fourohfour 10:14, 26 September 2005 (UTC)
Anyone else feel that this since the shells for the different tape-types aren't standardised, and what you can see of the tape inside looks the same, it doesn't actually add to the point it's meant to be illustrating?
OTOH, it's a fairly good technical shot (much better than the slightly-out-of-focus, non-colour-corrected, flash-reflections-showing, clutter-in-the-background stuff that sometimes appears on Wikipedia). Perhaps it should simply be rebadged as a general shot? Perhaps I'm nitpicking...
Fourohfour 10:03, 26 September 2005 (UTC)
I wouldnt mind seeing a timeline of casette types (as images), as they varied significantly over time. I've no images to offer though :( Tabby 16:01, 9 September 2007 (UTC)
There should be some mention of the 8-track in here. I'd like to know how the 8-track ever got off the ground if, in addition to an inferior design, it was launched five years after the aduio casette. - Litefantastic 23:58, 12 March 2006 (UTC)
I've noticed that in addition to the "Compact Cassette" logo, the branchlist was added, and now keeps getting moved around. The problem is that with the branchlist, contents table, cassette image and logo image, there isn't enough room for all of them near the start without the layout suffering. Frankly, the way the intro looks just now isn't good, and I wish we could just settle on something that is a bit more presentable than this. Fourohfour 12:28, 2 April 2006 (UTC)
This article doesnt know whether its about the cassete tape or cassette recorders. Which is it? It cant be both and needs to be split!-- Light current 01:08, 6 May 2006 (UTC)
From the article:
Really? I don't think so. Where does the author got this information from? —The preceding unsigned comment was added by 200.153.183.111 ( talk • contribs) 12:31, 14 May 2006 (UTC)
What, no mention of mixtapes? That was one of the biggest things about cassettes was that you could make a mix tape for your girlfriend. Ah, nostalgia. —The preceding unsigned comment was added by 204.87.87.170 ( talk • contribs) 08:21, 16 May 2006 (UTC)
Mixtapes were not exclusive to cassettes.
It has been done on reel to reel in the 50's & 60's.
When I was a kid in the late 70's & early 80's, I would make mix tapes on 8 Track to play on the portables in the cars.
And people now do it on CD-R.
So mixtapes have no exclusivity to cassette.
Teamgoon
12:29, 19 August 2006 (UTC)
True in theory, but not how it was in practice. For 99+% of the population, the cassette was for a long time the only accessible way to make music compilations. Tabby 15:59, 9 September 2007 (UTC)
In reference to the thumbnail of the Compact Cassette, Blainster's edit summary says he put it back to being large (300px) so that it's close to actual size. I've no wish to get into an edit war, so I've not made it smaller again, but I do think it wrecks the page for people with lower screen resolutions, by consuming potentially half of the available content space. (Some people still use 640x480!) This lack of accessibility is a shame. I'd see your point though, if it were possible to make things look actual size; that might be useful. But it's not what you've done here by specifying a number of pixels. The tape looks about half actual size on my widescreen laptop, for example, because it has very small pixels. If it looks actual size to you, that's a factor of your monitor. I'd suggest taking a new photo of a tape alongside a ruler or reference object. – Kieran T ( talk | contribs) 00:48, 11 June 2006 (UTC)
I wasn't aware that C46 was ever a common or popular cassette tape length, as the article suggests. C60, C90 and C120 have always been the standard lengths, but quite a few manufacturers produced "odd" sizes from time to time. AdorableRuffian 09:46, 21 September 2006 (UTC)
I recall TDK C46s from the 1980s (they always had the widest range). They were sold reasonably widely, but I still reckon their sales were dwarfed by C60s and C90s. Nor do I know if anyone else sold them. Common? It's all relative. As for data, wasn't that those short C12/C15 cassettes? Fourohfour 01:13, 6 November 2006 (UTC)
C45 & C96 are missing on that list. Maybe it would be helpful to split the list into 3 sections: most common (60,90), fairly common (45,46,74,120) and others also in use (all sorts) Tabby 14:57, 9 September 2007 (UTC)
I'm not clear how cassette's 60 mins a side beats CD's 80. CDs take up less space and cost less per minute than tapes - though this wasnt always the case of course. Tabby 16:20, 9 September 2007 (UTC)
In the section on "Cassette Types" it talks about 120 and 70 µS playback equalization -- does this mean microsecond? If so, isn't the recongnized SI symbol for second a lowercase s, which means it should say µs? It might be better to just say "microsecond" anyway for those who don't know what μS means.
I would change it myself but i want to be 100% sure that μS means microsecond.
"1 microsecond (1 μs) – cycle time for frequency 1 MHz, radio wavelength 300 m (AM mediumwave band)" -- Microsecond
-- Robert 19:41, 22 October 2006 (UTC)
OK, this one has always been a mystery, even to me. And converting 120 or 70 "microseconds" to Hertz taking them as period durations would give 8.33 KHz and 14.285 KHz accordingly, which means that if those were AC bias frequencies they should be pretty audible and far from "high frequency" standards (they would double if they were taken to mean semi-periods, though). Then again, tape bias works exactly by mixing a high-frequency AC signal to the recorded signal...only that those AC bias frequencies appear too low compared to the intended frequency response (20HZ-16KHz for Chrome tape, somewhat worse for Ferric tapes). Maybe then the "S" refers to some other kind of unit like e.g. magnetic induction? EpiVictor 14:55, 6 November 2006 (UTC)
The bias frequency should at least be 3 times the highest frequency to be recorded, better even 4 or even 5 times the value. Early reel to reel used 60-70kHz later types and cassette recorders for High Quality recording used 80-100kHZ. Although the whole Bias effect was never fully understood, the experiments and subsequent patents of B&O with the Dolby HX Pro system led to the conclusion that the higher frequencies of the audio signal also had a Bias side-effect. So clear separation of these signals in the frequency domain makes perfect sense. Donvr ( talk) 11:04, 4 February 2008 (UTC)
Magnetic tapes are recorded with constant magnetic flux indepent of the frecuency. Magnetic flux is directly related to the current through the coil in the recording head, so this current should be constant regardless of the frecuency being recorded. The recording head is inductive, this means that its impedance increases linearly with frecuency. Hence to achive the goal of a constant, frecuency-independent magnetic flux on the tape, the signal is pre-emphasized. This pre-emphasis, called NAB, means + 6dB/octave (=doubled frecuency) so increasing frecuencies are increasingly emphasized.
During playback, the higher the frecuency, the more magnetic flux lines pass under the playback head gap so the output voltage from the coil will increase with frecuency linearly, or +6 dB/octave. Hence a NAB des-emphasis is required. This is not a complement of that pre-emphasis at the recording stage, which was needed to get a magnetic tape recorded at constant-flux. This comes from Faraday's law. There will be a maximum response frecuency, but it will decrease beyond that, because the size of the playback head gap (around the micrometer); let's put it like this: frecuencies higher are recorded on tape as alternating positive and negative half-cycles; if two or more half-cycles of opposite polarities fill the gap the resulting flux will substract.
NAB equalization uses 'time-constants' rather than frecuencies to define 'zeroes' and 'poles'. Those are the micro-second values given with cassettes, and they are not the inverse of a frecuency as said in former commentaries: the relation is rather t=1/2.pi.f. So ecualization of 120 us stands for a pole at 1433 Hz and equaliation of 70 us stands for that pole at 2273 Hz. The 'zero' is at 3180 us or 50 Hz in both cases.
The NAB playback response decreases at the 6 dB/octave from 50 Hz to 1433 Hz, because of the zero to compensate the higher response with increasing frecuencies already explained . At this frecuency the pole compensates the zero so the frecuency response becomes plain. Change 1433 Hz to 2273 Hz for chromium and metal tapes.
There are many sources about this NAB equalization such as the datasheetss from chip manufacturers (check for example National Semiconductor's LM1897 section 'Application Hints/Design Equation', Sony's CXA1498S/M, KIA's KIA6225P/S and many more) or books devoted to magnetic recordings such as 'Consumer Electronics for Engineers' by Philip Hoff et Philip Herbert Hoff.
To record tapes a 'bias current' is added to the signal to linearize the hysteresis curve, which is S-shaped rather than linear. It relates the remanent magnetism left on the tape versus the applied magnetic field (directly related to the current through the head's coil). Low-end cassette recorders add a direct current so the recording only uses one branch of the hysteresis curve; better cassette recorders rather use a.c. at a frecuency well above the maximum signal recorded. Even the own signal's higher frecuencies have a biasing effect, Dolby/B&O HX recording system handles this effect to modify the a.c. biasing amplitude to improve the quality of the recording. Ferro tapes need lower amplitude of biasing than chromium and metal tapes, but these will provide a maximum-output-level higher if recorded with the proper, higher, biasing than ferro tapes.
Biasing current is not related with playback nor recording equalization. Perhaps the cassette tape makers used a very compact syntax to summarize the features of their products: material (= biasing current) and equalization poles.
—Preceding unsigned comment added by 194.224.48.91 ( talk) 22:28, 14 November 2008 (UTC)
![]() | This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | Archive 2 | Archive 3 | → | Archive 5 |
Data rates for cassettes as data storage are from Slashdot. Not verified. Hotlorp 05:09, 10 Jan 2004 (UTC) http://slashdot.org/articles/03/09/18/1857204.shtml?tid=126
The name "musicassette" or MC refers ONLY to pre-recorded cassette albums, not recordable cassettes. Lee M 01:57, 16 Apr 2004 (UTC)
Is it just me, or do the metric measurements in the article seem overly precise? Is the width of a single track really specified down to tenths of a micrometre? To help visualise it, a single human hair is about 100 micrometres thick. This means that the track width is specified to thousandths of a hair's width.
Conversions from imperial to metric shouldn't just be mechanic calculations based on conversion factors taught by rote in school. They should actually have some perspective to the real-life things that are actually being measured.
Or to put it another way, how would you feel if I (being approximately 186 cm tall) expressed my height as 6 feet, 1.228346457 inches? — JIP | Talk 10:08, 3 Apr 2005 (UTC)
The dimensions currently given in the article look not very plausible:
If someone has access to the standards that I added to the reference section, could you please look up the real dimensions and replace the current inch mess? Markus Kuhn 21:04, 18 December 2005 (UTC)
Here is an online example of cassette tape specifications. -- Blainster 10:22, 19 December 2005 (UTC)
It's very notable that 1/8 inch is far closer to 3.18cm than 3.81cm. Is the latter really correct? Perhaps it was typed in wrongly. Multiple sources don't necessarily mean much if they all got their wrong information from the same place. Fourohfour 10:46, 19 December 2005 (UTC)
The article's second sentence reads: "It consists of a length of magnetic tape from BASF inside a protective plastic shell." This could be interpreted to mean that all cassetes - sold by all companies - contain tape made by BASF. I'm pretty sure that's not the case, and the intended meaning may have been simply that BASF invented the type of tape used in cassetes, but it needs to be clarified.
-- 4.245.5.118 23:46, 25 Apr 2005 (UTC)
I removed a link that was essentially an ad. I saw the person who added it said it was a good example of the cassette in today's market, but it seems irrelevant. Anyone who feels different, feel free to explain why... methelfilms 03:00, 14 July 2005 (UTC)
Can somebody with more technical expertise that what I have write a section that talks about endless-loop cassettes? Although I think they've fallen out of favor, they were popular in answering machines and in music-on-hold systems. Some had a metalic piece of tape that served as a "marker" for the equipment to know when the tape had made a complete cycle. Any takers to write such a section? Joe 19:49, 3 October 2005 (UTC)
Am I right in thinking C15 tapes were widely available for a period? I seem to remember using them for ZX Spectrum data.-- bodnotbod 12:39, 26 October 2005 (UTC)
The Coleco_Adam computer used two high-speed drives based on compact cassette technology.
"the two stereo tracks lie adjacent to each other rather than a 1/3 and 2/4 arrangement. " -- could someone explain this for the layperson? -- Tarquin
I think it means that the tape is laid out like this:
1111111 2222222 3333333 4444444 <---> direction of tape
rather than:
1111111 3333333 2222222 4444444
Where 1 and 2 are the stereo channels of side one, and 3 and 4 are the stereo channels of side two. How to put that into simple language without taking up half a page is a different matter, however... -- Camembert
I read somewhere that a Stereo compact audio cassette, actually has five tracks... Tracks 1, 2, 4, and 5 have audio on them. Track 3 is silent... Side one is made up of Tracks 1, 2, and half of Track 3. Side Two consists of The Second Half of Track 3, plus Tracks 4 and 5. The Silent Track in the Center is there to help prevent crosstalk between the two sides.§ —The preceding unsigned comment was added by Garr1984 ( talk • contribs).
It was in some Handyman's encyclopedia, in a section discussing how to repair 8 Track and Cassette Tapes. Garr1984 04:09, 22 March 2007 (UTC)§
Discussion continued at New info on track & tape width dated 22 March, 2007 -- Blainster 20:48, 26 March 2007 (UTC)
Is 'compact audio cassette' a meaningful term? The official name is "Compact Cassette", and it's usually referred to as an "audio cassette" or simply "cassette" nowadays... so where did this hybrid come from? Has it ever been used except as a result of this article?
I'm not convinced that it's worth changing the title of the article now (too much hassle with links), but if this is simply a neologism, wouldn't it be preferable to discourage its use elsewhere?
Fourohfour 10:14, 26 September 2005 (UTC)
Anyone else feel that this since the shells for the different tape-types aren't standardised, and what you can see of the tape inside looks the same, it doesn't actually add to the point it's meant to be illustrating?
OTOH, it's a fairly good technical shot (much better than the slightly-out-of-focus, non-colour-corrected, flash-reflections-showing, clutter-in-the-background stuff that sometimes appears on Wikipedia). Perhaps it should simply be rebadged as a general shot? Perhaps I'm nitpicking...
Fourohfour 10:03, 26 September 2005 (UTC)
I wouldnt mind seeing a timeline of casette types (as images), as they varied significantly over time. I've no images to offer though :( Tabby 16:01, 9 September 2007 (UTC)
There should be some mention of the 8-track in here. I'd like to know how the 8-track ever got off the ground if, in addition to an inferior design, it was launched five years after the aduio casette. - Litefantastic 23:58, 12 March 2006 (UTC)
I've noticed that in addition to the "Compact Cassette" logo, the branchlist was added, and now keeps getting moved around. The problem is that with the branchlist, contents table, cassette image and logo image, there isn't enough room for all of them near the start without the layout suffering. Frankly, the way the intro looks just now isn't good, and I wish we could just settle on something that is a bit more presentable than this. Fourohfour 12:28, 2 April 2006 (UTC)
This article doesnt know whether its about the cassete tape or cassette recorders. Which is it? It cant be both and needs to be split!-- Light current 01:08, 6 May 2006 (UTC)
From the article:
Really? I don't think so. Where does the author got this information from? —The preceding unsigned comment was added by 200.153.183.111 ( talk • contribs) 12:31, 14 May 2006 (UTC)
What, no mention of mixtapes? That was one of the biggest things about cassettes was that you could make a mix tape for your girlfriend. Ah, nostalgia. —The preceding unsigned comment was added by 204.87.87.170 ( talk • contribs) 08:21, 16 May 2006 (UTC)
Mixtapes were not exclusive to cassettes.
It has been done on reel to reel in the 50's & 60's.
When I was a kid in the late 70's & early 80's, I would make mix tapes on 8 Track to play on the portables in the cars.
And people now do it on CD-R.
So mixtapes have no exclusivity to cassette.
Teamgoon
12:29, 19 August 2006 (UTC)
True in theory, but not how it was in practice. For 99+% of the population, the cassette was for a long time the only accessible way to make music compilations. Tabby 15:59, 9 September 2007 (UTC)
In reference to the thumbnail of the Compact Cassette, Blainster's edit summary says he put it back to being large (300px) so that it's close to actual size. I've no wish to get into an edit war, so I've not made it smaller again, but I do think it wrecks the page for people with lower screen resolutions, by consuming potentially half of the available content space. (Some people still use 640x480!) This lack of accessibility is a shame. I'd see your point though, if it were possible to make things look actual size; that might be useful. But it's not what you've done here by specifying a number of pixels. The tape looks about half actual size on my widescreen laptop, for example, because it has very small pixels. If it looks actual size to you, that's a factor of your monitor. I'd suggest taking a new photo of a tape alongside a ruler or reference object. – Kieran T ( talk | contribs) 00:48, 11 June 2006 (UTC)
I wasn't aware that C46 was ever a common or popular cassette tape length, as the article suggests. C60, C90 and C120 have always been the standard lengths, but quite a few manufacturers produced "odd" sizes from time to time. AdorableRuffian 09:46, 21 September 2006 (UTC)
I recall TDK C46s from the 1980s (they always had the widest range). They were sold reasonably widely, but I still reckon their sales were dwarfed by C60s and C90s. Nor do I know if anyone else sold them. Common? It's all relative. As for data, wasn't that those short C12/C15 cassettes? Fourohfour 01:13, 6 November 2006 (UTC)
C45 & C96 are missing on that list. Maybe it would be helpful to split the list into 3 sections: most common (60,90), fairly common (45,46,74,120) and others also in use (all sorts) Tabby 14:57, 9 September 2007 (UTC)
I'm not clear how cassette's 60 mins a side beats CD's 80. CDs take up less space and cost less per minute than tapes - though this wasnt always the case of course. Tabby 16:20, 9 September 2007 (UTC)
In the section on "Cassette Types" it talks about 120 and 70 µS playback equalization -- does this mean microsecond? If so, isn't the recongnized SI symbol for second a lowercase s, which means it should say µs? It might be better to just say "microsecond" anyway for those who don't know what μS means.
I would change it myself but i want to be 100% sure that μS means microsecond.
"1 microsecond (1 μs) – cycle time for frequency 1 MHz, radio wavelength 300 m (AM mediumwave band)" -- Microsecond
-- Robert 19:41, 22 October 2006 (UTC)
OK, this one has always been a mystery, even to me. And converting 120 or 70 "microseconds" to Hertz taking them as period durations would give 8.33 KHz and 14.285 KHz accordingly, which means that if those were AC bias frequencies they should be pretty audible and far from "high frequency" standards (they would double if they were taken to mean semi-periods, though). Then again, tape bias works exactly by mixing a high-frequency AC signal to the recorded signal...only that those AC bias frequencies appear too low compared to the intended frequency response (20HZ-16KHz for Chrome tape, somewhat worse for Ferric tapes). Maybe then the "S" refers to some other kind of unit like e.g. magnetic induction? EpiVictor 14:55, 6 November 2006 (UTC)
The bias frequency should at least be 3 times the highest frequency to be recorded, better even 4 or even 5 times the value. Early reel to reel used 60-70kHz later types and cassette recorders for High Quality recording used 80-100kHZ. Although the whole Bias effect was never fully understood, the experiments and subsequent patents of B&O with the Dolby HX Pro system led to the conclusion that the higher frequencies of the audio signal also had a Bias side-effect. So clear separation of these signals in the frequency domain makes perfect sense. Donvr ( talk) 11:04, 4 February 2008 (UTC)
Magnetic tapes are recorded with constant magnetic flux indepent of the frecuency. Magnetic flux is directly related to the current through the coil in the recording head, so this current should be constant regardless of the frecuency being recorded. The recording head is inductive, this means that its impedance increases linearly with frecuency. Hence to achive the goal of a constant, frecuency-independent magnetic flux on the tape, the signal is pre-emphasized. This pre-emphasis, called NAB, means + 6dB/octave (=doubled frecuency) so increasing frecuencies are increasingly emphasized.
During playback, the higher the frecuency, the more magnetic flux lines pass under the playback head gap so the output voltage from the coil will increase with frecuency linearly, or +6 dB/octave. Hence a NAB des-emphasis is required. This is not a complement of that pre-emphasis at the recording stage, which was needed to get a magnetic tape recorded at constant-flux. This comes from Faraday's law. There will be a maximum response frecuency, but it will decrease beyond that, because the size of the playback head gap (around the micrometer); let's put it like this: frecuencies higher are recorded on tape as alternating positive and negative half-cycles; if two or more half-cycles of opposite polarities fill the gap the resulting flux will substract.
NAB equalization uses 'time-constants' rather than frecuencies to define 'zeroes' and 'poles'. Those are the micro-second values given with cassettes, and they are not the inverse of a frecuency as said in former commentaries: the relation is rather t=1/2.pi.f. So ecualization of 120 us stands for a pole at 1433 Hz and equaliation of 70 us stands for that pole at 2273 Hz. The 'zero' is at 3180 us or 50 Hz in both cases.
The NAB playback response decreases at the 6 dB/octave from 50 Hz to 1433 Hz, because of the zero to compensate the higher response with increasing frecuencies already explained . At this frecuency the pole compensates the zero so the frecuency response becomes plain. Change 1433 Hz to 2273 Hz for chromium and metal tapes.
There are many sources about this NAB equalization such as the datasheetss from chip manufacturers (check for example National Semiconductor's LM1897 section 'Application Hints/Design Equation', Sony's CXA1498S/M, KIA's KIA6225P/S and many more) or books devoted to magnetic recordings such as 'Consumer Electronics for Engineers' by Philip Hoff et Philip Herbert Hoff.
To record tapes a 'bias current' is added to the signal to linearize the hysteresis curve, which is S-shaped rather than linear. It relates the remanent magnetism left on the tape versus the applied magnetic field (directly related to the current through the head's coil). Low-end cassette recorders add a direct current so the recording only uses one branch of the hysteresis curve; better cassette recorders rather use a.c. at a frecuency well above the maximum signal recorded. Even the own signal's higher frecuencies have a biasing effect, Dolby/B&O HX recording system handles this effect to modify the a.c. biasing amplitude to improve the quality of the recording. Ferro tapes need lower amplitude of biasing than chromium and metal tapes, but these will provide a maximum-output-level higher if recorded with the proper, higher, biasing than ferro tapes.
Biasing current is not related with playback nor recording equalization. Perhaps the cassette tape makers used a very compact syntax to summarize the features of their products: material (= biasing current) and equalization poles.
—Preceding unsigned comment added by 194.224.48.91 ( talk) 22:28, 14 November 2008 (UTC)