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Tape Biasing - Help Needed

Discussion in 'Tape' started by Bigerik, Jan 6, 2010.

  1. Bigerik

    Bigerik AK Subscriber Subscriber

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    Hi folks.
    Trying to dig up some information on what it actually means to adjust the bias on a tape. I have some articles from old audio mags which talk of it, but I find a lot of this information is falling by the wayside as there really are no good articles being published anymore about tape.

    Sooo, what I am hoping for is a good thread that will allow people really to understand what it means to bias a tape, and to that end, get a better understanding of how tape works. I know there are a lot of guys much smarter than me out there who could likely explain this very well. If we get a good thread going, I will make a sticky of it.

    Thanks!
     

     

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  2. Jonny Ramone

    Jonny Ramone Super Member

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    Bias is an ulta-high frequency, 80,000 to 100,000 hz (or higher), signal that is mixed in with the audio signal. Plain audio alone doesn't have enough energy to record smoothly on it's own. In practice, too much bias for a tape will dull the highs, not enough will have an exaggerated high frequency response. Some cassette decks have a rotary knob, or automatic system, to dial it in. I've actually been fiddling with the internal bias pots with my 1984 deck , with modern , affordable, tape, and getting FANTASTIC results- never sounded better.
     
    Last edited: Feb 12, 2012
  3. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    The idea of biasing the tape is this:
    When you record on a tape, the bias oscillator in the deck generates a signal of around 100KHz, at 100V peak to peak, to erase the tape. This is the erase current, and it goes to the erase head. To stir up the magnetic particles on the tape, to make it easier to record on it, a smaller portion of the erase signal is fed to the record head, or the record / play head on a 2 head deck. This is the bias signal. If the bias signal is too high, it will saturate the tape, (I think it actually is self erasing the recorded signal) and you will lose highs when you record. If the bias signal is too low, the highs will be accentuated and you will get higher distortion. The method of adjusting the bias is to record a 1KHz tone and a 10KHz tone at -20dB (so you will not saturate the tape and lose highs) and adjust the bias so that they are equal on playback. You can then sweep the signal up in frequency and see where it rolls off by -3dB. For the best high frequency extension, you can adjust the record equalization so a 15KHz tone at -20dB plays back at the same level as the 1KHz tone. If your deck doesn't have adjustable record eq, and you want to get a bit more high end extension, you can adjust the bias so that the 10KHz tone is +1dB from the 1KHz tone, but no higher.
    After adjusting the bias, you should adjust the internal record gain so that a 1KHz tone recorded at 0dB plays back at 0dB.
    Of course, the first thing you should have done to the deck is adjust the playback level so an alignment tape with a 0dB 400 Hz tone plays back at the 0dB level for the deck, and adjust the meters for 0dB.
     
  4. Bigerik

    Bigerik AK Subscriber Subscriber

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    Thanks Jonny.
    A couple of questions for you.
    Firstly, what deck are you using?
    When you say "Plain audio alone doesn't have enough energy to record smoothly on it's own", what do you mean by that?
     
  5. Bigerik

    Bigerik AK Subscriber Subscriber

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    Damn Warren.
    Impressive stuff there.

    When you say "If your deck doesn't have adjustable record eq, and you want to get a bit more high end extension, you can adjust the bias so that the 10KHz tone is +1dB from the 1KHz tone, but no higher.", how exactly does that work? Do you mean adjusting the bias internally?

    Lets just pretend for a moment that I am an idiot.... :D
     
  6. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    Yes, you adjust the bias internally. Even if the deck has an external bias adjustment, it is desire-able to set the bias up for the tape you intend to use most often, using the internal bias adjustments, and leaving the external bias trim in it's center detent position. Then, when you use your standard tape, you can leave the control in the center position. If you use a different tape and the recording sounds a bit lacking in highs, you can adjust the bias with your external control, a bit to the - direction for more highs. The + direction would give you less highs.
     

     

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  7. Jonny Ramone

    Jonny Ramone Super Member

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    From Feb. 1983 , HIGH FIDELITY magazine.:yes: Note Nakamichi BX-1, Revox B-710 Mk. II. ,etc.
     

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  8. Jonny Ramone

    Jonny Ramone Super Member

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    My main deck (for now) is a 1984 Technics RS-B18 I got for graduation. 2 head with Dloby B,C, and DBX NR. Note the bias pots- there are also pots for gain, playback level, etc. A test tape and Dr. Audios info would have served me VERY well back in the day. I adjust mine with the power off. On a 3 head deck, I believe a plastic flathead screwdriver would be used while monitoring, but I'm not, REPEAT, not an electronics technician, so don't electrocute yourself!:no: Clockwise to increase.
     

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  9. niklasthedol

    niklasthedol Super Member

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    Great thread!

    I will not stand forward, pretending I have enough technichal knowledge to explain biasing fullfillingly in theory.
    However, I believe I have success in real life alligning my decks to different tape formulations.

    On one of my decks I have Rec level and Eq adjustable for each channel, for each speed & for PB and for Rec and Bias adjustable for each channel & for each speed - all operational from the facia.

    I find it worth remembering that technologies making biasing a dynamic process were patented and used in some tape decks.

    HX-Pro is one of them. By some called a headroom expansion system and by some categorized as a noise reduction system (because it simply made conditions better for other noise reduction systems to be succesfull).

    Unfortunately I can not attatch *.djvu files as I have an article in djvu format describing the HX-Pro technology fairly ok.
    I will eventually think of a way to bring it up later.

    "dolph"
     

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  10. jan_stevns

    jan_stevns Super Member

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    I'll give the explanation a chance here (feel free to edit)

    ** work sheet 1 **

    To fully understand what it mean, you have to be familiar to the basics of how to record onto a tape.

    When the tape passes the recording head - the magnetism caused by the signal tries to arrange the particles on the tape to reflect the correct pattern, and since the are rather unvilling to change (*1) magnetic direction you would add a high frequent signal to ease this job.
    (this is called bias ~ helping signal. actually this extra signal pushed the particles into the "will accept changes" condition (*2)

    Du to the fact that not all material are the same, because tape formulas are different, there is/may be a need to adjust the amount of "helping" signal, but basically there are 4 significant levels for the 4 types of tapes.

    When you fine tune the bias - you are compensating for 2 things - one is to obtain minimal distortion, the other is to get the best possible levels of the high frequencies recorded. Bias has very little influence on the low frequencies.

    *1 magnetic particles on a tape are rather unwilling to change direction/polarity - this is also the reason why you can hear a tape after it has been recorded, and the reason for the really long term stabillity - particles can maintain their "pattern" for decades - you can play 50 years old tapes still today

    *2 if no bias is present recording would be very dull and muffled, and very distorted too, because some of the magnetic filed created by the recording head, is waisted on trying to arrange the particles on the tape, to reflect the desired sound pattern


    Maybe it's a good thing to use the term "sound pattern" ?? to clarify, that we wish to be able to playback the sound pattern recorded. (or use a better word for it ?)
     
  11. fortycoats

    fortycoats Super Member

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    Please correct me if I'm wrong but isn't it the case that it is the bias signal that actually gets recorded on the tape, after having been modulated by the audio signal (presumably amplitude-modulated)?
     

     

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  12. Jon_Logan

    Jon_Logan Addicted Member

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    What they said. Bias conceptually makes the magnetic particles very 'liquid' changing their polarity at 100k~120kHz rate. The audio leaves the latent magnatization. The bias filed overcomes magnetic particles hysterisis. The hysterisis causes the distortion as it takes a non linear energy to flip the polarity of the magnetic particle.

    There is a high frequency 'hump' that occurs in the bias vs. high frequency output plot. On the wrong side of the hump (low bias side), you are underbiased. You can have relatively flat frequency response, but you'll have high distortion. You want to tune the bias so that is after the hump (more bias), but where the high frequency response R/P is flat.

    All this being said. It is worthless to bias a deck where the PB response has not been verified.
     
  13. MarSanZ

    MarSanZ Simple Pleasures...

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    Dr. Audio, do you mean + for the VU meters when recording or + on the bias control?
     
  14. MarSanZ

    MarSanZ Simple Pleasures...

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    Jonny, I have a Technics RS 1500, do you know if it has the same bias adjustment? Do all R2Rs have internal bias adjustments?

    Thanks.

    m
     
  15. Jonny Ramone

    Jonny Ramone Super Member

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    The cover of my deck comes off with 3 screws- check yours and look. + on the bias knob=less highs. I would think most "modern" decks have adjustable bias internally- open reel and otherwise-maybe not more antique ones.You may not be able to easily access them, however. If you decide to experiment, take pics, or note position of pots(1 o'clock, 5 oclock, etc.), so you can go back to where you started if you screw up. For some reason the manufacturers rarely tell you the tape the deck was set up with. The pots are also somewhat delicate, so use a LIGHT touch.
     
    Last edited: Jan 8, 2010
  16. dr*audio

    dr*audio Fish fingers and custard! Subscriber

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    As I recall, on the RS-1500, you remove the bottom panel to access all adjustments. I think there is a label there for them, but it's been awhile since I've worked on one.
     

     

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  17. Nakdoc

    Nakdoc nakamichi spoken here

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    Bias adjustment is one step in the alignment of a tape deck. Technically, bias creates a large magnetic field whereby the much smaller audio signal will be "recorded" in the LINEAR region of the particular tape particles (and thickness of the layer).
    Why is bias adjustable? The first reason is to accomodate the different tape types. The second, is to allow for variations in record head tolerances. The third, most often used in cassette decks, is the provision for record frequency response balancing using fixed record EQ.
    Is there one proper method for setting bias? Not all by itself. In fact, setting bais and other tape deck alignments could be called an ART. (I like to think it is). Bias and record level must be adjusted together to achieve the highest performance, flattest record response, and maximum record headroom. Some here have talked about some of the methods used in some decks, but not all decks are the same.
    Decks often feature auto-cal or computer controlled bias and level calibration. On a practical service technician level thse circuits have their own adjustments, and are often subject to misalignment. A few decks do have built in multi-frequency oscillators and user accessable bias, level, and EQ controls, and if one has complete confidence in proper playback level, EQ, and meter alignment, each individual recorded tape can be optimized without a visit to the shop. Many, however, are mystified by the process, or end up frustrated when it "doesn't work".
    At the very basic heirarchy of tape adjustments, RECORD LEVEL calibration is the #1 contributor to good sound. Why? Dolby, DBX,ANRS, and their ilk create their own frequency dependent errors based on level changes. With perfect record level calibration these noise reduction processes have a fighting chance of achieving flat response.
    There are many DIY tricks one might use depending on the features and adjustablity of a particular deck. What might be most useful is to post tips based on specific features. I will try and follow this thread as you all post your questions.
     
  18. MarSanZ

    MarSanZ Simple Pleasures...

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    Thanks JR, I will check it out tomorrow.
     
  19. MarSanZ

    MarSanZ Simple Pleasures...

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    I took the bottom panel off when I first got it to blow out any dust, at the time I wasn't event aware of what bias was. I will take a peek this weekend, at this point the ATR Studio Master tape is working very well for me. I guess that means I should leave well enough alone:yes::yes:
     
  20. Bigerik

    Bigerik AK Subscriber Subscriber

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    Ok, I gotta know what kind of tips and tricks you are talking about here! :)
     

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