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Offline reelnut

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What are the effects of bias current?
« on: November 19, 2008, 12:19:16 AM »
Hello readers! The reason I have this question stems from the fact I am using an A810, which has a digital interface to calibration, and that I am not an expert. This kind of hardware makes it extremely easy to set all recording parameters for every single tape you might record. Thus, if you are a tweak freak (I am one), there is no reason not to check and set your bias, level and freq. response for every single tape you record. The manual shows a table listing different types of tape and lists an "over-bias" amount for each one. So how does an expert bias a tape that's not in the table, and what are the effects of an incorrect bias setting? I know some will say just put it wherever you think would be best, but I need to know specifically what to listen for in order to do this. My understanding (meaning I may easily be wrong) is that as bias current is decreased treble sensitivity increases, but IM distortion also increases, and vice versa. What is IM distortion? Does it translate into muddiness or distortion on the low end of the freq. spectrum? Thanks for any help on this matter!
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline sound signal

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Re: What are the effects of bias current?
« Reply #1 on: November 19, 2008, 05:08:36 AM »
Before you can arrive at a bias setting for any given tape, you first need a repeatably measurable parameter.  Otherwise even if you arrive at a setting which you consider optimum you'd never be able to repeat it.

The usual measurable parameter used is overbias drop-off at 10kHz.  That is, the bias is adjusted for maximum output at 10kHz, and then increased, causing a drop in the output level.  How much the output level drops in dB is the amount of overbias drop-off.

In the old days this used to have to be done at a recording level below 0dB VU even at 15ips because of tape saturation at 10kHz, but nowadays with, say, SM468 tape, I have found I can set the 10kHz overbias on my Revox B77 at either 7 1/2 or 15ips at 0dB VU without encountering saturation.  I even use a "hot" setting for 0dB VU, corresponding to a fluxivity of 355nW/m on the tape.  There's a lot of headroom that can be got from modern tapes.  For what it's worth, I use 4dB overbias drop-off at 10kHz with SM468 at 1ips on the Revox B77.

However, please note that you can't transfer an overbias setting from one type of recorder to another unless they use the same recording head (or at the very least, their recording heads have the same gap length).  On the Nagra IV-S I use a 10kHz overbias of just 1dB for the same tape at the same speed to achieve what I think is just about the same level of bias on the tape.

Having defined how to quantify the setting and how to achieve it repeatedly and consistently you can then set about finding what you consider to be an optimum setting.

Unfortunately this is not as straightforward, as just about every performance parameter is affected by the bias setting.  Briefly:

- Distortion.  As bias is varied, there is a value for which minimum distortion is achieved.  Unfortunately this varies with frequency.  Minimum distortion for a 1kHz signal needs rather higher bias than minimum distortion for a 10kHz signal.  So the setting is, unfortunately, a compromise.  Tape distortion for low frequency signals is usually quoted as 3rd harmonic distortion (tape overloads symmetrically, a bit like a push-pull amplifier, so any 2nd harmonic in the output is usually caused by single-ended record electronics rather than the tape).  Tape distortion for high frequency signals is quoted as intermodulation (IM) distortion, because the third harmonic of 10kHz is inaudible to humans at 30kHz, but, for example, the 1kHz intermodulation difference tone between 10kHz and 11kHz, and the various sums at 8, 9, 12, 13 kHz and so on are definitely audible.

- Saturation.  As bias is increased, there is a value for which the level at which the tape saturates (there is no further increase in output level for any increase in input level) reaches a maximum.  However, again, this varies with frequency; the saturation maximum is reached with a lower level of bias for high frequencies than for low frequencies.

- Sensitivity and frequency response.  As bias is increased, there is once again a maximum in sensitivity which, once again, varies with frequency.  In effect, variation of bias affects both recording level and frequency response.  The frequency response is corrected by adjusting the record equalisation after adjusting the bias.

So, in practice, and by my experience (there's probably a million ways to do this), you will choose a certain amount of overbias at 10kHz for your chosen type of tape on your machine, adjust the bias, record eq and record level and make some recordings.  Start with the machine manufacturer's recommended setting for that tape, or for a similar type of tape from another tape manufacturer.  If you find the sound is dynamic, especially in the highs, but seems to be a bit harsh, you probably need more bias.  If you find the sound seems smooth, but a bit lacking in dynamics and headroom, especially in the highs, you probably need less bias.  Make a measured change in the 10kHz overbias, re-set the eq and level and try again.

Whatever you do, don't try to arrive at a setting by varying bias only, because the change in frequency response and level will mislead you.  You must make a complete realignment of the record section of the machine to your chosen new bias level before making a comparison.

Have in mind that the setting you arrive at will also depend on what type of music you record.  My recordings of chamber music do not have the same frequency content as somebody else's recordings of highly percussive jazz, for example, or yet someone else's on-location film sound on a Nagra, complete with breaking glass, slamming doors, gunshots or whatever.  So it's likely that these different recordings will need different settings of bias and reference level.

An extreme example of this is Bay Area Studio Engineering's method for biasing a track on a multitrack machine speciallly for kick drum, by having the percussionist play only the kick drum, and listening off tape for minimum distortion.   Minimum distortion for the kick drum will probably require significantly more bias than usual, since the kick drum doesn't put out much high frequency content.  But other bias settings offering better high frequency headroom or less distortion at high frequencies are of academic interest only for a track with just a kick drum on it.

For further reading, you could do a lot worse than peruse RMGI's technical data for studio tapes.  These show clearly the variation in various performance parameters with bias level and usefully define these parameters.

You could also read Jay Mc Knight's technical paper on biasing.

I know this is a lot of information to absorb but the rewards are there to be had for putting in the effort.  Analogue open reel tape, having been the leading professional medium for half a century, has a long tradition of excellence behind it, backed by real know-how from people who put decades of research and practice into it.  We can only be grateful to them for so generously sharing their knowledge and experience.
« Last Edit: November 19, 2008, 05:44:29 AM by sound signal »
George Karaolides
Nicosia, Cyprus

Offline reelnut

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Re: What are the effects of bias current?
« Reply #2 on: November 19, 2008, 04:51:22 PM »
What a wonderful in-depth reply to my question! There is much there to think about- Thankyou George!

Re your 3rd paragraph: I do the same when I'm adjusting the bias. I usually input the 10kHz at a level which brings the VU meters up in the middle of the scale. It's then much easier to over-bias by the amount you want, since you can accurately see the change on the meters. (The A810 manual say to input 10kHz @ -20db, and I always wondered how they expected you to see for instance, a -4.5db change from -20db!). I am familiar with RMGI's website and refer to it when using their tape. Ive also read JM's paper, which, as you indicated is an excellent reference. However, it's so technical I found it really hard to digest!

Man, I gotta tell ya the reference to BASE's website at analogrules.com is a real gem! There is a TON of info there pertaining to R2R's! I've already spent literally hours going through it all! Thanks again for taking the time to provide us with these insights George!
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

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Re: What are the effects of bias current?
« Reply #3 on: November 19, 2008, 08:18:28 PM »
Hey George, I've gotta say that I'm with John in saying thanks for writing that great post on biasing and eq. Like John, I've done some biasing of my machines before and read and re-read everything I can get my hands on and while I can do basic biasing, I have to confess that I just don't understand why it works. I do have a little backlog of technical texts that were suggested to me by Jay McKnight so perhaps I should do things the right way for a change and read them first. But, if you get a chance, could you tell me what's in the last chapter?
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Offline sound signal

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Re: What are the effects of bias current?
« Reply #4 on: November 20, 2008, 11:03:46 AM »
Thanks very much John and Steve for the appreciation!

Steve, in the old days when tape limitations restricted you to making the 10kHz overbias adjustment at -20dB, you couldn't use the machine's own meters.  You were expected to have access to an audio millivoltmeter with a dB scale and use that.

In case you have to do it at -10dB, if you're lucky to have enough adjustment range on your machine's meters, maybe you can crank the meters up 10dB for the bias tests and re-set them 10dB down afterwards.  I used to do that on my B77 until I realised that, with SM468, I can do it at 0dB, even if my 0dB is a "hot" 355nW/m.

John, Jay Mc. Knight's paper explains why biasing works on the first page.  Since you asked, I'll try and summarise it for you here.

If a DC magnetising field is applied to a section of magnetic tape coating and then removed, the amount of permanent magnetisation that stays on the tape is not linearly proportional to the magnetising field, but highly non-linear.  But if an amount of AC magnetising field is applied at the same time, and the AC and DC fields turned off together, the magnetic material behaves differently.  The amount of permanent magnetisation of the tape becomes linearly proportional, over a useful range, to the DC magnetising field applied.  Therefore by this method the magnetic material becomes a useful recording medium, holding a permanent magnetisation directly proportional to the amount of DC magnetising field applied.  McKnight doesn't go down to molecule level and explain why the magnetic material behaves in this way, and we don't particularly care, because we are recordists trying to understand how different values of bias affect the recording we make, not scientists in a tape factory trying to design tape coating materials.

The amount of permanent magnetisation of the material also depends on the amount of the AC magnetising field applied. As the AC magnetising field is increased and the DC is kept fixed, a maximum is reached, beyond which increasing the AC field further results in a decrease in permanent magnetisation.  This is pretty much what we are familiar with from our tape recorders - there is a value of bias for which there is a maximum output for a given input.

Of course, with our tape recorders, we are not recording DC, but an AC audio signal of, say, 20Hz to 20kHz.  But in the case of pulling  magnetic tape past the record heads in our machines, the signal remains pretty much constant over the time it takes the tape to traverse the record head gap because the gap is so short - comparable to half the shortest wavelength of signal on the tape.  So the signal can be considered to be pretty much DC for our purposes.

While the tape is being exposed to the DC magnetising field due to the signal during that very short period of time, it is also taken through several cycles of the AC magnetising field due to the bias.  This is how the record heads of our tape recorders work.  Bias current which looks like AC to the tape while it's in the gap, is mixed with signal current which looks like DC to the tape while it's in the gap.  And when the tape exits the gap, the DC and AC fields die down together.  So an amount of permanent magnetisation remains on the tape which is linearly proportional, over a useful range, to the amount of signal current in the head.

As stated before, there is a value of AC bias for which a maximum permanent magnetisation is obtained for a given signal.  But since the signal isn't DC and the tape spends a small, yet defined, amount of time in the record head field, above a certain signal frequency less bias will be required to reach the maximum value of magnetisation for a given amount of signal, since the signal itself will be, in effect, supplying an AC bias.  McKnight shows on the first page of his paper some curves which illustrate how the value of bias for maximum output shifts downwards with increasing signal frequency - thus showing the effect of bias on frequency response.  He then discusses the effect of bias on distortion at high and low frequencies, and headroom at high and low frequencies.  My initial post in this thread is pretty much a simplified summary of those parts of his paper which discuss the effect of bias on recording parameters, given in empirical form related to how I set up a tape recorder.

I hope you found this useful.

With best regards,
« Last Edit: November 20, 2008, 11:10:33 AM by sound signal »
George Karaolides
Nicosia, Cyprus

Offline ironbut

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Re: What are the effects of bias current?
« Reply #5 on: November 20, 2008, 07:39:51 PM »
Yippie! I got it! Somehow I was missing the DC=signal in Jay's ideal recorder examples. Thanks a bunch for making that clear.
steve koto
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Offline reelnut

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Re: What are the effects of bias current?
« Reply #6 on: November 20, 2008, 07:58:32 PM »
Well, I know I haven't got it, but I get some of it!

Thanks again to George for his knowledgeable comments on this subject, and to all of the pros who are educating the rest of us!!
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

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Re: What are the effects of bias current?
« Reply #7 on: November 20, 2008, 08:56:07 PM »
The article that George and I are referencing is available at the MRL web site under "Technical Papers". I've read that one a few times (but obviously not closely). That site has tons of fantastic info regarding tape and machines. There's also a sticky at the top of the General forum with lots of other excellent links (but nothing really like Jay's).
http://home.flash.net/~mrltapes/
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Offline reelnut

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Re: What are the effects of bias current?
« Reply #8 on: November 20, 2008, 11:12:16 PM »
I thought this thread had been pretty well covered, until I came across what I've pasted in below. It's a direct quote from the bottom of page 11 of the MRL paper "Choosing and Using MRL Calibration Tapes for Audio Tape Recorder Standardization". (http://home.flash.net/~mrltapes/choo&u.pdf), one of my favorite papers to read. (I guess I should read it more often, since I had forgotten about this). The paper is a dandy, since it's written in a language that I can understand, and completely explains levels, eq, and more. The paragraph below prescribes a method of biasing for anyone (besides myself) who still is unsure of where to set the bias point when using an unfamiliar tape. I can't wait to try it. Read the whole paragraph- the best part comes at the end!

  "An old, simple, and effective method that we use for setting bias with all kinds of tape is to use a 1000 Hz test signal. First set the bias current for maximum recording sensitivity. Then, for 15- and 30-in/s recording, increase the bias current so that the recording sensitivity drops by 0.2 dB. For 3.75- and 7.5 in/s recording decrease the bias current so that the recording sensitivity drops by 0.1 to 0.2 dB. Another method, recommended by tape manufacturers, uses a 10 kHz test signal. We find this method more complicated ? you need toknow the gap length of the recording head, and the amount of signal reduction for each kind of tape and each speed ? and without any redeeming social value."
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline sound signal

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Re: What are the effects of bias current?
« Reply #9 on: November 21, 2008, 04:32:01 AM »
Greetings,

Yippie! I got it! Somehow I was missing the DC=signal in Jay's ideal recorder examples. Thanks a bunch for making that clear.

Steve, I used to teach Physics and loved it when I got that "a-ha!" from my pupils...  Thanks for giving me yet another moment to savour!

I thought this thread had been pretty well covered, until I came across what I've pasted in below. It's a direct quote from the bottom of page 11 of the MRL paper "Choosing and Using MRL Calibration Tapes for Audio Tape Recorder Standardization". (http://home.flash.net/~mrltapes/choo&u.pdf) [...]

That's one of my favourite papers too, but I'd forgotten reading that particular bit about bias.

I think McKnight objects to blindly following a tape manufacturer's recommendation for overbias at 10kHz, not to actually measuring the bias point by the overbias drop-off at 10kHz.  I think what he's saying can be rephrased as "For a tape manufacturer to be able to usefully recommend an optimum 10kHz overbias drop-off, they'd have to provide a chart listing values for each type of tape, at each tape speed it's likely to be used at, for each value of record head gap length it's likely to be used with, and even then you won't know whether the value they provide is for minimum distortion, minimum modulation noise, maximum HF headroom, a compromise, or whatever".

I also think a lot of confusion about tape recorder biasing arises from mistaking a method for repeatably and accurately measuring and setting any bias point, with a method for deriving and setting an optimum bias point.  The first is usually a pre-requisite for the second, but they are not the same thing.

Methods for repeatably measuring and setting any bias point include measuring the overbias drop-off at 10kHz, measuring the overbias drop-off at 1kHz, and direct measurement of the bias current either by built-in meters or external instruments.  This last method (with built-in meters) is used on professional multi-track machines where bias must be re-set quickly for many tracks, but it has to be occasionally referenced to another method as the characteristics of the record head change with wear, or when the record head is replaced, because then the actual bias applied for a given current to the head will obviously change.

Methods for determining the optimum bias point vary more widely.  For a few examples: There's Bay Area Studio Engineering, cited above, where they set the bias for particular instruments, especially low-frequency ones like the kick drum, by ear.  There's a method in the Nagra IV-S manual where they measure the bias oscillator voltage at two points, underbiasing and overbiasing for 1dB drop-off, then calculate a root mean square of the two and set the bias oscillator voltage as close to that as possible.  Then there's Jay McKnight's suggestion cited above, where he considers a 0.2dB overbias drop-off at 1kHz to be an optimum for all types of tape at 15 or 30ips, and 0.2dB underbias as an optimum for all types of tape at 3 3/4 and 7 1/2ips.  (But isn't 0.2dB a bit hard to read accurately or repeatably on any instrument?  On a linear scale instrument too, that's only a 2.3% change. And doesn't the level fluctuate randomly by about 0.1dB anyway, because of tape coating imperfections?)

Then there's also Bill Vermillion's alignment method, where he recommends recording a very low-frequency signal and listening for minimum modulation noise, which he considers the most annoying tape artefact.  But he notes that his method won't work for Agfa 468 tape (the predecessor of the RMGI 468 tape used by the Tape Project, and my own favourite type too) and has doubts whether it would work for more modern "high-energy" tapes than the ones he used "back in the day".

My own method is to use the drop-off at 10kHz to measure and repeatably re-set any bias setting.  I then use a combination of performance measurement (distortion, frequency response) on the bench, and trial and error while making and listening to my own recordings, to choose a setting which is optimal for the type of recording I make.  I then stick to that amount of 10kHz drop-off with that type of tape on that particular type of machine.  This works fine for me for now.  But I am very open to any suggestion and I also do see that my method might not be the best for everyone.
« Last Edit: November 21, 2008, 04:36:57 AM by sound signal »
George Karaolides
Nicosia, Cyprus

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Re: What are the effects of bias current?
« Reply #10 on: November 21, 2008, 04:37:08 PM »
Hi George-
I did come across Bill Vermillion's paper yesterday and read it completely- another really good article for tweak freaks! I was really intrigued by his method of biasing for minimum mod noise. Am going to try that, but for me it involves patching in a synth to my record setup to get the 7Hz tone, since I don't have a dedicated oscillator to use and I haven't gotten around to that yet because it involves physically moving some gear. It's more likely I'll record the tone to a CD first... (more playing around!). Most of my tape stock is newer, so I don't know if that method will work for me or not. I have recently been buying some lightly used Scotch 226 which comes from a film vault that I'll bet that would work great on. FYI I can shoot you a message letting you know my results if you like.

A little off the topic, but I was also really intrigued by the statement in that paper that using your Sync switch is a preferred and much more accurate way to set azimuth for the record head. Although the A810 I use is just a 2-track, it does in fact have the Sync ability. Sync isn't enabled on my machine, so that means I have to look at the manual and figure out which switch I need to turn on to get that going. I can see where that method would also be a time-saver, as opposed to setting record azimuth from the playback head. I use a $25 software oscilloscope when doing azimuth checks, which, if anyone is interested in can be found free to try for 21 days here: http://www.virtins.com/. It really is an incredible value for the money.
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline sound signal

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Re: What are the effects of bias current?
« Reply #11 on: November 23, 2008, 10:30:47 AM »
Hi John,

I did come across Bill Vermillion's paper yesterday and read it completely- another really good article for tweak freaks! I was really intrigued by his method of biasing for minimum mod noise. Am going to try that [...] FYI I can shoot you a message letting you know my results if you like.

By all means let us know how you get on with the minimum modulation noise biasing method.  I haven't tried it because I use SM468 tape exclusively for new recordings and Bill Vermillion specifically says it won't work for that.

Quote
A little off the topic, but I was also really intrigued by the statement in that paper that using your Sync switch is a preferred and much more accurate way to set azimuth for the record head.

The Sync function connects the selected track of the record head to the playback circuits.  Using Sync from both tracks of a two-track machine would let you set azimuth for the record head by playing the same calibration tape as for the playback head.  This is easier and more accurate than setting playback head azimuth with the calibration tape first, and then setting record head azimuth by recording a test tone and monitoring through the playback head.  I have no machine with a Sync facility, but I did do this once on a Revox G36 by unsoldering the leads to the playback head and moving them to the record head, just to see that it could be done.  It was indeed a bit more accurate and easier, but I don't think it's worth the trouble of unsoldering and re-soldering wires to the heads.  With Sync, you can do it by just pushing a button on the front panel and it's well worth it.

With best regards,
« Last Edit: November 24, 2008, 06:57:18 AM by sound signal »
George Karaolides
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Re: What are the effects of bias current?
« Reply #12 on: November 23, 2008, 02:41:58 PM »
John, that's a great suggestion on the virtual 'scope. I doubt that a cheaper one could be found. I have a software suite of meters that I use myself that I'm pretty happy with but it's $80. Cheap for what you get, but only if you need all the bells and whistles. One thing that I've found with software meters is that their accuracy depends a good deal on the quality of the AD converter. Most of them tend to roll off above 15khz. Of course for setting azimuth you don't have to worry about that. I think that it's well worth buying an analog function generator though. Using a test CD or software generator requires some calibration in most systems and makes doing the measurements a pain. A low distortion generator can be had for pretty cheap off of auction sites. Even the humble Heathkit ones can be upgraded and can cost less than $30.
steve koto
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Re: What are the effects of bias current?
« Reply #13 on: November 24, 2008, 02:36:05 AM »
Steve-
Good point about freq response dropping off at 15kHz. My soundcard can record up to 192kHz/32bit and the Virtins software accepts sampling rates up to 192/24. I usually input the azimuth signal at 96/24 and it seems to work well. Actually I do have a generator which I purchased last spring that can generate sine,  square and saw waves. But I haven't even tried it out because I don't have the test leads with a banana jack on one end! Is that ridiculous, or what? So I use my test CD.

Although only an amateur, I do want to comment that the very FIRST thing anyone needs to make sure of in a calibration procedure, and this is almost NEVER explained clearly by anyone writing their papers on the internet, is that when the VU meters read zero the machine is producing operating level at the outputs. Your machine will have a specification in volts as to what (in the case of the A810 it's 0.775v) this is. You change this setting by playing back the 1kHz tone on your test tape and then adjust the playback output amps to show operating level in volts at the tape outputs. After this is done you will then adjust the VU meter amplifiers (NOT the screw on the meter front!) so the meters read zero. Your meters are now displaying the reference level of your test tape (in nWb/m) as 0db and you are producing operating level, in volts, at the tape outputs. The syncing of the VU meters to the deck's specified operating level is normally a one-time-only adjustment. Maybe that's why noone ever mentions it, but when buying a used machine it's certainly the first thing to check! There is a Swedish person who has a website- http://home.swipnet.se/herbalifeinfo/Audio/reeltoreel.htm. It's the only one I know of that explains how and why to do this. Highly recommended reading for the novice. Reading this page a few years ago was the first time I ever said "Hey! I can do this!" Lots of explanatory pics there, as well. Hint: Just start reading from the 1st page. There is a link at the bottom of every page that takes you to the next one.

Steve, as you noted, calibration of most systems using a test CD can be a pain, since there isn't a way to easily change the line level without changing rec level as well. The A810 has a way to do this. The entire control area under the headblock is actually a hinged panel that folds up and can lock in a 45 or 90 degree position. This is where the cards and periphery controller reside. So the procedure is simply to set input level from the 1kHz tone on the CD to read 0db on the meters when the machine is stopped (which produces an operating level of 0.775v at the outputs). Then I put the machine into rec mode and monitor repro output. The VU meters now indicate that the output level changed when the machine started recording, since the rec level hasn't been set yet. It's a simple matter to now adjust rec level for each channel using the periphery controller. After setting the VU meters to again read 0db by using the rec level control of the periphery controller and not moving the input level control I am now recording reference level on the tape (in my case 355nWb/m) and producing operating level (0.775v) at the outputs. I know that's a lot of words, but you can see this calibration can be done in less time than it took to read about it! Steve, I thought it might help some of the really new guys out there to hear about this. Operating level, reference level- it can all get very confusing! Also, for those who are now asking "What the heck is a periphery controller", it's a digital interface to calibration of the A810, which uses pushbuttons and precise values instead of tuning potentiometers, and each parameter is divisible into 256 steps, which are displayed on the tape counter when calibrating. Recording & playback of the machine itself is 100% analog. This system allows for 2 complete sets of values (including eq, where the time constant is programmable for recording and/or playback) to be stored for each speed! In addition, it's possible to change the settings in real-time during recording and/or playback if that is desired, and all settings, including the counter reading are stored in non-volatile memory when the machine is powered down.

George-
I tried low-frequency biasing at 20Hz with a Q457 tape that I have. No results! At least I now know what to expect from a "newer" tape that this doesn't work on! Changing the bias current I could hear a difference in the sound of the noise, but it didn't vary enough in overall character so that I could say "Ah, I hear the point of lowest distortion". The sound didn't seem to change noticeably until I knew I was way away from the optimun bias point. Going to try it again as soon as I get some more of the 226 that I mentioned earlier. I have some on the way- should be here this week! I don't know alot about 226, having only one reel of it so far. I suspect it may also be of the "higher output" variety that Bill V. says this technique won't work on...
« Last Edit: November 24, 2008, 03:50:45 AM by reelnut »
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

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Re: What are the effects of bias current?
« Reply #14 on: November 24, 2008, 06:41:54 AM »
Hi John,

You're right, the playback level and VU meter sensitivity should be calibrated before everything else.  There was a question posed here about that very issue a while back, to which I posted a reply which you, or others, might find relevant.

Having once adjusted the playback level and VU meter sensitivity you should have no need to ever re-adjust them, just check them periodically to make sure they're still OK.  Once the flux is on the tape, nothing much changes on the machine that would cause a change in playback and VU meter settings.

Of course, you might want to set a different VU-meter sensitivity to help you get optimum level for your chosen tape type on your machine and the type of recording you make.  The value of reference fluxivity for 0dB given in your machine manual may be a good starting point but it's not carved in stone, it's up to you to choose what works best for your own recordings on your machine with your chosen tape.  See Choosing and Using MRL Calibration Tapes by Jay McKnight, section 6, "More on Choosing a Reference Fluxivity" for concise but comprehensive coverage of the issue.

The recording side of things requires more frequent attention, because of variations between tape types, or even between batches of the same type.

Please note that if you tweak the bias, you will certainly need to tweak the recording level again, even if you haven't changed tape types, because changing the bias also changes the recording sensitivity.

Thanks also for reporting the results on the minimum modulation noise biasing method.  Do keep us all posted with any more findings.

With best regards,
George Karaolides
Nicosia, Cyprus