ARG! I'm confused again...fluxivity vs. operating level

[sweetbeats]

My 58 was designed to operate with a fluxivity of 250 nanowebers. I thought that was equivalent to a +6 tape, as the 58 was also designed around 456 tape which is a +6 tape right?

Well, the folks at Tascam told me 456 is a +3 tape, and then I saw references that 250 nWb is equivalent to +3.

I decided to let it go as I figured "The deck was setup for 250nWb and 456 tape".

Last night, looking through the latest Full Compass catalog I see RMGI SM911 (a 456 equivalent) listed as a +6 tape...!

I can't let this go. I *think* I know that there isn't a direct relationship between fluxivity and operating level, but there is *some* relationship, right??

I should know this...if somebody tells me "Dude, this topic is worn out...go back and re-read the reference materials a the MRL site!" I'll do it, but no matter what, I gotta get my brain around this.

 

[Beck]

It can be confusing... even the person you talked to at Tascam has it wrong, unless they misunderstood the question.

456 is a +6 tape (370 nano-Webers per meter recommended max), but 250 nWb/m is the standard operating level for most machines in North America. +6 is short for “+6 dB over 185 nWb/m.” 185 is the old Ampex standard flux level, so is used as a reference for rating classes of tape. Tapes are rated on the basis of how much signal (magnetic flux) they can take before a certain level of distortion.

You could set your machine for +6, 370 nWb/m @ 0VU and just have 0 as an absolute redline. Machines are normally set lower, either 250 or 320 nWb/m to give you some space to work above 0VU.

Tascam machines are normally set at 250 and the TEAC test tape uses Quantegy 456 with tones recorded at 250 nWb/m.

406 is +3
456 is +6
499 and GP9 are +9

What can make it even more confusing is that there are two things people refer to as operating level. One is the line level of the machine, usually -10 or +4 dB. The other is the flux level. The two are independent, so a 250 nWb/m tone recorded @ 0VU on a machine with +4 dB line levels will play back at 0VU on a machine zeroed at 250 nWb/m with -10 dB line levels.

 

[sweetbeats]

the person you talked to at Tascam has it wrong, unless they misunderstood the question

I don't think they misunderstood as I even said "What? I thought 456 was a +6 tape..." and he answered "No, its +3."

What can make it even more confusing is that there are two things people refer to as operating level.

This whole bit I'm good with. I do understand the difference and relationship between the line signal level, and the signal to tape.

You could set your machine for +6, 370 nWb/m @ 0VU and just have 0 as an absolute redline. Machines are normally set lower, either 250 or 320 nWb/m to give you some space to work above 0VU.

Okay. I follow you, but how do you set the machine up for 320 or 370? You have to have a calibration tape was tracked at 320 or 370 right?

 

[Beck]

Okay. I follow you, but how do you set the machine up for 320 or 370? You have to have a calibration tape was tracked at 320 or 370 right?

No, you can set any flux level using any calibration tape by offsetting the VU meter by the difference. So for example if using a 250 nWb/m test tape for a 320 nWb/m calibration you would set the VU meter using the 1kHz tone to -2 VU instead of zero VU.

 

[sweetbeats]

mmm...

So...ah...when...ah...okay...for 320nWb/m........ . . . . . .

Hm. So the whole idea is to calibrate the deck (i.e. set the trimmers) to reflect a higher operating standard? Like, if I want to take advantage of the higher output level/input threshhold of a +6 tape, I would set the reference level lower on the deck during calibration so that when I'm setting levels at a session and lets say I'm setting my peak levels at around 0VU on the deck it is really going to print at +2VU (assuming I calibrated the deck to a 320nWb/m refence, which I'm assuming is +5...the MRL site shows +5 at 315nWb/m, but I'm assuming the 320nWb/m is +5 at 700Hz)???

So what is showing as 0VU on the deck is really printing hotter to the tape because I attenuated the record level trimmer to show -2VU on the deck...and...then...when...ah...setting the repro level with my 250nWb/m calibration tape, if I ALSO set the levels to -2VU that will ensure that what went to tape at 0VU (which was really +5 over the the 185nWb/m standard) is shown as reproducing at 0VU also...rrrr...rrright?

Am I tracking right? (um...no pun intended...)

 

[Beck]

Yep, you got it! And the whole flux level issue just goes on and on, so it’s best to stick with a standard level (or two if the machine is switchable).

What people refer to as +3, +6, etc, will depend on the frequency of the reference tone… 700Hz, 1kHz or even 400Hz in some cases. So 250/260 refers to +3 and 355/370 refers to +6. And then you have things like the European G320 (German 320), which by North American measuring methods is closer to 280 nWb/m, so it’s considered +4. And North American 320 is also called 315 and is +5.

And you’ll see tape manufacturers and tape deck makers recommending different flux levels. For example, if you’re not using noise reduction Tascam suggests 320 nWb/m as an alternative level, and some of their machines are switchable between two levels of your choice, like 250/320. But some people that are responsible for the standards, like McKnight recommends 355 nWb/m if not using NR… everyone has their own ideas. In one sense it all comes down to how you want to reference (or not reference) your VU meters. Some people tend to record with them buried in the red, so they might as well not have any.

One important point that may make all of this moot in a practical sense for a lot of people is if you’re going to be using dbx NR there’s no benefit in increasing flux operating level above 250 nWb/m. If not planning to use dbx NR with a Tascam ½” 8-track I would set it up for North American/Japan 320/315 nWb/m just as the Otari’s are set.

 

[evm1024]

Why 456 is a +6 tape

OK, My turn to cloud the thread....

Measurements show that a 1 kHz tone recorded on 456 at about 762 nW/m fluxivity will produce 3% distortion. Now this is not a number that you will find very often so hang in there.

If we look at Quantegy's spec sheet for 456

http://www.quantegy.com/specsheets/PDF/456.pdf

we see that the output level is 12.3 dB at 3% distortion of the 3rd harmonic. OK, 12.3 dB referenced to what? Well how about 185 nW/m.

So 10^(12.3/20) * 185 = 762.38 nW/m

Big deal, just numbers but wait! lets say that you want to give yourself 6.3 dB of headroom and operate your 456 at +6 dB over 185 nW/m. Let's plug in the numbers....

10^(6/20) * 185 = 370 nW/m (well 369.124 but who is counting?)

So 456 is a tape that can be safely operated with 0VU printing 6 dB over the 185 nW/m reference at 370 nW/m. All the while having 6.3 dB headroom from the 3% distortion MOL of 12.3 dB over 185 nW/m

If you set 0VU for a print of 250 nW/m you would have 9.3 dB headroom before reaching the 3% distortion MOL. With a 3 dB higher noise floor I might add.

As a suggestion if you were recording a highly compressed metal band sound and knew where your peak level was you could set your deck for 370 nW/m operating level and push the meters into the red a bit. Or a bit more if you wanted tape compression.

On the other hand if you were doing classical and did not compress and had no idea what the peak would be you might be better off with an operating level of 250 nW/m and try to stay our of the red all the while balancing the noise floor.

Or in my case you just might ask the Organist to give you a few moments Full Organ (all jokes aside!) and set your recording level 3 dB under the 3% MOL no matter what your operating level for 0 VU is calibrated to.

Whew!

Oh Looks like quantegy still has their recording level calculator online at:

http://www.quantegy.com/audiorecorder/

Regards, Ethan

 

[sweetbeats]

Beck,

Phew!

I actually think I really get it now.

I used to think that the reference fluxivity wasn't...ah...reference. Like something actually had to be different about the tape deck circuity to calibrate for a higher output tape, but it is as its namesake suggests, a reference level...In a way its like doing a meter calibration. You can dial in the meters to read whatever you want them to read at a given signal level. You can set the meter to read -3VU in reference to a +4dBm inpt signal level, only with calibrating the deck for tapes of differing output levels, you are setting the deck to display a certain VU meter reading for a given input signal strength depending at what flux level you want hitting the tape, which should not exceed and ideally leave some headroom from the maximum operating level of the tape you use. RIGHT?

This is cool.

Ethan

I *actually* follow what you are saying...kreepy. I have read those recommendations regarding claibrating the deck for a certain operating level based on the source material and whether or not noise reduction will be used.

Now it just makes a whole lot more sense, and I can tell I'm going to be needing to experiment to find what works best in general for my situation. Now I understand why manufacturers ever designed decks to have switchable calibration settings for quick changes to different tape types for different source material.

Wow. I got way more out of this than I thought I would.

So Beck, Ethan, any recommendation for what to start with on my 58? I'm pretty set on +6 tape for starters as that's what Tascam intended, and I figure its best to start there and go seeking later if I'm trying to find something different...plus the analog widget gurus at Tascam in CA. told me that the Tascam decks (like the 38, 48, 58, etc.), since they were designed to run +6 tape at 250nWb/m, the bias circuitry wasn't really designed to "excite" the heavier oxide coating of the high output +9 tape. Doesn't mean you can't run it...I hear many do, but it wasn't really what those decks were designed to run. So I want to start with the intended tape and go from there. But the fluxivity...

I'm typically going to be tracking pop, rock and dub drums and bass into the 58. I'll be anxious to try different source material. I was planning on printing with noise reduction...250nWb/m? 320nWb/m? 370nWb/m? What are others using and what kind of results do you get?

Thank you all. This is terrifically educational!

 

[evm1024]

250 has it's advantages

Tascam did very good engineering in picking 250 nW/m for the operating level (0 VU).

Up until 1966 or so it was thought that VU meters reported program material at about 6 dB below the peak levels. After 1966 the specs (DIN 445 406 specifically) were closer to reality in noting that with typical program material hitting 0 VU were in fact printing at +8 which was way over the 3% distortion MOL.

If you run 456 at 370 nW/m you have 6 dB head room. And unless you keep a careful eye get much more than 3% distortion with the meters reading 0 VU.

Tascam rightly set the operating level to 250 nW/m giving their customers 9 dB headroom well knowing that they would record with the VU meters hitting and in many cases exceeding 0VU and thus still keeping distortion at an acceptable level. (IMHO)

Now as to your 58.... I would think that if you calibrated it to 250 nW/m for 0VU and calibrated the peak LED (I assume that there is an adjustment for the LED) to light at 9 dB (the 3% distortion MOL for 465) you would have the best of both worlds.

Record with the meters hitting 0 VU when you wanted to minimize distortion or with the LED just flickering when you wanted those drums to thunder or let the LED light up when you wanted heavy distortion and compression. (Try not to bend the meters needles!)

Being an engineer more than an accomplished recording engineer YMMV....


Regards, Ethan

PS LED calibration - With 0 VU giving 0.316 mV output then +9 dB is 0.890 mV. For +4dBm outputs just multiply your 0 VU output by 2.82 to get the +9 output. 10^(9/20) = 2.81

 

[sweetbeats]

Ethan,

Thank you!

I'm assuming that your suggestions regarding peak levels for different outcomes is without noise reduction right?

I say that because my understanding is that meter deflection is halved due to the companding, and that would effect peak LED activity as well.

Yes the 58 has trimmers for the peak LED's. I have a handy chart for converting dBm into dBv etc. The peak LED's are suppoed to light at +12, but your logic makes sense with the distortion ratings.

What are your thoughts on that? I'll likely try it both ways, but wondering what makes the most sense for starters?

BTW I'm running I/O balanced +4dBm through dbx 150x n/r units.

 

[evm1024]

Cal without NR

Set those meters and LED without NR (as you know). I selected a +9 for the peak LED so as to tell me something that I want to know (3% distortion level). 12 dB peaks would print at about 995 nW/m and who knows what distortion and compression levels. In that case I think that Tascam was thinking that the peak LED represented the "extreme" upper end.

As for meter readings with NR, I don't typically use NR so others should speak up here. From what I know your NR peaks are compressed so the meters don't under read as much (0 VU meter peaks actually peak +5dB or so).

The "problem" with NR being that any distortion and nonlinearity introduced by the print levels on tape are returned to you maginfied by the decoders expansion.

456 is said to have 0.1 % distortion at a 250 nW/m operating level. So on decode that becomes 0.2%, not bad... and 3% becomes 6% which should be quite noticable. I'm not even thinking about pumping and other artifacts of the companding process.

Recording with VU meter peaks of -5 to -3 VU sounds like you will stay in the operating area where NR does the best. Only you can decide what souunds good to you.

Regards, Ethan

 

[Blue Jinn]

Revisited. I think I get the flux level/operating level/headroom thing. Question:

I have gathered a lot of data about various tape types from Scotch/3M, BASF/etc, Ampex. etc. Most of the data is coercivity and retentivity. There are a handful of x-reference tables, like 406 = 206 so +3 from 185.

Absent a cross reference, how do you deduce operating level from coercivity and reactivity, or do you?

Also, for Beck? Any idea what Memorex Type 2 is? I just picked up a reel 1/2" x ??? on a 14" reel (for the reel only) but ????