Help me understand my MRL tape.

  • Thread starter Thread starter Chris Shaeffer
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Chris Shaeffer

Chris Shaeffer

Peavey ROCKS!!!!!!!!!!!!!
So...

I was told the Tascam 48 is calibrated for 456 tape and it came with an MRL tape to calibrate it. I ran the tape through and sure enough- the meters are all at 0 (+/- 1dB).

So...

If 456 is +6 dB tape... what the hell do my meters mean? Is that MRL tape at actual 0dB and I have 6dB headroom, or... since the meters only go to +3, is 0 on the meter actually +3 and I have 3dB headroom? Or something esle.

I don't suppose it really matters- I'm just used to having 70 or so dB headroom on the meters and having 23 is an adjustment. It also an adjustment to get used to RMS meters again. Its easy to set peak meters on a digital system- just don't hit the red. But what does the peak light mean on the 48? Overloading the electronics? Going over the headroom on the tape?

:D It fun feeling like such a newbie.

Thanks,
Chris
 
I am sure you will get better answers than this one, but I'll give it a shot. I believe the "+6" on 456 has to do with the way the machine is biased, not levels. as far as clipping the machine, it can be done. if you clip the input electronics it sounds like ass. but I've recorded with meters way past the top of the red without any problems. you should be able to find a sweet spot on it if you don't mind burning a little tape. I wouldn't worry about the meters so much..just put the volume to where it sounds best.
 
FALKEN said:
I am sure you will get better answers than this one, but I'll give it a shot....
Click to expand...
Actually, that was pretty good. Nice one, Falken.
 
What reference fluxivity is your MRL tape? This is measured in nWb/m (nano-Webers per meter). This should be printed on you MRL box.
 
Close

The standard flux (magnetic intensity) at 0 dB is 185nWb/m. Take a look at:

http://www.quantegy.com/audiorecordersetup.asp

where you can find out what to set your meters to when playing your MRL.

The +6 for 456 means that the tape can handle a flux that is 6 dB greater than the 185 nWb/m standard as it's 0dB point (Recommended Maximum Operating Level)

Tape has a lot of headroom over the recommend max operating level, on the order of 8 to 12 dB before objectionable distortion happens.

You can adjust your deck so as to increase the headroom (0dB set for a lower fluxivity). This would be an advantage when recording something where you want to capture all the peaks and do not have a good idea where they will fall. (live recording)

You can adjust your deck for "normal" headroom which means that 0dB would be set for a greater fluxivity and that the noise floor would be (6dB) lower.

Or somewhere in between.

In my case I have a 200 nWb/m tape so to set the deck up for 456 I would play the tone and set the meters for -5VU (0dB = 370 nWb/m). Many set their decks up with 0dB = 320 nWb/m (-4 VU in my case) to get a little more headroom and a lower noise floor. (not all decks can adjust to all levels)

I should add that I do not count on the VU meters without checking their calibration with a multimeter. I have one deck that has a channel where the VU meter becomes insensitive at lower frequencies (below 400 Hz) THis is most likely due to a bad cap inthe meter amp. Someday I'll track it down and fix it.

Hope this helps.

Regards
 
FALKEN said:
I am sure you will get better answers than this one, but I'll give it a shot. I believe the "+6" on 456 has to do with the way the machine is biased, not levels. as far as clipping the machine, it can be done. if you clip the input electronics it sounds like ass. but I've recorded with meters way past the top of the red without any problems. you should be able to find a sweet spot on it if you don't mind burning a little tape. I wouldn't worry about the meters so much..just put the volume to where it sounds best.
Click to expand...


Sorry it is the levels not the bias level.
 
Chris Shaeffer said:
So...

I was told the Tascam 48 is calibrated for 456 tape and it came with an MRL tape to calibrate it. I ran the tape through and sure enough- the meters are all at 0 (+/- 1dB).
Click to expand...
Given that your MRL tape is 250nWb/m, then you're good to go!



Chris Shaeffer said:
So...

If 456 is +6 dB tape... what the hell do my meters mean?
... since the meters only go to +3, is 0 on the meter actually +3 and I have 3dB headroom?
Click to expand...
Yes,... that's exactly what it means!



Chris Shaeffer said:
But what does the peak light mean on the 48? Overloading the electronics? Going over the headroom on the tape? ?
Click to expand...
Yes! Both!



Chris Shaeffer said:
:D It fun feeling like such a newbie.
Click to expand...
You catch on fast for a "newbie"!!!! ;)
 
Thanks everyone. That helps me understand what the guy who sold me the machine was saying. The poor guy- he knew his stuff and was kinda proud of having a well maintained machine for sale and I could tell he was a little dissappointed that I didn't really understand what he was saying about how the machine was set up.

He did mention the nWb/m that the machine was calibrated to- something about being internally set so that the machine had the lowest noise floor.

So...if I have this right... if the tape is labeled X nWb/m (I'm out of town at the moment and can't check) and the meters are reading 0db wiht it... my meters are setup to treat that level of signal as 0db. And since that signal is a known level (nW/m) that translates into a known voltage and, checking the specs on the machine, I can find out what the electronincs can handle above that for clipping. Ideally the tape would clip at the same point the electronics clip.

Then the last variable is whether the meters are set differently than the internal levels. I'm not sure what all the controls are on the 48 (there's something like 8 or 10 little adjustment screws for each channel)- I'll have to check the manual to see if that's even possible.

Anyway- right now I get clip lights more often then I'd like when the meter gets up near the red. What seems like a good level on the meters can drive into a clip, say with a sharp strum on an acoustic guitar. Could just be that I need to get used to the meters. If 0 on the meter means +3 in terms of voltages, then I have more volume than I think.

Thanks again,
Chris
 
long winded, sorry about that.

Hi Chris, you are getting there.

The whole idea is that at the calibrated flux the VU meters read 0 VU and the output voltage level is -10 dBv (tascam unblanced). (1dBv = 1 volt and -10 dBv = 0.316 volt)

Perhaps the following discription is overboard but here it is in case it helps.

The trick is that input circuits, output circuits, meter circuits all have their own adjustments.

MRL tapes have a known flux so you can offset the indicated VU level up or down to cause 0 VU to be at the flux you want. It can ba a little crazy and decks do have different adjustments (I don't have a 488!)

Here is an example. I have a 200 nWb/m tape and I want to calibrate my deck for 456 at 370 nWb/m.

So I put the tape in and run the level calibration tone (more complexity left out RE tone frequency). I know that my tapes tone is recorded at 200 nWb/m which is 5 dB quieter than 370 nW/m. If I had a 370 nWb/m tape i would get -10 dBv (316 mV) at the output (it is the Tascam reference level). But because I have a 200 nWb/m tape I need to get -15 dBv (-10 + -5) which is 178 mV at the outputs. I stick my meter on the outputs and adjust each channel to read 178 mV. Poof, the playback is calibrated to 456 (at 370 nWb/m).

The VU meters may or may not read -5 at this point (remember that 200 nWb/m is 5 dB less than 370 nWb/m). In any case I adjust my meters to read -5 VU and they are now calibrated so that 370 nWb/m reads 0VU.

Then I can take my signal generator and apply a 316 mV 1 kHz signal (-10 dBv) to the line in jacks. Switching the source to line in my meters should read 0VU. If not I adjust the line in amps to make the meters read 0VU. I also check line out to be sure that they are also at 316 mV.

Poof, playback is set to 0VU = 370nWb/m.

Of course there is more (bias, record, sync) calibrations to do.
 
Chris Shaeffer said:
Anyway- right now I get clip lights more often then I'd like when the meter gets up near the red. What seems like a good level on the meters can drive into a clip, say with a sharp strum on an acoustic guitar. Could just be that I need to get used to the meters. If 0 on the meter means +3 in terms of voltages, then I have more volume than I think.
Click to expand...

You need to realize that peak meters are different than VU meters. VU meters have a much slower response time. VU's register levels much like out ears, they don't react to peaks as quickly. An acoustic guitar can be rather purcussive and contain peaks that are much higher than average.

Your peak LED are reacting to true peak levels, unlike the VU meters. When recording sources with a lot of transients, you should record at a lower level on the VU's than what you might think you should. For example, I keep drum overheads down around -10dB on the VU meters.

Now there may be an adjustment for your peak LED's. If so, you may want to check them out, to make sure they're not misaligned. Generally I would set my peak LED's to register about 8-10dB above 0dB on the VU meter.
 
Thanks, Eric. 8-10dB above 0 is a helpful starting point. Gives me a frame of reference for what's going on that the meters can't see.

Yeah, that's what I don't have much experience with, yet. The old VU meters I started with didn't have peak LED's but you could hear when you were overloading the tape. Not hard with a cassette.

10 years or so of peak meters on digital systems later and I'm pretty conditioned to set levels by the peaks (to avoid nasty digital clipping) and I don't really understand the relationship between the peaks and the VU meters RMS reading. I know the difference technically, I just don't have *any* experience setting recording levels with them since the 4-track days (when I didn't know what I was doing, anyway.)

Now I want to know where the tape starts to saturate and where its overloaded, as well as knowing where the electronics overload. There are just more stages and adjustments involved and I'm trying to figure out where they all are and what affect they're going to have on how the machine operates. Thanks to this thread, I'm starting to get a handle on them. It wouldn't matter so much if I wasn't purposefully playing around at the top of the range, but I figured I'd start with the most critical area to know.

For example, I tried slamming a pro tools mix pretty hard to the tape just to see what the tape did to it. I set the levels where I thought they were right (as high as possible with very infrequent clips). That turned out really well- a bit more crosstalk than I'd like, but that's another story. The "tape compression" was fairly obvious, though I couldn't really describe it: it sounded different and interesting. I did it again with a different mix that was a little bit hotter (i.e. more clips) and it sounded like *crap.*

The crap mix was probably overloading the electronics somewhere because that's what it sounded like. But I can't be sure because I've never had this much tape to try and overload and I wouldn't know what it sounds like. And what electronics were crapping out? The cheap Alesis 24 mixer I dug out to use with this at home? The input electroncs on the 48? The record heads of the 48? That's part of what I'm trying to figure out so I know what's going on there at the top of my headroom.

I want to *know* how the thing works! That's why I do this stuff!! :D It'll take some time and experimentation, but that's all time wonderfully well spent.

Take care,
Chris
 
measure the voltages

The crap mix could be clipping the electronics inthe mixer or in the deck. Typical limits for the deck are around 8 volts (check the decks spec).

Tape saturation can be figured. Here is a reference that may be of interest.

http://home.flash.net/~mrltapes/mcknight_tape-info-table.pdf

The Quantegy spec for 456 states that the 3% thd level is at +12.3 dB. But does not say referenced to what (all dB are ratios!). An older 456 spec had it calibrated at 0 VU = 320 nWb/m. Taking the remanence Fluxitivy from the URL listed above and doing the math gives:

0 VU = 370 nWb/m

Remanence Fluxivity @ 1950 nWb/m ===> 14.4 dB max
Rms Remanence Flu @ 1370 nWb/b ===> 11.37 dB

at

0 VU = 320 nWb/m

Remanence Fluxivity @ 1950 nWb/m ===> 15.7 dB max
Rms Remanence Flu @ 1370 nWb/b ===> 12.63 dB

So the 12.63 dB is right in the ballpark for the 3% levels.

Using 12.63 dB as the saturation point your decks output voltage at saturation would be 1.35 volt. And if all was calibrated it would also be your input voltage at saturation (so you do not need to clip at the electronics rails to reach saturation)
 
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OK- just to wrap up the subject and double check that I've absorbed everything correctly- and to say "Thanks again for all the help."

The MRL tape is actuall 355 nWb/m, or +3dB... uh, yeah... not sure what kind of dB. dBVU?

So that puts 0dB at -3 on the meter. And the actual saturation point of 456 (+6) at the the top of the meter (+3). If I was using 406 tape, 0dB on the meter (+3 in reality) would be the saturation point. Thus the guy who sold it to me saying that the machine is calibrated to 456 tape- the meters show me the actual headroom on the tape.

Doesn't that make it kinda hard to push 456 tape into saturation without the peaks hitting the rails? It seems like it was designed to NOT saturate. I suppose its easier with peaky material like drums... thus the idea that drums sound better on tape. Seems like you have to be *really* careful setting your levels and watch the peaks- just like a digital system- if you're going for that relatively narrow band of distortion before clipping.

Am I missing anything? Just plain wrong? :)

Thanks again.
Chris
 
I'm not gonna be able to challenge all the advanced math in the previous posts,...

it's after midnight, I'm tired & I'm sporting a beer as I write this.

This is my understanding of the ins and outs of calibration. All of you super-whizzes can feel free to pick apart what I'm about to say, if you like.
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The MRL tape reference fluxivity that corresponds to calibration for Quantegy 456 is 250nWb/m. 250nWb/m is +3db over 185nWb/m. 456 being referenced as a +6 tape is correct, but that's it's MOL (Maximum Operating Level). Recommended Standard Operating Level is below MOL. So, when you have a 250nWb/m reference tape, you'd set 0VU to reference 250nWb/m. That's +3db over 185. Then, when your meters hit +3VU you're at +6 (370nWb/m), the MOL of 456 tape. That's my understanding of it.
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A 355nWb/m reference tape is for setting up for a +9 tape such as GP9 or 499, where 0VU is set to 355nWb/m, which is APPROXIMATELY +6db over 185, and MOL would then be +9db over 185, (at +3VU on the meters).
~(+6db over 185 = 370nWb/m; +9db over 185 = 520nWb/m). I'm not gonna do the fractional math. Note the word APPROXIMATELY.
---
It's confusing, at best, but I acquired a pro-tape setup card that's like a sliderule, that I often refer to when deciphering these posts.
Thank you. :eek: ;)
 
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(sigh)

Just plain wrong it is..... back to the drawing board.

:D
 
Chris Shaeffer said:
(sigh)

Just plain wrong it is..... back to the drawing board.

:D
Click to expand...
Pls catch my late edits. Thanx. ;)
 
Mmmmmmmmmmmmmm,...

this is good beer! It goes right with calibration topics! ;)
 
In your case, Chris,...

seeing that the MRL tape you got was 355nWb/m, then simply set the 355nWb/m "0VU" reference tone to about +2 on the VU meters, and you'll then be set for 456 tape,... with +3db/185 as "Ovu" and +6db/185 as "+3vu".

Sorry if it's a very confusing issue. I get confused on it too, sometimes. ;)
 
Well... hmmm. I found where I got the reference for 355nWb/m being +3

http://www.digitalprosound.com/Htm/TechStuff/2000/Sept/AnalogPt2-2.htm

Clears this up a little for me, as well- apparently there are 2 (or more) general levels for zero- 185 as an older standard and 250 for a "pro" standard. 250 is the standard reference level in the manual for the 48, so that's what I've been going on. (The manual also mentions a 320 standard for European machines and high output tape.)

(deep breath) So 355 is +3 over the 250 standard... +6 over the 185... I see now why the tape info PDF is such a valuable tool- figuring things out with the tape's specs directly is a potentially less confusing than chasing around shifting dB standards.

Of course at this its all somewhat confusing. :) I'm certain you guys are saying it in plain english and there are just a few concepts I haven't grasped, yet. One that just dawned on me is that the levels coming into the tape machine are independent of the levels being sent to the tape heads- thus a machine can be set to produce 185nWb/m from a .316 volt input, or to produce 250nWb/m from the same .316 volt input. That's what evm was trying to say and I just now got it.

I think. :o

OK. Now I need a beer. I haven't crunched this hard on something since college. Loving it, even if I feel a little stupid. I'll get it eventually.

Thanks again,
Chris
 
Okay,... well,... I've taken issue with some of the online references I've seen,...

but I'm not up to all the fractional math at this time,... so some common sense approximations work just as well.

For some of the online references to say 456 is a +6 tape, that's fine, but they often go on to say "set the 0VU point to +6", and IMO that's wrong. The MOL of 456 tape is +6/185, and therefore your +3VU peak should equal +6/185. The Standard Operating Level is below the MOL.

So, using the estimates I have in my head,... if your MRL tape is 355nWb/m, then set the "0VU test tone" to +5Vu on the meters. That should give you proper 456 calibration.

Using the 355nWb/m reference tape, set the "0VU test tone" for -1 on the meters for calibrating to 499 levels.;)

Mmmmmmmm,... good beer, & pizza too! ;)
 
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