TP-027, Jerry Garcia / David Grisman wins a Writer's Choice Award from Myles Astor of Positive Feedback Online

Author Topic: VU meter calibration for Pro recorders  (Read 22946 times)

Offline Ki Choi

  • Sr. Member
  • ****
  • Posts: 314
    • View Profile
VU meter calibration for Pro recorders
« on: October 24, 2008, 01:02:42 AM »
Hi,

The Studer A80 has R18 in its VU meter amp board that would allow me to hit 0VU with
+4dBv jumper setting. I can get the AF generator to output 1.228v (connecting it
directly to the millivolt meter for amplitute ajdustment) for 0VU reading
(after zeroing the meter).  Isn't correct to expect I should also get 1.228v from deck's line
outputs?

I'll have to check the schematic to be sure that with Input mode in Studers
engaged from the meter bridge, the AF generator's 1.228v is going directly
from input connector to VU meters and coming straight out to line output
connectors without getting amplified in the deck's audio section. 

It is confusing to me when I cannot get the same reading from line output, and
the VU meter (showing past 0VU) when fed it with 1.228v from AF generator.

Is it correct to set the AF generator to 1.228v and adjust for 0VU at the meter adjustment pot?
Or is there a proper procedure for VU meter calibration?

Thanks,
Ki
Ki Choi

Offline sound signal

  • Full Member
  • ***
  • Posts: 46
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #1 on: November 06, 2008, 09:35:11 AM »
Hi Ki,

The level for professional recorders is usually set to a reference fluxivity on the tape (in nanowebers per metre, nW/m).  The procedure to set level is as follows:

- Choose a reference fluxivity to calibrate your machine to.  It may be a good idea to begin with the manufacturer's recommended level and then change this if you prefer.  For a Studer this is probably 510nW/m, but check your manual.

- Find an appropriate calibration tape.  I use MRL tapes.  Read their publication Choosing and Using MRL Calibration Tapes.  Refer especially to Section 1.2.6 and Table 3.

- Play the calibration tape on your machine.  If there is an output level control, set the output level presets to the reference level specified in the manual.  If you are using a calibration tape with a different level to that specified in the manual, make the appropriate correction, see Table 3 in the document mentioned above.

- Set the meters to the appropriate reading at reference level.  This depends on whether your machine has Peak Programme Meters or VU meters. The meters read the same on steady calibration tones, but PPM's will read an average of 8dB above VU meters for typical variable signals like music or speech, so compensation must be made.  You can start with the values given in the manual for your machine.

- Load a blank tape of your chosen type.  Make sure that the machine's meters are reading signal from the input, not off tape.  Feed a reference signal and set the level so that the meters read the same as when playing back the calibration tape.

- Switch the machine to record and make sure the output is switched to tape, not source.  Do not touch either the playback level presets or the meter presets. Adjust only the record level presets so that the output off tape is the same as for the calibration tape.

- Continuing to record, switch the meters between input and tape to verify the adjustment.  The level should remain constant when switching the meters between tape and source.

- If you then proceed to adjust the bias, you might need to make a fine re-adjustment of the level.

For example, to adjust a Nagra IV-S as per the manufacturer's recommendations: the reference level according to the manual for the Nagra IV-S TC is 0dB on the modulometer for 510nW/m RMGI468 at 15ips. I have a 355nW/m calibration tape, not 510nW/m, so according to Table 3 in the above document I should set the modulometer to -2dB with this tape.  The Nagra has fixed playback level so I only need to set the meter calibration and recording level.

Another example, to adjust a Revox B77 Mk. II to my own preference: I set the VU meters to read 0dB for 355nW/m with SM468 tape.  I find this gives me the best dynamic range with the music I record and the tape I use.

Suppose I want to re-set the Nagra to get the same level as with the Revox when modulating close to 0dB.  Assuming that the modulometer reads on average 8dB more than the VU meter, I would need to set the modulometer to read -8dB with the same 355nW/m calibration tape.  So that setting would be 6dB hotter than the factory setting for the Nagra.  Unfortunately, there isn't enough range in the Nagra's meter presets for that.  So for now I just let the meters read higher on the Nagra than I do on the Revox when recording, to get about the same dynamic range.

Note that I record chamber music.  More demanding material with more highs and transients would probably need a more conservative setting (higher reading for the same level).  This would be closer to the Nagra's conservative factory setting.  After all, Nagras were mainly used to record film sound - complete with slamming doors, breaking glass, whatever - which has much more peaks to overload the tape than chamber music.

So your own preference, judgement and circumstances has to come into it eventually.  The manufacturer's settings are not carved in stone.

Good luck!
« Last Edit: November 06, 2008, 10:53:29 AM by sound signal »
George Karaolides
Nicosia, Cyprus

Offline reelnut

  • Full Member
  • ***
  • Posts: 55
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #2 on: November 24, 2008, 04:18:18 PM »
Ki-

Why are you trying to set operating level at your outputs with an AF generator? (See the first sentence of Sound Signal's reply). In principle, the operating level at your tape outputs (in your case 1.228v) is set to a "reference" fluxivity recorded on the tape. If you don't do this the operating level of 1.228v has no meaning. To directly answer your question: NO, you should not expect to see 1.228v at the outputs when inputting 1.228v with AF generator. You need to completely forget about the voltage on the input side! A direct answer to your second question: NO, it is not correct to input 1.228v and adjust VU meter amp! Again, forget about the input voltage. The only time you will adjust VU meter amps to read "0" is after you have played the 1kHz tone on your test tape and used that tone to set the level of the playback amps so that 1.228v is produced at the outputs.

After you have made the above adjustments using your test tape you will be playing back at the reference level you have chosen (in this day and age with the high output tapes we have this will most likely be 320 or 355nWb/m), and producing operating level at the outputs. Here is a great link which covers this in a language I can understand: http://home.swipnet.se/herbalifeinfo/Audio/reeltoreel.htm.

Does your A80 have the "uncalibrate" pushbuttons next to your input and output knobs? I'm guessing from the pics I've been able to find of the A80 that it does! The ones next to input controls have no effect here, since we are concerned with playback only at this point. But the ones by the output knobs definitely need to be out to be released in order to avoid amplifying the output by an arbitrary amount. Machines that do not have "uncalibrate" switches, such as RS1500 and X-2000R will typically state in the manual that the output knob is to be placed in a certain position before performing the adjustments. I hope this helps!

John
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline Ki Choi

  • Sr. Member
  • ****
  • Posts: 314
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #3 on: November 25, 2008, 01:11:00 PM »
Thanks to both George and John for the info.


My error was to start the calibration from the Input level per AF Generator and adjusting VU meter amp instead of picking the operating level per reference fluxivity of the MRL tapes.  Having to replace both VU meters on one of the A80s that behaved a little differently than the original VU meters may have contributed to me going in the wrong direction.

Your messages are very helpful in realigning my calibration steps in my head (although it is hard to just forget about the input voltages...) ;-)

Yes, all Studers I have including the A80s have Uncal mode that bypasses all internal amps.

Thus, if I put the deck in its uncal mode and play the MRL tape for 1Khz tone, the A80 in 4dB operating mode should produce 1.228v at the line outputs (if not, adjust the line amp level pots to produce 1.228V) but if the VU meters are not at 0VU at line outputs putting out 1.228V, I should adjust the VU meter amps to read 0VUs.  Correct?

Thanks,
Ki
Ki Choi

Offline reelnut

  • Full Member
  • ***
  • Posts: 55
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #4 on: November 25, 2008, 03:57:04 PM »
Hi Ki-

First off let's double check a few things: Does your A80 manual specify that operating level at the tape outs is 1.228v?

Are your meters set to PPM or Peak? With my A810 there is a 6db difference between the two when calibrating. I would double check this in the A80 manual.

And what is the reference fluxivity of your MRL tape?
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline reelnut

  • Full Member
  • ***
  • Posts: 55
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #5 on: November 25, 2008, 06:52:29 PM »
Ki-
Hi, it's me again-
Which A80 do you have exactly? I have the complete service manual for the A80 VU MKII/MKIII, A80 VU MKIV, and A80 MRMKII on DVD. If your machine is one of these it would help me to know which one. I have realized today that although basic alignment procedures would be the same for any machine the process gets complicated when you consider all the different standards that are used, and different machines can be referenced in differing ways!

For one thing, I can't find a reference to 1.228V. All of my Studer service manuals reference line level to the NAB standard, which is based on this relationship: 0dbm = 0.775V at 600 Ohms, or the CCIR/IEC1 standard, which is based on this relationship: 0dbu = 0.775V at 200 Ohms.

Edit:
OK, I've just come across a table of some different levels. I can see that a line level of +4dbu = 1.23V, which would imply that your MRL tape is CCIR/IEC1, correct?
« Last Edit: November 25, 2008, 07:23:00 PM by reelnut »
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline Ki Choi

  • Sr. Member
  • ****
  • Posts: 314
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #6 on: November 26, 2008, 03:49:20 PM »
Hey John:

The A80s are RC MkIIs and they are set up for NAB eq. in both record and repro cards.  Thus, I use MRL test tape with NAB equalization standard at 250 nWb/m.
The VU meters are set for +4dB for recording industry standards  (1.228v) and in PPM mode.

Thanks,
Ki
Ki Choi

Offline reelnut

  • Full Member
  • ***
  • Posts: 55
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #7 on: November 26, 2008, 10:27:03 PM »
Ki-

I'm workin' on it! Answers to all your questions soon with very precise answers, per Studer!
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline reelnut

  • Full Member
  • ***
  • Posts: 55
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #8 on: November 28, 2008, 12:12:59 AM »
Hi Ki-

Your were confused because, from what I can see, the different Studer manuals vary considerably in what they explain and what they assume you already know! My A80 VU MKII manual does not explain the difference between PPM and VU meters. There is one example of a hypothetical calibration where they tell you to set repro to -6 before you set line level and zero, but they don't say why you do this! (More on this at the end of this post). My A810 manual does not explain the voltages associated with different operating levels (0dBm, +4dBm, etc.), so I had to read, read, read... (more reading), and learn. In the end I recalibrated my machine to the +4dBm (1.228V) line level because I couldn't read and say "Ah! Oh now I get it"- I wasn't getting it! I normally use 0dBm (0.775V) as my line level. The only way I finally got it was by doing it! And I set the meters to PPM from VU, which I had been using. Sometimes the only real test of what a person knows is to actually do it, and then you can say "Ah, now I get it"! The A80 Vu MKII manual has a long table which lists the voltages that correspond to different line levels. The voltage of 1.228 is correct for a line/studio/operating  level of +4dBm. These 3 terms are used interchangeably. (Notice I did not use the words "reference level", which is another thing entirely and always pertain strictly to the magnetic flux level that's going on the tape). There's so many "levels" I thought this might be worth mentioning again! The manual states this voltage is to be measured across a 600 Ohm load for NAB eq. For CCIR/IEC1 eq the resistance is 200 Ohm. If you don't have a 600 Ohm resistor, don't worry too much. It doesn't appear to have a large effect. I use a very normal Wavetek multi-meter and tried it both ways. Not using a resistor causes the machine's meters to read 1/2db higher than they read when the 600 Ohm load is connected across the positive and negative wires of the output, and, of course, the voltage remains the same in both cases, so you would still adjust to 1.228V.

So, to answer the last 2 questions of your original post:

The voltage output of the AF generator is irrelevant, because you are concerned with setting the deck's meters properly for the line level of 1.228V. So, input a 1kHz tone, usually from your MRL tape. It is possible to set line level with the AF generator and then adjust meter amps if you  do not need a complete calibration of the machine. You do not need a test tape for that. All you need is a multimeter to confirm 1.228V at tape outputs, and then you adjust the meter amps. The info presented below will take us through a basic level calibration, using your MRL test tape, and at the end you will know why I have been using the word "adjust" instead of saying"zero".

There is a 6db difference in the way the Studer PPM and VU meters react, due to the dynamics of the different types of meters and how they respond to a sine wave test signal. Here's the procedure for calibrating which is straight from the A80VU MKII manual:

Step 1:
"Mode selector at SAFE; Meter selector at REPRO; mount reference tape, select preferred tape speed; Line level of this example: +4dBm/600ohms"

Step 2:
"Reference tape, 1kHz reference tone section; Depress calibrate button;" (Yeah, that's right partner! They are, in fact, instructing you to engage your repro amp at this point! You'll see why shortly!). "Adjust playback level pot of the corresponding speed to obtain the following meter reading: -6db". (The manuals are vague here. I was able to determine from looking at A80 and A810 manuals that they are assuming you have meters set to PPM indication.  If you are using VU metering you need to set playback level pot to 0db).

Step 3:
"Reference tape, 1kHz reference tone section; Adjust line level with control REPRO" (this is referring to your main output knob) "to obtain the following reading on the external voltmeter: +4dBm". (The table in this book defines this value as 1.23V, but they don't carry out the decimal point past 1/100th of a volt).

Step 4:
"Reference tape, 1kHz reference tone section; Adjust meter calibration control REPRO/SYNC to obtain the following reading: 0db". (This is where you are "zeroing" your meters, but as you can see, a correct calibration depends on playback of the reference tone to be set at -6db for PPM meters)!!

Here's a test I'm going to reccomend to you so that you can see and compare the action of PPM and VU meters:

Perform the above calibration. Now, don't touch anything, except to switch one channel to VU metering. The VU meter will read precisely +6db higher than the PPM meter when displaying the 1kHz sine wave at reference level, as it should. But instead of seeing -6db on the PPM and 0db on the VU both readings will be changed by 0.5db since you no longer have the 600 Ohm resistor inserted. My suggestion is to now play a tape and observe how the different kinds of meters respond to the music. For me it was a real learning experience because the PPM meters move quite slowly compared to VU meters. But this is only because the PPM has a slow "fall" time in order to give a better indication of the actual RMS output of the music. You will notice as you continue to watch, that the PPM has an extremely fast rise time and will reveal peaks that the VU meter can't catch. And you will also see the VU meter falsely over-shooting, giving false peak readings where none exist. Here's a way cool link to a page that tells about PPM and VU meters: http://www.mediacollege.com/audio/monitoring/ppm.html

Having a properly calibrated machine is a great feeling, isn't it Ki? Happy reeling!
« Last Edit: November 28, 2008, 03:14:54 AM by reelnut »
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline Ki Choi

  • Sr. Member
  • ****
  • Posts: 314
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #9 on: December 02, 2008, 12:43:50 PM »
Hi John:

Thanks for your lenghty post.

As you said, it will come me to when I actually recalibrate the A80RC MkII this weekend.  I did double check the meter setting and I reported wrongly.  It is in VU mode but certainly +4dB setting for normal operating level.

I see from you signature that you are a fan of Yamaha CX-1 and MX-1.  After trying out many of the high $$$ power amps, I still like MX-1 also even over the newer MX-D1s.

Ki
Ki Choi

Offline reelnut

  • Full Member
  • ***
  • Posts: 55
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #10 on: December 03, 2008, 07:07:25 AM »
Ki-

I am learning here, as well. I ended up leaving the A810 at the +4db operating level, as it was instantly apparent to me that now the machine matches up level-wise to the mixer that I use to route various sources into it. I've also left the meters in a PPM state, as PPM seems to be better at indicating if L and R channels are matched properly.

Thank-you for your comments regarding the Yamaha CX-1 & MX-1! That pre-amp weighs more than many amplifiers, and for me, being a bona-fide recording nut, the ability to route 7 sources simultaneously to 3 tape inputs is waaay cool! I was pleased to hear that you would prefer the MX-1 over the new digital version also, since all of the manufacturers are constantly trying to tell us that the equipment we own can be replaced by something which is "better"! It is very quick on transients, but for me the main drawback of the MX-1's class A amplification is the fact that it idles on 500 watts. Great for running up the electric bill!
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline Ki Choi

  • Sr. Member
  • ****
  • Posts: 314
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #11 on: March 02, 2009, 01:26:35 PM »
Hi George:

I had spend long hours reading your message from few months back and on the phone with reelnut (John).
My original post was regarding calibrating the VU meters on Studers.
It seems your reply was regarding repro calibration of a machine with working VU meters.

The reason for my initial question was I had replaced VU meters on one of my A80s and needed to make sure the the new VU meters were calibrated properly before I calibrate its repro and record functions.

Thus, understanding of Studer Operating Level in dBv, dBu and dBm in relationship to 0VU in VU meter was necessary.
Per description on Studer manual, I started to make sure the deck was in Input mode bypassing all repro circuits.

Over the weekend, I got close in understanding +4dBv operating level of Studer recorders.  One
of the A810 that had been calibrated by a real pro had been used to varify my findings.

Per info in the Studre A810 manual's calibration section, I started with putting the
deck into Input mode with no cal button and feeding 1.228V unloaded 1Khz sign wave and adjusted
the small pot in the Line Amp Card to get the line output to be 1.228V in the
millivoltmeter (it was already pretty close).  I then saw the VU meter was pretty close to 0dB and adjusted the VU meter
amp pot to get right at 0VU.  All these steps made to me sense since the
deck was set for recording studio's normal +4dBv operating level and the VU meter reading of 0VU meant that it was in actuality +4dBv as read in the line output voltage.

As guessed, changing the operating level from the Prephery Control card settting in the A810 did
not make any changes to the VU meter or the line output level.  It only effected when I play the MRL tape to adjust the line output level in Repro mode.

However, when I load the voltmeter with 600 ohm resistor as decribed in the
Studer manual, I get voltage drop and the lower readings show in both VU meter and the line
output.  The manual says that I should feed the operating level signal
(1.228V in my case)  to the Input mode and should get 0VU in the VU meter and again
with the 600 ohm load in place at the millivoltmeter, I should see the operating level output of 1.228V
from the line output.  But I get about -1.5dB on VU meter and voltage value of lower than 1.228V
from the line output when I have the load resistor across the milivoltmeter.

According to the manual, I should increase the output voltage
output to 1.228V by adjusting the fine gain pot in the Line Amp
Card and to move the VU meter to 0VU by increasing the VU meter amp pot. 

Is this correc to make the adjustment with the load resistor accross the millivoltmeter?

One more confusing thing is that one of the sites on Internet talked of dBu
being unloaded level and reference voltage of 1.228V should result in 0
dBu which is what I have found so far ... until I introduced the 600 ohm load resister to the milivoltmeter.

So then what is the purpose of the 600 ohm load resistor?  As I understand it, the load resistor of 600 ohm is used when measuring the dBm milliwatt power at 0VU....

thanks,

Ki
Ki Choi

Offline Ki Choi

  • Sr. Member
  • ****
  • Posts: 314
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #12 on: March 03, 2009, 10:59:22 AM »
After posting the above question, I spent few hours experimenting and answered my own question.

It seemed the 600 ohm load resistor described in the manual doesn't apply to modern test equipments with high input impedences.

After removing the load resistor from the millivoltmeter input, all fell into places.
Feeding the Studer input with 1Khz tone at my operating level (+4dBu) with all UNCAL button released, I was able to dial in the Line Amp card to 1.228V then achieve 0VU in the CH1 VU meter and the CH2 VU meter's VU amp pot was adjusted to get 0VU.

Having perfectly calibrated line outputs and VU meters, rest of the Repro, Bias and Record cal was a breeze.

Ki
Ki Choi

Offline ironbut

  • Global Moderator
  • leader in spreading disinformation
  • *****
  • Posts: 2503
  • rs1500>repro amp#1
    • View Profile
Re: VU meter calibration for Pro recorders
« Reply #13 on: March 03, 2009, 06:52:45 PM »
Glad to hear you got that mystery solved. I was looking for a thread on the Ampex list that was very close to what you were going through (a poster was replacing his VU's) and Jay McKnight had walked him through the procedure. I wasn't able to find it but if I recall correctly, you found the correct answer.
This thread (and others like it) are a big help for me. I've read through most of the papers that Jay has on the MRL site but more often than not, I just understand what I need to know at the moment. There's so much fantastic info in those papers that I believe that if one were to really "get their head around" all those ideas, you'd have a real understanding of magnetic tape recording and playback. This thread sent me back there a couple of times. Thanks.
http://home.comcast.net/~mrltapes/
« Last Edit: April 17, 2009, 10:46:26 AM by ironbut »
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
 HE Audio Jades