Need Some Help Understanding and Converting MRL

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SteveM

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I have an IEC1 250 nWb calibration tape I want to use to calibrate the Ampex 440, which I believe originally used a 185 nWb NAB calibration tape to set up. I was hoping someone could help point out the differences to me and tell me if I can and how to use my cal tape on the Ampex.

The ampex calls for either a +4 or +8 output depending on the "strapping". :confused:

Which I guess depends on what resistors you have connected to your VU. :confused::confused::confused:
 
Hey Steve,

Get this document and print it out. http://home.comcast.net/~mrltapes/choo&u.pdf

it covers the conversions from one fluxivity to another as well as corrections for IEC1 and IEC2 (iec to nab). Read it and take the time to understand it.

Don't confuse the on tape fluxivity ( +3 tape etc) and the electrical reference level (-10 dB or +4dBV) as that they are not related.

0 VU for any fluxivity YOU select should have an output voltage of 1.23 volt
(see http://www.sweetwater.com/sweetcare/summits/dB.php ) For + 4 dBu or 0.316 V for a -10 dBV deck.

This is no matter what on tape flux you set your recorder for.

Back to tape, It you select 250 nW/m for your reference flux then the meters should read 0 VU when reproducing that tape (and the output should be -10 dBV or +4 dBu based on what your deck is designed to do)

If you want to set the tape flux 3 dB higher than a 250 nW/m tape then your meters should read -3 dB and the output of the recorder should be 3 dB lower ( regardless as to weather the deck is a -10 dBV or + 4 dBu machine).

Hope this helps and happy Halloween!

--ethan
 
The "strapping" just refers to whether you have a wire in the electronics jumpered between terminals 1 and 2 -OR- 2 and 3. There's a barrier strip inside the electronics...let's see...look at section 2.5.4 on page 2-4 in the "INSTALLATION" section of the manual I sent you. That talks about the level strapping. You'll also want to look at figure 6-28 in the "Parts-lists-and-drawings" .pdf...that's section 6 of the manual. Figure 6-28 is on page 53 of the .pdf. Its a drawing of the inside of the righ-hand panel of the electronics module, like as if you were inside the module looking at the inside of the right-hand panel (right-hand as referenced when looking at the face of the module).

I hope that makes sense.
 
Thanks Ethan! Will check that out. I had been poking around in there for a while before I posted here but what you mentioned here should hopefully help me get an understanding.

evm1024 said:
Hey Steve,

Get this document and print it out. http://home.comcast.net/~mrltapes/choo&u.pdf

it covers the conversions from one fluxivity to another as well as corrections for IEC1 and IEC2 (iec to nab). Read it and take the time to understand it.

Don't confuse the on tape fluxivity ( +3 tape etc) and the electrical reference level (-10 dB or +4dBV) as that they are not related.

0 VU for any fluxivity YOU select should have an output voltage of 1.23 volt
(see http://www.sweetwater.com/sweetcare/summits/dB.php ) For + 4 dBu or 0.316 V for a -10 dBV deck.

This is no matter what on tape flux you set your recorder for.

Back to tape, It you select 250 nW/m for your reference flux then the meters should read 0 VU when reproducing that tape (and the output should be -10 dBV or +4 dBu based on what your deck is designed to do)

If you want to set the tape flux 3 dB higher than a 250 nW/m tape then your meters should read -3 dB and the output of the recorder should be 3 dB lower ( regardless as to weather the deck is a -10 dBV or + 4 dBu machine).

Hope this helps and happy Halloween!

--ethan
Click to expand...
 
Thanks again for that reference direction Cory. I'll take a look.

sweetbeats said:
The "strapping" just refers to whether you have a wire in the electronics jumpered between terminals 1 and 2 -OR- 2 and 3. There's a barrier strip inside the electronics...let's see...look at section 2.5.4 on page 2-4 in the "INSTALLATION" section of the manual I sent you. That talks about the level strapping. You'll also want to look at figure 6-28 in the "Parts-lists-and-drawings" .pdf...that's section 6 of the manual. Figure 6-28 is on page 53 of the .pdf. Its a drawing of the inside of the righ-hand panel of the electronics module, like as if you were inside the module looking at the inside of the right-hand panel (right-hand as referenced when looking at the face of the module).

I hope that makes sense.
Click to expand...
 
SteveM said:
I have an IEC1 250 nWb calibration tape I want to use to calibrate the Ampex 440, which I believe originally used a 185 nWb NAB calibration tape to set up. I was hoping someone could help point out the differences to me and tell me if I can and how to use my cal tape on the Ampex.

The ampex calls for either a +4 or +8 output depending on the "strapping". :confused:

Which I guess depends on what resistors you have connected to your VU. :confused::confused::confused:
Click to expand...

0 VU at +4 was more of the studio norm, while 0 VU at +8 dBm was more of a broadcast norm, IIRC. It's just a choice of electronic standard operating level. I'd advise leaving it +4 dBm for most studio situations, because relatively few other bits of gear are ready to deal with levels at +8. There is no audio performance advantage in a studio context for running at +8.

Cheers,

Otto
 
Hey Ethan, according to that MRL chart there is no conversion needed for an IEC1 15ips MRL tape. So with the reference tone at 1k at 0db, I should just set the meters to 0db and the output to +4 dBV (1.58 V) ? Is that right?

In what instances would you want to set the tape flux higher or lower on the VU meters?

Man this is confusing. That voltage seems low if 0 is 1.23v. Or is the 1.58 voltage added to the 1.23? :confused:





evm1024 said:
Hey Steve,

Get this document and print it out. http://home.comcast.net/~mrltapes/choo&u.pdf

it covers the conversions from one fluxivity to another as well as corrections for IEC1 and IEC2 (iec to nab). Read it and take the time to understand it.

Don't confuse the on tape fluxivity ( +3 tape etc) and the electrical reference level (-10 dB or +4dBV) as that they are not related.

0 VU for any fluxivity YOU select should have an output voltage of 1.23 volt
(see http://www.sweetwater.com/sweetcare/summits/dB.php ) For + 4 dBu or 0.316 V for a -10 dBV deck.

This is no matter what on tape flux you set your recorder for.

Back to tape, It you select 250 nW/m for your reference flux then the meters should read 0 VU when reproducing that tape (and the output should be -10 dBV or +4 dBu based on what your deck is designed to do)

If you want to set the tape flux 3 dB higher than a 250 nW/m tape then your meters should read -3 dB and the output of the recorder should be 3 dB lower ( regardless as to weather the deck is a -10 dBV or + 4 dBu machine).

Hope this helps and happy Halloween!

--ethan
Click to expand...
 
OK what you want to do is set your test tape to read +3 on the VU and +3 on your voltmeter (1.73). Then when you calibrate the record side use a 1.23 V tone and set that to zero on the VU. Also if your outputs are balanced make sure to strap the voltmeter across the + and - or else your reading will be half. This will account for the 3 db difference, and is what I do when I calibrate my 355 MRL for 250 tape. A separate issue is that you need to convert IEC to NAB.....I'm not quite sure what you need to do, but I'm sure its online somewhere. I believe your 10khz signal will need to read somewhere other than 0 on the VU.
 
I'm still not sure how it works out that way but I'll take your word for it. :D

Thanks Falken.
 
Hi Steve, Falken is correct. Let me walk through some of the chain. This could be of use to you and to others though I suspect that you know all this.

dB is always a relative measurment. Here is the formula: dB=20 log(v1/v2) or 20 times the log of the 2 voltages your are comparing. In dbV as Tascam uses it v2 (the rederence voltage) is 1 volt. So -10 dBV is 20 Log(0.316/1). Tascam made it easy for us by using 1 volt.

A +4 dBu system is referenced to 0.775 volts and 4dBu = 1.23 volts which = 20 Log(1.23/0.775).

So far we have been discussing the line level (-10 or +4) and the references that they use. The line level has nothing to do with the calibration level on tape. THis line level reference is what you get on your output lines or what you need on your input lines to have your meters read 0 VU. (and you should calibrate the deck that way (0 VU = line level reference) -10dBV is 0.316 V and +4 dBu is 1.23 V (AC RMS by the way)

Part of the calibration process is to be sure that -10 or +4 voltages do read 0 VU.

Now jumping to tape level.... In the old days with low output tape 185 nWb/m was about as hot as could be reasonably printed to tape. It was Ampex's standard. Least we forget, those folks were hard core Engineers who were near the top of their fields.

In calibrating a deck we pick a tape reference level that the deck and tape can handle and that makes sonic sense (would not want to write to GP9 with 185 nWb/m or to 632 at +9)

No matter what tape level you decide on when you are at that level your meters should read 0 VU and the output lines (on input for that matter) would be at the line reference, 0.316 for -10 dBV or 1.23 for +4 dBu.

With the right calibration tape no problem. All tones should read 0 VU and the output wold be at line reference levels.

Now to your question.... IEC1 tapes need no conversion for speed changes. If you are changing the tape reference flux then you need to offset the VU meter readings (and thus output levels) to accomadate the differences in flux.

So to cal to 185 nWb/m using a 250 nWb/m tape you tape would read 3 dB higher on the VU meters and the line output would measure 3 dB higher or log^-1(3 dB/ 20) * 1.23 = 1.74 V (antilog of 3dB/20 times the reference voltage).

This meter and output offset is only for playback of the calibration tape. Once you have tape playing back at the correct voltage (for the differences in desired to on tape levels i.e 250 nWb/m to 185) your playback system is calibrated to 0 VU = 185nWb/m on tape. Then youu go to the record section and calibrate to 0 VU because 0 VU is 185 nWb/m.

THe twist is to use an IEC1 tape for NAB calibration. In the pdf I referenced you can see table 5.2 (30 ips IEC2 to NAB). This is the table to use because there is no correction for going from 30 ips IEC2 to 15 ips IEC1 (those crafty engineers made the 30 ips EQ curve match the 15 ips curve when the 15 ips tape is played at 30 ips)

Using the table you would set your playback at 1 kHz to be +3 dB for the tape flux conversion plus the correction factor at 1k of 0 to equal +3 dB. for 10 kHz you tape the +3 dB flux correction and add the 1.95 EQ correction factor to have your meters read 4.95 VU. (use the recorded freq part of the table) And the line output voltage would be +4.95 dBu which is (log^-1(4.95/20)*1.23) =2.17 volts


Let me know if this is making sense. Sorry about putting in some basics.

--Regards, Ethan
 
Conversion table and calibration

I dug around and found Jay McKnights full table of conversions at this place:

http://home.comcast.net/~mrltapes/eqtables.txt

This table gives the conversion factors for just about everything. Might be useful to stash a copy on a computer somewhere.

For 15 ips IEC1 to 15 ips NAB you want to find the table listed as:

Calibration Tape 10: 15 in/s, IEC (IEC1) Equalization
Desired Playback 11: 15 in/s, NAB (IEC2) Equalization

Which I've copied:

CONVERSION TABLE FOR DIFFERENT SPEEDS AND EQUALIZATIONS
Normalized to the 1000 Hz Recorded Fluxivity
Calibration Tape 10: 15 in/s, IEC (IEC1) Equalization
Desired Playback 11: 15 in/s, NAB (IEC2) Equalization
Recorded Playback Playback
Frequency/ Frequency/ Level/
[Hz] [Hz] [dB]
32 32 -5.56
63 63 -2.31
125 125 -0.83
250 250 -0.35
500 500 -0.18
1000 1000 0.00
2000 2000 0.48
4000 4000 1.43
8000 8000 2.33
10000 10000 2.51
12500 12500 2.64
16000 16000 2.74
20000 20000 2.79


This table says your IEC1 tape will read 5.26 dB too low at 32 Hz and 2.51 dB too high at 10 KHz.

I've done the math and have a table for you that shows the VU meter correction and output voltage.

Using a 250 nWb/m tape and setting the deck for 0 VU = 250 nWb/m:

Freq ---- VU reads ---- Output voltage

32 -5.56 0.648
64 -2.31 0.943
125 -0.83 1.118
250 -0.35 1.181
500 -0.18 1.205
1000 0.0 1.23
2000 0.48 1.321
4000 1.43 1.450
8000 2.33 1.608
10000 2.51 1.642
12500 2.64 1.667
16000 2.74 1.686
20000 2.79 1.696


If you want to set the deck for 185 nWb/m then you need to use this table which applies a 3 dB correction:


Freq VU reads Output voltage

32 -2.25 0.916
64 0.69 1.332
125 2.17 1.579
250 2.65 1.669
500 2.82 1.702
1000 3.0 1.737
2000 3.48 1.836
4000 4.43 2.048
8000 5.33 2.272
10000 5.51 2.320
12500 5.64 2.355
16000 5.74 2.382
20000 5.79 2.396


happy calibrating.

-_Ethan
 
Thanks for putting this up Ethan. So 1k IS indeed flat?

This table says your IEC1 tape will read 5.26 dB too low at 32 Hz and 2.51 dB too high at 10 KHz.
Click to expand...

Did you mean 32 Hz would be 5.56 too low?

I think I'll try setting it up for 250 nWb again. It was too hot before and distorting early. There's also no VU adjustment with this deck when calibrating output. You just lock in the output voltage with the reproduce level.

Just a question. How is +4 considered 1.23 volts? Just trying to get another understanding.

Instead of saying +4 why don't they just say what the voltage should be? I know it's because it's in relation to something else but.....:confused:





evm1024 said:
I dug around and found Jay McKnights full table of conversions at this place:

http://home.comcast.net/~mrltapes/eqtables.txt

This table gives the conversion factors for just about everything. Might be useful to stash a copy on a computer somewhere.

For 15 ips IEC1 to 15 ips NAB you want to find the table listed as:

Calibration Tape 10: 15 in/s, IEC (IEC1) Equalization
Desired Playback 11: 15 in/s, NAB (IEC2) Equalization

Which I've copied:

CONVERSION TABLE FOR DIFFERENT SPEEDS AND EQUALIZATIONS
Normalized to the 1000 Hz Recorded Fluxivity
Calibration Tape 10: 15 in/s, IEC (IEC1) Equalization
Desired Playback 11: 15 in/s, NAB (IEC2) Equalization
Recorded Playback Playback
Frequency/ Frequency/ Level/
[Hz] [Hz] [dB]
32 32 -5.56
63 63 -2.31
125 125 -0.83
250 250 -0.35
500 500 -0.18
1000 1000 0.00
2000 2000 0.48
4000 4000 1.43
8000 8000 2.33
10000 10000 2.51
12500 12500 2.64
16000 16000 2.74
20000 20000 2.79


This table says your IEC1 tape will read 5.26 dB too low at 32 Hz and 2.51 dB too high at 10 KHz.

I've done the math and have a table for you that shows the VU meter correction and output voltage.

-_Ethan
Click to expand...
 
Last edited:
Steve, note that the -10 is in dBv, while the +4 is in dBu. The voltage in dBu at -10 is different than the voltage in dBv at -10.
 
Thanks for reminding me Cory. Trying to remember which is which is part of what adds to the confusion. So what makes the +4 is in reference to the .776 v which is 0 dBu?
 
dBV dBu have different references

Steve, looks like you are on the right track. Good info from Cory.

It boggles my mind that there is no VU adjustment. Perhaps the VU meters are "precision" and read 0 VU at 1.23 volts output. That wold be OK and something you can check.

Do try to calibrate to 250 nWb/m. An IEC1 tape has a flat EQ curve from 300 Hz down where the NAB curve increases the EQ. So the Nab tape is 5.9 dB hot at 32 Hz. Way hotter than IEC1.

Here is a document that shows the EQ curves. Thanks again to Jay Mcknight

http://home.comcast.net/~mrltapes/iec94-flux-v-frequency-table.pdf

IEC1 id the t1=50 column and NAB is the t1+t2 50+3180 column.

Good luck.

--Ethan
 
evm1024 said:
It boggles my mind that there is no VU adjustment. Perhaps the VU meters are "precision" and read 0 VU at 1.23 volts output. That wold be OK and something you can check.
Click to expand...

Yeah, it is weird. It must equal itself out somewhere. There is a VU adjustment with biasing and when setting up record, but nothing in reproduce.
 
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