1/2" MRL 7.5 NAB to 15 IEC1 Correction

[evm1024]

I picked up a set of 2 older 1/2" MRL tapes off of ebay the other day (for $15) figuring that if nothing else that the reels would be worth that. They arrived tonight and are both sticky (as expected). They are in the "oven" right now baking.

One is from 1975 and the other from 1984. The older is 200 nW/m and the newer is 250 nW/m. Only one has a color dot and appears to be Ampex 406. The older MAY also be 406....

They are 7.5 ips NAB tapes. Now at first glance this appears to be a problem but reading through "Choosing and Using MRL Calibration Tapes for Audio Tape Recorder Standardization" http://home.flash.net/~mrltapes/choo&u.pdf we find tables "5-7 Calibration Tape: 7.5 in/s, NAB (IEC2) Equalization
Desired Playback: 15 in/s, NAB (IEC2) Equalization" and "5-4 Calibration Tape: 15 in/s, NAB (IEC2) EqualizationDesired Playback: 15 in/s, IEC (IEC1) Equalization" which give us the correction factors for the speed and EQ mismatch.

I plugged the corrections into a spreadsheet and came up with this table:

7.5 15 7.5 Speed 15 EQ 15 Output
NAB correct NAB correct IEC1 mV

31.5 63 -10 2.25 -7.75 2.31 -5.44 0.090
63 126 -10 0.45 -9.55 0.83 -8.72 0.042
125 250 -10 -0.55 -10.55 0.35 -10.2 0.030
250 500 -10 -0.84 -10.84 0.18 -10.66 0.027
500 1000 -10 -0.71 -10.71 0 -10.71 0.027
1000 2000 -10 0 -10 -0.48 -10.48 0.028
2000 4000 -10 1.63 -8.37 -1.43 -9.8 0.033
4000 8000 -10 3.48 -6.52 -2.33 -8.85 0.041
8000 16000 -10 4.51 -5.49 -2.74 -8.23 0.047
10000 20000 -10 4.67 -5.33 -2.79 -8.12 0.049


1000 2000 0 0 0 -0.48 -0.48 0.283


7.5 --- Freq at 7.5 ips
15 --- Freq when played at 15 ips
7.5 NAB --- Level recorded on MRL tape
Speed correction --- dB correction when played at 15 ips
15 NAB --- VU when played at 15 ips NAB Eq
EQ correction --- dB correction to get IEC1
15 IEC1 --- VU when played at 15ips IEC1 Eq
Output mV --- Output voltage (where 0 VU = 0.316 mV)

I've not put in a correction to recalibrate at a different fluxivity but that is a straight change based on table 4 in the paper.

Regards, Ethan

PS sorry about the sucky formatting. I've added it in as a word doc.

 

[FALKEN]

give it a shot...you'll know if its in the ballpark or not.

 

[FALKEN]

though I think you want playback to = .316 @ 1000 hz for -10.

 

[evm1024]

though I think you want playback to = .316 @ 1000 hz for -10.

Ouch, caught in my own typos.... Thanks for the catch. I messed up the formula to calculate the output voltage in the spreadsheet....

Yes, 0 VU should read 0.316 mV (at any frequency not just 1 kHz)

Here is an updated doc file

 

[evm1024]

OK I see more of the confusion....

The MRL tapes I have have a 1 kHz tone recorded at 0 VU at reference fluxivity. As well as the other tones recorded at -10 dB level.


0 VUwill give 0.316 mv output on a machine whos output level is -10 dBV and a different output voltage on a +4 dBu machine. (-10 dB of 1 volt is 0.316v)

So having a -10 dBv output level machine and having some of the tones recorded at -10 db of 0 VU (0.1v output on a -10 dBv deck) made for some confusion.

But still I did typo the formula....

 

[FALKEN]

I'm still not sure about your right column. the one that is 500 hz converted to 1000 hz should read 316 mV. I am sure you know this...I think your figures look good otherwise.

 

[evm1024]

Column headers

The first column is the frequence at 7.5 ips, the second the freq when played at 15 ips. Doubles of course.

The 4th column (7.5 NAB) is the level that the tone is recorded on my MRL tape. I could not say what other MRL are recorded at.

The 5th column is the correction in dB for the speed change. And the 6th (15 NAB) is the VU reading when played at 15 ips with NAB eq.

The 7th column is the correction factor for using IEC1 eq and the 8th (IEC1) is the VU reading when played at 15 ips with IEC1 eq.

The 8th column is the expected output in mV.

For a tascam deck with 0VU outputting >>> -10 dBV <<< we expect to see 0.316 mV. 10^(-10 dBV/20) * 1 Vref = 0.316

A signal recorded at -10 dB (not -10 dBV) below 0VU would output 0.1 V. 10^(-10 dB/20) * 0.316 (0VU ref) = 0.1 v output.

In the table the 500 Hz tone was recorded at -10 VU. Only the 1 kHz tone listed in the last row was recorded at 0 VU....

Both the 1/2" MRL tapes I picked up are recorded this way (0 VU ref at 1 kHz then tones at -10 VU) I do not know about other MRL. These are quite old and as that they are 7.5 ips NAB I expect them to be a bit odd anyway.

Regards, Ethan

 

[evm1024]

New correction table

I was trying to cal up my 38 on those NAB MRR that I have but tings were not coming out right. SO I checked the math and re-read the paper. (and repeted the cal a number of times)

And then it hit me that I was over thinking the problem. Correction for 7.5 to 15 ips and then NAB to IEC1. The key was in the paper where it says that an IEC1 MRL needs no correction when played at other speeds.

I redid the math and did a quick test and discovered that was the problem. Just to be sure I exchanged emails with Jay McKnight who confirms the table.

So here it is:


CONVERSION TABLE FOR DIFFERENT SPEEDS AND EQUALIZATIONS
Normalized to the 1000 Hz Recorded Fluxivity
Calibration Tape 8: 7.5 in/s, NAB (IEC2) Equalization
Desired Playback 10: 15 in/s, IEC (IEC1) Equalization
Recorded Playback Playback
Frequency/ Frequency/ Level/
[Hz].........[Hz]........[dB]
32...........64.........5.00
63..........126........1.75
125.........250.........0.28
250.........500........-0.17
500........1000.......-0.23
1000........2000........0.00
2000........4000........0.68
4000........8000........1.64
8000.......16000........2.25
10000.......20000.......2.35
12500.......25000.......2.42
16000.......32000.......2.47
20000.......40000.......2.50