Otari calibration without an external tone oscillator

M

Muckelroy

Member
I've run this through my head several times, and it seems pretty dang accurate to me. SOMEBODY correct me if I'm wrong. ALSO, I have a digital multimeter. WHAT ac voltage level is the equivalent of +3dB???

OTARI CALIBRATION 101:

When calibrating, the first thing to do is to clean & demag the heads and clean the guides and pinch roller. Make sure all SRL buttons on the front of the machine are depressed. Set the output and input level switches to HIGH. Play your calibration tape (I believe the Otari manual calls for a 500Hz tone........) and hook the output of your track-in-question to an AC voltmeter. (I didn't have one, so I ran it into my mixer's SUB stereo input, so I could use the master meters on my mixer to see the ACTUAL OUTPUT LEVEL.) If you measure the output of each track using an external meter, make sure that each one is of equal level on your external meter. If there's a discrepancy between tracks, you have a problem, and you need to adjust the SRL. If you’re using a 250 nWb/m calibration tape, then be sure to set the SRL level to read +3dB on your external meter. (exactly HOW TO DO THIS is a mystery to me, because I do not know the actual VOLTAGE level that +3 dB is. All I have is a digital multimeter.) In order to adjust this level, you adjust the SRL repro pots on the back of the machine.

The METER calibration pots are totally seperate. Once you have consistent output levels, then calibrate each meter to read +3dB. (this is all while playing back your 500Hz cal tape tone.) At this point, your meters should be dialed in, and you can calibrate your repro hi-F EQ using the cal tape's 10K tone. ( My guess is that this level should ALSO be +3dB.) Do the exact same above procedure, while monitoring from your SEL-REPRO head, and adjusting the SEL-REPRO calibration pots.

Now, to calibrate the machine's oscillator levels correctly, is a heck of a trick to me........After the meters are dialed in, you still need a reference to dial in the input level. You cannot use the internal oscillator to accurately dial in the input levels, because the oscillator is normalled to the input and it has its own level setting, which means it can be altered before it goes to the input level circuit. Your best option is to find an external oscillator, use your AC voltmeter to make sure it’s outputting 0dB, and plug that bad boy into your inputs to calibrate it. OPION 2 -....…..WHY didn’t I think of this before?

HERE'S THE INTERESTING PART!!!

Given that your output SRL (standard reference level) above is set to +3dB correctly, play your 500Hz calibration tape tone, and monitor say……..track 3 from the repro head. Then run a cable from the output of that track’s tape out, to the input on your track in question, like, track 1 tape input. Set this track in question to monitor input, and adjust the input calibration pot until the meter on that track’s tape machine meter reads +3dB.

Do this same adjustment for every track, to correctly set the input levels. (of course, pick a different track to monitor the calibration tape from when you set track 3, er whatever.)

NOW, ideally, all input levels should be dialed in. Monitor input on ALL tracks, and turn on the internal oscillator. Adjust every oscillator level pot until it reads 0dB.

You can then set your bias. IF using 456 tape to record with, throw on a blank reel, and record a 10kHz tone onto all tracks, monitoring all from the repro head. Adjust the bias screw until the meter peaks. You may have to fish around for this peak reading. It does not matter WHERE this peak reading is. Then, turn the bias pot clockwise until the meter is 3 dB lower than that peak reading. Do this for all tracks.

NOW, throw on a blank tape, keep the oscillator on, and record on all tracks, while monitoring from the repro head. Now, you can set each track’s record level until each one reads 0dB (on the tape machine’s meters, monitoring from the repro head of course). Then turn the oscillator to 10K, and do the same procedure to adjust the record EQ pot.

I think that about covers it. Tell me if that bit about the lack of an external oscillator is true or not????

-callie-
 
As usual, i've confused myself, and everyone who'se read my stupid post.....

I basically have 3 questions:

1 - is my calibration procedure more-or-less correct?

2 - what is the AC voltage level of +3dB?

3 - is the whole bit of calibrating the input level without an external oscillator accurate? will it work?

-callie-
 
I've just talked with one of my technical gurus, and he has informed me that +4dBm = 1.228Vrms

The equation used to find this was: dBm = 20log(Vrms/0.775) , and solve for Vrms.

I asked what dBm level a 520 nWb/m calibration tape should read at, and he said it would be +10 dBm, (as opposed to +7, which is what I thought.) because apparently doubling nWb/m adds +6dB of level. (+4dBm + 6 = 10dBm)

He then told me that I should stick with a +4dBm reading, despite the fact that I'm using a 520 cal tape, simply because he thought that +10 would be too hot for a machine with that narrow of track width...............then I thought.....what does it matter? It's just output levels. I want it to match the type of tape I'm using, right? If 456 tape gives 6 more dBm of headroom than 407 tape, hell, I'm gonna take advantage of it..
 
Yes, you NEED an EXTERNAL OSC. Get one on google. Search for HCH TONE GENERATOR. GEt the demo. This way you can just run it out of your 1/8" out from your soundcard. These are the adjustements that will include the TEST OSC:

And Yeah, +4 dbm is what you're supposed to set the input levels to. And yeah, it's 1.228 volts RMS. Be sure your meter reads in RMS (most if not all do, but some MAY read PP, but I doubt it)

And make sure your meter reads in milivolts, I had to REALLY look hard to find one, all the ones in radioshacks and hardware stores only do like 750 and 900 AC voltage or something.

It's not a good idea to try and use the internal OSC to calibrate the inputs. This is coming from someone who has had a reel (hehe) hard time figuring all this out, but now am able to do it all with ease, and have done it on both my otaris.

See, the TEST OSC. can be calibrated too, so it can be off too. And secondly, the VU meters can be off too. The best thing to do is to first adjust the input levels.

This is done by sending a -18 dbm into the TEST OSC. Download the NCH Tone Generator demo (google it) And don't just set the generator to -18 db, because it wont give you a good reading. you have to hook it up to your voltmeter untill you can get as close to 18 dbm as possible (.0975 Volts)

Then run that -18 dbm signal into the TEST OSC. jack and set all input levels (the knobs next to the SLR buttons, not the cal. pots) fully clockwise. What this does is basically bypasses the pot (resistor) so there is an unaffected signal clear through to the XLR outs. At the XLR outs you put the + lead of the meter to pin 3, and - to pin 1. Adjust the Input levels (VR 106) until you get 4 dbm, or 1.228 V AC.

So basically

send a -18 dbm 1 khz tone into the TEST OSC while all iput knobs near the meters are fully clockwise. Measure the XLR outs while adjusting vr 106 until you get + 4 dbm.

Then do the VU meter adjustement. This too you need to use the demo OSC you download. Set the osc untill you get as close to + 4 db as you can (1.228 V) It's ok if you're a few mVs off.

Then run that through the TEST OSC. while the input knobs are still fully clockwise. Adjust the VU meter pots (VR 107) until they read 0.

And you need to adjust the SLR levels too. This time you send it a -8 dm

(.31 V AC) into the machine WHILE THE SLR BUTTONS ARE IN and adjust the 206 pot for a 0 VU level on the meters.

You should adjust the tests osc. level too. Do this AFTER you've done the input levels. If you want me to help you through that too, just ask. In fac,if you have any questions about all this stuff ask.
 
WHY THE CRAP DIDN'T I THINK OF THAT????

All this time, I coulda just used my computer as a friggin tone generator

I have sound forge, and a multimeter, which should do the trick!!

Sound forge will generate tons of stuff, so sine wave test tones should be no problem. Wow. I feel dumb.

One more question -- when you say to run these signals into the "test osc" inPUT, are you talking about the "EXT" connector on the front of the machine, near the oscillator seciton?

thanks!
-callie-
 
yeah, that's the one, it should be right under the osc pot.

Be sure when you send a tone out of your comp, to put the sine wave on continuous, if not, the sine wave may eventually die out. This may already be a programed feature though.
 
antispatula said:
Yes, you NEED an EXTERNAL OSC. Get one on google. Search for HCH TONE GENERATOR. GEt the demo. This way you can just run it out of your 1/8" out from your soundcard. These are the adjustements that will include the TEST OSC:

And Yeah, +4 dbm is what you're supposed to set the input levels to. And yeah, it's 1.228 volts RMS. Be sure your meter reads in RMS (most if not all do, but some MAY read PP, but I doubt it)

And make sure your meter reads in milivolts, I had to REALLY look hard to find one, all the ones in radioshacks and hardware stores only do like 750 and 900 AC voltage or something.

It's not a good idea to try and use the internal OSC to calibrate the inputs. This is coming from someone who has had a reel (hehe) hard time figuring all this out, but now am able to do it all with ease, and have done it on both my otaris.

See, the TEST OSC. can be calibrated too, so it can be off too. And secondly, the VU meters can be off too. The best thing to do is to first adjust the input levels.

This is done by sending a -18 dbm into the TEST OSC. Download the NCH Tone Generator demo (google it) And don't just set the generator to -18 db, because it wont give you a good reading. you have to hook it up to your voltmeter untill you can get as close to 18 dbm as possible (.0975 Volts)

Then run that -18 dbm signal into the TEST OSC. jack and set all input levels (the knobs next to the SLR buttons, not the cal. pots) fully clockwise. What this does is basically bypasses the pot (resistor) so there is an unaffected signal clear through to the XLR outs. At the XLR outs you put the + lead of the meter to pin 3, and - to pin 1. Adjust the Input levels (VR 106) until you get 4 dbm, or 1.228 V AC.

So basically

send a -18 dbm 1 khz tone into the TEST OSC while all iput knobs near the meters are fully clockwise. Measure the XLR outs while adjusting vr 106 until you get + 4 dbm.

Then do the VU meter adjustement. This too you need to use the demo OSC you download. Set the osc untill you get as close to + 4 db as you can (1.228 V) It's ok if you're a few mVs off.

Then run that through the TEST OSC. while the input knobs are still fully clockwise. Adjust the VU meter pots (VR 107) until they read 0.

And you need to adjust the SLR levels too. This time you send it a -8 dm

(.31 V AC) into the machine WHILE THE SLR BUTTONS ARE IN and adjust the 206 pot for a 0 VU level on the meters.

You should adjust the tests osc. level too. Do this AFTER you've done the input levels. If you want me to help you through that too, just ask. In fac,if you have any questions about all this stuff ask.
Click to expand...

I HAVE SCOURED THE INTERNET FOR WEEKS AND FINALLY FOUND EXACTLY WHAT I NEED ASSISTANCE WITH! Please still be active here and willing to reply!...

I recently acquired a MKiii 8 and have been banging my head against the wall trying to use the manual to adjust input levels with the "+4dbm," reference level; which I've now come to understand is used primarily academically and has no direct correlation with the db scale I've been using with the 1kHz signal generator plugin out of Pro Tools. You're instructions here are so concise and very much appreciated but I've seemed to still run amiss somewhere along the way.

I was able to use my Fluke digital true-RMS multimeter to measure the 1Khz tone output from Pro Tools to .0975 volts (which is roughly -34.2 db on the signal generator plugin). Since I'm coming directly out of Pro Tools with an xlr cable, I use a Hi-z unbalanced adapter to plug the signal into the EXT TEST OSC jack on the front of the Otari. Turned all the INPUT knobs fully clockwise (the VU meters are slammed into the PEAK), and connect the same Fluke true RMS multimeter onto the OUTPUT xlr pins in the back of the Otari. The "Output Level," switches are in "H," position and the "INPUT LEVEL SWITCH," are all in "L," position.....BUT!!! The voltage reading from my multimeter reads 11.20V and adjusting the (VR106) only gets me down to a modest 9.50V level. (The levels are all roughly 11.20 V...so at least they are all equal as of now).

Where am I going astray?! Am I missing some fundamental step? Do I need to adjust other levels on the back of the Otari before I try and adjust the inputs?! Does using the hi-Z adapter somehow change the (.097 Volts)

Thank you so much for your posts already, they are already so helpful and literally the only place on the entire that I've come across that details this process so articulately and straightforward. Any additional insight and assistance would be tremendously appreciated! I grew up in a primarily digital recording environment and this is my first foray into incorporating a tape machine into my workflow and I couldn't be more excited, nor more novice, haha. Thanks again!
 
Jdoo said:
I was able to use my Fluke digital true-RMS multimeter to measure the 1Khz tone output from Pro Tools to .0975 volts (which is roughly -34.2 db on the signal generator plugin). Since I'm coming directly out of Pro Tools with an xlr cable, I use a Hi-z unbalanced adapter to plug the signal into the EXT TEST OSC jack on the front of the Otari.
Click to expand...
Did you check the bandwidth capability of your Fluke meter? I have one that only measures up to 500hz, it gets extremely inaccurate at higher frequencies.
 
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Well this is an old thread but still people can learn from this answer as it does not expire. First all professionals use a made for audio dB meter like a Leader LMV181A or even higher models. Next you have to select a correct test tape not the one used for super expensive Studer machines as this is NOT that. This machine has flux densities of 185, 250 and 320nWb Meter flux as set by the level switch- this is not the same as the output switch that selects between 0 dB and +4dBm. The internal oscillator is garbage and for use in quick set up but a real Technician will use spot frequency generator or in they are able an Audio Precision One Plus Portable. This will allow different levels and will do sweeps. The Otari manual starts out with recording which is ridiculous as all other tape decks are correctly done playback reference first. This is how I do it in all cases. I use the center level which is the 250nWb/M flux as they use on Tascam decks. Operating level is set and the required outputs obtained- the Ap as we call it has XLR inputs You can select what level you want to use 0 dBm or +4 dBu. It is all just a few button pushes away for setting this on the Ap. Once operating level is had and the azimuth is correct with the 16KHz tone then play EQ is corrected using 10 or 12.5KHz signals to be the same as the 500 Hz refernce tomne at the start of frequency test. After this is obtained then it is off to record section. I commonly use SM911 tape and the results are usually pretty good the SM900 or ATR Master can be used as well. The record section is all front panel adjustment that the customer can goof up as soon as he puts a driver into the hole.
 
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