Need some calibration help

Whoopysnorp

Whoopysnorp

New member
Okay, I ponied up the dough for an MRL tape, and I was finally able to attempt some calibration on my Tascam ATR-60-8 last night. I didn't get very far, though...the manual prescribes the use of an oscilloscope and an AF level meter. I don't have either of those, but I do have a PC with Visual Analyzer 8 installed. I did manage to adjust the azimuth on the sync and repro heads to be in phase (or at least close) using the oscilloscope function of that software, but I'm having trouble adjusting my levels. The manual says that when I apply a +4 (0 VU) signal to each channel, I should be reading +4 on my AF level meter. I can successfully get a 0 VU signal to each channel by using a software tone generator, passing the signal into my Presonus Eureka, and making sure the output level of the Eureka is 0 VU. However, I'm at a loss as to what my levels should be reading on the PC (that's really all I have as a measurement tool). The sound card is an Audiophile 2496 whose input levels are fixed at whatever M-Audio calls 'Consumer'. The tape machine is putting out a +4 dB signal. With all of these variables, and me not really understanding dB scales, I have no idea what readings I should be shooting for. I did notice that if I took the trim pot as far over as it could go without sending it careening into clipping the input on my sound card, I was getting readings of -7.6 dB every time. Can anybody help?
 
The specs I have say the peak output level of the 2496 in Consumer mode is +2dBV. This translates to +4.2 dBu.* If you want a +4dBu signal, set your generator to -0.12dBFS, and I think you'll have it.

(You should set the 2496's output level to "consumer" in order to match the fixed input levels.)

* Calculations courtesy of http://www.sengpielaudio.com/calculator-db-volt.htm
 
Thanks for the help (and that link--that will come in handy, I'm sure). I'm not so much having a problem with output levels, though...I control that via the gain and master controls on my Eureka channel strip. Where did you find those 2496 specs? Can I assume the maximum input level is also +2 dBV, and that a +4 dBu signal from my tape machine should read -0.12 dBFS on the PC's meters?
 
Whoopysnorp said:
Need some calibration help
Click to expand...

An AF level meter just measures AC voltage. If you want to be sure about your levels all you really need is a good digital multi-meter like model 22-811 from Radio Shack. I have a more expensive Beckman that I won’t take out of the house, but the RS model is what I’ve ended up using the most. It is very accurate and dependable.

http://www.radioshack.com/product/index.jsp?productId=2103175

With all the variables you have, the way you are trying to measure now there is a lot of opportunity for error. To be certain of where your devices are in relation to a standard measurement… in this case +4dBu, it’s best to measure voltage, rather than depend on the level meters of your recording devices, since they may themselves be off.

With the multi-meter you can be sure of what you have coming in and going out of all your gear. Even with a fixed pocket tone oscillator it’s a good idea to have a meter to check it before use.

Using the Multi-meter you are measuring to see that you have a reading of 1.23 Volts with a 1kHz tone coming from your oscillator. 1.23 Volts equal +4dBu.

So you measure the output of the oscillator; then patch it in and check the output of the recorder to see that they are the same… both 1.23V while the meters on your ATR at 0 VU.

:)
 
Beck said:
If you want to be sure about your levels all you really need is a good digital multi-meter like model 22-811 from Radio Shack.
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I agree wholeheartedly! But I figured he was broke after purchasing the MRL tape.

Do inexpensive DVM's have pretty good frequency response these days? I think my old one (over 20 years old and going strong) is down about 3dB at 10kHz.

Don
 
Yesterday a friend at work suggested I use a volt meter, which I already had, and that worked quite well (and revealed that Presonus put +4 dBu way past 0 and into the red on the Eureka's VU meter). Thanks to everyone for the help. I made some good progress last night before needing to call it an evening, only I remembered this morning that I forgot to compensate -3 dB because I have a 250 nWb/m tape and will be recording to +6/185 tape. Guess I'll have to redo a bunch of it.
 
DonF said:
I agree wholeheartedly! But I figured he was broke after purchasing the MRL tape.

Do inexpensive DVM's have pretty good frequency response these days? I think my old one (over 20 years old and going strong) is down about 3dB at 10kHz.

Don
Click to expand...

That's a good question... some are only fair in that department, so I recommended one I know works well for measuring frequency and level. Once you have an MRL tape all you really need is a DVM that will be right on @ 1kHz at your operationg level. That particular RS model is a good buy. It exceeds the listed specs, which are plenty adequate for these purposes. I can't say the same about any old DVM... there will be differences.

:)
 
Whoopysnorp said:
I made some good progress last night before needing to call it an evening, only I remembered this morning that I forgot to compensate -3 dB because I have a 250 nWb/m tape and will be recording to +6/185 tape. Guess I'll have to redo a bunch of it.
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On the ATR-60 with Quantegy 456, RMGI 911, etc, you might want to try 320 nWb/m, which is +5/185. It is a common standard and gives you a little more headroom to play with. For 320 you would set for –2 VU if using a 250 nWb/m calibration tape.


:)
 
Thanks for the advice Beck, that sounds solid. I like the idea of leaving some headroom. I have a reel of Ampex 499 as well; when I decide to use that do you recommend recalibrating or just leaving as is for the extra headroom?

I also have a reel of Agfa 468 and BASF 468, which based on what I've found has an operating level not far above Ampex 456. Do you think I'm good to interchange the 468 and 456 on my machine without recalibrating?
 
Bias will be an issue with 499, as it is not bias compatible with the 456 class. You can use 499 at any standard level. Of course you get the most out of it if you can hit it harder and keep your recording levels further from the noise floor, but without distortion. If you use +9 class tape like 499 or GP9 it's best to bias for that tape and stay with it. 456 will sound dull if used to record on a machine with bias set for 499

As for BASF 468... technically it requires a bit more bias for optimal performance, but people have always used it interchangeably with 456. My machine has bias and levels set for 456 and I use 468 and 911 on it without any issues.

Unless your machine can switch between different operating levels and bias settings with the press of a button (some can) I recommend staying within the so-called +6 class of tape. You have the added benefit of being able to use AMPEX/Quantegy 406 as well because it is bias compatible with 456. Your levels will just be down a couple dB from where you record them.

There are other issues with +9 tapes, like reel tension, which would need to be adjusted to prevent excessive head wear. (no, not that kind of headwear!)
:D
 

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Whoopysnorp said:
Thanks for the advice Beck, that sounds solid. I like the idea of leaving some headroom. I have a reel of Ampex 499 as well; when I decide to use that do you recommend recalibrating or just leaving as is for the extra headroom?

I also have a reel of Agfa 468 and BASF 468, which based on what I've found has an operating level not far above Ampex 456. Do you think I'm good to interchange the 468 and 456 on my machine without recalibrating?
Click to expand...

Next time you have some $$$, you would be well served to get an Oscilloscope off of the want ads (or eBay if you like).

They will go for pretty cheap bucks. Look for a 20-50mHz Tektronix 2-channel. They are VERY dependable and last a good long time.
 
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