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Thread: Weird calibration question

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    Weird calibration question

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    Sorry to be a pain, but since I'm chronically unable to figure out the necessary calculations myself, perhaps someone else can help.

    This may be a fool's errand, but I'd like to try out some SM900 on my mastering deck, but I don't have a suitable MRL tape for this level and ordering one is unlikely to arrive this year.

    The idea is to try and use my 200nw/M MRL tape to line the machine up for +9 operation, or failing that +5 if it can't handle it properly. The catch is that I don't have an accurate DB meter, so I'm trying to do it by output voltage.

    Right now the machine outputs 1.23v on each channel which I believe is correct for 0vu at +4.

    What voltages should the machine output if it was playing back that same +1 tape, but if the deck was set up for +9, and for +5?

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    AFAIK,

    The voltage output should be +4dBu (1.23v) no matter what the operating level of the tape is, just that 0VU is 520 nW/m (+9dB) instead of 200nW/m (+1dB or so) I don't have a chart for a 200nW/m tape, here is one for 250nW/m, which is per this http://www.digido.com/quickfaq/n/nan...tml?format=pdf 2dB higher than 200. So the math should be -8VU for +9 or -5VU for +6 taking into account the difference.

    See e.g.
    Attached Images Attached Images 250nw-jpg 
    Last edited by Blue Jinn; 11-10-2010 at 12:08.

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    jp,

    does your deck have noise reduction? do you plan to use it? I am going to assume for mastering on +9 tape that you will not be using any NR (you don't really need it). In this case, I believe if you want a +9 signal, your tape should be playing back about 6 db under 0.

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    Quote Originally Posted by Blue Jinn View Post
    AFAIK,
    The voltage output should be +4dBu (1.23v) no matter what the operating level of the tape is, just that 0VU is 520 nW/m (+9dB) instead of 200nW/m (+1dB or so) I don't have a chart for a 200nW/m tape, here is one for 250nW/m, which is per this http://www.digido.com/quickfaq/n/nan...tml?format=pdf 2dB higher than 200. So the math should be -8VU for +9 or -5VU for +6 taking into account the difference.
    Yessss, but that's assuming I have a reliable VU meter. I don't, it's an LED ladder which is good enough to watch the peaks and average output but you'd have to be mad to calibrate against it. The machine is an A807, without the meter bridge.

    I do have an accurate voltmeter, so if I had a 520 reference tape then yeah, I'd adjust the repro level until it matched 1.23v. But without one of those, I figured the sensible approach would be to play back the 200 tape, but line it up for -8 or whatever, which is going to be a lower output voltage.
    Then I'd calibrate the record level by increasing the drive until the output matched 1.23v.

    Is there a flaw in this reasoning? Or are you suggesting that I calibrate the tape to the lower output and just utilise the extra headroom..?

    ...one idea might be to hook the thing up to the B77 and use the input meters on that (with an adjustment to compensate for the -10/+4), but it's a borderline insane way of working :P

    Falken, there is no NR on this deck, except for the HX system which modulates the bias during record depending on the input frequency. Fortunately this deck is digitally-controlled so I can just re-enter the settings for SM911 if it all goes pear-shaped.

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    I think Blue Jinn got it....I dont know if the extra 2 db is actually 2 db or just the difference between calibrating at 700 hz or 1000 hz....either way with the B77 you can probably push it to 8db.

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    Quote Originally Posted by jpmorris View Post
    Yessss, but that's assuming I have a reliable VU meter. I don't, it's an LED ladder which is good enough to watch the peaks and average output but you'd have to be mad to calibrate against it. The machine is an A807, without the meter bridge.

    I do have an accurate voltmeter, so if I had a 520 reference tape then yeah, I'd adjust the repro level until it matched 1.23v. But without one of those, I figured the sensible approach would be to play back the 200 tape, but line it up for -8 or whatever, which is going to be a lower output voltage.
    Then I'd calibrate the record level by increasing the drive until the output matched 1.23v.

    Is there a flaw in this reasoning? Or are you suggesting that I calibrate the tape to the lower output and just utilise the extra headroom..?
    That makes sense. I just can't say off hte top of my head what -6VU or -8VU translates into volts, with 0VU at 1.223. Is -8VU = -4dBu????? if so then you'd measure .448 volts....

    http://www.sengpielaudio.com/calculator-db-volt.htm

    This would seem to suggest .489v with 0VU = 1.228v and the "UK VU scale":

    http://www.sengpielaudio.com/MeterComparisonTable.htm
    Last edited by Blue Jinn; 11-10-2010 at 17:08.

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    Quote Originally Posted by Blue Jinn View Post
    That makes sense. I just can't say off hte top of my head what -6VU or -8VU translates into volts, with 0VU at 1.223. Is -8VU = -4dBu????? if so then you'd measure .448 volts....

    http://www.sengpielaudio.com/calculator-db-volt.htm

    This would seem to suggest .489v with 0VU = 1.228v and the "UK VU scale":

    http://www.sengpielaudio.com/MeterComparisonTable.htm
    Thanks, that calculator is just what I was looking for, and yes, I agree with your figures. Thanks very much!

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    Quote Originally Posted by jpmorris View Post
    Sorry to be a pain, but since I'm chronically unable to figure out the necessary calculations myself, perhaps someone else can help.

    This may be a fool's errand, but I'd like to try out some SM900 on my mastering deck, but I don't have a suitable MRL tape for this level and ordering one is unlikely to arrive this year.

    The idea is to try and use my 200nw/M MRL tape to line the machine up for +9 operation, or failing that +5 if it can't handle it properly. The catch is that I don't have an accurate DB meter, so I'm trying to do it by output voltage.

    Right now the machine outputs 1.23v on each channel which I believe is correct for 0vu at +4.

    What voltages should the machine output if it was playing back that same +1 tape, but if the deck was set up for +9, and for +5?
    I got 0.490V and 0.691V, respectively. But that was using 1.23V as +4 dBu. Probably the 0.489V answer is correct if you use 1.228V.

    Cheers,

    Otto

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