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dBu vs dBV


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#1 thinkchronicity

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Posted 08 August 2019 - 10:33 AM

Question for those with a signal generator please. Looking at various calibration tests in boombox service manuals,they indicate for example inputting -20dB line in @ 1kHz to check the vu meters. I understand all about the two different scales of reference and how you convert to millivolts...but which one do boombox manuals from '78 to '85 actually use? dBu I'm guessing...
Thanks to anyone who knows!

#2 Superduper

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Posted 08 August 2019 - 11:12 PM

db is a ratio, not a value with 0db being the reference.  The question is what value is 0db?  I would go with .775V (dbu) for audio line-in, although dbV references at 1V which in all honestly, probably isn't going to make that much difference for your boombox calibration anyhow.  In any event, if you have a signal generator, it probably doesn't allow you to change the db reference, especially if you use the internal source, so just choose -20db on your generator and go at it.



#3 thinkchronicity

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Posted 09 August 2019 - 01:06 PM

db is a ratio, not a value with 0db being the reference.  The question is what value is 0db?  I would go with .775V (dbu) for audio line-in, although dbV references at 1V which in all honestly, probably isn't going to make that much difference for your boombox calibration anyhow.  In any event, if you have a signal generator, it probably doesn't allow you to change the db reference, especially if you use the internal source, so just choose -20db on your generator and go at it.


Thanks for replying on this. My SG, just acquired, is about 40 years old and has no dB markings on it, only coarse buttons and a fine knob to vary the voltage. So out with multimeter first I guess, and then hookup to the box. Everywhere I read they bang on about dBV, specifically -10dBV 316mV being the line out voltage for domestic audio stuff, and +4dBu 1.228V being the pro studio out level. But so far it seems that as far as cassette goes they've stuck to dBu as the scale of reference like you suggest. My friend's Marantz deck, which I recalibrated the output for, was specced at 387.5mV, so -6dB(u) from 775mV. I wondered till now why they chose that funny figure..
Bloody nightmare this dB malarkey can be. The essence is simple enough, but then they go and have three systems for VU scales, two ways of measuring flux on CAL tapes, and now this!

#4 thinkchronicity

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Posted 11 August 2019 - 09:22 AM

Update: checked through a few service manuals from the library...

Marantz pms7000 uses dBu

Crown csc950 dBV

Sharp 9494 dBV

Sanyo 9998 dBV

Pioneer sk900 dBu

 

My Sanyo m9990 appears to use 0dB as 1.5V, so going it's own way on that model, unless the voltage is meant to be pk-pk, that would make it the dBV standard.

 

Out of interest, classic era Nakamichis use dBV and my old Akai reel to reel uses dBu, so clearly there's no hard and fast rule. You're at the whim of the service manuals whether they specify what's what.

 

oh well, makes life interesting :-)