cjacek said:
Tim,
How silly it may be after I have a good night's rest is open to debate but here goes: Would it be feasable and I'm not suggesting in any precise way (but inexpensive and workable) to do as "goldtopchas" suggested and run a bunch of test tones (and other sounds) and tweak all parameters (by ear) to more or less callibrate a machine for one's tape choice, less of course the mechanical adjustment, so that the "source" and "from tape" monitoring sounds matched ? No MRL tape and no other tools than a blank tape, service manual and a source of test tones and sounds. A poor man's callibration perhaps ? No laughing please!
*** LONG ANSWER ALERT ***
(short answer in bold below)
Don't worry, a lot of people have a hard time with this wide awake.
The calibration tape is necessary for tape operating level, EQ, and as you mentioned mechanical head alignment. It can be confusing because there are two different levels to deal with and people call them both operating level.
The amplification line level of the machine (what’s going in and coming out) will either be -10 dB or +4 dB (usually). However, the magnetic flux level applied to the tape is the same no mater what the line level is. So for example, a tone recorded @ 0 VU on a semi-pro half-track set for 250 nWb/m will playback at 0 VU on a pro half-track set for 250 nWb/m. It doesn’t matter that one machine has a -10 dB line level and the other has a +4 dB line level.
So, there are two independent systems to be adjusted, though they interact. The key is you can’t determine a deck’s flux level by measuring voltage at the outputs, because that voltage will be (should be) the same for a given machine no matter what the flux level is.
For most semi-pro machines you use a 1k tone @ -10 dB to confirm the signal level coming from the RCA outs is the same as that going into the RCA ins.
After that is established you play a calibration tape with a known flux level (e.g. 250 nWb/m) to make sure the output reads -10 and the VU meters read 0 for that flux level.
If you use a machine of unknown condition to record your own calibration tape, that tape will be off to the degree the machine is out of spec.
Making your own calibration tape is like making your own scale without knowing the weight of the objects you are using to balance the scale.
However, once your machine is calibrated you can make a reference tape for checking and adjusting flux level in the future… but, once your heads become significantly worn from that state you must have a full-track MRL tape again.
If you buy a typical deck off of eBay, all bets are off – it might be at factory spec, but it could be set @ 250, 355, or even 500 nWb/m for +9 tape. Also, if any of the channels are low, you can’t tell if it’s on the record side or the repro side without an MRL tape.
(By the way, if you buy from a knowledgeable seller that knows the machine and spend a little more, you can be hundreds of dollars ahead in the long run. The ending bid price is just the beginning)
Buying or borrowing a standard calibration tape is even more important now since most decks are pre-owned.
~Tim
