General Category > General Discussion

HOW TO USE THE CALIBRATION TAPE

<< < (3/3)

High and Outside:
And here's part two:

These are the instructions for using the test tape published by The Tape Project. The process is similar to what you would do with an alignment tape from a standards lab, but with one or two differences.
We have included two signals on the test tape which allow you to confirm the correct polarity of your system.

All musical sounds (except some synthesized sounds) are asymmetrical to some degree. Assuming that the recording has been made in a way that preserves all sounds in the same polarity, and assuming that the release format presents them in the correct polarity, you will be able to be sure that they are being reproduced in your listening room at the same polarity with which the sounds were originally produced. (Both of the above assumptions are correctly realized sometimes, and sometimes not.)

The first polarity signal is a tone of approximately 1Khz which has had the bottom of the waveform clipped, while the top of the waveform is intact. Displayed on an oscilloscope the difference is obvious. Check the signal at different points as the signal flows through your chain and you will easily see each reversal of polarity. (If you listen to this tone you may think it sounds distorted. It IS distorted: half of the waveform has been severely clipped.)

If you want to be sure of getting correct polarity all the way through to your speaker output, you are better off getting a popper and using the second signal on the tape. But in the absence of a popper, here is the best you can do with this tone and a 'scope: If you get to the point where your amp connects to the speaker and the tone is IN polarity, then the speaker output should be IN polarity if it is wired correctly. If you see the tone at this point OUT of polarity, reverse the connections to both speakers.

For some years polarity test sets have been made which consist of two parts: a "pop" generator, and a receiver. These are commonly called "Polarity poppers" (or by people who don't value precision in their technical communications, "Phase poppers"). Our second polarity signal on the tape is merely the recorded output of one of these pop generators. Assuming you have one of these polarity test sets (highly recommended, very very useful and only about a hundred bucks) you can just play this portion of the tape and measure the output of the speakers. The number of polarity reversals through the chain is immaterial. When it comes out of the speakers, either it's IN polarity or you reverse the connections at both speaker terminals.

It's really just as straightforward as it seems, except for one thing: measuring the polarity of loudspeakers is fraught with hidden traps. It is beyond the scope of this paper to do an exhaustive review of the technique of using this instrument, but we'll give you two valuable pointers. First hint: hold the receiver in front of one driver, say the midrange driver. Your reading may be unequivocal, or it may bounce back and forth between positive and negative. If it doesn't give you a steady reading, try different positions until you find one that does. Then check the woofer and the tweeter. Again you may have to look for a position that gives you a steady reading. You might expect that all the drivers will give you the same reading, either positive or negative. Your expectations might be confirmed, or you may be in for a rude shock. Some speaker/crossover designs reverse polarity at each crossover point. If you find this to be the case with your speakers, you're on your own as far as deciding which polarity to settle on for the system.

Following these procedures will assure that you are getting the correct polarity when listening to tapes from The Tape Project. But what about your other sources? The first thing to do would be to check the polarity from your other line inputs using both halves of your test set: plug the pop generator into each line input in turn and check it downstream with the receiver. If they are all the same as the reading you got from the tape, that's a good first step. It doesn't guarantee that the components you plug into those inputs follow the polarity conventions, but you can hope.

Wind the tape back to the beginning and hit play. ALWAYS store your test tape (in fact any important tape) flatwound at play speed. This assures that the tension is even within the pack, and that edges are smooth, which will prevent edge damage.

Wimdo:
Maybe a simple question with a more complicated answer...
"Which trimmers to use on the Nagra T to adjust the levels ?"

Thanks for the all the good music on tape !

Regards from Belgium

Navigation

[0] Message Index

[*] Previous page