HOW TO USE THE CALIBRATION TAPE

[mep]

Maybe I missed it, but I sure did look.  Are there any instructions on how to use the calibration tape?  The Otari MX-55 manual says they assume you want everything calibrated to +4dBm and gives instructions on how to do that using the calibration tape.  Is that the right answer?  I received 6 tapes yesterday, and so far all I have done is set my machine's flux to 250 and set the equalization to IEC and let 'er rip.  Judging by the Otari VU meters reading the playback head with the 3 tapes I have played so far, all appears right in the world.  I also have really good readings on my Ampex 350 VU meters.

[docb]

We did a brief instruction in past newsletters, which are not very easy to find:

Part One
http://www.tapeproject.com/August2007/August2007.htm

Part two
http://campaign-archive.com/?u=4f23616eda4b05232b75ece4e&id=1J2ghkjcZI&e=71aa28f943

[mep]

Thanks Doc, I appreciate the links.  What is not clear to me is that if I end up having to tweak my deck in order to get 0 on the VU meters at all the different frequencies, will that only effect the IEC playback or will it have changed my NAB playback as well?  For the here and now, when I make a recording and play it back, I believe it is very accurate to the source.  And this is true using various different tape formulations.  I would hate to make any adjustments that would change something for the worse when I play back tapes I have already made.  Any thoughts on this would be appreciated.

[ironbut]

How the calibration effects all playback on a machine depends on the machine itself and whether you're using the internal playback electronics of that machine or not.

If you're using the stock playback electronics of any machine, you should check first to see if you have settings for IEC and NAB or just IEC or (quite often) just NAB. If you have a switch that allows you to choose between these two different playback equalization curves then that switch is connecting one of two circuits (either active or passive). So when you do adjustments to one circuit, it doesn't effect the other. Some machines have an actual circuit board that has to be exchanged to switch from IEC to NAB. Others with microprocessor controls can be switched with the touch of a button.
In any case, NAB and IEC are different enough that a second circuit is needed.

[mep]

My Otari MX-55 has a switch for IEC or NAB.  The manual doesn't address performing calibrations for each standard which made me wonder if I adjusted the calibration for IEC if it would somehow affect the NAB settings.  Since I don't see where there are different trim pots to adjust for IEC vice NAB settings, I can't help but think if you change those settings it will affect both IEC and NAB.

[docb]

If our tape is used for physical head alignment only and the IEC playback EQ setting is used that head adjustment will be valid for either EQ. If you are using the tone tape to tweak some adjustments like trimpots in the EQ circuit you really should have a tone tape for each EQ you are interested in.

[ironbut]

All the setting may not be adjustable. You may just have fixed resistors for some or all of the eq. The only way to be sure is to switch from one to the other and see what goes in and out of the circuit. The NAB should have a high frequency and a low frequency filter while the IEC just has a high frequency filter.
Also, if you're happy with the sound of both Eq's, there's no reason that you have to tweak this unless you change heads or something.

[mep]

OK, here is the deal.  I joined the TP under the limited subscription deal and I have received all 6 of my tapes.  I thought the purpose of the calibration tape was to set up your deck so that it will be set as closely as possible to the deck that actually made the tape in terms of playback levels at discrete points across the audio band for the type of tape used as well as azimuth alignment of the playback head.

I haven?t made my way through all 6 tapes yet as I just received them on Monday night.  So far, I am very pleased with what I am hearing.  I can?t imagine the midrange or high end being any better than what I am hearing right now nor the bass on TP-08 and TP-09.  My favorite tape so far is TP-09.  When Mose Allison sings Seventh Son, it sounds like you are there watching him sing. The acoustic bass throughout TP-09 just sounds right.  It always surprises me how much bass energy comes from an upright acoustic bass.  I have only listened to Side A of TP-03 and TP-05 and I haven?t heard any bass drum whacks yet that knocked me over if there is supposed to be any (and don?t get me wrong, there is bottom end there and I am not complaining-I am just not familiar with either one of these albums).  If there is some extreme bottom end on side A of either TP-03 or TP-05, please tell me which cut to listen for.  And I am only saying this as I want to understand if I am missing something which would dictate the necessity of going through the calibration process.   The bottom end on TP-09 sounds fantastic.  Ditto for TP-08.  The top end punch on TP-05 is amazing.

The bottom line is that my deck sounds so good with recordings I make and playback and so far with the TP tapes that I am loathe to change anything, especially if changes I make on the IEC side would cause changes on the NAB side.  When I looked at the calibration procedure in the manual for the MX-55, I saw nothing to indicate there are different trim pots for IEC and NAB although it seems there should be.  Maybe I overlooked it and I will study it again.  Part of being an audiophile is being slightly neurotic.  You would hate to think there is possibly more on these tapes than what you are hearing if you only did some tweaking.   Based on what I am hearing so far, it?s hard to imagine it could get much better.

[docb]

Mark, it will do no harm to put the tone tape on and see what the response is. Without doing this we can't really advise on what your machine's frequency performance is. Just put the tone tape on, set the playback eq to IEC, adjust the level controls so the VU meters read 0dB on the first 1kHz tone on both channels, and then let us know what dB you read for each of the following tones of 10kHz, 15kHz, 90 Hz, 60 Hz and 30 Hz. If there are any frequencies that are reading higher or lower than 0dB tell us how much and we can advise on what might be done to improve the machine's performance.

[High and Outside]

The first newsletter linked above is no longer available online (so if you have one, put it up on eBay for big bucks! Vintage! Extremely Rare!!)

So I am posting the text of it here, slightly edited:

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.

Before you begin, the tape machine must be in good mechanical condition. You can never achieve proper playback unless the tape path is working as it was designed, without excessive wear of any parts, including the heads. Additionally everything must be scrupulously clean.

The tape contains tones for setting levels, the EQ curve and azimuth. The levels and EQ are electrical adjustments. Azimuth is a physical adjustment of the relationship between the playback head and the tape. The test tape also has two sections for checking polarity.

You will need meters for doing the basic setup of level and EQ. An oscilloscope is highly recommended for adjusting azimuth, and necessary for using the polarity tone. A popper is necessary to take advantage of the polarity pops. This will be explained in Part 2, the polarity section of these instructions. If your tape machine has VU meters these may be accurate enough for the level and EQ adjustments. If not, you will have to use an external meter.

In most parts of the procedure, it is important that the steps be done in a certain order, as each adjustment builds on one which came before it. So please pay close attention to the sequence of the steps and perform them in the order described. A few of the adjustments interact with others, so occasionally you will be directed to back up and repeat a few steps.

Consult the manual for your machine, and make sure you know where the adjustment points are for level, high frequency EQ, low frequency EQ, and azimuth.

Now, with the preliminaries out of the way, let's begin.

Your test tape is supplied tails out, so please place it on the takeup side of the machine, which is generally on the right. Thread it as required for your machine and rewind it to the beginning. Play the first tone, which is 1Khz. Adjust the output level control to "0" VU or any other convenient reading on your meter.

The second tone is 10Khz. Play it and adjust the high frequency equalizer to the same level you previously set at 1Khz. (This is an example of the order of adjustments being significant. You are setting the HF EQ in relation to the level you set at 1Khz, so obviously the 1Khz must be set first. The same is true when you set the Low Frequency EQ.)

Now it's time to check azimuth. Ideally you will have the two channels feeding an oscilloscope in X-Y mode. Play the 15Khz tone and adjust the azimuth for the highest and steadiest reading on the two VU meters. Be careful—you should not have to move the adjustment very far. If it seems that you have to move it quite a ways, or if the optimum is hard to find, stop and figure out why. Enlist the aid of a technician if necessary. You don't want to get so far out of adjustment that you wind up on one of the false peaks. Then looking at the Lissajous pattern on the oscilloscope, fine adjust the azimuth until the pattern resembles a straight line.

Azimuth can affect the previous adjustments, so wind back to the beginning and repeat the 1KHZ and 10Khz tones, tweaking if necessary. While playing the 10Khz tone, check the Lissajous pattern on your scope. If all is well, it will look as good as the trace at 15Khz did. If it's way off, it is an indication that you got the azimuth onto a false peak (since the two tones are not harmonically related their false peaks won't be at the same place.)

We have provided three frequencies for adjusting the Low Frequency EQ. This is because there are always inconsistencies in the LF range, commonly known as head bumps. Having these three frequencies will allow you to find the best compromise for your machine. As a starting point play the second LF tone, 60 Hz, and set the LF adjustment for the same level you previously set at 1Khz. Then play all three LF tones, noting their readings. If it suggests you may be able to get it flatter overall, try trimming it again, and check the readings of all three tones. Continue until you are satisfied that you have the flattest LF response that you can achieve with this machine.

This completes the alignment for frequency response and azimuth. If you are not going to be checking polarity at this time, 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.



Part 2 will appear in the next issue, and cover the use of the polarity pops included on the Tape Project Alignment Tape.

[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