From: jeh@cmkrnl.com
Newsgroups: sci.electronics
Subject: Re: Cassette recorder questions
Date: 9 Sep 92 19:41:27 PDT
Distribution: usa
Organization: Kernel Mode Consulting, San Diego, CA

In article <1992Sep9.115347.18897@panix.com>, schuster@panix.com (Michael Schuster) writes:
> In article <1992Sep9.021229.20646@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes:
>>(1) Are the counters on cassette tape recorders standardized?
>>That is, do they all read the same numbers for the same positions?
> 
> Not at all.

Unless of course the decks in question have real-time counters which read
out in minutes and seconds.

> 
>>(2) Where can I get a test tape for adjusting alignment and speed?
>>Minimally, it should have a tone at a known frequency recorded on it.
> 
> There are many of these available, ranging from relative inexpensive to
> very expensive. The tapes made by Magnetic Reference Laboratory
> (415-965-8187) are reputed to be the best.
> 
> A more cost effective alternative is the TEAC MTT-114 which is used by many
> Japanese cassette deck manufacturs for azimuth setup at the factory.
> It costs $38 from TEAC's parts department (213-726-0303 x720 or x757).
> It contains a 10 KHz tone.

This is fine for head alignment (actually I think the current model is the
MTT-256U -- that's what they sold me when I called and asked for the -114;
they said the -114 had been replaced by the -256).  For speed calibration
and flutter checks you want the MTT-111N, which contains a 3 kHz tone of
much longer duration than the 10 kHz tone on the head alignment tapes. 
Reason is that the 10 kHz tone is out at the edge of the deck's response and
is recorded at -20 dB besides, so doesn't provide a reliable trigger for a 
freq counter.  Or, at least, the 3 kHz tone is much more reliable.  

> Be aware that to use this properly you need an oscilloscope, and preferably
> one having an x-y mode. However, you can get pretty close using your
> hearing and a couple of tricks.

I disagree.  The best way to set head alignment on a stereo deck is indeed with
a two-channel scope, but in the two-channel display mode, triggering from only
one channel.  For a coarse adjustment you get the levels to be pretty much the
same in the two channels, and then you go for minimum phase difference between
the two.  

It is very tempting to use X-Y mode for the phase adjustment, just like they
taught you in school (Lissajous patterns), and in fact some tape deck service
manuals recommend that.  The idea is that a diagonal line at 45 degrees will
indicate zero phase shift between the channels.  The trouble is that not just
phase differences but also level differences will affect the slope of your
diagonal line.  Most deck's channels will differ in output level by at least a
few percent, perhaps even ten percent or so.  Your scope's channels may differ
a bit too. Unless you do a complete end-to-end alignment of the deck, so that
the output levels of the two channels are identical, the X-Y method will give
misleading results. 

Besides, to my eye, it is lots easier to judge when one row of sine waves
matches up with the other, than it is to judge a 45 degree line. 

Someone in rec.audio once suggested connecting a headphone across the two "hot" 
sides of the deck's headphone output and adjusting azimuth for minimum output.
The problem of differing output levels applies to this method as well.  

If you DO want to do a complete alignment, plan on buying a few more test tapes
(for one, you'll need a Dolby level calibration tape, MTT-150 from Teac),
and be sure you have access to a precision audio generator and to a true-RMS AC
voltmeter that goes down to 10 mV full scale, with bandwidth of at least 200
kHz (for the bias adjustments).  As well as the scope.  

I'd suggest simply taking the deck to a good shop, but finding a good shop is
decidedly non-trivial.  

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
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