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From: klieb@tekigm2.MEN.TEK.COM (Kurt Liebezeit)
Newsgroups: rec.audio,sci.electronics
Subject: Cassette Bias Trim
Keywords: recording cassette bias
Message-ID: <5482@tekigm2.MEN.TEK.COM>
Date: 30 Jul 89 05:01:40 GMT
Sender: klieb@tekigm2.MEN.TEK.COM
Reply-To: klieb@tekigm2.MEN.TEK.COM (Kurt Liebezeit)
Followup-To: sci.electronics
Distribution: usa
Organization: Tektronix, Inc., Vancouver,  WA.
Lines: 78

Some questions from an audio neophyte:

I recently purchased a portable tape deck that happens to have a bias trim
adjustment available.  The owner's manual is somewhat vague on how to use
this feature; it says to monitor the tape during recording of program
material with lots of high frequency sound, and adjust to my liking (it's
a three head deck, by the way).  I've tried this method, A-B'ing between
source and tape while recording some hammered dulcimer music. At one end
of the trim's range the tape was muddy compared to the source, and at the
other end of the range it sounded bright.  It was hard to choose a setting
in the middle that was clearly the best.

I thought there probably ought to be a more scientific way, and so I brought
the recorder into work.  A review article said that the line input expected
70mV.

<Question #1: is it customary to quote rms? pk-pk?>

I hooked up the line in input to a function generator, and set it for 70mV
peak to peak, 440 Hz sine wave.

<Question #2: should I have used a DC-blocking capacitor?>

The VU meter barely registered the tone, though I could hear it easily
through the monitor speaker.  I adjusted the recording level to get a
couple divisions showing on an oscilloscope I hooked up to the line out
jack.  Then I set the function generator to 12.5kHz, still at 70mV.  The
VU meters jumped higher, the tone got louder, and the signal filled the
screen on the scope!  Clearly, the recorder plus my setup did not have
flat frequency response!

<Question #3: the function generator has a 50 ohm output impedance, and I
initially reasoned that it would look like a reasonably ideal source to the
deck inputs.  Perhaps the line in expects a higher output impedance source
that would roll off the high frequency response?  Or does the output of the
line out expect some sort of equalization to be supplied by the amplifier?
Why didn't 70mV produce something close to 0 VU?>

Perhaps someone with experience in this area could suggest a method of
adjusting the bias trim.  Is it possible to measure frequency response with
simpe generators and scopes?  I have access to good T&M equipment thru work;
I suspect that impedance matching is my problem.  I'd also appreciate any
pointers to magazine articles that might help.  The recorder in question
is a Marantz PMD430.

This is my second tape deck, BTW.  I'd like to use both in conjunction with
my amplifier, but the amplifier has a limitation: one can dub from TAPE 1
to TAPE 2, but not vice versa.  The two decks have incompatible noise
reduction systems, so I can't simply dedicate one deck as the source.
Radio Shack sells a little switch box that can mechanically connect either
tape deck to the amp, or dub either way.  There is no buffering in the box,
just switches that connect the line in and line out of each deck to each
other in various ways.

<Question #4: are line in and line out directly compatible?  The review
article says that line in is 70mV, and line out is 400mV.  On the face of
it I would be wary that the line out of deck #1 will swamp the line in of
deck #2.  Is this a problem?>

For convenience, I'd like to monitor the source deck through the amplifier
line in jacks, while the signal also goes to the destination deck (the
amplifier won't allow me to pass the signal from source to amp to destination,
unfortunately).

<Question #5: is it a OK to expect the source deck to drive cables and the
impedances of two destinations (amplifier input and destination deck input)?>

<Question #6: this whole setup will involve longer RCA-RCA cables than I
am used to using. Although they are standard shielded cables, will the build-
up of capacitance cause high-frequency roll-off problems? Are fancy audio
cables significantly better in this respect?>

Thanks in advance for any advice you may have.  I suspect that most people
share my ignorance in these matters, so posting may be more appropriate
than e-mail.

Kurt Liebezeit               (views expressed are those of the author only)
klieb@tekigm2.MEN.TEK.COM           ...!tektronix!tekigm2.MEN.TEK.COM!klieb