Re: Measuring Frequency Response of Cassette Tapes

"Chris F." <zappymanNOSPAM@xxxxxxxxxxx> wrote in message
news:466aed30$0$4298$9a566e8b@xxxxxxxxxxxxxxxxxx
I've been fascinated by audio cassettes ever since early childhood, and
today thanks to Ebay I've collected many new-old-stock tapes which are now
considered collectors items. With many different tapes to compare, I'd
like
to do my own frequency response tests, like those printed on the back of
many cassettes. I know that the basic idea is to compare the input signal
to
output signal, and measure the difference in dB at a range of different
frequencies. But all I have for measurement is a DMM, with a range down to
about 10mVAC. Is this enough? How to I convert mVAC to dB? And is it
possible to do this with any accuracy, without spending thousands on
lab-grade test equipment?
Some of the comparisons I'd like to make are of different series of the
same tape (say, TDK SA) to see how they improved over the years, or to
compare the results of the obscure FeCr format to other formats of the day
(and yes I do have a deck with an FeCr setting). If the results are
interesting enough I may eventually post them for others to see.

Hey, everybody needs a hobby.

Here's what's possible. First, you can make frequency response measurements
like the ones on the cassette wrappers: get a piece of graph paper and a
pen, and draw whatever response you like, freehand or with the aid of
drafting tools. What you get will be as realistic as what's on the wrappers;
those responses are created by the marketing department, and bear no
resemblance to engineering data.

Can you make real response measurements without spending a fortune? Yes. The
DMM is almost certainly useless for this purpose, as most DMMs don't have
flat frequency responses across the audio band themselves, and in any case
cassettes have enough variation in level due to dropouts that the numbers
will jump around so much that you won't get any meaningful numbers.

So what you need is a good audio voltmeter and a good audio-frequency signal
generator. You can get both from e-bay, probably cost you about $100-200 for
decent ones. Hewlett-Packard is the name to look for. Oh, you'll need some
sort of oscilloscope to adjust azimuth; you can get real cheap ones for $50.
Total outlay: $150-250.

Now you have your audio voltmeter and oscillator, you can test responses,
right? Well, first you need to know that your test deck is responding
properly, which requires an alignment tape to set the azimuth correctly and
to set the playback frequency response and level (so Dolby tracking won't be
messed up). Once those steps are taken, you're ready to start measuring
frequency responses. You'll do that at about 20dB below the 0dB mark on the
deck's meters, because cassettes can't record the full frequency spectrum at
0. And when you do that, if your deck is without external bias-adjusting
controls, you'll discover that your deck is underbiased for some tapes
(which will have a peak in the highs), overbiased for other tapes (which
will have highs that roll off too early) and just right for others. Rather
than testing the actual characteristics of the tapes, you're really testing
their compatibility with your machine's bias points. Fun but still kind of
pointless.

So you go out and buy a deck with adjustable bias. You set it up carefully
for each tape, and *now* you can make measurements which actually mean
something. Of course, it's likely you'll find most cassettes have a
different frequency response at different parts of the tape. Or the two
sides are different.

Like I said, everybody needs a hobby, and you can spend quite a few
delightful hours at this if you are so inclined. Total outlay to do
meaningful tests: the aforementioned $100-200 for test equipment, maybe $50
for a cheap oscilloscope, and maybe $300 for a clean 3-head Nakamichi with
adjustable bias.

Have fun!

Peace,
Paul


.

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