Re: Measuring Frequency Response of Cassette Tapes

The deck I'm using is a Sony TC-K670, and it does have adjustable bias.
However it does not have a FeCr setting; for that, I was going to use an old
TC-FX2.
As for alignment, I currently do it by ear using a prerecorded tape. I
once tried the oscilloscope / test tape method, but it did not work because
my scope was not calibrated properly or something. Perhaps using a computer
is the best and cheapest idea - I'll give it a tr.
One other thing.... how does one go about measuring other things like
signal-to-noise ratio, or background noise level?
"Paul Stamler" <pstamlerhell@xxxxxxxxx> wrote in message
news:1LEai.70155$Sa4.2634@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
"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




.

Relevant Pages

  • Re: Sluggish Tape Deck
    ... >>I have a Pioneer CT-F8282 tape deck that acts really sluggish. ... and the idler which drives it. ... >of the swinging arm type, which rolls to the right to drive takeup and FF, ...
    (sci.electronics.repair)
  • Re: 1993 Olds Cutlass Ciera Delco AM/FM/Cassette
    ... The cassette deck of my Delco model 16129923 AM/FM/Cassette deck plays ... had hoped replacing the belt would also solve my tape speed problem, ... that moves the tape head into the cassette might need lubricated. ...
    (sci.electronics.repair)
  • Re: DV Tape Playback Problem
    ... come up when I play other tapes, but I ran my tape cleaner anyway. ... use the firewire or other output on the camera to transfer what's on the tape to a known good support. ... Then get the deck fully serviced. ... This could be due to a problem with the cassette housing, a problem with the camera heads being dirty when you made the recording, or the camera and deck alignment being just out of line with each other enough that a slightly weak signal drops below the acceptable threshold. ...
    (rec.video.desktop)
  • Re: tape deck setup question
    ... >some problems with intermittent channels (rec and play) and took it apart to ... >honest, I hadn't used the deck in quite a while, so it may be the way it ... Tape formulations are such that they are simply not 100% capable of ... It would run a test tone in three basic ranges ...
    (sci.electronics.repair)
  • Re: Measuring Frequency Response of Cassette Tapes
    ... investigating the relative frequency response of various ... tape forumulations in his collection. ... like the Nakamichi T-100, which can measure frequency response, noise, ...
    (rec.audio.pro)