Some Cassette Gremlins
One reader tells me that, although he
sets the recording levels on his cassette deck so that the signal peaks at 0dB, he still
finds the output is very low and very noisy. To achieve a level approximately the same as
the source, he has to peak at +7 to +10dB. This doesn't seem to result in high distortion
levels, but something is obviously wrong.
I suspect that miscalibrated meters are at fault. If so, he
could simply continue to make recordings that peak at +7, but I wouldn't advise it. The
cassette system works within fairly close tolerances, and ideal recordings require levels
to be set as high as possible to achieve maximum signal-to-noise performance, but not so
high as to introduce gross distortion. It's a delicate balance at the best of times; in
this case, reliance on guesswork may produce recordings that are acceptable, but they're
unlikely to take full advantage of the tape deck's capabilities. A trip back to the
manufacturer may be necessary, but this should pay dividends in better recordings.
Several cassette decks a friend owns exhibit the same
annoying problem: During quiet passages, the heavy beat of the music on the other side of
the tape is clearly audible in the right channel. Tapes made on friends' machines do the
same thing, while tapes he makes sound fine on other systems.
To some extent, this sort of leakage is inevitable because
the tracks on a cassette are very close together, and low-frequency information has a
tendency to spill onto the next track slightly. The inner tracks on the tape correspond to
the right channel in each direction, and so such crosstalk is likely to occur there.
Normally, the levels are low enough that the correct signal masks the spurious one, but it
can be audible when the "forward" signal is quiet and the "backward"
one loud.
Since the effect in this case is not limited to one
cassette deck, however, I suspect that something else in the system is to blame. I have
encountered one situation where such leakage became annoyingly prominent. The owner of the
system in question was fond of lots of bass, and not only had powerful subwoofers but
habitually equalized the low-frequency portion of the sound to truly gut-wrenching levels.
In this case, the phantom bass became much more audible than it would have been had the
frequency response been closer to flat.
To preserve older recordings, another correspondent has
been copying them onto cassettes. On one of them, he can clearly hear the sound of his dog
barking during the quiet passages of a Beethoven sonata. That seemed impossible, as there
was no microphone, but the fact remained that the barking was on the tape.
In fact, he did have a pretty effective microphone feeding
into the system. A vinyl record, particularly if it's not in firm contact with the
turntable platter, can be very sensitive to airborne sounds, and a phono cartridge, which
doesn’t care whether the oscillations it detects are caused by the record groove or
the physical movement of the record itself, easily picks this up. Warped discs are
particularly prone to this effect, as large parts of them are floating in air. Turntables
that only support the record at a few points can cause problems of this sort as well.
The ideal situation is for the record to be supported in
the playing area, but not at the raised edge or label area. If the phenomenon persists
even with the right sort of mat, there are clamps or weights designed to stabilize
records. The dust cover should be closed as well.
As a last resort, put the dog out when dubbing.
...Ian G. Masters
ian@mastersonaudio.com
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