What does bias eq mean?

[Kevin]

What does the bias 120 microsecond EQ mean on audio cassettes mean? Thanks,
Kevin.

[dB King]

 
Kevin wrote:

What does the bias 120 microsecond EQ mean on audio cassettes mean?  Thanks,
Kevin.

That is the Record Bias Equalization which the tape deck is set to, according to tape type.
The 120µS "normal" tapes are used for recording voice.  Music material should
be recorded with the 70µS "hi-bias" tapes.  I use metal bias tapes for critical recording, but when I went shopping for metal tape recently, I noticed it's becoming harder to find locally.  :(

The reason for equalization in the deck is to have the record amplifier boost high and low ends of the freq. range following a curve for a given tape type.  The different tape types require different bias current levels at the head in order to properly align the oxide particles on the tape.  There are tape type switches on some tape decks.  On other decks the switching is done automatically -- there are switches which "read" the hole configuration on the cassette.  If you look at the tab end of a metal tape shell, a hi-bias shell and a normal-bias shell and compare them, you'll see what I mean.
By contrast to the record head, there's a fixed amount of bias current applied to the erase head.
 
 

P.S.  remove one "n" from addy when emailing replies...

[Gary Pearcey]

Kevin wrote in message <7bhb3b$oqh$1...@remarQ.com>...

>What does the bias 120 microsecond EQ mean on audio cassettes mean?

Bias and EQ are two different things.

To help counteract hiss on cassette recordings, manufacturers
boost the highs on recordings. Upon playback, the tape player attenuates the
highs an equal amount.
The 120 refers to the time constant of the EQ network. Most cassettes use
120 microsecond pre-emphasis (EQ) but metal tapes generally use 70
microsecond.

Now as for bias...
Tape particles do not like to have their magnetic state changed. If you try
to record just sound on a magnetic
tape you get distortion. An early trick was to magnetize the tape slightly
with trickle DC applied to the record head. It cut distortion but is also
tended to erase all the high frequencies. Then they tried a high frequency
tone, a tone outside of human hearing. This works pretty good but will also
erase highs if applied too strong.
AC bias (ultrasonic tone) is adjusted so just enough is applied to reduce
distortion but not enough to erase
the high frequencies. Modern recorders often continuously adjust bias based
upon the highs in the recorded
material. Different tape types (oxide, Cro2, metal) require different
levels of bias.

Usually bias and EQ are adjusted automatically based upon tabs cut into the
cassette shell.