What does tape bias mean ?
Fuji Sartono
Hello,
I'm just wondering what Tape Bias means ? There's Tape Bias 70uq and Tape Bias
100uq etc.etc. Would someone mind explaining this to me.
Please mail.
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"The World Is Not Enough"
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Gary L. Sanders
The curve of the magnetic recording on tape of flux vs. amplitude
recorded, is non=linear, so a high-frequency bias signal is applied with
the audio signal added to it. The bias signal must be a pure sine wave
for low distortion, and the amplitude is critical to minimize distortion.
The frequency should be 100kHz or so, so it can be filtered out without
affecting the audio. Record equalization is also applied to the audio
prior to recording, and there is play equalization too. The equalization
is different for different speeds. The time constant of the break
frequency is used to describe the equalization, hence the terms 100uS or
such, which is the time constant. Generally, the bias is adjusted by
recording and playing back simultaneously (with 3-head machines) using a
test signal of 1kHz at -10dB. The bias is first turned down to a minimum,
and then increased until the playback level peaks and further increased
until the playback level falls by 1dB. This is called the overbias point,
and is used to guarantee that the distortion will be minimized without
adversely affecting frequency response.
In VHS video recording, the chroma is recorded at a low frequency of
around 700kHz as an analog signal. The luminance is modulated on a high
frequency FM carrier of 3.8-5.4MHz, which is a constant amplitude signal.
This FM carrier acts as the bias signal to make the chroma recording more
linear, just like in audio recording.
--
Gary L. Sanders, V.P. Engineering
Sanders Media Adventures, Inc.
Video Design Consulting
C-MAC/SMA Active, Hybrid Video Filters
http://ourworld.compuserve.com/homepages/gls_sma
phone/fax (408) 279-5118
Richard Kuschel
I've wanted to say this--Read the FAQ, but to answer your question. Bias
is a high frequency signal applied to tape to linearize the frequency
response of the recording system DURING RECORDING ONLY . Different tape
formulations require different bias currents. The 70 microsecond and 120
microsecond markings do not refer to tape bias, but to tape equalization
standards which are as follow--
Type I tape = Ferric oxide foumulation, is reddish in color, EQ is 120
micro second and requires a low bias current.
Type II Tape= Chromium dioxide Tape(CrO2) or Cobalt formulation Tape is
black in color uses 70 Microsecond EQ and a higher bias current.
Type III is a FerriChrome tape which never caught on and is obsolete
Type IV is "pure metal particle" tape, is black in color, uses 70
microsecond EQ and a very high bias current.
It is a common practice in the cassette duplication industry to make a
tape using Chrome formulations and recorded using high bias but with a
120 microsecond EQ curve, and put the tape in a type I shell so that the
tape can be recorded hotter and will play back correctly on portable and
car stereos that do not have 70 microsecond (Chrome or Metal) playback EQ.
-
RICHARD KUSCHEL EXY...@prodigy.com
"I canna' change the law o'physics."-Scotty
-
RICHARD KUSCHEL EXY...@prodigy.com
"I canna' change the law o'physics."-Scotty
Adrian Bartel
"Gary L. Sanders" <gls...@ix.netcom.com> wrote:
>The curve of the magnetic recording on tape of flux vs. amplitude
>recorded, is non=linear, so a high-frequency bias signal is applied with
>the audio signal added to it.
--snip--
Nice answer. There's also some info in the FAQ - does any post
pointers to the FAQ? - it took me a while to find it.
Cheers
Adrian
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"It does not do to leave a live dragon
out of your calculations." -- Tolkien
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SGreene123
To answer your question more thoroughly, Richard Kuschel correctly stated
that {mixing, actually} a high-frequency signal to the audio input signal
prior to the signal going to tape is required to overcome "coercivity"
effects of recording tape.
During World War 2, just prior to the advent of recording tape, the
Germans used a "wire recorder" to record and then playback allied radio
transmissions, in an effort to decipher coded messages for the purposes of
military intellegence.
The recording medium was actually a ferrous wire run past a crude version
of today's recording and playback heads. It was a bulky and inconvenient
affair. What they had to deal with was a great deal of "crossover"
distortion. Crossover distortion occurs as the signal passes through
"zero volts," or "zero webers",( a measurement of magnetic flux. FYI with
regards to the recording process, flux is expressed in nanowebers.) Due
to the properties of magnetic media, either the media is magnetized, or it
isn't, so the waveform jumps from zero to the lowest level the magnetic
media can become magnetized, or "coerced" into magnetization. The
threshold at which this occurs is somewhat "north" or "south" of zero
nanowebers. Therefore, it is impossible to accurately record a pure sine
wave, as the waveform becomes distorted every time it passes through
"zero" on the recording medium. To overcome this, a very high
(ultrasonic) signal (in some recorders almost 200 KHz) is added to the
audio signal to "bias" the waveform. By so doing, the magnetic media is
seeing "zero" for a much smaller time than it would, "looking" just at the
audio spectrum input signal. Furthermore, the chances of the bias signal
and the audio signal crossing "zero" sumultaneously is so mathematically
remote, that the distortion caused by the coercivity effect drops to an
acceptable level. When the signal is played back, a "bias trap" circuit
is used in series between the playback head and output circuitry to remove
the ultrasonic signal component, and preventing "intermodulation
distortion". As you further your knowledge, you will become more
familiar with frequency and amplitude modulation (am & fm, get it?) and
how these basic concepts figure into a very large portion of signal
communications, including, but certainly not limited to, audio recording.
Good luck!
-Steve Greene
Jay Rose
In article <4sotla$a...@newsbf02.news.aol.com>, sgree...@aol.com
(SGreene123) wrote:
[an excellent explanation of hf tape bias, snipped for space, but including]:
...
|: Therefore, it is impossible to accurately record a pure sine
|: wave, as the waveform becomes distorted every time it passes through
|: "zero" on the recording medium. To overcome this, a very high
|: (ultrasonic) signal (in some recorders almost 200 KHz) is added to the
|: audio signal to "bias" the waveform....
An historic side note: before there were all-in-one chips, cheaper tape
recorders skimped on circuitry and eliminated the bias oscillator.
Instead, they used DC bias techniques -- the same as you'd use to move a
tube or transistor into its best operating range -- and ran the tape head
in an always-positive or always-negative mode. This had the same effect on
zero-crossing... but you can imagine what it did for s/n as the head
became magnetized.
Some early tube recorders (e.g., 1954 AC-operated Ampro) and transistor
ones (mid-60's $30 Lafayette portable) had this arrangement.
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