Acoustics 424

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Acoustics 424Unit IV, Session 3 Electronic
Sound Recording
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4. More Examples of Transducers

• a. Magnetic Recorders

1) Wire Recorder On December 1, 1898, Danish
electrical engineer and inventor Valdemar Poulsen
(1869-1942) patented the first practical magnetic
sound recorder, the 'Telegraphone', using
magnetized piano wire as the recording medium.
A later version of a wire recorder
Poulsens wire recorder 1898
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Poulsen realized a better version of his machine
could be made by using metal or some type of thin
tape. His wire recorder won first prize at the
Grand Prix exposition in Paris in 1900, which was
the same year that he received a U.S. patent for
his invention. However, his invention was almost
lost. He started the American Telegraphone
Company, which sold dictating machines and
telephone recorders. These new products were not
well received by the American public and his
company was forced to shut down and he went into
broadcasting.
During the 1930's, the Germans developed
Poulsen's invention further, in an attempt to
support their war effort. Hitler was very fond
of this technology which he used to great
advantage during World War II. The magnetophone
recorder allowed him to address the entire
country without leaving his home base, also this
proved effective in confusing the Allies, who
could not pinpoint his exact location.
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• Tape Recorder
• The Germans further developed the
  electromagnetic recorder by changing the thin
  steel wire to a thin plastic tape coated with a
  magnetic film.
• On one side of the tape particles of powdered
  iron oxide were deposited. The particles on this
  tape would be magnetized in varying patterns by a
  small electromagnet connected to the audio input
  circuit. When this tape was played back across
  an electromagnet set up to detect the magnetic
  signal, the input sound was reproduced.
• This new invention provided better sound
  quality, and could be erased and reused several
  times.

Captured German Magnetophone 1945
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After victory in WWII, the USA wanted to develop
the recorder for it's own use. The Ampex
corporation of California was commissioned to
build the new recorder. Originally intended for
Government use only, the recorder quickly found
it's way into the entertainment industry. In
1948, the first recorder was put to use to
playback a tape delay performance of a Bing
Crosby concert.
Captured German Magnetophone 1945
Bing Crosby with a home version of the Ampex
Recorder
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How the Tape Recorder Works
The tape itself is actually very simple. It
consists of a thin plastic base material, and
bonded to this base is a coating of ferric oxide
powder. The oxide is normally mixed with a
binder to attach it to the plastic, and it also
includes some sort of dry lubricant to avoid
wearing out the recorder. Iron oxide (FeO) is
the red rust we commonly see. Ferric oxide
(Fe2O3) is another form of iron oxide. (Hematite
is the common name for this substance.) This is
the form of iron oxide typically used in magnetic
recording media.
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• Record Mode

The basic principle of tape recording involves an
electromagnet that exposes the moving oxide tape
to a constantly changing magnetic flux (flow).
The oxide particles act like miniature magnets,
permanently aligning themselves to the
frequencies of the magnetic flux.
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The tape recorder's magnet (record head) is a
very small, circular electromagnet (about the
size of a flattened pea) with a small gap in the
middle. It consists of an iron core wrapped with
wire, as shown in the figure. During recording,
the audio signal is sent through the coil of wire
to create a magnetic field in the core. At the
gap, magnetic flux (flow) forms a fringe pattern
that bridges the gap. This flux is what
magnetizes the oxide on the tape.
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• Playback Mode

During playback, the motion of the tape pulls a
varying magnetic field across the gap. This
creates a varying magnetic field in the core and
therefore a signal in the coil. This signal is
then amplified to drive the speakers.


The head on the left is a bulk erase head used to
wipe the tape clean of signals before recording.
Bulk Erase Head
Play/Record Head
The head in the center functions as both the
record and playback head. In a normal cassette
player, there are actually two small
electromagnets. Together these magnets are about
as wide as one half of the tape's width.
The two heads record the right and left channels
of a stereo program.
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• Playback Mode

Capstan
On the right are the capstan and the pinch roller,
closeup of capstan assembly
Pinch Roller
The capstan revolves at a very precise rate to
pull the tape across the head at exactly the
right speed. The standard speed is 1.875 inches
per second (4.76 cm per second). The roller
simply applies pressure so that the tape is held
tight against the capstan.
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Four Common Types of Magnetic Tape
Type 0 The original ferric-oxide tape. It is
very rarely used now. Type 1 Standard
ferric-oxide tape, also referred to as "normal
bias. Type 2 This is "chrome" or CrO2 tape.
The ferric-oxide particles are mixed with
chromium dioxide. Type 4 This is "metal" tape.
Metallic particles rather than metal-oxide
particles are used in this tape.
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Generally speaking, sound quality improves as you
go from one type to the next, with metal tapes
having the best sound quality. A normal tape deck
cannot record onto a metal tape (the tape deck
must have a setting for metal tapes in order to
record onto them). However, any tape player can
play a metal tape. The controls on the tape deck
let you match the recording bias and signal
strength to the type of tape you are using so
that you get the best sound possible.
Std, Fe,Cr selections
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Bias
The controls on most tape decks today let you
match the recording bias and signal strength to
the type of tape you are using so that you get
the best sound possible. Bias is a special 100
kHz signal that is added to the audio signal
during recording. Early tape recorders simply
applied the raw audio signal to the electromagnet
in the head. This will work, but it produces a
lot of distortion with low frequency sounds.
The bias moves the signal being recorded up
into the linear portion of the tape's
magnetization curve. This movement means that
the tape reproduces the sound recorded on it more
faithfully.
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Dolby Noise Reduction
Dolby NR is a noise reduction system developed by
Dolby Laboratories for use in analog magnetic
tape recording. It works by companding, i.e.
reducing the dynamic range of the sound during
recording and expanding it during playback. It
is not the only system that works in this way,
but it is the most widely used. It was one of
the most important innovations that made high
fidelity practical on cassette tapes which
normally have high noise because of the slow
speed and narrow tape format created initially
for compact voice recorders, and is common on
stereo tape players and recorders to the present
day.
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How Dolby NR Works - Recording
Dolby B (and C which is similar) is a form of
dynamic pre-emphasis. The background hiss of a
tape (white noise) is unnoticeable if it is
masked by a stronger audio signal, especially at
higher frequencies. This is called
psychoacoustic masking. When the tape is
recorded, the amplitude of the signal above 1 kHz
is used to determine how much pre-emphasis to
apply - a low level signal is boosted by 10 dB
(Dolby B) or 20 dB (Dolby C). As the signal
rises in amplitude, less and less pre-emphasis is
applied until at the "Dolby level" (3 VU), no
signal modification is performed.
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How Dolby NR Works - Playback
On playback, the opposite process is applied
(de-emphasis) based on the signal level. Thus,
as the signal level drops, the higher frequencies
are progressively filtered more strongly, which
also filters the constant background noise
level. The two (pre and de-emphasis) processes
cancel each other out as far as the signal is
concerned. During playback, the de-emphasis
curve is applied to the signal and the noise.
After playback de-emphasis is complete, noise in
the output signal is reduced, but the signal
itself is not appreciably affected.
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Dolby Noise Reduction Systems
Several types have been developed A - operates
in four bands providing 10 dB noise reduction
across the full frequency spectrum. It was
intended for professional users. (1966) B - a
single band system providing up to 10 dB noise
reduction on frequencies above 1 kHz. It
provides far less effective noise reduction than
Dolby A by a factor of about 3 db or more.
This is the most common Dolby system used in
most consumer electronics (1968) C - Dolby C
provides up to 20 dB noise reduction in the high
frequency range, but the resulting recordings
sound much worse on equipment that does not have
Dolby C noise reduction. It is only used on
high-end equipment. (1980)
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SR - a much more aggressive noise reduction
approach than Dolby A. It attempts to maximize
the recorded signal at all times using a complex
series of filters that change according to the
input signal. As a result, Dolby SR is much
more expensive to implement than Dolby B or C,
but Dolby SR is capable of providing up to 25 dB
noise reduction in the high frequency range.
Only found on professional recording equipment.
S - The Dolby S system is basically a cut
down version of Dolby SR and uses many of the
same noise reduction techniques. It is capable
of 10 dB of noise reduction at low frequencies
and up to 24 dB of noise reduction at high
frequencies.
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HX HX Pro - This method further increases the
dynamic range of a cassette tape by dynamically
adjusting the level of bias. Dolby developed
this system in 1982 calling it Dolby HX (later
marketed as HX Pro). While not a classic noise
reduction system per se, Dolby HX Pro does
provide a cleaner original recording. It works
by modifying the ultrasonic bias signal, used by
all analog tape decks, to increase the headroom
for high- frequency audio signals. (HX stands
for "headroom extension".) Because tape is
magnetic, it is inherently non-linear in nature,
due to the hysteresis (slow reaction time) of the
magnetic particles. If an analogue signal were
recorded directly onto magnetic tape, its
reproduction would be very distorted due to this
non-linearity. To overcome this, a high
frequency signal is mixed in with the recorded
signal, which "pushes" the envelope of the
signal into the linear region.
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Dolby Noise Reduction - Epilogue
Dolby's analog noise reduction systems, though
still used in some professional applications,
have been made obsolete by the widespread
adoption of digital audio (in the form of compact
discs, MP3s, MiniDiscs, and to a lesser extent
DAT) in the home for entertainment and
professional studios for recording. Dynamic Noise
Reduction (DNR), (often confused with Dolby) is a
playback-only signal processing system that
doesnt require the source material to first be
encoded (it is non-complementary). Thus it can
be used to remove background noise from any audio
signal by as much as 10 dB. DBX noise reduction
system is based on "linear decibel companding" -
compressing the signal on recording and expanding
it on playback. DBX was eclipsed in the general
consumer market by Dolby, and then by digital
sound.