Analog Audio
Analog audio, in the form of the vinyl LP record, the compact cassette, and open-reel-to-reel tape, continues to interest young and gray-haired music enthusiasts.
An understanding of how this stuff works can lead to improving analog & even digital musical enjoyment.
An intro to analog basics begins with the microphone and the speaker.
The Microphone
There are several types of microphones – dynamic, ribbon, condenser, & piezoelectric.
Each converts sound waves to mechanical movement and then to electrical voltage.
I limit comments to the dynamic microphone.
The Dynamic Microphone
The dynamic microphone handles the acoustical-to-mechanical step via a light diaphragm that shadows and responds to sound wave compression/rarefaction (squeeze/expand).
The mechanical-to-electrical step attaches a light coil of wire to the diaphragm.
The diaphragm mounts over and slides the coiled wire into the gap of a magnet-pole assembly.
The magnet-pole-assembly surrounds the attached coil.
Sound waves modulate (compress/rarefact) the diaphragm-coil assembly.
Its modulating magnetic field produces a corresponding modulating voltage.
The voltage, an analog of the sound waves, is used to modulate a speaker cone or feed the input of a recording device.
The Speaker
The speaker is the inverse of the microphone.
It is an electrical-to-mechanical-to-acoustical transducer.
As the microphone, there are several types of speakers – dynamic, ribbon/planar, electrostatic, and piezoelectric. As the microphone, I limit comments to the dynamic design.
The Dynamic Loudspeaker
The dynamic loudspeaker is the prevailing loudspeaker technology.
A tight coil of wire, the voice coil, is connected to an amplifier.
The voice coil, attached to a speaker cone, is placed and aligned in the gap of a magnet-pole assembly.
As the dynamic microphone, the assembly surrounds the attached coil.
The modulating output voltage of an amplifier continually reverses polarity.
Reversing electrical polarity creates a mechanical push/pull reaction to the speaker’s fixed magnet.
The push/pull modulates the speaker cone, reproducing an analog of the original sound waves captured by the microphone.
The Vinyl LP Record
The archival of reproduced sound began with the Edison phonograph and continued through the
20th Century with its ultimate successor, the LP record. The following illustrates how the LP works.
Voltage from a microphone or studio master tape feeds an assembled coil & magnet.
It’s similar to a speaker coil/magnet but attached to a cutting stylus rather than a speaker cone.
The modulating stylus etches a spiral groove of its movement onto a rotating flat circular master engraving.
The groove begins at the outer edge and terminates near the center.
The engraved master is used to create/press vinyl LP copies.
During playback, a turntable’s stylus riding in the groove of the rotating LP record traces the mechanical movement. The stylus modulates an attached small/magnet/coil assembly, which generates a small voltage.
The voltage of the moving magnet/coil recreates an equivalence of the original microphone voltage.
The voltage routed to an amplifier connected to a speaker reproduces the original recorded sound waves.
Stereo From One Groove?
The LP’s V-shape groove provides two modulating side surfaces.
The single stylus connects to two independent magnet/coil assemblies.
The modulating stylus simultaneously traces each surface.
The modulating voltage of each magnet/coil assembly creates the left & right stereo voltages.
The stylus and magnets/coils are housed in the phono cartridge.
Select this Vinyl Recorder animation Link to observe how it works.
Magnetic Tape
Analog magnetic tape was the principal record medium of the last half of the 20th Century.
The most popular formats were open-reel-to-reel and the compact cassette.
The tape recorder magnetically imprints a microphone’s modulating voltage onto a coating of metal oxide glued to the surface of Mylar tape.
The process begins by applying the microphone’s voltage to a coiled wire within the recording head
of the tape recorder. The electrified coil generates a modulating magnetic field that leaves
magnetic imprints on the Mylar tape drawn/pulled across the recording head.
Playback reverses the process.
The magnetically imprinted Mylar tape, drawn/pulled across the coiled wire in a playback head, generates a small voltage.
This voltage is an analog of the microphone’s original voltage.
The voltage is amplified and then routed to a speaker.
Select this Magnet/Academy Link to observe how tape works.
Scroll down their page and wait a moment for the animation to load.
Analog Loose Ends
The Phono-preamp
Vinyl LP record playback requires a phono-preamp
Legacy audio receivers, integrated amplifiers, and preamps include the phono-preamp. However, many current products do not, which results in inadequate volume, weak bass, and distorted high-frequency audio. Many current turntables include a built-in phono-preamp engaged via a sometimes hidden switch.
If absent the phono-preamp must be added as a separate component.
Why is the phono-preamp required?
The phono-preamp solves issues regarding LP manufacture and the phono cartridge playback level.
LP Manufacturing
If left as-is, the master cutting stylus can create a groove so deep and wide that it does not allow playback.
Wider grooves reproduce low-frequency bass.
Too wide and deep can cause the stylus to leap from the groove.
In addition, if left as-is, noise generated by the vinyl surface will distort high-frequency sound.
The Recording Industry Association of America (RIAA) standardized a solution.
• Low-frequencies grooves are reduced/narrowed by a predetermined amount.
This modification allows the stylus to navigate the record groove.
• High-frequency grooves are increased/widened by a predetermined amount.
This modification increases high-frequency volume levels well above surface noise.
The phono-preamp reverses the level adjustments of RIAA modifications.
It restores low & high-frequency audio to its original levels, thus circumventing the manufacturing issues.
The Phono Cartridge
A high-fidelity phono cartridge/stylus voltage is too low to produce a desired volume level.
In addition to implementing the RIAA solution, the phono-preamp increases the voltage/volume to a desired level.
Analog Tape Bias & Equalization
Much as the LP, analog magnetic tape also needs assistance.
It comes in the form of record bias & equalization.
As-is, high-fidelity tape recording cannot maintain linear accurate frequency response.
Adding an AC bias voltage to the input audio signal solves the problem.
I will not take you into a deep dive into how it works.
However, be aware that high-fidelity cassette and open-reel-to-reel decks include different record bias and equalization options for each type of magnetic tape.
The options are manually selected via front panel switches.
Select the tape manufacturer’s specified bias/equalization settings for best results.
That it’s for today
Links for Audio Fun
Paul McGowan
A YouTube audio gold mine.
Michael Fremer on YouTube
The ultimate vinyl expert.
The Absolute Sound
Open Reel Primer
Library Of Congress
Timeline of Sound.
Ed’s AV Handbook
I lifted most of this blog’s content from my AV hobby website.
I use it as a personal easy access reference to review audio/video subjects.
It originates from notes I once used to conduct sales trainings.
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