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SOUND II
Michael Drolet -- 2006
Home Audio Recording


INTRODUCTION:

 

Analog Systems

Until recently, most audio systems were analog.  Although simple to implement, maximising performance required careful design and constant maintenance.

Digital Systems

Digital systems are more complex to implement as they have more components.  The initial design is critical, but they tend to be more stable and require less maintenance.


ANALOG RECORDING:

In analog recording, we set up an analogy between parameters (variables) of the original sound, and parameters of the recording medium.  We vary some parameter of the medium in correspondance to changes in the sound wave.  The two sound parameters we usually use are time and amplitude.

 

Vinyl Disk:

In phonograph recording, changes in the amplitude of the original sound, cause changes in the depth of the groove we cut into the surface of the disk.  The louder the instantaneous amplitude of the sound, the deeper we cut.
Time is represented as distance along the spiral groove.  As the stylus moves from the outside edge toward the centre; the diameter decreases.  Since the disk rotates a constant speed -- 33- 1/3 rpm -- the linear speed decreases near the centre.

Scratches and dirt on the disk surface, disrupt the amplitude analogy; causing noise (pops and crackle) in the playback.  Warping of the vinyl material disrupt the time analogy; causing changes in pitch (warble or wow).  Irregularities in the composition of the vinyl disrupt the amplitude analogy randomly;  causing surface noise.

Film Sound:

In photographic or optical  film sound tracks, changes in amplitude of the original sound cause changes in the width of the clear area of the optical track.  The louder the instantaneous amplitude of the sound, the wider we make the sound track.
Time is represented as distance along the film.  Film speed is constant  -- 18 in./sec for 35 mm. film.

Scratches and dirt on the film surface, disrupt the amplitude analogy; causing noise (pops and crackle) in the playback.  Shrinkage of the film with age disrupt the time analogy; causing changes in pitch (warble).  Inadequecies in the photo-chemical process disrupt both amplitude and time analogies; causing distortion.

Magnetic Recording:

In magnetic recording, changes in amplitude of the original sound cause changes in the strength of the magnetic field stored on the recording tape.  The louder the instantaneous amplitude of the sound, the greater the magnetic field.
Time is represented as distance along the recording tape.  Tape speed is constant -- 7.5 or 15 in./sec for reel to reel;  1- 7/8 in./sec for cassette.
On a professional reel to reel recorder, such as the Revox PR99,  the tape moves from the left supply reel, past the tape guides and head block, between the capstan and pinch roller and onto the right take up reel.

In the recording mode, the erase head erases the previously recorded signal.  The record head records a new signal on the tape; and finally, the playback  (replay) head plays the just recorded signal.  The replay head serves to provide confidence playback to ensure the quality of the recording. Because of the spacing between the record and play heads, there is a noticeable delay in the confidence playback; dependent on tape speed.

In the play mode, the erase and record heads are not used.
The erase, record and playback heads are electomagnets; coils of wire wrapped around pieces of iron (or other magnetic material).  A space in the magnetic material, called the gap; concentrates the magnetic field.  The erase head has a very wide gap to provide the maximum erasure.  The record and play heads have narrower gaps to maximize reponse to high frequencies (short wavelengths).
Stray magnetic fields, from loudspeakers, electric motors or computer monitors can demagnetise (erase) parts of the tape; disrupting the amplitude analogy; causing drops in signal level.  Stretching or shrinking of the plastic tape disrupts the time analogy; causing pitch changes (wow and flutter).  Loss of the magnetic coating (oxide shedding) dsirupts the amplitude analogy; causing drops in signal level (dropouts).  The random variation in magnetic particle size also disrupts the amplitude analogy; cauing tape hiss.

The tape heads, guides, capstan and pinch roller must be cleaned regularly with alcohol to remove glue from splicing tape, finger print oils and wax pencil grease.

The moving magnetic field of the tape can magnetise metal parts in the tape path and these must be periodically de-magnetised; otherwise noise will be added to your tape during each playback.


DIGITAL RECORDING:

In digital recording, rather than setting up an analogy to represent the signal parameters, we represent their instantaneous values as a series of numbers or digits.  These numbers can be stored on tape or disc, then later used to reconstitute the original signal.

In English we call this a digital system, but this is really a misnomer.  It is in fact a numerical sytem.  In French -- numérique.

The signal to be recorded is processed by an Analog to Digital Converter (ADC).

We sample or measure the signal often enough to get a smooth representation of its shape.  Sampling theory dictates that we must sample at twice the highest frequency we want to represent.  For professional audio, we consider the highest frequency to be 20 kHz.

For compact disc, the sampling frequency is 44,1000 times per second or  44.1 kHz.  In other professional systems we use 48 kHz.
This sampling gives a series of pulses whose amplitude changes with the amplitude of the original signal.
The amplitude of each pulse is measured and converted into a binary number.  For CD and DAT, these binary numbers have 16 digiits or bits. 

A 16-bit number can represent 65,536 different levels or steps of amplitude. Digital signal processors and consoles use 24  or 32 bit numbers internally to minimise errors.
These numbers are then stored on tape, or disk.  Extra data is included to permit detection and correction of errors, and to identify the individual recordings.  On playback, the extra information is stripped off.

The sampled numbers are passed to a Digital to Analog (DAC) converter which  reconstitutes the basic shape of the wave. 
As you can see,  the shape is rather jagged, because the individual steps are wider than the original sampling pulses, representing a false time value.  The DAC output is resampled as shown below.
The pulses are passed through a filter which removes all frequencies above 20 kHz.   This restores the smooth shape of the original signal.  This is the signal which appears at the analog output of the recorder or CD player.
The standard parameters for CD Audio are :
  • stereo (2 channels left/right)
  • 44,100 samples per second/channel (44.1 kHz)
  • 16 bits per sample (65,536 steps -- dynamic range 6dB/step, 96 dB total.)