Sound Recording and Reproduction

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2

Phonograph Recording

The phonograph uses a different type of analog technology, in which the waveform physically exists on the surface of a record. To make a phonograph recording, electrical signals from a microphone are sent to a small cutting tool resting within a magnetic field. The electrical signals influence the magnetic field and cause the cutting tool to move accordingly. When a soft plastic material or a soft metal is passed beneath the cutting tool, a groove is cut in the material. This groove reflects the mechanical oscillations of the cutting tool and is a replica of the waveform of the recorded sound. The groove begins at the outer edge of the record and spirals inward toward the center.

A record is played by placing it on a turntable. A special needle, called a stylus, rides in the groove of the record as it is spun underneath the needle at a regulated speed. Long-playing records, or LPs, spin at 33 1/3 revolutions per minute, or rpm, whereas smaller records, called singles, spin at 45 rpm. Part of the stylus rests in a magnetic field. The grooves in the record cause the stylus to move within the magnetic field. These movements are converted into electric energy and sent to loudspeakers.

Monaural and stereophonic records have different types of grooves. In mono records, both the frequency and amplitude of the signal are stored as side movements in the groove. Only one channel is present. In stereo records, two separate channels are present in one groove. The side movements in the groove store the information for one channel and the vertical movements in the groove store information for the other. The grooves are angled to allow a phonograph with a mono stylus to follow the side movements of a stereo record groove. Compatibility between mono and stereo formats was important when stereo phonograph technology was introduced to a mono market in the 1960s. This compatibility allowed listeners to keep their old mono recordings and yet upgrade to the new stereo phonograph.

The phonograph continues to be important today. Once an early form of playback for recordings, the phonograph has become a musical instrument. Mixing together short passages from different phonograph recordings by manually moving turntables is popular with DJs (disc jockeys) in dance clubs to create spontaneous rhythms or musical effects. The phonograph itself can be used for making musical, rhythmic, and percussive sounds, at times by scratching the stylus across the grooves and at other times by dragging the record across the stylus.

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Optical Recording

Optical recording stores sound as a series of light and dark sections on a strip of film. This technique was the first method used to put sound on motion-picture film, and it remains common today. The soundtrack is printed photographically on the film near the sprocket holes. It appears as a narrow strip of varying light and dark areas. As the film is run through a projector, a beam of light is focused on the strip. On the other side of the film, a photosensitive cell records the changes in light intensity as the film moves and converts those changes into an electrical signal. The signal is then sent to loudspeakers. Some movies use a magnetic soundtrack attached to the film, but this process is expensive. The latest optical soundtracks store the sound signals digitally or use the optical signals to synchronize a separate compact disc containing the movie soundtrack. The move toward all-digital motion pictures that are not projected from film may eventually make optical recording obsolete.

B

Digital Recording Systems

Digital recording systems transform the changing values of sound waves into a series of numbers. During playback, the numerical sequence is transformed back into electric energy, which is sent to loudspeakers. Digital data can be stored on CDs, DVDs, videotape, computer memory, computer hard drives, and any other computer-related storage medium, such as a Data DVD or a memory stick.

Digital recording revolves around two concepts: sampling rate, which is the number of times per second that a waveform is measured, or sampled, and quantization, which refers to the numerical size of each sample. Higher fidelity, or sound quality, results with faster sampling rates and increased sizes of samples.

Digital recorders have an internal clock, which allows a recorder to function in evenly spaced segments of time. At every tick of the clock, the device takes a “snapshot” of the waveform. The “snapshot” is then defined as a binary number. This number represents the sound's amplitude at that moment and is a single sample of the waveform. The size of this number, delineated in bits for the number of 1s or 0s, determines how close this amplitude is to the original. The more bits available, the more accurate the value of the “snapshot.” How a sample relates to the preceding and following numbers (samples) ultimately determines the frequency and amplitude of the waveform. The sampling rate is measured in kilohertz (kHz), or thousands of cycles per second. At low sample rates, a digital reproduction will sound incomplete or choppy. A sampling rate of 44,100 samples per second (44.1 kHz) is used for commercial CDs, which are the most popular format for digital music. If the sampling rate is lower than this, the human ear can begin to hear differences between digital and analog sound.

Digital information is stored as a series of 0s and 1s, known as bits (short for binary digits). The number of bits used to represent a digital sample is called the quantization. With a high quantization, a greater range of amplitude values can be represented. A common quantization value is 16 bits per sample, which is the standard for commercial CDs. A 16-digit binary number can represent any of 65,536 different levels of sound, ensuring a high-quality sample. Digital recorders compile lists of samples and store the binary number equivalents in order. To play a digital recording, a computer within a playing device reads the numbers and creates an electronic signal that corresponds to the value of each binary number. When the signals are played in order, the original sound is reproduced. This process is called digital to analog conversion.

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1

Compact Disc (CD) Recording

A CD is encoded with tiny pits that are arranged to represent the binary code of the recorded material. A laser beam is used to read the binary code off of the reflective surface of the disc. The light beam is either reflected back from the flat surface or dispersed when the beam strikes a pit. A sensor records whether the light is reflected or dispersed, and based on these changing values, the CD player reconstructs the original binary code of the recorded sound. Commercial CDs have a sampling rate of 44.1 kHz and a 16-bit bit depth for quantization. These technical specifications for recording CDs are called the “Red Book Audio” standard and are set by the recording industry.

The digital audio format of a CD allows for more than one hour of stereo music (74 minutes). The diameter of a disc is 12.07 cm (4.75 in). The data are recorded from the inside of the disc outwardly in a continuous spiral of tracks.

Recordable CDs allow the consumer to burn their own CDs using their computer’s CD burner or a stand-alone model. For a recordable CD (CD-R) to be playable on a commercial CD player it must conform to the Red Book standard. This requires certain table of contents, indexing, timing, and other information to be encoded along with the music.

CDs can also be used to store digital audio files (music, voice, and other recordings) in other formats, such as MP3 and AAC). These files may or may not be playable on a certain CD player or on a computer, depending on the unit. Digital audio formats are different in sample rate, bit-depth and other important ways that make them incompatible. CDs can also be used as CD-ROMs for digital storage of photographs, film and video footage, software, or any combination of these.

Digital Video Discs appeared in 1996. They have quickly been rethought to be “digital versatile discs” (DVDs) because of their many potential uses. They are a common format for viewing movies and also popular as a format for large multimedia programs for computers and many other uses. DVDs are the same size as CDs but have the capacity to contain much more data. They have a number of levels of storage capacity: for example, the single-layer DVD has a capacity of 4.7 billion bytes, equivalent to seven CDs and the double-sided, dual-layered DVD has a potential to hold 17 billion bytes, the same as 26 CD-ROM discs. For sound recording and reproduction, the DVD has brought the potential of higher quality recordings and more audio channels for surround sound.

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DVD-Audio, Super Audio Compact Disc, DualDisc, and Surround Sound

The DVD-Audio (DVD-A) disc, Super Audio Compact Disc (SACD), and DualDisc formats allow surround-sound recordings to be contained on a disc the same size as the CD. These channels can have improved sound quality from the CD, as “high resolution” sound with sampling rates up to 192 kHz and bit-depths of up to 24-bits are possible. The three formats are not compatible, and each has its attractive qualities. These discs often include content other than music: photos, music videos, lyrics, interviews, links to websites, and additional material.

The DVD-A is formatted to be playable on any commercial DVD player. It can contain 86 minutes of 5.1 surround-sound music at 96 kHz/24-bits in single-layer, and 156 minutes in dual layer. In order to navigate many of its controls, the user must often turn on a video monitor (television). As DVD players are now common in homes, this format is intended for the consumer with a home theater system or a high interest in the quality of their music listening experience. The DVD-A is not compatible with CD players, and some DVD-A discs will not play on some early DVD players. Therefore, you would need to purchase a DVD-A and a CD of the same recording if you want to use it in both ways.

SACDs are intended for the audiophile market, those that seek a very high quality experience when listening to music recordings. It has exceptional sound quality with a potential frequency response of 100 kHz and a loudest-to-softest sound dynamic range of 120 dB, compared to the CD’s 20 kHz frequency response and approximately 45 dB dynamic range. It is typically a hybrid disc that has two layers: one high-density layer and one standard audio CD layer that allows playback on standard CD players. The high-density layer is used for surround sound in high resolution, in a proprietary format called direct stream digital (DSD). This format encodes a digital signal into a 1-bit format at 2.8 MHz sampling rate, resulting in the equivalent of 29-bits at 96 kHz. DVD players must contain a DSD decoder in order to play an SACD. There are some SACDs that only contain high-resolution two-channel recordings, and some that contain only high-resolution surround sound recordings.

DualDisc has a similar principle as the hybrid SACD, in that each “CD” provides material that can be played back on a typical CD player and materials that can be played back in high-resolution on a DVD-Audio player. The DualDisc is different in that it has two sides. One side is Red Book CD format, 44.1 kHz/16-bit and is the same as the standard release of the CD. The other side has a host of possibilities, along with high-resolution 5.1 surround sound mixes of the other side; it can also contain photos, videos, behind-the-scenes footage, and other content. The DualDisc is slightly thicker than a standard CD or DVD. This difference can create problems for individual players, and especially for computer drives.

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