AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Week 9 - Consumer Media |
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Here I am leaving a more open approach for your research. I wish you to take each topic and investigate and provide suitable description for each point. Imagine you are explaining to someone who does not understand the area.
Describe and explain consumer media:
• Compact disc
• DVD-Video, as related to digital audio
• DVD-Audio, SACD and similar media of current relevance
• Mastering
• Manufacturing
• Physical and optical operation of the player
• Organisation of data and metadata
Describe and explain digital systems:
• System configuration
• Interconnection and routing
• Synchronisation
• Clocking
• Sample rate conversion
• SMPTE/EBU timecode in digital audio systems
• MTC in digital audio systems
_________________
It's all in the ears. - Learn the concepts not the software.
Audio Courses is a way into the music business for you
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Mon Oct 24, 2005 8:30 am |
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Polarman
Joined: 24 Jun 2005
Posts: 55
Location: Barbados |
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Hi!
I hope this is the format you wanted on the answers this week...
Describe and explain consumer media:
Compact disc
CD stands for compact disc which is a general term for all formats of CD media. CD formats available on the market now include CD Audio, CD-ROM, CD-ROMXA, VideoCD, CD-I and others (Parsons & Oja, 2000).
A Compact Disc is a plastic (polycarbonate) disc with several layers. The diameter of a CD is about 12 centimetres and is about 120-mm thin. Information on a CD can only be stored in a digital form; even music must be converted to digital information (DATA). The 120-mm disc can hold 74 minutes of audio. There are versions holding 80 and 90 minutes too. The 80-mm discs are used as "CD-singles" or "business-card CDs" and they hold about 20 minutes of audio. (WeetHet, Tips, Tricks and Insight to Computer Technology, 2005)
The laser and its sensor follows the Track (also called helix). This works similar as the record-player, the needle moves through the track until the end of the track has been reached. The track on a CD can be up to 5km in length. An audio CD consists of up to 99 stereo tracks stored using 16-bit PCM (pulse code modulation) coding at a sampling rate of 44.1 kHz. (Wikipedia - The Free Encyclopedia, 2005).
DVD-Video, as related to digital audio
DVD stands for “digital video disk” or “digital versatile disk”. It is a variant of CD technology that was designed to provide enough storage capacity for a full-length movie (Parsons & Oja, 2000).
DVDs are made from a 0.6 mm thick plastic disc coated with a thin (reflective) aluminium layer. Two such discs are glued together to form a 1.2 mm double-sided disc. The physical dimensions of a CD and a DVD are the same—12cm or the mini 8cm. However, a single-layer DVD can hold more data than a CD. It stores 4.7 GB which is roughly seven times as much a standard CD-ROM. The DVD format is 47 percent more efficient with respect to CD-ROM, which uses a "third" error correction layer (Wikipedia – The free encyclopedia, 2005).
The audio data on a DVD movie can be of various formats: PCM, DTS, MP2, or Dolby Digital (AC-3). In countries using the NTSC standard any movie should contain a sound track in (at least) either PCM or Dolby AC-3 formats. Also, any NTSC player must support these two; all the others are optional. This ensures that any standard compatible disc can be played on any standard compatible player (Wikipedia - The free encyclopedia, 2005).
DVDs can contain more than one channel of audio to go together with the video content. Often sound tracks in more than one language track are available. With Various channels of audio from the DVD, the cabling needed to carry the signal to an amplifier or TV can sometimes be a bit frustrating. An optional digital connector for this task is often included in most systems. This is then paired with a similar input on the amplifier. The selected audio signal is sent over the connection in its original format to be decoded by the audio equipment. When playing compact discs, a different signal is sent (DVD Demystified, 2005).
DVD-Audio, SACD and similar media of current relevance
DVD-Audio is a format for delivering high-fidelity audio content on a DVD. It offers various channels, from mono to 5.1 surround sound, at different sampling frequencies and sample rates. The DVD format allows for the inclusion of much more music (when it comes to to total running time and number of songs). It also allows for higher audio quality which is reflected by higher linear sampling rates and higher vertical bit-rates, and/or additional channels for spatial sound reproduction. Inspite of DVD-Audio's superior technical specifications, there is debate as to whether or not the resulting audio improvements are obvious to human ears. Currently, DVD-Audio forms a niche market, probably due to its dependency upon new and relatively expensive equipment (Wikipedia - The free encyclopedia, 2005).
Super Audio CD (SACD) is a read-only optical disc developed by Sony and Philips and introduced in 1999. This disc is designed for the provision of higher fidelity digital audio reproduction than the CD (DVD Demystified, 2005).
The SACD might contain a 2-channel stereo mix, a surround mix or both. The surround mix is usually the 5.1 layout, but it doesn’t necessarily have to be in this format. The old 4.0 format (quadraphonic) will do as well. The correct description for the surround part of a SACD is “multi-channel”. Its own logo “Multi-Ch” is usually printed on the back cover. Three types of SACDs are available: Hybrid, Single layer and Dual Layer (DVD Demystified, 2005).
Mastering
Mastering is the process of preparing and transferring recorded audio to a medium for future duplication. Depending on the intended release format of the final product, the specific medium that receives the mastered audio varies. This medium is then used as the master copy, from which all further production of the audio material will be based (Katz, 2002).
Manufacturing
Manufacturing is when the information is placed on the disc all together with its creation. For this the CD needs sophisticated manufacturing processes and rigorous quality control in order to guarantee a satisfactory produce. The manufacturing process is made up of 3 general steps: 1) premastering; 2) disc mastering and 3) disc replication. Premastering can be done in a properly equipped studio while special equipment, which is found only in disc manufacturing plants, is needed for the latter two (Pohlman, 2005).
Physical and optical operation of the player
When a CD is inserted into the loading port of a CD player, its presence is normally sensed by an optical sensor when the disc blocks the beam from a lead. The voltage on the sensor changes when the beam is blocked which tells the control chip to start the loading motor. The loading motor drives either 1 or 2 rubber coated rollers which pull the disc into the player. When the disc is fully pulled into the mechanism (to a point where the hole in the center of the disc is directly over the spindle table), the rollers will separate and the disc will be clamped down onto the spindle table (spindle). The spindle table is usually mounted directly on the shaft of the spindle motor (the motor which spins the disc) where the disc may be clamped by spring tension or by magnetic attraction. When you the player is told to play the CD, it moves the laser pickup to its innermost position. Power is then applied to the spindle motor and focuses the laser on the layer of the disc where the information is stored. The first information retrieved from the disc is its table of contents (TOC). The TOC has all of the information needed to play the CD, including number of songs, time for each song and total time. It will then play the first track on the disc. As the program on the disc is played, the laser pickup is moved outward. The speed at which the disc is moving, directly above the laser, remains constant this means that the disc spins more slowly as the laser moves outward. A more detailed description of some of the aforementioned functions will follow (www.bcael.com, 2005).
Organisation of data and metadata
Data on CD and DVD is organised with tracks laid after each other. With this the determination of the order of the tracks is determined. However, data on a hard disk drive is organised different to CDs and DVDs where data is recorded with the use of a technique called magneto-optical technique. A hard disk drive is organised by formatting the disc into sectors on different locations on the disk (DAISy, 2005).
Metadata ensures that you can find what you want, even when you've got masses of records. Metadata is information about a particular data set which may describe, for example, how, when, and by whom it was received, created, accessed, and/or modified and how it is formatted. A table of content (TOC), also known as metadata, is written for each track on a CD and DVD (Wikipedia - The free encyclopedia, 2005).
Describe and explain digital systems:
System configuration
System configuration is the process of setting up hardware and software devices and assigning resources to them so that they work together without problems. A properly-configured system will allow avoid bad resource conflict problems, and make it easier to upgrade a system with new equipment in the future. An improperly-configured system will lead to strange errors and problems, bad performance and make upgrading a difficult.
Interconnection and routing
Different types of interconnection are used in digital systems. One of these is AES/EBU. This is a digital audio transfer standard developed by AES and EBU in the early 1980s. The specifications for the standard have been revised twice since then. AES/EBU is the most common alternative to the S/PDIF standard and the most common AES/EBU physical interconnect is AES Type I Balanced - 3 conductor, 110 ohm twisted pair cabling with an XLR connector (Sweetwater Expert Center, 2005).
An extension of the AES/EBU format is MADI (Multi-channel Audio Interface), which is the standard digital interface channel for multi channel audio. It provides the simplest method of transmitting “sample accurate” audio channels over long distances. MADI can also be used to transmit high sample rate audio signals. Technically, MADI keeps all AES/EBU signals in serial, while allowing the sample rate to still vary by +/-12.5%. The MADI format is able to transmit up to 64 audio channels (at standard sample rates of 44.1 or 48 KHz) on a standard 75ohm coaxial or optical fibre cable (Sythax.com, Undated)
Additionally, the S/PDIF (Sony/Philips Digital Interface Format) is considered a consumer format which is mainly based on the AES/EBU standard and the two are compatible in many cases. However, there are the two formats differ in the channel status and user bits. For transmission S/PDIF uses unbalanced high impedance coaxial cables or fiber optic cables (Sweetwater Expert Center, 2005).
Other means to interconnect and route digital systems is via Serial Digital Interface (SDI). This is standardised in ITU-R 656 and SMPTE-259M, and is a digitised video format used for broadcast grade video. It typically uses 75 Ohm BNC coaxial cables (which makes it easily upgradeable from analog video setups, which use the same cables). Using equalisation at the receiver, it is possible to send SDI over 300 metres, but shorter lengths are preferred (Wikipedia - The Free Encyclopedia, 2005).
Synchronisation
All digital audio equipment samples the incoming analog audio at a rate called the sampling rate. Most digital audio equipment generates this rate from an internal clock If the equipment is not interconnected digitally with other digital audio equipment this works fine. However, if it is digitally interconnected, some means must be provided to precisely synchronise the digital equipment's sampling rates so they can transfer audio without distortion. Here are a number of standards that can be used in a digital system. These include SMPTE, MTC, MIDI Clock and SPPs (Song Position Pointer represents the position in a stored song, in terms of MIDI beats from the start) and the Frequency Shift Keying system which records a series of electronic tones. It is used to connect drum machines together with an audio recorder (Peak Audio, 2005).
Clocking
All equipment in a digital audio system is synchronised by a master clock “clocking”. This synchronisation controls all components from a master clock. A stable, accurate, jitter-free clock is a prerequisite for proper A-D conversion (Robjohns, 2003). Digital signals are made up of individual samples and a clock signal. The clock tells the system exactly when each sample starts. The clock should be stable, must run at the proper sample rate, and there must be the only clock driving the system (Towne, 2005).
The major difference between high-end and budget converters is the quality, stability, and consistency of the internal clock circuitry. This is the part that determines when a sample is taken. The interval between the samples must be precise. If the reference clock is unstable then the interval between samples will vary. This problem is known as jitter, and it affects many different aspects of digital audio systems (Robjohns, 2003)
Sample rate conversion
Digital Audio is sound represented by numbers. It is categorized by an audio signal that is processed as binary data. Binary data is information stored in a format used by electronics (zeros and ones, positive/negative, true/false). Digital audio mediums such as CDs, DATs, computer memory, and hard disk recorders all use a binary system to represent an audio signal with numbers (The Sonic Spot, 2002).
A digital audio recording is stored as a set of amplitude (volume) samplings. Each sample represents the amplitude of the audio signal at the time that it was recorded. The amount of time between audio samples determines the quality of the reproduction. This is called the sample rate, which is the rate at which each sample is recorded. The sample rate is usually described in terms of cycles per second (Hertz/Hz) or thousands of cycles per second (Kilohertz/kHz). Compact Discs use a sample rate of 44.1 kHz, so it contains 44,100 samples for every second of digital audio. In fact, it is actually double that because CDs are in stereo so they must contain two channels of audio samples (The Sonic Spot, 2002; Watkinson, 2001
There are various sampling frequencies, but 44.1 and 48 kHz are the most common. If the sampling rate differs between devices, they can normally not be connected. Therefore they need to be synchronised to an exact common frequency. Synchronous sample ate converter converts one rate to another. Sample rate conversion occurs when a digital signal is passed through a D/A converter, the analog signal lowpass filtered, and then passed through a D/A converter operating at different sampling frequency (Pohlman, 2005).
SMPTE/EBU timecode in digital audio systems
SMPTE timecode is a set of cooperating standards to label individual frames of video or film with a timecode defined by the Society of Motion Picture and Television Engineers. Timecodes are added to film, video or audio material, and have also been adapted to synchronize music (Wikipedia - The free encyclopedia, 2005).
Timecode is necessary for editing, but the standards of timecode for digital audio recorders have been troubled by the variety of standards in video. An SMPTE timecode signal can be coded with a data rate of 2.4 kbps. The SMPTE timecode is necessary for NTSC recording. Due to a 0.1% slip between the actual field rate and 60 Hz the SMPTE doesn’t relate easily to digital audio sampling rates. On the other hand, the EBU timecode relates to digital audio sampling rates of 48 kHz, 44.1 kHz and 32 kHz (Watkinson, 2001).
There will be a conflict between the needs to lock the different sampling rates in the system with the need to lock the time codes, if timecode and the sampling rate isn’t synchronised. This synchronisation problem can only be resolved with synchronous timecode (Watkinson, 2001). An SMPTE/EDU timecode reader enables the recorder to operate synchronously with an external video synchronisation signal. Playback starts automatically when licked to the receiver timecode in normal mode. In the rechase mode, the recorder continually chases, keeping synchronisation despite variations in the master clock (Pohlman, 2005).
The time code used in most equipment follows the SMPTE standard for 525/60. The EBU timecode is in essence similar to the SMPTE code except for it being designed for 50 Hz frame rate systems (Watkinson, 2001).
MTC in digital audio systems
MTC simply put, is LTC SMPTE that has been converted to travel down a MIDI cable. This is how DAW and sequencer software is able to "chase" or sync to video. Many devices that may be needed in order to sync will only have MIDI interfaces and will not be able to receive audio timecode. MTC has a number of advantages and disadvantages. The main advantage is that MTC can be used with many devices such as video devices and it is more understandable to non-musicians because it is tied to absolute time (seconds, minutes and frames) and not to Bars and Beats. Conversely, it does not send tempo information. Therefore, MTC cannot be used by devices that are trying to sync to tempo rather that to a particular moment in time. Some devices that need tempo information is the LFO on a synthesizer, or an outboard time delay effect. The MIDI clock would be used instead of MTC if tempo information needs to be sent (audioMIDI.com, 2005).
__________________________________________________________________
SOURCES
audioMIDI.com (2005). Available: http://www.audiomidi.com/. Accessed: October 28, 2005).
BCAEL (2005). Available: http://www.bcae1.com/. Accessed: October 29, 2005.
DAISy: Digital Audio Industrial Supply (2005). Available: http://www.daisy-laser.com/technology/techsacd/techsacd20.htm. Accessed: October 29, 2005.
DVD Demystified – Home of the DVD FAQ. (2005). Available: http://www.dvddemystified.com. Accessed: October 29, 2005.
Katz, R., A. (2002). Mastering Audio: The Art and the Science. Focal Press: Burlington.
Parsons, J.J., & Oja, D. (2000). New Perspectives on Computer Concepts, 4th ed. Thomson Learning: Cambridge.
Peak Audio. (2005). Available: http://www.peakaudio.com/CobraNet/licensee/Sync.html. Accessed: October 29, 2005.
Pohlman, K.C. (2005). Principles of Digital Audio, 5th ed. McGraw Hill: New York.
Robjohns, H. (2003). Digital Clocking Explained. Sound on Sound, Available: http://www.soundonsound.com/sos/Apr03/articles/digitalclocking.asp. Accessed: October 29, 2005.
Synthax.com (Undated). What is MADI (Multi-Channel Audio Digital Interface)?. Available: http://www.synthax.com/MADI_what.html. Accessed: October 30, 2005.
Sweetwater. (2005). Glossary: The Most Comprehensive Music Technology Glossary on the Planet. Available: http://www.sweetwater.com/expert-center/glossary. Accessed: October 28, 2005.
The Sonic Spot. (2002). Available: http://www.sonicspot.com/guide/digitalaudio.html. Accessed: October 27, 2005)
Towne, J. (2005). Digital Audio Basics. Available: http://www.transom.org/tools/basics/200207.digitalbasics.html. Accessed: October 28, 2005.
WeetHet, Tips, Tricks and Insight to Computer Technology, (2005). Available:
http://www.weethet.nl. Accessed: October 29, 2005.
Watkinson, J. (2001). The Art of Digital Audio, 3rd ed. Focal Press: Burlington.
Wikipedia - The Free Encyclopedia (2005). Available: http://en.wikipedia.org. Accessed: October 29, 2005. |
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Sun Oct 30, 2005 4:20 pm |
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rachelh
Joined: 16 Jan 2005
Posts: 35
Location: Trinidad WI |
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WEEK 9 - CONSUMER MEDIA
DESCRIBE AND EXPLAIN CONSUMER MEDIA:
• Compact disc
The Compact Disc or CD is an optical storage medium that incorporates digital signal storage and optical scanning. The disc is 12 cm in diameter; information is stored in pits that are impressed into one side of the disc’s plastic polycarbonate substrate. The data surface is metallized so that the laser beam is reflected which is used to read the data from underneath the disc.
Each disc stores a stereo audio signal, which comprises of 16 bit data words sampled at 44.1 kHz. The channel bit rate, which is the rate from which data is read from the disc, is 4.3218 million bits per second.
A standard CD has a playing time of 74 minutes, 33 seconds; but playing times of over 80 minutes can be achieved by varying CD standards. [1]
• DVD-Video, as related to digital audio
Taken from: http://en.wikipedia.org/wiki/DVD-Video
DVD is an optical disc storage media format that can be used for data storage, including movies with high video and sound quality. DVDs resemble compact discs: their physical dimensions are the same—12cm or the mini 8cm—but they are encoded in a different format and at a much higher density. DVDs contain a file system, called UDF, which is an extension of the ISO 9660 Standard used for CD-ROMs.
A single-layer DVD can store 4.7 Gbyte, which is around seven times as much a standard CD-ROM. A DVD can contain:
· DVD-Video (containing movies (video and sound))
· DVD-Audio (containing high-definition sound)
· DVD-Data (containing data)
The disc medium can be:
· DVD-ROM (read only, manufactured by a press)
· DVD-R/RW (R = Recordable once, RW = ReWritable)
· DVD-RAM (random access rewritable)
· DVD+R/RW (R = Recordable once, RW = ReWritable)
· DVD-R DL (double layer)
· DVD+R DL (double layer)
The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity.
· DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB)
· DVD-9: single sided, double layer, 8.5 GB (7.92 GiB)
· DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)
· DVD-14: double sided, double layer on one side, single layer on other, 13.3 GB (12.3 GiB)
· DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB)
DVD-Video discs require a DVD-drive with a MPEG-2 decoder (e.g. a DVD-player or a DVD computer drive with a software DVD player). Commercial DVD movies are encoded using a combination of MPEG-2 compressed video and audio of varying formats (often multi-channel formats as described below). Typical data rates for DVD movies range from 3–10 Mbit/s, and the bit rate is usually adaptive. The video resolution on NTSC discs is 720 × 480 and on PAL discs is 720 × 576. A high number of audio tracks and/or lots of extra material on the disc will often result in a lower bit rate (and image quality) for the main feature.
• DVD-Audio, SACD and similar media of current relevance
SACD or Super Audio CD standard was developed in 1999 by Sony and Philips to combat the increasing need for more storage than could be attained by the regular CD whose capacity was 680 Mbytes. SACD supports both two channel and multi-channel audio recordings and utilises the Direct Stream Digital DSD coding method {DSD utilises a high sampling frequency while achieving a flat frequency response to 100 kHz and a dynamic range of about 120 dB in the 0 to 20 kHz band.}
SACD has the same dimensions as a regular CD that is; 12 cm diameter and 1.2mm thickness. A standard single-layer SACD disc holds 4.7 GB of data – for two channel stereo this equates to about 10 minutes of playing time. However the capacity varies in accordance with the number of layers being utilised.
The SACD format supports three disc configurations:
1. Single-layer SACD
2. Dual-layer SACD
3. Hybrid- SACD
A SACD disc can hold text and graphics but not video and follows the Enhanced CD or Blue Book standard, SACD itself is known as the Scarlet Book – dubbed after the colour of the book containing its specifications.
In comparison to the DVD-5 disc, SACD holds similarities in the mechanical and optical property specifications, however the data format, logical layout of contents and copy protection measures are not compatible with the DVD format. It is vital to note that SACD format is not playable on DVD standard drives, but, some players are specially designed to play both SACD and DVD discs. [1]
Taken from: http://en.wikipedia.org/wiki/DVD-Audio
DVD-Audio is a format for delivering high-fidelity audio content on a DVD. It offers many channels (from mono to 5.1 surround sound) at various sampling frequencies and sample rates. Compared to the CD format, the much higher capacity DVD format enables the inclusion of either considerably more music (with respect to total running time and quantity of songs) or far higher audio quality (reflected by higher linear sampling rates and higher vertical bit-rates, and/or additional channels for spatial sound reproduction).
Audio on a disc can be 16, 20 or 24 bit, with sampling rates of 44.1, 48, 88.2, 96, 176.4 or 192 kHz. (The highest sampling rates of 176.4 and 192 kHz are limited to two channels.) Different sampling sizes and frequencies can be used on a single disc. Audio is stored on the disc in LPCM format (uncompressed or losslessly compressed with Meridian Lossless Packing). The DVD-Audio player may downmix surround sound to two-channel stereo if the listener does not have a surround sound setup. The downmix capability is limited to two-channel stereo, not to other configurations, such as 4.1. DVD-Audio may also feature menus, text subtitles, still images and slideshows. Inclusion of DVD-Video also is possible. Such discs commonly contain Dolby Digital or DTS versions of the audio (with lossy compression, usually downsampled to lower sampling sizes and frequencies) in the DVD-Video section. This is done to ensure compatibility with DVD-Video players.
A "hybrid" DVD-Audio disc (HDAD) contains a standard audio track (e.g., 24 bit/96 kHz) on one side, playable on any DVD-Video player. The other side contains the same content with a higher sampling rate, such as 24 bit/192 kHz, playable only a compatible DVD-Audio player. High fidelity DVD-Audio is never output in digital format by DVD-Audio players, to discourage digital copying. DVD-Audio players have six analog outputs, one for each audio channel (left, right, center, left surround, right surround, and Subwoofer). DVD-Audio players requires a compatible Surround sound amplifier, with the same six analog inputs, to play program material in surround. Otherwise, it is quite possible to use the format to play stereo recordings at high resolution with an ordinary stereo amplifier.
The introduction of the DVD-Audio format angered many early adopters of the DVD format. While DVD-Audio discs are capable of higher fidelity sound than CDs, there is debate as to whether or not the fidelity above 48 kHz is distinguishable to typical human ears. However, 24 bit recordings at 44.1 kHz and 48 kHz are widely accepted to be of substantial improvement to the 16 bit recordings currently available on Compact Discs. DVD-Audio currently forms a niche market, probably due to its dependency upon new and relatively expensive equipment and a lack of education regarding higher fidelity and multichannel music. DVD-Audio is currently in a format war with SACD. DVD-Audio might find more success in the form of DualDisc and CD/DVD packages than as a standalone format, although newer formats such as HD-DVD and Blu-ray may render the format obsolete.
• Mastering
Pre-mastering is the culmination of the recording process and the prelude to replication. In this process, an audio mater media is prepared before a glass master disc is prepared. This media contains edited version of the media and should be recorded at the highest bit and sample rate possible. In CD mastering, in most cases a photoresist process is used to create the master disc. “A glass plate of about 240 mm in diameter and 6 mm thick comprising of simple float glass is washed in alkali and freon, lapped and polished with an optical polisher”. The plate is then prepared in a clean room with extremely stringent dust filtering, after inspection and cleaning the disc is tested for optical dropouts with a laser, any burs dropouts in reflected intensity are cause for rejection of the plate. To prepare the disc for photoresist mastering, an adhesive is applied followed by a coat of photoresist applied to the spin coating. The depth of the photoresist is critical as it determines the overall pit depth. The plate is cured in an oven then stored with a shelf life of several weeks. The plate is then ready for mastering”. The metallized master disc is transferred to an electroplating room where the plating process produces metal stampers. The disc is made eelectrically conductive by spraying it with a silver or electron-less nickel layer in a process known as Electroforming.
[1]
• Manufacturing
“Mass production of discs can be accomplished with injection of moulding to produce disc substrates- a polycarbonate material being used. Polycarbonate pellets are heated to about 300 degrees Centigrade and in its molten state injected into the mould cavity faced on one side by a metal stamper- hence the disc substrate with pits is created. The centre hole id periodically formed. After moulding is finished a metal layer is placed over the pit surface to provide reflectivity in most cases aluminium is used but silver and gold can also be utilised. The reflection coefficient of this layer inclusive of the polycarbonate substrate us specified to be at least 70 percent. A cold solid target is bombarded with ions which release metal molecules that coat the disc. With the utilisation of high voltages, a discharge is formed between a cathode target and an anode. Metallization may take approximately 3 seconds. The metal layer is then covered with an acrylic layer with a spin coating machine, and cured with an ultraviolet light. This layer protects the metal layer from scratches and oxidation; the label is printed directly upon this layer.
The final step in CD manufacture is inspection and packaging. The finished discs are inspected for continuous and random defects using automated checking procedures. Discs can be scanned to check for physical defects in the substrate such as bubbles or inclusions, missing metallization, and staining to evaluate the replication process. “[1]
• Physical and optical operation of the player
The function of the player is to recover the data that is encoded into the disc medium. The task begins at the laser pickup, which is used to read data. For instance, the data is recovered from a CD via the use of an optical pickup, which moves across the surface of the rotating disc. The optical pickup has the functions of focusing, tracking, and reading data with sub-micron precision. [1]
• Organisation of data and metadata
Metadata is a relatively new digital practice that is supposed to combat the over reliance of compression which is necessary to allow television and radio audio so that they will have widespread reception. It allows the user to define the sonic composition of the signal received.
Essence refers to content such as audio, video, still pictures, graphics as well as text whilst metadata refers to content such as edit lists or other related data, which describes the data. Metadata can hold parameters such as sampling frequency, down mixing, and channels all of which describe how the essence should be decoded. Metadata can be also used to search for essence and contain intellectual property information such as copyright and ownership, which in turn is, needed to access the essence. Metadata can also play an essential role in storing data that provides insight into how certain elements should be assembled – also known as ‘composition’ as well provides information for synchronisation. [1]
DESCRIBE AND EXPLAIN DIGITAL SYSTEMS:
• System configuration
Digital system configuration would be determined by the use; more namely configuration would be implemented by the user at levels, which aim to attain the desired results. Each device can be configured so that it will run at its optimum level or at a pace that relates to the user specifications.
• Interconnection and routing
Digital systems can be interconnected via the use of cables, they can be synchronised together, or daisy chained. The master device in a synchronised system would be in charge of signal routing and device operation. The daisy-chain describes the process by which one MIDI device is linked to the next by re-transmitting data that is received by a device’s MIDI in port to another device through its thru port [or MIDI echo] port. The disadvantages of daisy chaining stem from the chaining itself, which would become cumbersome if many devices are being used – cables would be literally everywhere and it would be hard to decipher where the problem is- if there is a problem in the chain. [2]
• Synchronisation
In digital systems internal or external sync between compatible devices is often obtained by using a clocking pulse that is directly embedded within the digital data line itself. Synchronisation relies on the timing relationship between two or more devices, it follows that the easier method of achieving sync is to have one or more devices [slaves] follow the relative movement of a single transport device [master]. The basic rule is to note that there can only be one master in a synchronised system and an unlimited number of salves. [2]
• Clocking
In digital systems internal or external sync between compatible devices is often obtained by using a clocking pulse that is directly embedded within the digital data line itself. MTC or Musical Instrument Digital Interface Time Code was developed for electronic music studios, project studios and all other production environments to have a cost effective and easily implemented way to translate time code into time-stamped MIDI messages and back. It allows for time code to be distributed throughout the MIDI chain to devices that are able to understand and execute MTC commands. The MIDI Clock is not related to SMPTE time code or any external reference. Ideally it is the built in and often transparent protocol for synchronising all of the timing elements of each MIDI device within a connected system to a master timing clock. This protocol operates by transmitting real time messages over standard MIDI cables. As with all types of synchronisation, one device must be designated the master device in order to provide timing information to which all other slaved devices are locked. MTC is more accurate than the MIDI clock because it is universally accepted under the Society of Motion Picture and Television Engineers protocol. [2]
• Sample rate conversion
Taken from: http://leute.server.de/wilde/resample.html
Three methods of sample rate conversion are:
1. Linear Interpolation: missing samples are linearly interpolated from existing samples.
2. Band-Limited Interpolation: missing samples are interpolated using a Kaiser-windowed sinc function from existing samples.
3. Polyphase Filtering: traditional DSP interpolation and decimation mechanisms implemented efficiently.
The polyphase filter has the best interpolation performance, closely followed by the bandlimited interpolation, both of them leaving linear interpolation far behind.
“Under ideal mathematical conditions, sample-rate conversion from a lower sampling rate to a higher sampling rate creates the original spectrum of the signal at the new sampling rate with no energy in the frequency domain between the original highest frequency (Forig / 2) and the new highest frequency (Fnew / 2), where Forig and Fnew are the orignal and new sampling rates. There is no loss of information from the original signal.
Sample-rate conversion from a higher sampling rate to a lower sampling rate removes signal energy from Fnew / 2 to Forig / 2, thereby reducing the signal content, and creates a new signal at the new sampling rate.
To evaluate different sample-rate conversion techniques, the output of each is compared to this ideal. The key features for up-conversion are:
· Amount of signal energy above Forig / 2 relative to maximum signal energy.
· Amount of original spectrum retained from 0 to Forig / 2. ’
• SMPTE/EBU timecode in digital audio systems
SMPTE abbreviates for the Society of Motion Picture and Television Engineers whilst EBU abbreviates for the European Broadcast Union. SMPTE, much like the Audio Engineering Society and other organizations, provide a coherent place for keeping professionals (in this case television and film audio engineers) up to date with current information as well as formalizing and documenting necessary standards from time to time. It was SMPTE who devised the classical method of measuring intermodulation distortion, but one of their most noteworthy achievements is the formalization and standardization of SMPTE Time Code, which is the standard method of interlocking audio, video and film transports that makes use of a code developed by the Society of Motion Picture and television Engineers [STMPE]
. The use of this STMPE time code allows for the identification of an exact position on a tape or within a media program by assigning a digital address to each specified length. This address code cannot slip, and always retains its accuracy between 1/24th and 1/30th of a second [dependant on the media type and the standards being used]. The specified tape segments are called frames, a term taken from film production. And the time code address is the tag given to every audio and video frame and is represented by an 8 digit code highlighting hours:minutes:seconds:frames i.e. 00:00:00:00. For monochrome video signals the frame rate is 30 frames/sec whilst for colour the rate is 29.97 frames/sec.
EBU time code utilises SMPTE’s 80 bit codeword but differs in that it uses a 25 frame/ sec frame rate. Both monochrome and colour video run at exactly 25 frames/sec hence no drop-frame code is necessary. [2]
• MTC in digital audio systems
MTC or Musical Instrument Digital Interface Time Code was developed for electronic music studios, project studios and all other production environments to have a cost effective and easily implemented way to translate time code into time-stamped MIDI messages and back. It allows for time code to be distributed throughout the MIDI chain to devices that are able to understand and execute MTC commands. [2]
MTC contains an address, which in essence a time based code with commands. MTC does not contain a clock - Timecode is the standard method of interlocking audio, video and film transports that makes use of a code developed by the Society of Motion Picture and television Engineers [STMPE]. The use of this STMPE time code allows for the identification of an exact position on a tape or within a media program by assigning a digital address to each specified length. This address code cannot slip, and always retains its accuracy between 1/24th and 1/30th of a second [dependant on the media type and the standards being used]. The specified tape segments are called frames, a term taken from film production. And the time code address is the tag given to every audio and video frame and is represented by an 8 digit code highlighting hours:minutes:seconds:frames i.e. 00:00:00:00 [2]
MTC comes in all the frame rates of SMPTE/ EBU timecode, which in turn equates to four rates:
I. 30 fps
II. 25 fps
III. 24 fps.
IV. 29.97 fps (30 fps drop frame)
REFERENCE:
1. PRINCIPLES OF DIGITAL AUDIO – KEN C. POHLMAN
2. MODERN RECORDING TECHNIQUES – DM HUBER, R RUNSTEIN |
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