AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Week 9 - Consumer Media and Digital Systems |
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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 May 02, 2005 3:58 am |
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Rico1210
Joined: 03 Aug 2004
Posts: 39
Location: Newcastle, UK |
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Hi,
answers for this week.
Describe and explain consumer media:
Compact disc
The early 80's saw the introduction of the CD(Compact Disc) which was a massive hit as a consumer product and is still the most widely used listening format. A standard CD can hold up to 74 minutes of 16bit digital audio at a frequency of 44.1kHz. In the 90's the CD progressed into the personal computer industry as the CD-ROM (Read Only Memory), with the capability to hold 700MB of data. CD's are widely used in the personal computer industry as storage media and are considered to be a safe option for long-term storage. Advancements in technology have seen CD re-writers appear as standard with many PC builds, allowing the user to create their own musical compilations aswell as transfer data for storage or transit. The CD-R has become the most widely used digital media in both music and personal computer industries. A CD-Recordable WORM(Write Once Read Many) disk can only be recorded once. Once a recording has been made on a WORM disk it can't be edited or erased. CD's also come with re-writeable capabilities for frequent use.
DVD-Video, as related to digital audio
Digital Audio in video recording systems is recorded in stereo at a sample rate of 48KHz. The sample rate of 48 kHz is adopted from the entire film, video and broadcast industry and also DVD is based on the standard of 48/96 kHz. DVD's use the MPEG2 perceptual coding technique to reduce the bit rate of a signal and improve the representation. The perceptual coding removes the parts of video and audio that can't be recognised by the human senses, therefore reducing it in size which enables hours of video and audio to be recorded onto a disc.
DVD-Audio, SACD and similar media of current relevance
DVD audio offers at least 74 minutes of very high quality, surround sound, plus additional features that are not available on CDs. These features include text, pictures and video content. In addition the disc can accommodate the same audio encoded as Dolby Digital for playing on existing DVD-Video players.
A Super Audio Compact Disc(SACD) is a high resolution audio CD that uses Direct Stream Digital(DSD) to encode audio. DSD digitally encodes the audio information as 1-bit data words sampled at 2,822,400 times per second(2822.4kHz) . SACD can support high-resolution stereo recordings as well as high-resolution multi-channel(up to 6) audio channels. SACD discs can be played back on dedicated SACD players or combination SACD/DVD-Video players. Certain SACD discs feature a conventional audio CD layer which can be played back by any CD player or computer with a CD-ROM drive.
Mastering
The process of creating a DVD master, or DVD authoring is as follows: The first step is to plan how the various elements of the project are going to work together. The source materials to be used are then gathered, these may include video or stereo/multichannel audio plus menus. The source material is then encoded into a format that is compliant with DVD, and navigational instructions are added for end-user control. The encoded material and navigational instructions are then multiplexed into a DVD-compliant stream, which is then burned to a DVD video or DVD Audio master.
Manufacturing
CDs or DVDs are manufactured from a master disc. Once the mastering process is completed, the outcome is a master disc that all copies will be produced from. There are a few different methods of writing the data to a disc. One type writes the data to the disk by melting spots on a thin layer of metal with a powerful laser. Surface tension creates a hole with a raised edge on the metal layer that can be read by a low-power laser. The laser is reflected by the metal, but is able to pass through the hole. Another method uses a very thin layer of metal on the surface of the disk, which is heated by the laser causing a raised spot to appear. The plastic beneath the metal layer is heated to the point of decomposition producing a gas that raises a bubble on the surface. A third method uses a chemical dye that darkens with the high radiation of a writing laser, creating a pattern of dark areas that can be read by the read laser. Once a pattern has been written to disk using one of the above methods, it is permanent and cannot be reversed.
Physical and optical operation of the player
The data pickup of a CD player is a near-infrared 780nm laser that reads the data off the pits and lands of the disc. The laser shines through the plastic of the CD and reflects off the metal layer back to be read by a photosensor in the head. It reads the start and end of a pit as 1, and all other areas as a 0. The laser in the CD drive refracts through the polybycarbonate layer over the metal film, allowing the laser to be finely focused. The laser can focus 800um at the bottom of the polycarbonate, to 1.7um at the metal surface. The effects of dust and scratches are reduced as the focus width of the laser is reduced when it hits the CD.
Organisation of data and metadata
The data on a CD and DVD is organised in a linear fashion, which means the data is a continuous chain and tracks will be laid after each other. This also determines the order that tracks are played. A hard disk drive or Sony's MiniDisc media organise data in a completely different fashion to CDs and DVDs. The disc is formatted into sectors and data is recorded using a magneto-optical technique, which allows data to be organised or erased very easily. Tracks can be recorded in any order on the disc, in fact tracks themselves do not need to be completely stored in one area of the disc. Half of a track can be recorded in one sector and the other half in another sector somewhere else on the disk. Each track is written with a Table of Contents(TOC) to instruct the reader where to look for the data. This Table of Contents(TOC) is known as metadata. Meta is a prefix that means an underlying definition or description, therefore metadata can be described as data about data. Metadata is the background information which describes the content, quality, condition, and other appropriate characteristics of the data. For instance, the title, subject, author, location, and size of a file is metadata, and the index of a book or the indexes of books in a library are metadata.
Describe and explain digital systems:
System configuration
A digital system may consist of a digital mixing console and a number instruments, drum machines, samplers or synthesisers. Digital Mixing Consoles come in all different sizes. 24, 36, 56, and 72 channel models with moving fader automation and come with a choice of working at 44.1 kHz, 48 kHz, 88.2 kHz, or 96 kHz. The preferred operation being 24-bit/96kHz. They come with auxilary/insert sends for effects processors, surround monitoring capabilities and motor driven, touch sensitive faders. Input channels usually have independent compression and gating/ducking processors for dynamics control, as well as 4-band parametric channel equalizers. Digital mixers usually come with an LCD display, expensive models may have touch screen technology. Digital Consoles often come with a preset library of effects, compression, gating, EQ, I/O patching, and channel setups. User setups can be added to the libraries for instant recall whenever they are needed. Studio Manager software is also usually included, which makes it easy to relate on-screen controls to the corresponding console functions. Digital Mixing Consoles also tend to have a small number of user-defined keys that can be assigned to software control functions. Instruments, drum machines, samplers and synthesisers are connected to the line-level inputs of the channel. Performers microphones are connected to microphone preamplifiers of the channel. Auxiliary/insert sends can be used to route the signal to effects units and processors, or to performers headphones for foldback. Ouptputs can be sent from each channel to multitrack software or a multitrack recorder.
Interconnection and routing
The following are all types of interconnection that can be used in digital systems:
AES/EBU Audio Engineering Society / European Broadcasting Union is the standard digital audio interface for professional equipment that was introduced in 1985. It was introduced to enable professional digital audio equipment to be linked irrespective of origin. AES/EBU is generally implemented using 3 pin XLR connectors and transmits 2 channels on a single cable.
S/PDIF Sony/Phillips Digital Interface is the standard digital audio interface for consumer equipment. It was introduced as a standard of interconnection between consumer digital audio equipment, but also has compatibility with the professional interface. S/PDIF can be be used with an electrical RCA Connector or Optical cable.
MADI Multi-channel Audio Interface is the standard digital interface for multi-channel interconnections. It is an extension of the AES/EBU format that can transmit 56 channels on a coaxial cable. MADI requires a master synchronisation clock.
SDI and SDTI Serial Digital Interface is the standard for digital video transmission. SDI uses standard coaxial cables that are generally used in Television installations. An SDI signal can contain up to four independent digital audio signals, aswell as the video signal.
Digital audio can be transferred between devices via USB (Universal Serial Bus)
IEE1394 is another name for Firewire technology. The interface IEEE1394 has a bandwidth of 400 Mbit per second, which is a lot better than USB and comparable to SCSI.
Computer systems interfaces. The digital interface to computer systems is represented by the connections on the sound card. These can be optical connectors or RCA connectors.
Synchronisation
It is very important to have equipment working in synchronisation in a digital system. Synchronisation enables equipment/instruments to be started, stopped and played together at the same time. One of the pieces of equipment will be the master that all other equipment in the system will follow. The followers are known as slaves, as they slave to the master machine's clock. The following standards of synchronisation can be used in a digital system:
SMPTE - Society of Motion Picture and Television Engineers is based on real time measured in hours, minutes and seconds.
MTC - MIDI TimeCode is a MIDI interpretation of the traditional SMPTE (Society of Motion Picture and Television Engineers) protocol.
MIDI Clock and SPPs - Song Position Pointer represents the position in a stored song, in terms of MIDI beats from the start.
FSK - Frequency Shift Keying is a system which records a series of electronic tones. It is used to connect drum machines together with an audio recorder.
Clocking
It is often preferred to use a Master Clock to synchronise all equipment in a digital audio system. The Master Clock synchronisation system of a digital signal chain controls each individual component from a master clock. This stops any accumulation of sample frequency jitter if the clock was transferred sequentially through the chain of equipment.
Sample rate conversion
Digital Audio is simply an alternative way of transmitting a waveform. The information is transmitted using binary code at separate intervals rather than as a complete waveform. This is known as sampling. The samples in a digital system are actually a representation of an analog waveform. Some authorities prefer to use the term numerical audio as this appears to be a contradiction. If a system uses a sample rate of 44.1kHz, this means that there are 44100 samples per second taken from a waveform. These samples are converted to binary to be transmitted then converted back to analogue to reconstruct the waveform. The higher the sample rate, the more accurate the representation of the analogue waveform will be.
SMPTE/EBU timecode in digital audio systems
SMPTE - Society of Motion Picture and Television Engineers is based on real time measured in hours, minutes and seconds. SMPTE/EBU is the standard system used to synchronise digital audio equipment. The information in a digital SMPTE timecode signal is held in binary format. Within the word the numbers of the SMPTE timecode frame address are in binary coded decimal representation. Binary Coded Decimal(BCD) is a method of representing decimal numbers using binary codes. Each decimal digit occupies four bits of a binary code. For instance, 0000 = 0 in decimal, 0001 = 1, 0010 = 2, 0011 = 3 etc.
MTC in digital audio systems
MIDI TimeCode is a means of synchronising MIDI-controlled equipment to a real time reference. MIDI TimeCode is a way of transferring longitudinal timecode (LTC) around a MIDI system. Longitudinal TimeCode is a different format to MIDI messaging, therefore the signal must be converted into a MIDI compatible format ie. status byte followed by data bytes
Reliable synchronisation of audio is not possible using MIDI timecode(MTC) because it does not contain a clock pulse. MTC consists of messages sent every quarter-frame that build up over two frames into a complete frame address. MIDI timecode is not as reliable as SMPTE timecode. If MIDI TimeCode is mixed on the same cable as musical data, it will cause delay(or latency) in the timing of the notes. This is because MIDI TimeCode will take up a sizeable chunk of the bandwidth which will enable less musical data messages to be transmitted simultaneously. |
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Sun May 08, 2005 5:24 pm |
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