AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Week 2 - Basic Principles |
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Consider the following and then answer the questions below in your own words:
Why Digital?
Outline of Analogue Signal Processing
Analogue audio is a mature technology - all of the great breakthroughs in analogue have been made
Analogue technology is moving towards its limits, governed by the laws of physics.
The law of diminishing returns applies- as short comings in performance are studied, the equipment needed to counteract them becomes complex and costly.
Weaknesses of Analogue
Degradation's cannot be separated from the original signal-
at the end of a system a signal carries the sum of all the degradation's introduced at each stage through which it has passed.
There is a limit to the number of stages through which a signal can be passed before it is useless.
What is Digital Audio?
Digital audio is simply an alternative means of carrying information, although there are a number of ways in which audio can be represented digitally, there is one system, known as Pulse Code Modulation (PCM), which is in universal use.
PCM Key Points
The waveform is not carried by continuos representations, but by measurement at regular intervals (sampling).
The frequency with which samples are taken is called the sampling rate or the sampling frequency Fs.
If every effort is made to rid the sampling clock of Jitter, or time instability, every sample will be made at an exactly even step.
The length of the sample, which will be proportional to the voltage of the audio waveform, is represented by a whole number (quantizing).
By describing audio waveforms numerically, the original information has been expressed in a way which is better able to resist unwanted changes.
The rate at which the voltage is measured (sampled) and the accuracy of the measurement are the only factors which determine the quality, because once a parameter is expressed as a discrete number, a series of such numbers can be conveyed unchanged.
Why Binary?
Binary has only two digits, 1 and 0.
With only two states, there is little chance of error.
These are readily conveyed in switching circuits by an 'off' and an 'on' state.
In a binary number, the digits represent increasing powers of two from the LSB.
The bits represent 1,2,4,8,16, etc.
A multi digit binary number is commonly called a word, and the number of bits in the word is called the wordlength.
A word of 8 bits is called a Byte.
The capacity of memories and storage media is measured in bytes.
The whole number representing the length of the sample is expressed in binary.
The two states change at predetermined times according to some stable clock.
Further points to note
The bandwidth of the system effects slew rate (Fourier).
Noise added to a sloping signal can change the time at which the slicer judges that the level passes through the threshold.
This effect is eliminated when the o/p of the slicer is re-clocked.
However many stages the binary signal passes through it still comes out the same, only later.
Audio samples which are represented by whole numbers can be reliably carried form one place to another by such a scheme, and if the number is correctly received, there has been no loss of information.
Transmission
Two ways binary signals can carry information are:
1. Parallel
2. Serial
When each digit of the binary number is carried on a separate wire it is a parallel transmission.
However, using multiple wire is cumbersome.
A single wire can be used where successive digits from each sample are sent serially. This is the definition of Pulse Code Modulation.
Clearly the clock frequency must now be higher than the sampling rate.
A single high quality audio channel requires around 1 million bits per second.
Data rates needs to be handled economically.
Advantages of Digital Audio compared to Analogue
The quality of reproduction of a well-engineered digital audio system is independent of the medium and depends only on the quality of the conversion processes.
The conversion of audio to the digital domain allows tremendous opportunities which were denied to analogue signals.
Wow, flutter, noise, print-through, drop-outs, intermodulation noise, HF squashing, azimuth error, interchannel phase errors are all history.
When a digital recording is copied, the same numbers appear on the copy: it is not a dub, it is a clone. If the copy is indistinguishable from the original, there has been no generation loss.
Digital recordings take up less space than analogue recordings for the same or better quality. Tape costs are far less and storage costs are reduced.
Digital circuitry costs less to manufacture- switching circuitry which handles binary can be integrated more densely than analogue circuitry. More functionality can be put into the same chip.
Disk drives and memories developed for computers can be put into use in audio products- there seems to be little point in waiting for a tape to wind when a disk head can access data in milliseconds.
High performance manipualtion - the difficulty of locating, and the permanence of an analogue edit make it hardly worth considering when the waveform can be viewed on screen, trimmed and auditioned before making it permanent.
Communication networks developed to handle data can happily carry digital audio over indefinite distances without loss.
Digital equipment can have self-diagnosis programs built in-
the machine points out its own failures.
An organisation will still need maintenance staff, but they will be fewer in number and their skills will be oriented more to systems than devices.
Debates about sound quality are academic; analogue equipment can no longer compete economically.
References and further reading:
1. John Watkinson, "The Art of Digital Audio", Pub. Focal press, 1995
2. Bloom, P.J., "High-quality digital audio in the entertainment industry": an overview of achievements and challenges. IEEE Acoust. Speech Signal Process. Mag., 2, 2-25 (1985)
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1. What are the characteristic of analogue audio?
2. What is digital audio? - include the term PCM.
3. Explain how the binary number system works. Include the terms MSB and LSB along with wordlength.
4. Briefly compare the differences between serial data transfer and parallel data transfer, mention such terms as clock frequency.
5. Outline the history and background of binary code; development of digital audio and the use of binary.
6. Outline the growth of the digital audio industry since the early 80's; - leading bands and pioneering musicians that have exploited the technology, explosion of digital audio.
7. What is quantisation and how many quantising levels are there in a 16 bit digital audio system?
8. What is oversampling?
9. Explain aliasing.
10. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
11. Briefly describe and explain these methods of digital interconnection:
AES/EBU
S/PDIF
SDIF2
MADI
SDI and SDTI
Dolby E
Proprietary interfaces
Digital audio via USB
Digital audio via IEE1394
Computer systems interfaces
Other methods of digital interconnection of current relevance
_________________
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 Mar 14, 2005 6:21 am |
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Rico1210
Joined: 03 Aug 2004
Posts: 39
Location: Newcastle, UK |
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Hi,
Here are my answers to Basic Principles. Text Book is very helpful 
1. What are the characteristics of analogue audio?
Analogue audio conveys information on an infinite scale as opposed to digital audio which takes a sample of the information at set intervals. A value can be found for the recorded signal at any point along it's length.
Analogue audio is degraded the more stages it is passed through. These degradations cannot be separated from the signal, therefore nothing can be done about them. Eventually the signal will become useless the more stages it passes.
If the speed is not constant, a time based error can occur. This time based error cannot be detected in an analogue system as the waveform will still be valid.
2. What is digital audio? - Include the term PCM.
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.
Digital Audio information is transmitted as a serial binary signal known as Pulse Code Modulation (PCM). PCM is the universal system for digital audio representation.
3. Explain how the binary number system works. Include the terms MSB and LSB along with wordlength.
Binary only has two states, on and off represented by 0 and 1. A multi-digit binary number is known as a word, with the number of bits contained in the word being the wordlength. A binary word with a wordlength of eight bits is known as a byte. The name byte is contracted from 'by eight'.
In a binary word, the digits represent an increase by the power 2 from the Least Significant Bit (LSB). The LSB is the binary number farthest right which represents 1, the second bit represents 2, the third bit represents 4, the fourth bit represents 8, and so on. The last number in the word which is situated farthest left is known as the Most Significant bit (MSB).
In terms of memory, an address range can be found by raising two to the power of the wordlength. Therefore a four bit word has sixteen combinations, which can address sixteen locations.
4. Briefly compare the differences between serial data transfer and parallel data transfer, mention such terms as clock frequency.
There are two ways that binary signals can be transmitted, parallel and serial. Parallel transmission carries each digit of the binary word on a seperate wire. Serial transmission carries the binary word all on the same wire and sends the digits in succession. The serial transmission is defined as Pulse Code Modulation (PCM). Serial transmission is the preferred option as the information can be sent all on one wire, whereas parallel transmission can require lots of wires, especially if a long wordlength is being used. When serial transmission is used the clock frequency must be higher than the sample rate.
5. Outline the history and background of binary code; development of digital audio and the use of binary.
Binary code dates back to the 1800's where it was developed in a similar way to morse code, which also only has two states. Binary calculation is based on rules known as ''Boole algebra'' from the name of the English mathematician who formulated them for the binary systems in the 1800s.
The two symbols of binary code are 1 and 0. These are known as a BIT (Binary Digit). The space that the symbol occupies represents the growing powers of 2. Binary is different to the commonly used numerical system of the decimal, which has ten symbols that represent the growing powers of 10.
6. Outline the growth of the digital audio industry since the early 80's; - leading bands and pioneering musicians that have exploited the technology, explosion of digital audio.
In the 1970's the digital audio industry produced technology in the shape of the digital tape recorder or DAT(Digital Audio Tape). The DAT recorder was originally aimed at the consumer market, but did not gain wide recognition. It did however gain recognition in professional studios around the world. MIDI(Musical Instrument Digital Interface) was a very important technolgical advancement that was welcomed by artists and producers as it turned the Personal Computer into a Digital Audio Workstation. Allowing multiple instruments to be connected and controlled. 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. The CD has since been followed by technology such as DVD (Digital Versatile Disc), MP3(MPEG-1 Audio Layer 3), MPEG (Moving Picture Experts Group), DAB radio(Digital Audio Broadcasting), MiniDisc (MD). All of this technology is wqidely used in the music industry by artists and producers.
Had trouble finding pioneering bands etc. that first took advantage of digital technology. Got any links?
7. What is quantisation and how many quantising levels are there in a 16 bit digital audio system?
Quantisation is an approximating process that records a value by rounding up or down to a nearest point. Quantisation is used to convert an analog waveform into a series of numbers. With reference to digital audio, the voltage of the audio waveform is measured at seperate intervals and represented (quantised) as a whole number.
I've read that there are 65,536 quantising levels in a 16 bit digital audio system. I've worked out how to get from 16 to 65,536 (16 x 16 = 256, x 256 = 65536), but i don't know why!
8. What is oversampling?
The oversampling process is when a higher sampling rate is used than is necessary.
Oversampling in Analog to Digital conversion involves sampling audio at a higher rate than the Nyquist rate (twice the highest audio frequency) this can result in better sound quality.
Oversampling in Digital to Analog conversion can be performed in order to convert at a higher sampling rate. New samples need to be calculated and inserted between the Nyquist rate samples. These samples are then converted back to analog at the higher rate which avoids the need for reconstruction filters.
9. Explain aliasing.
Aliasing takes place when the sampling rate is inadequate, or when the frequency is higher than half the sampling frequency. This creates 'spurious' frequencies within the audio band when the higher frequencies are inproperly encoded. These 'frequencies' are totally unmusical errors.
Aliaising can occur for any frequency above one half the sampling rate.
10. Explain the limiting parameters of a hard disk drive that restrict operation in a digital audio workstation.
One of the limiting parameters of a hard disk drive that restricts operation is the speed of the disk. The number of tracks of which a system may be capable depends on how fast the disk can access data, and how fast the rest of the system can process it. Hard disk recording systems can play back much slower than normal speed, but only a little faster because they are restricted by the speed that data can be accessed from the hard disk. It is easy for the data to be read at a slower rate than normal, but reading data faster than normal is difficult and can cause delays or audible errors during playback.
Another limiting factor of a hard disk drive is the size. The size of the hard disk will limit the number of tracks that can be recorded onto the disk and the sample rate of the tracks
It is not advisable to use a hard disk for long-term storage, as they are a mechanical medium that contains perishable parts. If a part of the hard disk e.g. the arm were to seize, then the data contained on the disk would be lost.
11. Briefly describe and explain these methods of digital interconnection:
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.
SDIF2 Sony Digital Interface
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.
Dolby E. 'Dolby E is a digital audio compression technology designed for use by TV broadcast and production professionals in and among their facilities. It allows an AES/EBU audio pair to carry up to eight channels of digital audio and Dolby Digital metadata.'
http://www.transtec.nl/downloads/Pro/dolby/DolbyEFAQ.pdf
Proprietary interfaces. An ADAT digital interface has sync in/out ports that use 9 pin female serial ports for Alesis proprietary sync for timing and machine control.
Digital audio via USB. Digital audio can be transferred between devices via USB (Universal Serial Bus)
Digital audio via IEE1394. 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.
Other methods of digital interconnection of current relevance. Ethernet, ISDN, and internet |
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Sat Mar 19, 2005 1:58 pm |
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AUdIoCoUrSeS
Joined: 31 Oct 2002
Posts: 2014
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| Good Start |
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quote:
Had trouble finding pioneering bands etc. that first took advantage of digital technology. Got any links?
Have a start here http://directory.google.com/Top/Arts/Music/Styles/By_Decade/1980s/ and then start to look at how musicians began to use MIDI equipment to automate sounds and essentially sequence sounds. The Atari computer has an awful lot to answer for.
quote:
I've read that there are 65,536 quantising levels in a 16 bit digital audio system. I've worked out how to get from 16 to 65,536 (16 x 16 = 256, x 256 = 65536), but i don't know why!
what do you not understand about this?
Good start Rico!
_________________
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 Mar 21, 2005 12:13 pm |
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