MPEG
Introduction
Digital audio is the result of sound reproduction. It makes use of pulse-code modulation and digital signals. This includes analog to digital conversion and also digital to analog conversion; its storage and transmission. Digital audio came as a result of its usefulness in the recording, manipulation, mass production and the sound distribution. Modern distribution of music across the internet through online stores depends on digital recording and digital compression algorithms. The distribution of audio as data files rather than as physical objects has significantly reduced the cost of distribution, over the time the data has this technology has grown and grown from digital microphone to digital mpeg.
Digital microphone
Sound is definitely in analogue form. This means that the pressure waves in air or other mediums are not grouped into steps which are discrete. The vibrations of sound are not quantized and if their intensity is measured it would be very unlikely to find that it corresponds exactly to any form of regular scale.
In this sense the in the operations of a microphone it has to incorporate some kind of analogue phenomenon to the digital phenomenon converter. This is the form in which the digital microphone should be: It should be a device with a conventional analogue transducer which is connected at close quarters to an onboard converter of analogue to digital. This seems reasonable.
Most of the audio application will have to convert the input audio signal into a digital representation so as to process the information further. Since the real world audio signals are small, the analogue signals which is located at the input of the systems has two separate functions: pre-amplification and the analogue to digital conversion.
System partitioning
Some several factors constrain a cost effective partitioning of blocks which are practical to manufacture and flexible. An optimal partitioning of the blocks can be defined with satisfaction of the mentioned constraints.
User adaptation and customization
Most of the applications require that there is a lot of customization. This can be either in form calibration or real time adaptation and the user customization of the audio signal. This falls into the pre-amplification block or the digital filtering block. The latter can be essential to shape the audio response such that the resulting digital representation is most suitable for more processing. For example in sigma-delta Analog to digital conversion, the decimation filter determines the ratio of signal to noise as well as the bandwidth of the audio. Different filters can result in the hi-fi audio quality or highly compressed signals of voice. Signals that are not wanted like the echo can be suppressed by the through a digital filtering. Therefore having user or control system over this filter is very important. This makes the hard wiring a specific filter inside the ECM impractical. (Pohlmann, 2000).
Overall complexity and standardization
A high number of I/O signals from ECM to a digital signal processing unit can result in a complex interface between the microphone and the system of processing digitally. If the transmission of data in complete words with error correction association, synchronization and other signaling required for robust communication is implemented. A system of this form is said to be more complex, both in the count of pin as well as in its functionality. To find and to agree on a common standard will also delay market acceptable of such a microphone.
This partitioning is chosen is for a digital microphone depending on the above description. Inside the ECM a single cell will implement the preamplifier and the sigma of the delta modulator. The output of this block is an over-sampled single bit stream and the input a signal of clock. The modulators run free: the conversation will start as soon as the signal of the clock is present and stop when the clock is low. This figure shows a block diagram of this system.
Application
Digital microphone is applicable to many audio applications. Since the design and the manufacturing of this ECM is not considerably different from conventional JFET ECM, high volume, cost sensitive applications can be addressed. An application which can benefit from this technology is. (Pohlmann, 2000)
Digital mobile communication devices
These devices can benefit from this ECM due to the robust nature of the interface. Most of the audio input today suffers from low levels of voltage requiring extensive design effort and external filtering components. When one has high digital signal it eliminates the need for filtering and then it makes it easy for replacement of the microphone with respect to the baseband ASIC and RF section.
The elimination of the ADC inside the baseband IC opens the way to future all the digital baseband processors which take advantage of high performance digital technologies that are sometimes suitable for processing of sensitive analog signal. For the audio processing to be completely digital, a digital speaker driver is needed. Nowadays there is already a trend to use more efficient drivers which take a digitally coded signal from an all-digital baseband.
Block system of an all digital audio system
Digital Mpeg
Reasons for increased focus on MPEG
MPEG is a family of the ISO standards for digital video and audio compression which optimize the match between quality and the requirements of storage. These standards are established and maintained by a group called Moving Picture Expert Group. This word MPEG is a synonym for this group. There is MPEG 1 which is more established and MPEG 2 which is targeted for broadcast-quality in television because it requires significantly higher data transmission rates and is not practical in use of instructions. There is MPEG-4 which has a broader standard for the interaction with the multimedia with the inclusion of graphics and text in addition to video and audio encompassing a greater range of data rates. This remains an emerging standard.
MPEG-1 video and audio (Middleton, 2003).
It provides very acceptable full motion video which is a standard image format with pixels of 352 x 240 which can be rescaled to full screen, at 30 frames per second. It has a stereo audio at data rates up to 1.5 Mbits per second; this permits storing a video which is one hour long in a CD-ROM, or in real-time delivery over a LAN through the connection of internet.
$1· MPEG-1 audio can be used alone, without the video; audio layers 1,2 and 3 refers to the encoding schemes with the degrees of compression.
$1· Software for playing back MPEG is typically included with today’s graphics cards and the web browsers
$1· The tools for conversion and capture of MPEG are now many and cheap.
$1· MPEG is an open standard, not subject to random changes in the marketing strategy of a single firm
The lower bandwidth DAV technologies are less suitable than MPEG for purposes which are more instructional and they are appropriate only when they are dictated by the limitations in capacity of storage or the channels of distribution. They lack the MPEG video quality which includes: small size of image, usually 160×120, with insufficient visual detail to be resized to full screen with low frame rates which result in motion that is jerky and an audio that is tiny sounding. Uncompressed alternatives are unnecessarily wasteful of bandwidth resources; the quality that is perceived is not enough better to justify the high requirements of storage. The higher-bandwidth alternatives such as broadcast-quality MPEG-2 simply exceed the requirements of typical applications, and impose costs that are unnecessary for the distribution and storage. (Leider, 2004).
Reference
Leider, C. N. (2004). Digital audio workstation (1st Ed.) USA: McGraw-Hill/TAB electronics
Middleton, C. (2003). Complete guide to digital audio. USA: Course technology PTR
Pohlmann, K.C. (2000) Principles of digital audio: USA: McGraw-Hill professionals
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