Analogue and Digital Signals Flashcards
Analog
continuously changing waveform
Digital
“Samples” the waveform at specific intervals and describes content in bits
Sampling Rate (3)
- how many times, per second, a digital sample is taken of the analog signal
- determines the digital signal’s accuracy, which is how closely it captures the frequency of the original signal
- The greater the sampling rate, the greater the resolution of the digital copy
Bit Depth (3)
- the number of bits used to describe a sampled voltage level
- Bit depth is also known as “quantization” because it involves assigning a quantity to the signal being measured
- Greater bit depth gives a more accurate representation of the sampled signal.
Pulse Code Modulation (PCM)
the sampling rate of an audio CD is 44.1 kHz
Nyquist-Shannon Sampling Theorem (2)
- an analog signal can be reconstructed if it is encoded using a sampling rate that is greater than twice the highest frequency sampled
- For example, since the range of human hearing extends to 20 kHz, the sampling rate for digital audio should be greater than 40 kHz.
resolution of the converter (4)
- the number of discrete values it can produce over the range of analog values
- values are usually stored electronically in binary form, so the resolution is usually expressed in bits
- the number of discrete values available, or “levels”, is a power of two
- For example, an ADC with a resolution of 8 bits can encode an analog input to one in 256 different levels
least significant bit (LSB) voltage (2)
- Resolution can also be defined electrically, and expressed in volts.
- The minimum change in voltage required to guarantee a change in the output code level
Digital audio bit system
Digital audio uses a 16, 18 or even 24-bit system.
Digital Audio Sampling
the base sampling rate is 44,000 times per second (44KHz), which provides for more than 2 samples to describe the highest frequency encoded by the system = 20 KHz, thus keeping in line with the Shannon-Nyquist theorem.
Mp3 Bit Rate
128-160 Kbit/s
AudioCD Bit Rate
1411.2 Kbit/s
VideoCD Bit Rate
1 Mbit/s
DVD Bit Rate
5 Mbit/s
HDTV Bit Rate
20Mbit/s
Signal Compression (2)
- the more you compress a signal, the more you affect its quality
- If you remove too much data, the effects can be seen in image files and heard in audio files.
Compression and Digital Video
Standard DV (digital video) cameras usually compress at a ratio of 5:1. Some video formats offer much higher compression rates
CODEC (Coder/Decoder)
an electronic device that converts analog signals, such as video and audio signals, into digital form and compresses them to conserve bandwidth on a transmission path.
Container
the structure of a file where the data is stored. It defines how the data is arranged to increase performance and which codecs are used.
Containers
- File structure
- Defines how data is arranged
- MOV, AVI, WMV, MPEG-2
Codecs
- Compression algorithms
- COmpression/ DECompression
Digital Formats (12)
- MP3
- CD
- WAV
- WMA
- MPEG
- WMV
- AVI
- MOV
- ASF
- MJPEG
- DivX
- Sorenson
MP3
MPEG1-Audio Layer 3
CD
Compact Audio Disc
WAV
Waveform Audio Format (Microsoft)
WMA
Windows Media Audio
MPEG
Moving Picture Experts Group
WMV (Windows Media Video)
Collection of Microsoft proprietary video codecs that uses a version of MPEG-4
AVI (Audio Video Interleave)
Commonly contains MJPEG or DivX, but may contain a variety of codecs.
MOV (Quicktime(R))
Quicktime(R)) In most cases, this is used with the locked Apple(R) codec. Quicktime can also contain other codecs such as M-JPEG
ASF (Advanced Systems Format)
Early Microsoft implementation of an MPEG-4 codec- intended primarily for streaming, it is a subset of WMV
MJPEG
Motion-Joint Picture Experts Group
An early encoding method, it creates a JPEG image from each frame of video. Motion effect is achieved when the images are run in series. MJPEG is still found in some consumer digital cameras.
DivX
began as an ASF codec inside an AVI container. Version 4 and later versions are variations of the MPEG-4 codec.
Sorenson
Apple’s proprietary codec. Apple started implementing versions of the MPEG4 codec with the release of QuickTime 6.
MPEG-1
No longer in widespread use
MPEG-2
Lowest and common denominator for most equipment and the most common MPEG codec
MPEG-4
Group of codes of which some are proprietary Microsoft and some are open.
Lossless Compression
- a process that retains the original quality of a file after it has been compressed and decompressed.
- WinZip(R), a popular file compression program, is an example of a program that uses lossless compression.
Lossy Compression
gives an approximation of the original data by eliminating redundant or unnecessary information.
