Dale & Lewis Chapter 3 Data Representation

Analog and digital information The real world is continuous and finite, data on computers are finite  need to approximate real-world data for our computational needs Analog data: information represented in a continuous form Digital data: information represented in digital form

Noise in signals

Digitizing a signal Sample the signal in time within discrete levels The pieces are numbered The binary number system is used to represent the numbers n bits can represent 2 n numbers Q: how many bits are needed to represent m numbers? Actual number of bits that can be easily addressed in a computer sets some constraints

Storing sound on media Grooves in a record Analog: a record (45 minutes in an LP) – about 10 3 sound levels (depending on wear) Digital: typically up to 24-bit Pits in a CD

Sampling & reproducing sound – the y axis The higher the number of levels used to sample the y-axis of a waveform, the higher the dynamic range  range of sound volume levels The human ear can distinguish a factor of 10 7 in sound levels, but perception in a concert hall is about 10 4 levels CD quality sound is recorded in 16 bits, though producers tend to compromise quality for “loudness” 4-bit sampling 32-bit 8-bit

Sampling & reproducing sound – the x axis The higher the frequency of sampling intervals the x-axis of a waveform, the better are high-frequency sounds reproduced The human ear generally distinguish frequencies between 20 Hz (rumble) and 20,000 Hz (mosquito buzz) Sounds are sampled at 44,100,00 Hz to −ensure the capture of peaks and valleys in a 20,000 Hz wave −allow some flexibility in the electronics to filter out aliasing effects kHz 8 kHz

Audio formats WAV – compressed or not AU – begun with 8 kHz sample rates AIFF – uncompressed and lossless (10 MB per minute) VQF – compressed, marketed as alternative to MP3, but didn’t become popular

MP3 format MPEG-2 Audio Layer III (Moving Pictures Experts Group) Lossy compression algorithm 128 kb/s deemed a sufficiently faithful reproduction, 1/11 th the size of a CD quality audio file “perceptual encoding”, loose information where the listener will likely not notice Can contain audio file metadata in a section of the file (ID3 tag) Uncompressed, Vorbis compression, MP3 compression