files and compression Flashcards
define audio data compression and audio level compression
amount of data in the recorded waveform is reduced for transmission
- data compression reduces the number of bits needed to represent a word length
- makes a file take up less space while retaining the same or perceived information
dynamic range of an audio waveform is reduced
what does each letter mean in terms of value when put in front of Bytes?
K
M
G
T
KB = 1000
MB = million
G = billion
T = trillion
how many bits in a nibble
4
how do you calculate kbps for data compression
no. of channels ( sample rate x bits )
for 16 bit depth the bits would be 16 for 1 second of audio
how do you calculate KB of a storage used by the compressed audio file
no. of channels ( sample rate x bytes )
for 16 bit depth, bytes would be 2 for 1 second of audio
gives an answer in B
then divide by 1000 for KB
give 5 ways in which data can be compressed to make files more manageable
1 lower audio quality, lower kHz, bit depth
2 use non-linear quantisation
3 adaptive differential pulse code modulation (ADPCM)
4 exploit predictability (& redundancy) of audio waveforms
5 take advantage of human hearing using psychoacoustics
describe ADPCM
adaptive differential pulse code modulation
measures the differences between samples which are often smaller then the amplitudes themselves, so fewer bits are needed to encode
more efficient
define lossless compression
compression that allows the audio file to expanded back to a bit for bit file identical to the original file
- looks for patterns and probabilities and so does not need to repeat
- reduces file to 50-60% of original size
- .zip and FLAC files use this
define lossy compression
compression that causes information to be lost, expansion results in something similar but not identical to the original
- 10% of original file
- includes MP3, Ogg Vorbis, AAC
how does MP3 work
takes audio in 26ms chunks and analyses each chunk for its freq and amplitude values.
if they are low, then fewer bits can be assigned to that chunk and so saves data, no loss in perceived sound
the ultimate size of a file is driven by….?
bitrate
how many kbps is PCM sound format of CD audio?
1,411 kbps
pulse code modulation takes 1,411 kilo bits per second
define
CBR
ABR
VBR
constant bitrate - each sample of encoded file must go thru the same number of bits
average bitrate - like CBR but has a reservoir, if number of bits is greater than the average/target, some can be taken from the reservoir
variable bitrate - bitrate is reduced for more silent, low freq parts and saved for more busy parts, less file size
