Advanced Digital Audio  

PCM • Pulse-code modulation (PCM) o Digitally represents an analog signal that is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric (usually binary) code. • Standard digital method used for music audio playback of ALL music CDs and DVD's. o DVDs have a greater volume so they use Linear PCM o Higher sampling rate — up to 24-bit at a sampling rate of 96 kHz • Several varieties in use today, including linear, non-linear, floating point, and differential o Vary in dealing with quantization, and handling values that fall "between" the digital signal bits. • Grandaddy of digital audio and digital video. o Developed by the phone company in the early 1960's for ease of combining line transmissions

Digital Audio • The first 16-bit PCM recording in the United States was made by Thomas Stockham at the Santa Fe Opera in 1976 on a Soundstream recorder. o No mixing stage involved - used unaltered as the master tape for subsequent commercial release o Called DDD because pure digital from recording to disc. o Unmixed analogue recordings are ADD to for a single generation of analogue recording • Used in Electronic keyboards

Advanced Digital Audio  

Digital Video • In the late 70s to the early 80s - time base correctors (TBC) and digital video effects (DVE) units (two of the latter being the Ampex ADO, and the NEC DVE) were introduced that operated by taking a standard analog video input and digitizing it internally. o Made it easier to either correct or enhance the video signal and the digitized and processed video from these units would then be converted back to standard analog video. o Digital video first introduced commercially in 1986 with the Sony D-1 format evolved into the Sony Betacam o 1990 - Computer digital video first appeared in QuickTime, Apple Computer's architecture for time-based and streaming data formats

Codecs • Short for either Coder/Decoder or Compressor/Decompressor o Refers to optimization or sample conversion of audio information o Compresses audio file sizes while either maintaining, decreasing or increasing sample resolution and frequency. o Software CODECs utilized in creation of all of the popular web formats today • mp3, RealAudio, Windows Media, and AIFF, to name a few. o to digital and vice versa • Analogue to Digital Converter (ADC)and DAC, or A/D and D/A Converter

Bit Depth • Describes how many bits a digital recording or digital device uses. o 16-bit recorders produce digital files with a bit depth of 16 o More bits capture quieter data more accurately.

Advanced Digital Audio  

o 8 bits make up 1 Byte of storage, 16 bits - two bytes, 24 bits use 3 bytes of storage.

Quantization • The process of converting a continuous range of values into a finite range of discreet values. o Function of analog-to-digital converters o Creates a series of digital values to represent the original analog signal. • The bit depth (number of bits available) determines the accuracy and quality of the quantized value. o Think of as digital rounding up of a number since you can't have 1/2 in a binary system.

Audio Quantization • Digital audio takes an analog waveform (i.e. sound waves) and converts it to a series of individual samples, each of which has an amplitude value. o The range of possible amplitude levels are defined by the bit depth • 8-bit quantization = 256 possible values; 16 bit quantization = 65,536 possible values, etc. o Important because if you add an extra bit to a word length -the possible combinations of values are DOUBLED in order to round up the level • The level of the audio has to be rounded to the nearest value allowed in a bit. o 16 bit audio has 65,536 possible values, 24 bit audio has 16,777,216 possible values. o Gained 256 TIMES the accuracy of the word lengths (samples). More dynamic range and clarity of signal. • Can also mean the process of correcting the timing of a musical performance. o The music track is analyzed and stretched in time so that beats are evenly distributed, eliminating timing errors. o Some manufacturers refer to quantizing features as autocorrect.

Advanced Digital Audio  

Video & Image Quantization • Digital image quantization is the process of determining which parts of an image can be discarded or consolidated with minimal subjective loss. o Image quantization is inherently "lossy" meaning that the image quality is reduced due to the loss of some information. o Color quantization is a method of reducing the number of colors required to represent an image. o Converting a photograph to GIF format reduces the number of colors to 256. o For image scanners, quantization is the process of forcing similar gray levels to the same level as a result of limited tonal resolution. Usually seen in the shadow areas of scanned images.

Sample Rate • How many times per second the source material is being "sampled" or recorded. o Sample- a snapshot of sound o Determines the upper limit or ceiling of audio bandwidth that can be digitized. o The number of bits in each sample determines the theoretical maximum dynamic range of the audio data regardless of sample rate. • Like movies, if the samples are not played back fast enough your ears will hear the gap. o Highest frequency which is accurately sampled is 1/2 the sample rate. • Called the Nyquist frequency - named for the scientist who developed the math - known as the Nyquist theory. o Cd's - Highest frequency - 22.050 khz, sample rate - 44.1khz • In general, the higher the sample rate, the better the sound quality. o Best sample rate depend on use and application, equipment, and amount of storage available (more rate = more storage) o CDs - 44.1 kHz sample rate

Advanced Digital Audio  

o DAT/DVD recorders - 48 kHz. o Various codecs like MP3, WMA use lower rates for maximum efficiency.

Difference Between File Format and Codec • The codec does the encoding and decoding of raw audio data • The audio format is the specific type of file where the data itself is stored. Multiple codecs can be supported, like AVI. o Uncompressed Audio •

WAV, AIFF Lossless Compressed Audio



FLAC, Monkey's Audio (APE), WavPack (WV), Shorten (SHN), lossless Windows Media Audio (WMA), lossless Apple format



Lossy Compression Formats MP3, MP4, Ogg Vorbis, Musepack (MPC), AAC, lossy Windows Media Audio (WMA), ATRAC (Sony).

Quality of Recording High •

Uncompressed



WAV - Standard CD quality – best for archiving original recording o

Large, approx. 10 MB per minute.

o

Can store any combination of bitrates (sampling rates).

o

Encodes all sounds and total silence with the same number of bits per unit of time

o

minutes of music is the same file size as 2 minutes of silence

o

Need lots of processing power

o

Lossless Compression - similar to zip files for audio

o

FLAC, MPC, APE, Lossless WMA, Apple -2:1 ratio

Advanced Digital Audio  

Low •

Compressed o



 

Called "Lossy" due to removal of information

Mp3 best example o

Variable quality

o

Range from 1/4 to 1/10th original file size

o

Small file sizes make for easy storage and distribution