17Digitizing Sound Sampling The selection of the sampling rate If limiting of hearing is 20 kHz, a minimum rate of 40 kHz is required by the Sampling Theorem.The sampling rate of audio CDs is 44.1 kHz22.05 kHz is commonly used for Internet kHz for speechDAT (digital audio tape): 48 kHz
18Digitizing Sound Quantization It’s usually quantization levels for CD audio16 bitsUnder-sampling a pure sine waveAn analogue signal will be coarsely approximated by samples that jump between just a few quantized valuesDitheringWhen a small amount of random noise is added to the analogue signal before sampling
30Audacity Recording Click on the red Record button to begin recording. Click on the blue Pause button to pause the recording. Press it again to continue.Click on the yellow Stop button to cease recording. The cursor will return to its previous position, before the recording was started.
37Digitizing Sound Recording and Importing Sound Sampling rate and sampling sizeIf level of signal is too low, then resulting recording will be quiet.If level is too high, clipping will occur.FigGain control can be used to alter level.Automatic gain control
38CompressionUse a bank of filters to split signal into bands of frequencies; 32 bands are commonly used.The average signal level in each band is calculated, and using these values and a psycho-acoustical model, a masking level for each band is computed.
39Compression MPEG Audio 3 layers Layer 1: 192 kbps for each channelLayer 2: 128 kbps for each channelLayer 3: 64 kbps for each channelMP3 = MPEG-1 Layer 3 compression rate = 10:1
40frequency specified (kHz) CompressionBlock Diagram for MP3Input audio samples are fed into the encoder.The mapping creates a filtered and subsampled representation of the input audio stream.A psychoacoustic model creates a set of data to control the quantiser and coding.sampling_frequencyfrequency specified (kHz)'00'44.1'01'48'10'32'11'reserved
41CompressionThe quantiser and coding block creates a set of coding symbols from the mapped input samples.The block 'frame packing' assembles the actual bitstream from the output data of the other blocks, and adds other information (e.g. error correction) if necessary.
42CompressionBlock Diagram for MP3SMR: signal-to-mask ratio