Archivio Storico:- ex Dipartimento di Musica e Spettacolo - Universita' di Bologna
Diapositiva di PowerPoint
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Note:
Digitisation Procedure Quality Control: Real time sound analysis of the analogue and digitised audio stream is performed during digitisation in order to control the analogue to digital conversion process. For this reason, digital audio workstations provide several useful features such as:
- · automatic detection of start and end position of the audio signal
- · automatic detection of pauses during the recording
- · automatic detection of the noise floor level
- · automatic detection of clicks and impulsive distortions
- · automatic detection of analogue media drop-outs
- · average value of signal to noise ratio of the audio signal
- · average value of frequency bandwidth of the signal
- · average value of stereo correlation
- · average value of level dynamics
Signal parameters extracted by means of digital signal processing as listed above are used to control the sound quality of the digitised waveform. Any errors occuring during digitisation of analogue recordings should be detected on the fly. Standard quality criteria, obtained from long term statistics are matched against actual values measured. A transfer (quality) protocol is usually added to the technical metadata set.
Sample Rates and the 24 – 20 –16 bit Story: Sample rates in professional audio and video environment range from 32 kHz to 96 kHz and 192 kHz (DVD audio), 44.1 kHz and 48 kHz being used most frequently. Other sample rates occasionally used are: 44.056 kHz, 47.952 kHz, 64 kHz, 88.112 kHz, 88.2 kHz, 95.904 kHz and 176.4 kHz.
It is advisable for sound archives and collections to maintain audio workstations capable to select sample rates ± 10% off the nominal values. At least one unit serving arbitrary sample rates between 5 kHz and 48 kHz should be available in order to handle non-standard recordings from several sources; otherwise a sample rate converter as a separate functional unit is necessary.
High end DAWs use accurate, discrete, multi-bit A/D converters. The A/D converters operate at a sample frequency of 192 or 176.4 kHz and employ sophisticated digitally subtracted dither to produce both low noise and distortion components below -120 dB FS, or less than one part per million. The 192/176.4 kHz signal is decimated to 96 or 88.2 kHz, 24 bits using optimised filtering.
In spite of all technology progress CDs are still in 16 bit audio. Several solutions for handling both the 24 and 16 bit domain have been proposed. In order to reduce the 44.1 kHz, 24-bit signal to 16-bits while retaining many 24-bit audio benefits, soft limiters are applied which allow the increase of the peak signal level up to 6 dB without overloading. The peaks are reconstructed when decoded increasing dynamic range by 6 dB. For undecoded playback the units work as standard limiter. Some DAW units provide a low level range extension which gradually increases the gain on low-level signals (approx. starting at -45 dB FS) by 4 dB over a 20 dB range.
As one of several possibilities the final step in the reduction to 16-bits is to add high-frequency weighted dither and round the signal to 16-bit precision. The dither can be applied to the frequency range of 16 kHz to 22.05 kHz leaving the noise floor flat below 16 kHz without influencing the psychoacoustic relevant frequency range for the perception of tonal signals.
Psychoacoustically designed noise shaping filters are controlled by the spectral range of the time varying audio signal. Some audio systems introduce, as part of the final quantisation, a pseudo-random noise hidden code as needed into the LSB of the audio data. The hidden code carries the decimation filter selection and peak detection and low level range parameters. The hidden code is completely inaudible and is only inserted 2-5% of the time, effectively producing 16-bit undecoded playback resolution. The result is an industry standard 44.1 kHz, 16-bit recording which should be compatible with all CD replication equipment and consumer CD players.
Archivio Storico:- ex Dipartimento di Musica e Spettacolo - Universita' di Bologna
