Digital Signal Processing Reference
In-Depth Information
8 Compression of Audio Signals to MPEG and
Dolby Digital
8.1 Digital Audio Source Signal
The human ear has a dynamic range of about 140 dB and a hearing band-
width of up to 20 kHz. Therefore high-quality audio signals must match
these characteristics. Before the analog audio signals are sampled and digi-
tized, they have to be band-limited by means of a lowpass filter. Then ana-
log-to-digital conversion is performed at a sampling rate of 32 kHz,
44.1 kHz or 48 kHz (and now also 96 kHz) and with a resolution of at least
16 bits. The 44.1 kHz sampling rate corresponds to that of audio CDs,
48/96 kHz are studio quality. While the 32 kHz sampling frequency is still
provided for in the MPEG standard, it is in fact obsolete. A sampling rate
of 48 kHz at 16 bit resolution yields a data rate of 786 kbit/s per channel,
which means approx. 1.5 Mbit/s for a stereo signal (Fig. 8.1.).
Right
16 bit
A
...768 kbit/s
~1.5 Mbit/s
D
15...20 kHz
Bandwidth
32/44.1/48 kHz
Audio sampling
frequency
Compression
16 bit
Left
A
...768 kbit/s
D
100...400 kbit/s
15...20 kHz
Bandwidth
32/44.1/48 kHz
Audio sampling
frequency
Fig. 8.1.
Digital audio source signal
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