Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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Maintaining Audio Quality
Assuming perfect hardware, it can be shown that this debate comes down entirely
to the audibility of a given anti-aliasing filter design, as is discussed below. Nevertheless,
in a marketing-driven push, the record industry attempted to change the
consumer standard from 44.1 kHz to a higher sampling frequency via DVD-A and
SACD, neither of which succeeded in the mass marketplace. The industry is trying
again with Blu-ray audio and it remains to be seen if they will be more successful
than they were with DVD-A or SACD.
Regardless of whether scientifically accurate testing eventually proves that this is
audibly beneficial, sampling rates higher than 44.1 kHz have no benefit in FM stereo
because the sampling rate of FM stereo is 38 kHz, so the signal must eventually be
lowpass-filtered to 17 kHz or less to prevent aliasing. It is beneficial in DAB, which
typically has 20 kHz audio bandwidth, but offers no benefit at all in AM, whose
bandwidth is no greater than 10 kHz in any country and is often 4.5 kHz.
Some A/D converters have built-in soft clippers that start to act when the input
signal is 3 – 6 dB below full scale. While these can be useful in mastering work, they
have no place in transferring previously mastered recordings (like commercial CD). If
the soft clipper in an A/D converter cannot be defeated, that A/D should not be used
for transfer work.
Dither is random noise that is added to the signal at approximately the level of the
least significant bit. It should be added to the analog signal before the A/D converter,
and to any digital signal before its word length is shortened. Its purpose is to
linearize the digital system by changing what is, in essence, “crossover distortion”
into audibly innocuous random noise. Without dither, any signal falling below the
level of the least significant bit will disappear altogether. Dither will randomly move
this signal through the threshold of the LSB, rendering it audible (though noisy).
Whenever any DSP operation is performed on the signal (particularly decreasing
gain), the resulting signal must be re-dithered before the word length is truncated
back to the length of the input words. Ordinarily, correct dither is added in the A/D
stage of any competent commercial product performing the conversion. However,
some products allow the user to turn the dither on or off when truncating the
length of a word in the digital domain. If the user chooses to omit adding dither,
this should be because the signal in question already contained enough dither noise
to make it unnecessary to add more.
Many computer software volume controls do not add dither when they attenuate
the signal, thereby introducing low-level truncation distortion. It is wise to bypass
computer volume controls wherever possible and if this is not possible, to maintain
unity gain through the volume control. Microsoft Windows Media Player and Adobe
Flash Players should be operated at 100% 0 dBFS at all times, and level control
should be done either at the amplifier volume control or console fader.
In the absence of “noise shaping,” the spectrum of the usual “triangularprobability-function
(TPF)” dither is white (that is, each arithmetic frequency increment
contains the same energy). However, noise shaping can change this noise spectrum
to concentrate most of the dither energy into the frequency range where the
ear is least sensitive. In practice, this means reducing the energy around 4 kHz and
raising it above 9 kHz. Doing this can increase the effective resolution of a 16-bit