Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

in the Broadcast/Netcast Facility 35
system to almost 19 bits in the crucial midrange area, and is standard in CD mastering.
There are many proprietary curves used by various manufacturers for noise
shaping, and each has a slightly different sound.
It has been shown that passing noise shaped dither through most classes of signal
processing and/or a D/A converter with non-monotonic behavior will destroy the
advantages of the noise shaping by “filling in” the frequency areas where the original
noise-shaped signal had little energy. The result is usually poorer than if no noise
shaping had been used. For this reason, Orban has adopted a conservative approach
to noise shaping, recommending so-called “first-order highpass” noise shaping and
implementing this in Orban products that allow dither to be added to their digital
output streams. First-order highpass noise shaping provides a substantial improvement
in resolution over simple white TPF dither, but its total noise power is only 3dB
higher than white TPF dither. Therefore, if it is passed through additional signal
processing and/or an imperfect D/A converter, there will be little noise penalty by
comparison to more aggressive noise shaping schemes.
One of the great benefits of the digitization of the signal path in broadcasting is
this: Once in digital form, the signal is far less subject to subtle degradation than it
would be if it were in analog form, although in fixed point form it is still subject to
clipping for reasons discussed earlier in this book. Short of being clipped or becoming
entirely un-decodable, the worst that can happen to the signal is deterioration
of noise-shaped dither, and/or added jitter.
Jitter is a time-base error. The only jitter than cannot be removed from the signal is
jitter that was added in the original analog-to-digital conversion process. All subsequent
jitter can be completely removed in a sort of “time-base correction” operation,
accurately recovering the original signal. The only limitation is the performance
of the “time-base correction” circuitry, which requires sophisticated design to reduce
added jitter below audibility. This “time-base correction” usually occurs in the
digital input receiver, although further stages can be used downstream.
Sample rate converters can introduce jitter in the digital domain because they resample
the signal, much like A/D converters. Maintaining lowest jitter in a system
requires synchronizing all devices in the audio chain to a common wordclock or
AES11 signal. This eliminates the need to perform cascaded sample rate conversions
on the signals flowing through the facility. Good wordclock generators have very
low jitter (also known as “phase noise”) and allow the cascaded devices to perform
at their best.
There are several pervasive myths regarding digital audio:
One myth is that long reconstruction filters smear the transient response of
digital audio, and that there is thus an advantage to using a reconstruction filter
with a short impulse response, even if this means rolling off frequencies above 10
kHz. Several commercial high-end D-to-A converters operate on exactly this mistaken
assumption. This is one area of digital audio where intuition is particularly
deceptive.
The sole purpose of a reconstruction filter is to fill in the missing pieces between the
digital samples. These days, symmetrical finite-impulse-response filters are used for