OPERATING INSTRUCTIONS FOR THE INT202 FIREWIRE INTERFACE
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11
Date: 03/10
I have given the example of a 24 bit recording which has
to be truncated to 16 bits. Where is the application in
High-End Hi-Fi audio? More and more signal processing is
implemented in the digital domain. Think of digital
equalizers, digital volume controls, upsamplers, digital
pre-amplifiers, decoders for encoded signals on DVD etc.
All those applications perform some mathematical
operations on the digital audio signal. This in turn causes
the wordlength of the signal to be increased. E.g. an
input signal to an upsampler may have a wordlength of
16 bits (off a CD), but the output signal of the upsampler
may have 24 bits or even more. This comes from the fact
that the mathematical operations employed in such
devices increase the word length. E.g. a multiplication of
two 2 digit numbers results in a four digit number. So
after the upsampler the word length may be higher than
the subsequent processor may be able to accept. In this
example, after the upsampler there may be a D/A
converter with a 24 bit input word length capability. So if
the upsampler generates a word length of more than 24
bits it should be dithered to 24 bits for maximum signal
fidelity.
I hope these excursions into the theory and practice of
audio engineering have been useful for you. If you would
like to dive further into those issues I recommend to visit
the website of Mr. Bob Katz, a renowned Mastering
Engineer and a Weiss Engineering customer. He
publishes articles on Dithering and Jitter and many other
topics at http://www.digido.com
Firewire vs. USB
Firewire is a peer-to-peer protocol, meaning that every
device on a Firewire network is equally capable of talking
to every other device. Two video cameras on a Firewire
network can share data with each other. A Firewire audio
interface could save sound data directly to a Firewire
hard drive. Your computer is just another peer on this
network, and has no inherent special status.
Firewire is always implemented in hardware, with a
special controller chip on every device. So the load it
puts on your CPU is much lighter than USB
communications load, and you're much less likely to lose
any sound data just because you're running fifteen things
at once. Specialized hardware usually makes things
faster and more reliable, and this is one of those times.
But the real reason Firewire is more reliable than USB is
more fundamental than that. It's because Firewire allows
two operating modes. One is asynchronous, similar to
what USB uses. The other is isochronous mode, and it
lets a device carve out a certain dedicated amount of
bandwidth that other devices can’t touch. It gets a
certain number of time slices each second all its own.
The advantages for audio should be obvious: that stream
of data can just keep on flowing, and as
long as there isn't more bandwidth demand than the wire
can handle (not very likely) nothing will interfere with it.
No collisions, no glitches. From a practical perspective,
