Eventide H8000FW, Operating Manual

Page: 90 / 206

84
In effect, the H8000FW has two "speeds," a bit like gear
ratios on a car. In the normal (lower) speed range, all
presets can run, but only sampling rates between 32kHz
and 50kHz may be used.
In the higher speed range, some presets cannot run (those which do not have the "
9
6
"
symbol beside their name on the program screen (e.g.,
Quad*16 Grafic Eq
and
Stereo*32 Grafic Eq
in this screen shot), but the whole sampling rate range may
be used.
Note that because the ADAT protocol does not support the higher sampling rates, ADAT inputs and outputs
are unusable at 88.2kHz and 96kHz on the H8000FW and H8000FWA.
The H8000FW supports the SMUX protocol, allowing the ADAT inputs to pass four
channels at 96kHz.
The System Sampling Rate – A Digital Audio Primer
All digital audio systems operate by expressing the value of the audio signal at any instant
in time as a (digital) number. This number is sampled (recalculated) at regular very short
intervals. The rate at which the value is sampled is known as (wait for it) the
sample rate ,
sometimes known as a clock .
All the internal parts of the H8000FW family (including the A/D and D/A converters
and the digital outputs) run at a single sample rate, known as the
system sampling rate . This
rate may be determined either by an internal clock, or by a single external input. If any
external input is used as an audio source, it must have exactly the same sample rate as the
system sampling rate, or things will get out of step, causing one or more of the digital
values to be lost. This will result in clicks or distortion – a bad thing. For example, at a
sample rate of 96kHz, an 0.001% difference (10 parts per million – more accurate than
most clock sources) will cause a (probably faint) click about every second.
A very small difference in sample rates, as in the example above, results in slipping ,
meaning that the sample points “slip” past each other in time. Depending on how severe
the sample rate difference is, the results may be inaudible (slight clicks will be ‘masked’ by
the signal) but will be avoided by all those interested in audio quality.
Since it is unlikely that pure chance will allow any two or more external signals to have
exactly the same sample rate, it is necessary to
synchronize (lock) them, meaning that the
external signals derive their sample rate from some common reference. This may be either
a signal from the H8000FW itself, or an external signal feeding the H8000FW. It is good
studio practice to establish a central high quality clock source, to which all the digital
audio equipment is synchronized.