Behringer SRC2496 - Manual (Page 11)

11
ULTRAMATCH PRO SRC2496
4. TECHNICAL BACKGROUND
boost step involved prior to recording. This treble boost is undone
during playback. A specific bit (emphasis bit) in the digital data
stream contains the information whether or not the signal has been
processed with this technique. It is however NOT a modification
of the audio signal, but only of the emphasis marker contained in
the digital data stream.
So much for the theory. In practice, a lit EMPHASIS LED does
not necessarily show that the emphasis function has really been
applied. In several situations the emphasis bit was set during the
mastering and editing process due to defective hardware/software—
without any treble boost applied. This error usually remains
undetected when the material is transferred, because modern DAT
recorders no longer have an emphasis indicator for reasons of
rationalization. When the master tape—usually only monitored briefly
on the DAT recorder due to lack of time—arrives at the customer’s
end, it gives him a big surprise: The treble range has been cut in
the DAT recorder by more than 10 dB, which cannot be undone
and results in a dull sound that lacks brilliance in the treble range. It
is for this reason that we equipped the ULTRAMATCH PRO both
with an emphasis status LED and with an additional switch for
manual correction of the emphasis bit setting.
3.5 Line booster, signal refresher
Longer cable lengths or the use of a digital patchbay lead to
deterioration of the signal quality, which may result in a higher
noise floor or even connection failures. Using special input
circuitry your ULTRAMATCH PRO removes jitter, interference
and level loss, thus restoring the signal to perfect quality. As a
consequence, you can also use the SRC2496 to refresh the
signal over longer cable lengths or to restore attenuated and
distorted signals.
3.6 Correcting incorrect sample rates
The sample rate of any device is subject to fluctuation due to
temperature and ageing effects. Normally, an automatic locking
circuit ensures reliable operation even with varying or slightly
fluctuating clock signals. If such fluctuations exceed a certain level,
however, the receiving device cannot be synchronized any longer
to the clock rate supplied and therefore fails to function properly.
Your ULTRAMATCH PRO is totally free from such restrictions,
as it works over the entire bandwidth from 31 to 100 kHz, rather
than only within a small range around the actual sampling rate. The
signal output by the SRC2496 is always synchronized correctly to
the value selected. Thus, you can use your ULTRAMATCH PRO
to “rescue” and work with sampling rates that have been modified
deliberately (vari-pitch function on sampling keyboards or CD
players) or fail to fall within the adjusted range due to rate fluctuations.
3.7 Vari-speed application
There are several reasons for modifying the speed of a digital
playback, e.g. to change the pitch or synchronize the playback to
other devices. In such cases, the ULTRAMATCH PRO can track
the sampling rate as it changes and then output the signal with a
fixed rate of 44.1 kHz (or any other selectable or wordclock-defined
value). Thus, it reliably removes sampling rate fluctuations, which
could otherwise make it impossible to process the digital audio
material any further in the digital domain.
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At its digital input, the ULTRAMATCH PRO accepts
sampling rate fluctuations of up to ±12% from the
adjusted sampling rate, without producing any distortion
at its output. When things get worse, the connection
will not be interrupted, but temporary drops in the signal
quality (distortion) can occur.
3.8 Format converter
Digital data transfer is frequently affected by problems with the
format required. While some devices specifically need S/PDIF
signals on the input side, others can only output AES/EBU. With
the SRC2496 this problem is now history. The input of your
ULTRAMATCH PRO readily accepts any standard used today.
On the output side, you can select either AES/EBU or S/PDIF, so
that it is just a matter of one key press to realize the successful
transfer of your audio material.
3.9 Format interface
Many CD players have no coaxial but only an optical output. Some
DAT recorders, on the other hand, only have a coaxial input. Digital
interface cards for personal computers are usually equipped either
with optical or coaxial connectors. So, when you try to transfer data
from one device to another you are likely to encounter incompatibility
problems, in that one device has an optical connector, while the
other has a coaxial RCA connector. Connection to professional
devices is difficult because of the XLR connectors used there, or
even impossible if they feature optical interfaces.
Such incompatibilities between inputs and outputs are definitely
a thing of the past with the ULTRAMATCH PRO. For example, if
the CD player only has an optical digital output, while the
DAT recorder is equipped with a coaxial RCA input, your SRC2496
will easily eliminate the resulting connection problems with its three
inputs and outputs in all formats. Your ULTRAMATCH PRO allows
for interconnecting virtually any devices that are available on the
market today!
3.10 Patchbay/splitter
The digital outputs of your ULTRAMATCH PRO can all be
operated at the same time. In this case, the input selector functions
as a kind of miniature patchbay determining which signal is sent
to the output. It is not necessary to switch-over the output, because
you can freely select on the down-stream devices which signal to
accept (example: recording active/inactive). Therefore, the outputs
of the ULTRAMATCH PRO carry the identical signal all the time
and are operative simultaneously.
The parallel operation of all three outputs enables you to use
your SRC2496 as a splitter, which is useful for copying material to
several DAT recorders. With some special devices in particular,
which do not allow for looping through the digital data from the
input to the output, this useful function makes it possible to split
up the signal.
4. TECHNICAL BACKGROUND
4.1 A short digression into digital sample rate
conversion
In the past, the conversion of various sample rates into a fixed
value necessitated a whole rack full of components and even then
had some disadvantages that could be measured as increased
noise, distortion or undesirable mirror frequencies (high-frequency
chirping). On the other hand, the sampling rate converter chip
installed in the ULTRAMATCH PRO converts in real time and with
24-bit precision. Its processing is absolutely inaudible and can
only be identified with state-of-the-art, extremely expensive
measuring instruments.
The functional principle is difficult to comprehend, even for
professionals. That is why we are presenting you with an equivalent
model, which describes the processes taking place in the
processor in an easily understandable manner. The processor
carries out an oversampling at the input signal. Between each
sample, more samples are inserted, thereby significantly
increasing the number of sampling points by filling in the gaps.
Subsequently, the signal passes through a variable low-pass filter,
which ensures that the correct limit, below which no problems
with mirror frequencies (aliasing) occur, is observed. Then, the
number of sampling points is distributed in such a way as to obtain
the desired sampling rate at the output. This enormous
oversampling allows the SRC processor to achieve excellent
precision in the complete conversion range. Moreover, it also
simultaneously eliminates any jitter present in the input signal.