Prism Sound Titan Operation Manual page 55

Prism Sound Titan Operation Manual Revision 1.00
mass-interconnection the system grounds using heavy gauge cable so as to minimize the hum
voltage resulting from the current.
Obviously many items of analogue audio equipment only have unbalanced connections; this is
especially true of consumer equipment, which is often used for monitoring even in professional
studios. If you must use unbalanced connections, keep them as short as possible and use
good-quality cables with substantial screens. If you have a choice, keep the signal level as high as
possible on the interconnection, since this will make any interference proportionally less noticeable.
Instrument connections are often particularly vulnerable to hum and other interference, since they are
usually unbalanced and low-level, and frequently employ a long cable not selected for its
interference-immunity qualities. Also, the source impedance is often high, making the connection
particularly vulnerable to interference.
Some digital audio and computer equipment with switched-mode power supplies can cause
particularly troublesome interference problems, especially for low-level, unbalanced signals. This is
discussed in the following section.
Interference
The increasing use of low-cost digital equipment and computers in the audio production process
results in various potential problems for the remaining analogue devices. It is well-known that the
hostile power and EMC environment inside a typical computer is likely to be the limiting factor
governing the audio quality of an internal analogue sound card. A solution to this is the use of external
'sound cards', such as Titan, with their own enclosures and power supplies allowing adequate space,
power and electromagnetic peace and quiet for the well-being of studio-quality analogue circuits.
However, even the sound quality of external devices can be compromised by the proximity of some
types of digital equipment. Many low-cost switched-mode power supplies emit interference which can
compromise system audio quality even at a distance. The hostile mechanism is usually 'conducted
interference', wherein the high-speed switching action of the power converter results in voltage and
current transients being conducted back down their power cords. If the equipment is connected to
mains safety-ground, transients can also be conducted down the ground connection. Radiated
emissions (airborne radio interference) can also be a problem, but it is less common that this will
have such a serious effect on audio quality.
Conducted power-line interference can cause problems in analogue equipment within the installation
if its own power supply allows the transients to pass through to the audio circuits. However,
conducted ground interference can be even worse since, if the ground connection of the analogue
equipment is modulated by switching interference, there is nothing that the designer of the equipment
can do to combat it.
How much any conducted ground interference affects audio quality depends on many factors, mostly
to do with how the various analogue boxes in the system are interconnected and grounded. Where
possible, high-level balanced connections should be used, just as in the case of hum-loops as
discussed in the previous section.
Where ground-potential variations are caused by switching power supplies, the effect can be more
difficult to resolve, since the signals can occur at more noticeable frequencies: although the supplies
usually switch at frequencies too high to hear, the frequency is often modulated by variations in the
load current over time, resulting in a continuous modem-like chirping in which can be heard particular
events such as computer screen updates, disk activity etc.). Another problem is that even heavy
ground cabling may not reduce the effect of the interference, since high-frequency currents may not
see much less resistance in a thick conductor than a thin one.
How do the equipment manufacturers get away with this? Surely there are stringent regulations
covering conducted and radiated emissions? Well that's true, but the level of emissions which can
result in audible degradation of low-level, unbalanced audio interconnections are well below
legislation levels. Unfortunately, computer power supplies (and especially the switching wall-warts
and line-warts which power notebook computers and other small items) are amongst the worst
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