Processing Card
Main Features
Although the aim was to
design a preamplifier which did not modify the audio signals...the Motherboard
has been designed to allow the use of a Processing Card, to alter the audio
signals as they pass through the pre-amp between the Dual Input Cards and the
Dual Output Cards. A Processing Card could be used, for example as a "Tone" (or
Equalisation) control, or for dynamic range compression.
Card Slot 9 of the
Motherboard is unique because it is the only slot which presents the audio buss
tracks on the back of the Motherboard PCB, allowing them to be cut so that the
busses are broken into two sections - one half for the Dual Input Cards, and
the other half for the Dual Output Cards - with the Processing Card providing
the bridge/intercept point between the two halves. (This is shown on the
schematic on the Motherboard page.)
This facility is provided to
allow users to implement their own designs of Processing Card according to
their requirements, although standard designs may be available in the future
(no designs are currently available). Alternatively, users can ignore this
facility, and just use Card Slot 9 on the Motherboard for a Dual Output
Card.
A Processing Card can have
four high impedance unbalanced inputs to connect to the audio busses fed by the
Dual Input Cards on the Motherboard, and low impedance unbalanced outputs to
feed the audio busses received by the Dual Output Cards.
The key features of this
facility are:
- General purpose configuration for
modifying the audio signals as they pass through the preamp
- Users can design their own cards, or
standard card designs may be available in the future
- Audio signals are processed on the
card and returned to the Motherboard busses (no input or output XLR sockets on
the card)
- Any standard card designs will be 4
layer PCBs, on a Eurocard 160mm x 100mm (6.3" x 3.94") size
- Provision for on-board ±15V
regulators for guaranteed low noise DC supplies on the card, to match the
standard card designs.
Possible
Uses
A few possible uses (and potential design
ideas) for a Processing Card are given below.
Frequency Response
Filters - Controlled
A Processing Card with
electronically controlled (e.g. by DC controls or by microprocessor) frequency
response filters could be used as a high quality "Tone Control", to improve the
sound of non-ideal recordings.
Frequency Response
Filters - Fixed
A Processing Card with fixed
frequency response equalisation could be used to correct for the low frequency
response, etc of loudspeakers. Note that in this configuration, all Dual Output
Cards connected to the output buss fed by the Processing Card will have the
equalisation in circuit and so may only be applicable to some users' system
designs.
Dynamic Range
Compression
A Processing Card with a
compressor (effectively an automatic volume control) can be used to control the
dynamic range of audio signals, which can be useful occasionally for listening
to classical music etc in noisy environments (such as kitchens/bathrooms). The
compressor settings can either be fixed, or electronically controlled. As the
Dual Input Cards allow the user to match the audio levels between different
sources, a compressor used in this preamp system will offer higher perceived
quality than might be expected from a simple automatic volume
control.
Miscellaneous
Facilities
A Processing Card with
miscellaneous facilities such as the ability to invert the phase of one/both
channels or to swap the channels in a stereo pair, could be used to check the
quality of a recording. This may have uses in a professional audio/recording
environment for quality assessment, etc.
Connection
Options
Overview
Three connection
configuration options are shown for using a Processing Card with the
Motherboard. Note that the sketches only show one channel of both stereo busses
on the Motherboard for clarity.
The three different options
offer advantages and disadvantages, so making each more suitable for particular
uses. Note that different configuration techniques can be implemented on Busses
1 and 2.
Buss Intercept - with
Bypass
This technique requires the
audio buss PCB tracks to be cut so that the input buss audio signal (i.e. the
buss from the Dual Input Cards) is fed through the Processing circuitry. A
bypass switch is shown allowing the output buss (i.e. the buss feeding the Dual
Output Cards) to be fed from either the input buss directly or from the
Processing circuitry.
The bypass switch could be
mechanical (e.g. used occasionally or for alignment purposes) or electronic.
This configuration may be preferred as it will maintain high audio quality
where the Processing circuit has, for example, higher distortion than the rest
of the audio chain even in a "flat" or "bypass" mode when not in
use.
Buss Intercept - No
Bypass
If the Processing circuit
has the equivalent of a "no processing" setting, such as "flat response" for a
tone/equalisation control, then this can be used in circuit all of the time.
This option is, effectively, the same as that above but without the bypass
switch.
This configuration will be
most appropriate for fixed frequency response filters (e.g. loudspeaker
correction) where they are always required to be in circuit (possibly only on
one buss).
Audio Fed to Other
Buss
The third option does not
break the audio busses on the Motherboard, as signals are received from one
buss and then fed to the other buss. This allows some Dual Output Cards to
carry the signal direct from the Dual Input Cards, with others carrying the
Processed signals. This configuration may be most appropriate where the
Processing facility will only be used rarely (such as occasional use of a
dynamic range compressor, or tone/equalisation control). This design offers the
highest quality when the Processing is not required.
Note that this configuration
(feeding an alternate buss) allows a Processing Card to be used in any slot of
the Motherboard. The first two configurations must use Card Slot 9 as it is the
only one where the busses can be cut, allowing a card to intercept the signals.
Card Slot 9 also has 16 control lines available (Card Slots 1 to 8 only have 8
control lines).
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