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audio -bus to end
'spaghetty' at the radio table (published in Electron #2, 2000) Introduction It this a familiar situation? In a QSO your
party suddenly doesn't react anymore. When he returns after a few calls he
tells you he took the wrong microphone..... Or in a different version: you have been a
radio-ham for quite some time and hate to part with your first transceiver,
that nostalgic radio-system, that robust military set. To keep everything
operational all are equipped with a microphone, Morse-key, headphones /
speaker because plugs are different for each set, as are impedance levels,
internal voltages etc. This picture may be painted in almost every
radio-amateur shack, including mine. With yet another transceiver to install
and thereby completely loosing track through all cables on the table and,
more important, loosing view to all the beautiful dials and displays, I
decided to take action. The following discussion is about my ham-bus,
that is operational for quite some time now and really ended the 'spaghetti'
at my radio-table once and for all. Operational
principles
Basic operational principles may be explained
in a few simple lines: -
all audio-outputs are 'translated' into a current source, allowing
signals to be added and and this way to listen to
more radio's at the same time. -
all audio-inputs are connected to the same, low impedance microphone
channel that is still allowing local levels to be tuned separately. -
all 'PTT' inputs are controlled by a single line, still allowing
complete separation between transceivers. In this control system one bus line (with
four wires) is connected to a small control box at the back of each
transceiver and all control boxes are identical and are connected in
parallel. The connection between the control box and the transceiver is
dedicated and is equipped with the required connectors for that transceiver.
The connection cable is connecting to a line-level input/output connector,
most transceivers are equipped with. The four-wire line bus-line connects to
the central ham-bus cabinet to control all sets. Since the ham-bus cabinet is central to all
equipment, all audio-processing is concentrated in this position and is
acting on all transceivers. All your nice idea's as to an equalizer, notch,
(DSP) filtering, microphone signal processing/compression, metering and
digital mode I/O (FSK, packet) may be designed into the central ham-bus cabinet. The central cabinet may also
connect to all your peripheral equipment for recording an play-back and TNC /
computer for all digital communication modes. The central ham-bus cabinet is
further connecting to only one microphone, one speaker and a convenient send
/ receive switch (maybe foot controlled) to control all transceivers. In principle the number of connected
transceivers is unlimited. In practice this is limited to some extend by the
power of the central microphone amplifier and the PTT control line. In the
concept as below over ten transceivers may easily share one bus-line. Block
diagram
In figure 1 the ham-bus block diagram may be found.
The 'match-box' is at the back of each transceiver and is showing all
internal components (6). It is clear this box may be very small indeed. In my
boxes a 5-pin DIN-connector is handling all connections to each transceiver,
so all matchboxes will be the same; only the connection cable (and
transceiver plug(s)) is typical to the transceiver. Each box is equipped with
a contract strip to 'splice-in' the 4-wire bus-cable. Figure 1. Audio-bus block diagram The Morse-key symbol at the transceiver is
depicting the PTT switch. Since all transceivers are in parallel, it sometimes is more convenient to not allow
all transceivers to switch to the transmit mode at the same time (e.g. in
cross-band operation). To this extend a tumbler is added to the match box. The speaker symbol at the transceiver is
depicting the audio-output of the transceiver (headphones, speaker,
line-output and the like). Try using an output as close as possible to the
detector to prevent phase distortion when in FSK-mode. This output preferable
is also just behind the volume control of the transceiver to enable
controlling the output level to the bus to balance with other transceivers. A
dummy headphone plug at the transceiver may silence the internal speaker, if
any. The 47 kOhm resistor in the match-box is
changing the audio-output into a current source. The amplifier at the ham-bus
is very low impedance at the input so all transceiver outputs may be put in
parallel without mutual interference. The microphone symbol at the transceiver is
depicting the audio input to the transceiver. I noticed these input to be
very different as to impedance and sensitivity. To this extend the ham-bus
will put a constant signal level onto the bus, to be monitored by a level
meter at the ham-bus cabinet. The match-box is containing a potentiometer to
(once) set the bus level to the required sensitivity of the specific
transceiver (e.g. at microphone level or line level). PTT switch
In figure 1 also the complete (simple)
diagram of the PTT switch may be found (upper right). A general purpose PNP
transistor is directly connected to the bus-line through a 100 Ohm resistor
(1 Watt) to current-limit an accidental short circuit (my supply voltage is
12 V.). PTT bus-line connects to the switch at each match box (to exclude a
switched-on transceiver) and to a voltage divider that is controlling a
general purpose NPN transistor. I noticed PTT inputs to be very different
between transceivers as far as input voltages and currents were concerned.
Since all needed a contact to ground for switching, the match-box transistor
takes care of all other requirements while separating the transceiver from the
bus at the same time. The input filter section at the PTT switch
will also be applied extensively at all other inputs at the ham-bus and is
keeping HF currents out of the cabinet. This filtering actions should be well
taken care off since bus-lines are running along all transceivers thereby
acting as a (not so) nice pick-up antenna. Filters should be connected as
close as possible to the ham-bus inputs. When still some HF is leaking in, a sleeve
choke will do a good job (run cable a few times through a ferrite toroide of
4A11 of '43' material). The type and number of centralized PTT
switches is unlimited. For manual control I have a three position tumbler at
the ham-bus with a click and a spring-loaded action and an additional foot
controlled switch. Also the PTT control for all digital modes is acting
through this circuit. Audio amplifier
The audio amplifier may be found in figure 2.
Inputs and outputs are again equipped with the now familiar filter circuit. The audio-input begins with a capacitor since
some transceivers are presenting also a DV-voltage at the line output. This
is followed by the virtual ground input of the first amplifier, at a gain of
one (47 kOhm at the match-box). Directly at the output of this amplifier is a
second amplifier at a gain of three to take care of all digital modes and
also a direct output for (tape-) recording purposes. The outputs are filtered
again against HF break-in. All operational amplifiers are of a general porpose type, in my system: NE5512 (dual) or NE5514
(quad.
speaker headphones Figure 2. Audio output amplifier The central filter unit may be any type you
like. I advice to make the filter switch-able as in the diagram. In my ham-bus variation a have a selection of
several filter types at my disposal. One is an order twelve, controllable,
active high and low pass section to allow a variable band-pass section at any
position in the audio band. Also two active notch filters are present as well
as a DSP processor for speech control, also discussed at this web-site. Some of these (digital) filters are allowing
switching noise to seep through, which is why a filter is connected around
the potentiometer. The output section is straight forward again
and constructed as a single-chip, two channel audio-amplifier. Many more
solutions are possible and currently I am using a abondoned
audio amplifier with small speaker boxes, that is directly connected to the
potentiometer. More audio-sources are hanging around in the shack like FM-
broadcast tuner, CD-players, that may all be controlled through this
channel. Microphone
amplifier
As already presented in the block diagram,
the microphone amplifier is consisting of four distinguishable parts. The
pre-amplifier, an (equalizing) filter, an automatic level control (AGC, 'processor')
and a summing amplifier / line-driver. The communication microphone is a much
discussed topic. Some microphones are supposed to enhance your voice or give
it a 'natural' sound and many more 'qualifications' may be heard on the
waves, with microphone prices to range from a few bob to hundreds of Euros
and more. In my experience a simple, very cheap electret microphone may do a
very good job as well as surplus conference microphones that are offered in
abundance at ham-fest. It is a good idea to start with as 'neutral' a
microphone as possible since sound-coloring may always be performed
afterwards. The latter is the basic idea behind this microphone channel with
equalizer. Figure 3 is presenting the microphone channel
with the low-noise NE5533 as the pre-amplifier, designed for application with
a dynamic (conference) microphone (output a few mili-volt)
at a gain of about 30 dB. A small supply network is providing a power for an
electret microphone. The amplifier is bringing the microphone signal to a
convenient level of 50 mV. Figure 3. Microphone amplifier and equalizer Behind the preamplifier, the audio spectrum
is sub-divided into three frequency ranges, with the highest frequency band
(1,5 - 3 kHz.) and lowest band (20 - 500 Hz.) at a variable level control.
The mid-band section has a fixed gain. The resistor network at the output
summing amplifier ensures each channel to arrive at a gain of one with
controls at the center position. The switch allows to cut-out the equalizer. With this equalizer, many microphone
characteristics may be selected (or corrected!). In my set-up I made many
QSO's especially at difficult conditions, and fiddled the equalizer controls
for best intelligibility at the other side for my particular voice type. In
the end a setting has been found that is giving my voice the punch-through
effect that is bringing 'the message' even with the S-meter not moving to the
signal at the receiving side. This setting has not been changed ever since. The opamps are general-purpose type again,
e.g. NE5532 (dual) or NE5534 (quad). All filter inductors are of the
micro-choke type, not much bigger than 1/4 W. resistors. These small indictors
will exhibit some series resistance, that is part of the design; changing
these micro-chokes for 'better' inductors therefore will yield 'less'
results. Automatic level control and summation / buffer amplifier Figure 4 is showing the automatic microphone
level control (in some transceiver advertisements referred to as the
'processor') and the summation amplifier / line driver. Figure 4. Automatic level control and line
driver The automatic level control is operating in
the following way: -
With the FET switched-off (gate at maximum negative voltage with respect
to the source), the input voltage is attenuated by 100 x, followed by an
amplifier at a gain of 1x. Total input to output 'gain' therefore is 0,01x. -
With the FET switched hard on (gate voltage equal to the source), the
input 10 kOhm resistor is connected to the virtual ground pin of the opamp.
The total input to output gain therefore is at 15 / (10 + 15) = 0,6 x. Dynamic control range of the amplifier is 0,6
/ 0,01 = 60 x (36 dB) which is convenient at keeping 'normal' audio speech
within limits. The FET will be controlled negative as soon
as the voltage at the output of the lower amplifier is more negative than 0,6
Volt (diode step voltage). This opamp is set at a gain of 200 x which may be
calculated back to the input of the AGC circuit: 0,6 / 200 V. = 3 mV. This
translates back to the input to 3 / 0,6 = 5 mV. Level control will be at the end-of-range
with the FET completely switched off. With more input signal, output will
follow linearly. This level is reached with the input level 60 x higher, at
60 x 5 mV. = 300 mV. (the control range). It is advisable to apply a FET with a low
gate voltage range. This gate range is controlling the compression range. Time constants for this circuit have been
selected at 0,1 second for attack and 1 second for decay to smoothly control
the output level without the 'pumping' effects often heard at the
radio-bands. With the nominal level from the pre-amplifier
at 50 mV., whispering hams at -20 dB. and 'shouters' at + 16 dB will remain
within control of this automatic level. Within this control range, output
will vary between 45 mV and 300 mV to still allow for some voice dynamics. A
switch will provide nominal signals without level control. The potentiometer is set to a convenient
audio level at the bus. The output buffer is also the right place to insert
the digital signals and the play-back information from the recorder. Level
indicator
In the block diagram it was discussed the
audio level at the bus should be set to a pre-conditioned level for all
transceivers to reference to the same standard signal. Therefore we need an
indicator to check for this standard level and control the level-setting when
out of range. I first planned to apply a big level meter,
but finally decided I needed no more than a quick indication to see the
signal is off, too low, just right or too high. This is conveniently designed
around a two colored LED, showing none, green, yellow and red (yellow is red
plus green). This small indicator may easily find its position at any crowded
front panel. The indicator circuit may be found in figure 5. Line-level signals are detected at the upper
right opamp, that is wired as an active detector (no diode set-up
voltage). The (DC) detector output
voltage is connected to the three level-detecting opamps at the left-hand
site, the upper two as a detector with some hysteresis. The other input of
each detector is connected to a resistive ladder network, setting the trigger
voltages. With the input voltage at 0,7 V. the first
detector will trigger, lighting the green part of the combi-LED.
At 1,2 V. input, the second detector will
trigger, also lighting the red part of the combi-LED.
With red plus green lighted, the LED will show yellow. At 2,4 V. at the input, the last detector
will trigger, 'un-triggering' the upper detector so the green LED will
extinguish with only the red light remaining. While communicating, you may watch the yellow
light for the right, nominal voltage at the bus line, with an accuracy of a
few dB. Any type of opamp may be selected for this
function, that is capable of delivering a few milliamps to drive the LED's. A
convenient type is housing four opamps in a single package. Supply voltage for the ham-bus is set at +12
V. and - 12 V., but may easily be adapted at your convenience, provided you
recalculate the reference voltages for the level detector. Bob J. van Donselaar, on9cvd@veron.nl |
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