STM Dispatcher Service Manual

Dispatch system showing one radio
Right-click and select View Image to see at full size. Hub has two user inputs (at top). Inexpensive headset shown had to be modified for stereo.
The version 2 prototype shown above was replaced by version 3, which returned to a sloped mini-console similar to version 1 (see parent page) but it now has a total of 12 buttons on it, plus four volume controls (see picture below). The following operating description is for version 3. Version 2 above works internally the same but lacks audio activity indication and channel set, and the two volume controls are for selected and unselected only.

New system diagram


The STM Dispatch System allows two or more operators to conveniently transmit and receive on four radios by grouping the push-to-talk buttons into one small desk-top box. Each operator has a keying box and a headset or a computer-style desk microphone and headphones. Common amplified computer speakers may be used instead of headphones. The radios can be located remotely if the operator does not need to change channels on any of them, or any of the radios can be placed near either of the operators if they need to be accessible for changing channels or other functions. (Channel functions are available on the latest design.) One earpiece or speaker provides receive audio for the "selected" channel, which is the last channel keyed, while the other side carries combined receive audio from the other three "unselected" channels.

The depiction shown to the right reflects the new appearance described below, showing the channel set buttons and the four independent volume controls (the black dots across the top of the beige desk units). Right-click and select View Image for a better view. Kenwood radios and Eventide logging recorder shown as examples. The channel set buttons may be round on the final design, or you may be allowed to specify 12x12mm square (shown), 9x9mm square, or 12mm round.

New system photo

The system controls up to four radios (not all paths need to be connected). The number of desk units can be increased by paralleling them with network splitter jacks. So four dispatchers sitting by four radios could be interconnected so that each person has access to all four.

The hub has four balanced line level outputs for log recording. The recorder can be located anywhere the audio lines can be sent.


When first powered up, the channel selections may need to be synchonised: set a channel for each radio by hand. If this step is not done the radios may be on different channels than indicated at the desk. Only one desk set needs to take this step; the others will register accordingly. To use, briefly key to set a radio as "selected," sending its receive audio to one ear (of a headset) or one speaker. All the unselected radios receive audios are mixed to the other ear/speaker. Receive audios are passed though automatic gain control (AGC) circuits which keep reception from being too low, within reason. At each desk, each radio has its own volume control, so you can turn any radio completely down there.

If all the radios are close by the user(s), the headphone can be left off, as the users will hear the radio speakers directly. In that case there need be no selected/unselected feature or headset jack. Nor would there need to be channel set, as they could do that on the radio face. Only the basic PTT buttons would be used, and the system would only serve to connect one mike (per desk unit) to all the radios. Although useful for that alone, the system is more appropriate for radios that are remoted to another part of the building. The control line is not intended for runs of more than about 100 feet, although it should work for somewhat more. The desk units can be bridged together, then one cable sent to the radio room. In that case, however, the power for all the desk units will be supplied by a single cable. Each desk set draws between 50 mA (standby) to 100 mA (four radios keyed together) at 10 volts. A local power plug-in can be tapped onto the cable with a cable splitter/power tap block. The tap block can supply several desk units. A common filtered nominal 12v walwart plugs to the power tap.

To transmit, press and hold the push-to-talk (PTT) button for the radio desired and speak into the mike. The green LED above the button is the "go ahead" signal. The red LED will flash briefly on key-down. Although this does not stop you from speaking as soon as you key, it is good practice to pause a fraction of a second before starting to talk, so letting the red light go out before you speak is proper usage.

The transmit audio is modestly AGC'd, so it is not necessary to speak very close to the mike, but doing so will ensure better audio going out. The AGC action will then act to reduce background noise. If the mike is not close to the person speaking, the AGC will attempt to give sufficient output by increasing the gain, but that also amplifies any background sounds present in the room. The red LEDs are the audio indicators, both when transmitting and receiving. The LED above the key in use should flash red on your louder words. This does not mean you are talking too loud. Just the opposite, if you never see a flash, you are talking too softly. The red flash occurs at the optimal voice level. When receiving, the flashing will help you identify which radio is the source, particularly between the unselected radios. The selected radio sound comes from its own speaker or earpiece and is identified that way.

If you wish to transmit on more than one channel at a time, just press the appropriate buttons together. Your voice will go out equally on all of them. The unit sets the last channel keyed to 'selected', so in this case the selection will be the last button released. If perchance it selects a radio you do not want, just tap the correct button. Selection happens faster than keying, so you do not need to actually transmit. The green LED remains dimly lit to indicate the selected channel. Other operators select their own radio, which need not be the same as yours.

Once a radio is selected, its receive audio will be sent to the other ear or speaker. You can put the headset on either way if it is the type that allows the microphone to be left or right, so you can designate 'selected' to be right ear or left as you prefer. If using speakers, just position them accordingly. Non-amplified external speakers should not be used for very long, if at all, as the headphone driver in the unit may overheat; it is rated for only about one-third of a watt. That is plenty loud in a quiet office using efficient speakers, but some cheaper computer speakers are not efficient at all.

When you key a radio, the red LED above the corresponding button on the other operators' boxes light, meaning "radio in use, do not key." If they try anyway, nothing will happen. They are blocked from that radio while you are using it. If another person keys a radio first, they block you the same way. You hear what they say as they keys, and they hear your transmits, in their unselected ear if they have a different channel selected. Your own selected ear is muted when you key, but you will continue to hear any unselected traffic in the other ear, unless you turn their volumes down.

The buttons below the PTT keys are monitor/mute/intercom buttons. Pressing any of them for at least 1/3 second will turn on the corresponding red LED dimly. This indicates that the tone-coded squelch of that radio has been defeated, so you will hear any traffic on that channel, coded or not. The indication is passed on to the other operator, so he can see which channels are set to monitor mode. While you hold the button, that channel is silenced, so it can be used to temporarily mute undesirable traffic. Your mike is live on the line while you hold any monitor button down, so they also serve as Intercom keys. You can hold down all four if you can spare the fingers. These buttons are blocked during transmit. To return to coded squelch, tap the same Monitor button again briefly (less than 1/3 second). Keying the corresponding PTT button will also reset monitor (no minimum time required). If desired, set Mon again after keying. The red LED will indicate the end condition.

The channel-change option allows access to up to four channels per radio. Each radio has a single bicolor LED to display the set channel. When the LED is dark the radio is on its default or "free" channel: the radio can be set to anything by its front panel. Red means the first fixed choice (nominally channel 2, but the radio can be pointed to anything when it is programmed), green the second choice (channel 3), then yellow (red and green both lit) the third choice (channel 4). When set to a color channel by the controller, the radio's front panel channel switch is overridden. To change the channel, press and hold the desired channel button, then tap the PTT button of the radio to be set. Its channel indicator will change color to match the button you are holding down. The corresponding indicator on the other desks will change color also. While you are holding the number button down, all the other users are blocked from keying, except that any keydown already underway is not interrupted; but if they unkey while you are holding the number button down, that radio also becomes key-blocked. This allows you to select any radio to change channel. Obviously, responsible operation would preclude blocking everyone like that for more than the second or so it would take to set a channel.

If you do not need 4-channel capability on all four radios, just program the radio to ignore any channel command. For example, one of the four radios might not respond at all, another might respond only to 1 and 2, and so on. The controller will not know this and will dutifully indicate the color chosen, whether the radio changed or not. You would probably want a cheat sheet posted in any case, listing the channels available for each radio, but it need not be a full list of 16.

The channel set feature simply presents the radio with a binary pattern: 11 for 1, 10 for 2, 01 for 3, and 00 for 4. (Note that this is inverted binary, so that a broken cable does not lock the radio; on Kenwoods, an open input floats high.) If you do not need to use these for setting channels, a savvy technician could repurpose these bits for other functions, with suitable radio programming or added interface relays/transistors. The chip that drives the output data can supply at most 25mA per bit (both low and high), and high is 5 volts. Of course the buttons would have to be appropriately labelled for the purpose installed.

For example, to turn scramble on and off: the simplest way would be to assign that to its own channel. Just set the radio to the scramble channel, and set back to the clear channel when done. But if you want, the binary data could set scramble on/off directly for one of the radios. That would however not allow it to also be used for channel setting on that particular radio.

Repeater setup: The system can optionally allow a repeater arrangement to be set up between the connected radios. The full-featured version has eight slide switches on the hub, two for each radio. A lesser option would have only four switches, for one-way repeating, or the system can be supplied without any switches. The switches are marked "T/off/R". For no repeat, leave the switch centered in the off position. To set up repeat, set the intended Receive radio(s) to R and the radio(s) to retransmit what the first radio(s) receive(s) to T. You can set one receiver to go back out on all three of the others, or three receivers to all be repeated on the 4th one, or you can set up two independent repeaters.*

The repeater arrangement acts like another desk set, so during a repeat, others are blocked from speaking on the keyed radio(s) at the same time, but they can key the receiving radio, interrupting the repeat. Of course, the audible traffic should alert the user not to do this.

If the receiving radio is to be used only for repeating, its dispatch-system PTT can be disconnected by deprogramming the "EXT PTT" pin and disabling channel changes. Or a 5th radio could be dedicated for repeater Rx and hard-wired externally to key one of the four connected to the dispatch system. Then the desks could hear repeater traffic on the transmitting side, and also talk out on it. In the last setup they would not be blocked from co-keying unless the external wiring sent the blocking signal (nominal +5v) to the audio line. This could be done with a single transistor and one resistor. Matching the Rx audio to the Tx mod input is the more difficult part. This external connection could be done in a small box plugged in series with any desk line.

(Click on message to dismiss.)

The eight-switch option permits two-way repeating. For each radio involved, set one switch to T and the other to R. The first one to receive traffic will key the other(s). The simplest example is one pair set up as a cross-band repeater. Whatever comes in on one goes back out on the other, both ways, simplex. If both radios are in the same band, a notch filter tuned to the other radio’s transmit frequency should be placed between each radio and its antenna (or use a duplexer to a shared antenna) to prevent the transmitting radio from desensitizing the receiving radio. Since the notch filter will be quite narrow it will then not be advisable to change the transmit frequency, as it would then bypass the notch and desense the receive side. You can instead use pass filters, but these by nature tend to be broader so they will help only if the transmit and receive frequencies are fairly far apart. Finally, you might have successful in-band repeater operation without any added filters if you can place the antennas so that they are in the other’s null zone. This is fairly easy using directional yagis (assuming your operation involves a specific direction), but it can be done with omnidirectional verticals by placing one above the other. The isolation obtained this way is not as good as a notch filter provides, but it does allow for frequency changes and does work if the transmit power is kept modest.

Even when no repeater is involved, just four independent radios, these principles of mutual interference should be kept in mind. When one is keyed, injudicious antenna placement may make others unable to receive for that interval. Or they may receive noise, which would be passed back down to the ear of the operator, especially for any radio that is not code-squelched.


Radio modification: The accessory in/out port must provide, or be programmed/modified to provide, the following:

  1. Squelched, unscrambled receive audio between 0.1 and 1.0 volt pp (nominal 0.2v pp), DC blocked:
  2. Monitor, active low input:
  3. Tone-operated relay (TOR), low output when a signal with the correct code is received:
  4. PTT active low input:
  5. Audio modulation input, electret mike level (0.1-0.25v pp):
  6. For channel change option:



  1. Presently the Rx path is non-adjustable; it assumes a certain approximate audio level from the radio and then applies automatic gain control to supply the system with a predictable level internally. Only rather significant deviation from the expected level would necessitate changes in resistance values at the Rx input.
  2. The line outputs to the logging recorder provide approximately standard line level (+4dBu) or more, that is, between 4 and 5 volts pp. This will survive lengthy cabling if need be, such as telephone line. It is 600Ω balanced and isolated and can be connected to a dedicated leased phone line (not one with DC used for dialing, unless blocked with a capacitor), or your own in-house cable. But it has no lightning protection, so that should be provided (usually at the punch block) if sending it out of the building. At the other end, adjust your recorder's input accordingly. If necessary, pad it down. A typical computer audio line input is looking for only -10dB, so a 15dB pad will match the levels. A computer's line input is unbalanced; the line can be unbalanced by grounding one side directly, sending the other leg (through the pad, a 5:1 resistor pair, for example series 4700Ω followed by a shunt 1000Ω, to use standard values) to the input high side. If there is hum picked up you may need another 600:600 isolation transformer at the recorder end. Most logging recorders have balanced inputs to prevent that need. A balanced pad would simply be three resistors in series, such as 2700-1200-2700, with the recorder input across the center 1200. The 2700's should be very nearly equal; any mismatch could show up as hum. Most recorders should be able to accept the full level without padding, but they will need to be adjusted for it.
    The logging recorder at the first installation had loud cooling fans, so it was placed in another room away from the dispatcher. Yours may not need to be placed so far away. The circuit can be modified to balanced impedance output (omitting the output transformers), suitable for shorter cable runs, but hum pick-up could appear as the recorder and the hub will then share a ground connection.
  3. Using a service monitor, measure the transmit deviation of each radio in turn as you key down and adjust VRT1 through VRT4 while speaking in the expected manner into the microphone to obtain full but undistorted modulation. If you speak up close and loud, adjust for full deviation. Then try a more moderate voice level; the AGC should give you nearly full deviation. Then try a softer voice; the modulation should still be clearly readable, but no longer full deviation. If your end user is likely to prefer the soft voice (or tends to lean back and mumble) you may want to increase the setting a little. The mike AGC stage in the desk unit is designed for a typical computer-style mike and has no adjustment. If you need more sensitivity the best solution is move the mike closer or get a better mike. As a last resort the input resistance can be altered.
    If you do not have a service monitor, you should be able to just plug and play, as the factory settings are approximately correct. The radios themselves have modulation limiting, which is not the same as AGC; trying to drive the radio hard will only cause distortion. So the "full deviation" mentioned above is a judicious balance between the deviation meter and the audio quality as judged by the ear, assisted by the scope on the service monitor.

Here is the User/Service manual in pdf format. It is a little out of date in that I decided not to offer an internal speaker driver, instead just use common amplified computer speakers, which have their own driver. Also the STM Radio mailing address now 117 Primrose Lane (else same).

If you examine page 2 of the desktop schematic you will see that four audio lines are used, with commands sent as DC levels superimposed on the audio. I considered using a seperate single serial data line to convey commands for all four radios, but that would require a custom programmed microcontroller in each unit. I prefer to design for simplicity and repairability, and came to the conclusion that the serial-data choice would not be much simpler in layout and considerably less repairable, especially with age. The DC-level design is easy to understand and repair, hindered only by the size of the SMD components, which are chosen to be as large as is commonly available.

If you are interested in having one of these built for you please contact us at STM Radio. The cost is not yet firmly established as the design is still dynamic, with only proof-of-design prototypes built. The desk box will be a small slope-face unit (the one used in the first working unit, which is still in service, either in black, or 'bone' as in the photo). The final hub design will also be a larger box, with all the jacks and switches on the ends instead of on top.

Design philosophy: make it repairable. The parts most likely to fail (the switches and driver IC's) are through-hole mount and replaceable. No proprietary software needed. In fact no software at all, no computer needed. Any competent technician should be able to repair it. No esoteric parts used (though some uncommon ones are, such as dual digital transistors, but these are less likely to fail.)

Created on September 8th, 2013