If and when the proposed, fully operational 10kHz frequency spacing system on the 36MHz band is introduced with both 10kHz and 20kHz keys in use simultaneously, operation of the Silvertone Keyboard system of frequency control will for the first time in nearly 30 years revert to the operational mode for which it was designed. That is as a variable bandwidth system of frequency control.

For the past 20 years or more, the keyboard has functioned essentially as a simple fixed bandwidth system in which the key width and the slot spacing correspond. Thus for many years we ran a simple 20kHz slot spacing with 2" keys. The pegboard system is another fixed bandwidth system.

A variable bandwidth system calls for the mixing of R/C systems of various bandwidths. This means that keys of various widths will occupy the board simultaneously and a natural consequence of this being that a wider key will block out one or more of the narrow keys or slots. This means some apparent inconvenience for operators of narrow band systems or odd or even slots and calls for a more gracious approach to frequency usage.

The key to this more gracious approach is to consider the 10kHz allocation as a means of increasing the number of flights per hour on any flying field. Secondly the more frequencies available and in use, the less likely is an accidental shoot-down or frequency clash, for in most clubs each member should be able to have their own personal frequency. We now have 86 frequency spots on 29 and 36Mhz and there are not too many clubs with that many active members. True, sometimes it will be difficult to insert a key however there are ways to alleviate some of these problems. Grouping the narrow band sets or odd and even frequencies at one end of the board is one such method.

As the MAAA will impose certain restrictions on the operation of 10kHz spacing, especially with regard to high speed and heavy models, it makes sense for clubs to organise themselves in such a manner that club activities operate under the old 20kHz, restriction free frequency spacing, reserving 10kHz operation for busy interclub and national events. One way to achieve this is for clubs to adopt the "Odds" and "Evens" system. Thus if 50% of clubs adopt the "Odd" frequencies for their club activities and 50% the "Even" frequencies, then when clubs mesh together at large events where 10kHz operation will have clear advantages, there will be a minimum of frequency clashes. Plug-in crystals will sort out those clashes that do occur.

The Issue 7 keyboard has been designed with this system in mind. Whereas the Issue 5 keyboard was laid out with the paired slots matched together in the more technically correct Odds paired with Evens slots, the Issue 7 keyboard has Odd slots paired with Odd slots and Evens with Evens, thereby making possible the introduction of the Odds and Evens system in Australia. This system of club organisation is very widely used in the UK for example, where 10kHz is approved for club use by the BMFA. Many UK clubs however chose to do their day to day flying on the more relaxed 20kHz spacing, reserving the more demanding 10kHz operation for busy events such as the National etc. The UK frequency band is not sufficiently wide enough to introduce 455kHz intermodulation therefore the Silvertone UK keyboard (Issue 6) does not feature paired slot for this reason.

NB. Please note here that paired slots can only be used with 20kHz keys as the intermodulation product of two transmitters 455kHz apart is offset from the 36MHz, 10kHz frequency spacing by 5kHz and therefore falls in the middle of the two frequency slots closest to the intermodulation product. Thus the operation of paired slots depends entirely upon the extra width of the 20kHz key to block off both of the effected slots. R/C systems operating with 10kHz keys cannot use paired slots.

SILVERTONE KEYBOARD DESCRIPTION.

So what is the Silvertone Keyboard system of frequency control and how does it work?

The Silvertone Frequency Control Keyboard system is essentially a graphic representation of a frequency band laid out on a grid of X (inches or millimetres) = 1kHz. From 1969 until the release of the Issue 5 Keyboard, the standard grid was 0.1" = 1kHz. In the case of the Issue 5 Keyboard a new standard of 1.27mm (0.05") = 1kHz was adopted to overcome objections raised about the old 0.1" = 1kHz standard 36kHz Keyboards. This standard has been retained on the Issue 7 Keyboard. Briefly, the keyboard operates in this manner;

Frequency keys of a width proportional to the bandwidth of their R/C systems are issued to all club members. These keys are slid into the appropriate slot on the keyboard in order to reserve the frequency block required by the operator for safe operation of his system and the TX must not be turned on until the key is in the keyboard. Only when the TX is turned off is the owner of the TX allowed to remove the key from the board.

For those wondering about the 1.27mm standard instead of the apparently more logical 1mm here are the reasons. One inch (25mm) key moulds already exist. Also with the looming release of 10kHz bandwidth systems a 10mm key would be called for if the more logical 1mm = 1kHz standard were adopted. The window in this key would be 8mm or less, thus the channel numbers would be tiny and the keys themselves a little too dinky. A 12.5mm key is small enough without going any narrower. Finally what we end up with by using 1.27mm (0.05") = 1kHz is essentially a half scale keyboard.

ANALYSIS OF SHORTCOMINGS OF THE 0.1" STANDARD KEYBOARD.

During our investigation into the validity of the aforesaid objections, it became obvious that the standard that has served us so well for 30 years has finally become unsuitable for modern conditions on the 36MHz band. The 27, 29 35 and 40 MHz bands are not effected and the old Issue 1 - 4 36MHz Keyboards are still current. Issue 4 Keyboards will remain available for clubs preferring the 0.1" standard and 2" keys will remain available for older Keyboards. The Issue 7 Keyboard uses a 25mm key for a 20kHz system. Perhaps a few words of explanation regarding the problems relating to the 0.1" 36MHz Keyboard are in order here.

Over time, as new 36MHz frequencies have been released, the length of the 36MHz keyboard has gradually increased. Eventually a single Keyboard spanning 600kHz became too large and mechanically unstable and the more practical approach of splitting the board into two halves was adopted. This worked well with 20kHz frequency spacing for many years, however when the MAAA released the new 10kHz spacing but for use with 20kHz keys, many complications arose. Further complications arose as a better understanding of intermodulation effects found their way into the R/C fraternity.

The objections to the 0.1" Keyboard centred on the use of a guard key on channels 630 and 631 and 645 and 646 if paired slots are used. Price was another major consideration. The argument over paired slots/open slots is a controversial one and not so easily settled. The guard keys on 630/631 and 645/646 are required because of the break in the boards at these points. Due to the mechanical anomaly caused by the break it is possible to operate two 20kHz systems 10kHz apart, with one 20kHz key at one end of the keyboard and another 20kHz key at the other end of the second board. This anomaly thereby breaching the MAAA safety guidelines. The guard keys prevent this safety breach from occurring.

Incidentally these complications are perfectly normal in a frequency control system designed to cope with R/C units of various bandwidths and deal with intermodulation considerations as well. Even when we go to the full 10kHz system with sets approved for use with a 10kHz key there will still be many of the older 20kHz bandwidth sets on the flying fields using 20kHz keys so the guard key will be with us for some time.

As two of the foregoing objections are related to size of the boards it became obvious that reducing the grid size would be a distinct advantage. In effect we now have a half scale Keyboard which accommodates all 59 slots on a single Keyboard. Reducing the grid size removed the need for a guard key on 630/631 and slashed the price of the 36MHz Keyboard by almost 50%. There is still the need for a guard key on 646/647 if paired slots are used for this is unavoidable as long as 20kHz keys are in use.

OPERATIONAL CONSIDERATIONS.

In operation all R/C systems are tested for bandwidth by Model Aeronautical Association of Australia (MAAA) approved testing stations and a frequency key is issued proportional in width to the system bandwidth. Hence a 20kHz system is issued with a key twice as wide as a 10kHz system. Therefore when the key is inserted into the Keyboard, a block of frequencies are reserved for that R/C system equalling the system bandwidth. In this manner R/C systems of various bandwidths may operate on the same field with complete safety. First introduced in 1969, the Silvertone Keyboard system of frequency control has since become the accepted national standard for frequency control at all officially sanctioned contests in Australia.

It is interesting to note the progress in technology. Our first 27MHz Keyboard featured 40kHz, 30kHz, 25kHz, 20kHz and 15kHz keys. We are now using 20kHz keys as standard and are considering the introduction of 10kHz keys in the near future.

Here in Australia we are allowed full access to a complete block (36.0 - 36.6MHz) and how we divide that block is a matter for the R/C modelling governing body (MAAA) to decide. The current ruling is that 59 frequencies may be used, spaced at 10kHz separation, however for safety reasons 20kHz bandwidth must be reserved for each system. This has resulted in some serious complications in so far as frequency control is concerned.

A second complication arises due to the fact that the band spans 600kHz which means that there are fourteen pairs of frequencies spaced 450/460kHz apart. These frequencies when operated simultaneously give rise to Receiver Mixer Intermodulation (RMI), a phenomenon in which the two incoming transmitter signals mix together in the receiver mixer and produce a signal 5kHz removed from the receiver intermediate frequency (IF) of 455kHz. This can result in interference under some conditions, particularly in receivers with a bandwidth wider than 5kHz.

RMI does occur when two transmitters are allowed to operate simultaneously and separated by 450/460kHz apart and it will occur in all single conversion receivers operating on that field regardless of frequency of the receiver or the frequencies of the intermodulating pair of transmitters. Thus a single pair of transmitters operating 450/460kHz apart will effect all 59 channels to some extent. The two receivers most at risk are the two separated by 450/460kHz.

However the practical effects will vary widely depending upon a host of very subtle factors. (See Silicon Chip Magazine articles on intermodulation for a more complete explanation.) Foremost in this list is capture effect. Capture effect locks out interfering signals that are weaker than the controlling signal. FM receivers have a better capture effect than AM receivers and it is this factor that gives the FM receiver the edge over AM receivers on 36MHz. FM receivers are relying entirely on capture effect and IF bandwidth to mask the effects of RMI. This is what makes demonstrating RMI on the flying field so difficult. If however conditions arise which favour the RMI signal over the correct transmitter then capture effect will lock out the correct transmitter and control will be lost. Alternatively a receiver with wider than normal bandwidth may operate on a field at some point. This is why RMI presents a potential hazard and should not be allowed to occur on model flying fields. Nobody can predict when conditions will become unfavourable for safe operation.

AM receivers have not been very successful on 36MHz because they are more seriously effected by RMI. Secondly most AM receivers are not as highly developed as the modern FM RX and do not feature the expensive narrow bandwidth ceramic IF filters. The main effect of RMI in AM receivers will be reduced range under some conditions. To capture an AM receiver is a more difficult proposition than to capture an FM receiver but the AM receiver is more susceptible to interference from a lower power signal. Glider fields where the transmitters are widely scattered around the flying field present the most serious problem in this regard. The MAAA has never placed a blanket ban on AM receivers on 36MHz, even though the issue has been discussed. Silvertone recommends that AM receivers not be used in locations where two transmitters are allowed to operate simultaneously 450/460kHz apart. This stipulation does not apply to frequency bands less than 450kHz wide such as the 27, 29, 35, and 40MHz bands. On these bands AM receivers give excellent results and can be used with complete safety. They can also be used quite safely on 36MHz in clubs running Keyboards with paired slots.

Thus some clubs have opted to open all 59 slots as per MAAA guidelines and allow the use a second (lockout) key in the channels 450/460kHz removed instead of operating paired channels. In effect the MAAA is leaving the choice of operating system in the hands of each modeller.

Using two keys relies on each operator to do the right thing, a somewhat risky procedure. How does a tyro R/C flier know which slot to put his second key into? And why carry a second key? And why state that there is no problem with RMI but if any flier is unsure then use a second key to be on the safe side. Either there is a problem or there is not a problem and to err on the side of caution is surely the wisest procedure. Many clubs run paired slots and the issue of intermodulation is a controversial one. As we gain more experience with the new frequencies we will eventually gather sufficient evidence of a practical nature to settle the matter completely.

In the meantime Silvertone does not agree with the two key policy as we believe that RMI does present a possible interference risk, especially with AM receivers and that two transmitters 450/460kHz apart should not be allowed to operate simultaneously on any R/C site. Closing slots 646 - 659 and forcing these transmitters to use paired slots absolutely guarantees that only one of the intermodulating pair can operate at any given time, thereby completely eliminating the potential risk of RMI.

However here at Silvertone we understand that we may be a little too conservative and as an aid for clubs who wish to stay in line with MAAA guidelines, we have given clubs the choice of paired slots or two keys on the new Issue 7 Keyboard. As an aid to users of the second key in clubs adopting the all 59 slots open policy we have provided prompts which indicate the correct slot for that key

There are two methods of setting up the Silvertone Issue 7 Keyboard.

The method recommended by Silvertone calls for the closing of slots 647 - 659 and forcing the two intermodulating transmitters to use a single slot. In this system a guard key must be used to eliminate the risk of two 20kHz transmitters spaced 10kHz apart operating simultaneously on 646/647, thereby breaching MAAA bandwidth safety guidelines

The method recommended by the MAAA calls for opening all 59 slots, allows simultaneous use of the two intermodulating transmitters however with a Lockout key if so desired. This system requires no guard key.

1. Block off slots 647 to 659 by using a small screw or gluing a piece of perspex, balsa etc in the opening of these slots.
2. Place a 25mm key in one of the two slots marked "G". The second "G" is located under the 646 slot. This key stays in the board at all times and becomes the guard key. It is a good idea to paint or otherwise mark this key so that it is obviously very distinct from a normal key. The Silvertone Issue 7 Keyboard is now ready for use.
3. In use the transmitter operator simply walks up to the board and checks to see that the required slot is clear. If it is clear then the key may be inserted into the appropriate slot. Only after the key is safely in the slot is the operator allowed to switch on his transmitter. If the slot is in use then he must not switch on his transmitter.
4. In the case of operators using frequency slots 647 to 659 then the key is inserted into the paired slot with the appropriate frequency number marked in Red. These slots are located on the left-hand side of the Keyboard.. A Red key is available with a window correctly located to block out the lower black number and show only the correct Red slot number. It is strongly recommended that operators using the frequency slots 646 to 659 use the Red key.
5. Operators using the frequency slots 646 and 647 must use the guard key to prevent inadvertent operation at 10kHz spacing. For example operators using 646 will insert their key into slot "646/G" thus blocking use of 645 but not 647 as this slot is still physically open at the left hand end of the board. However this slot is electronically only 10kHz away and if an operator on 647 inserts a key into 647 a safety breach will occur. Therefore the Primary key must be inserted into the 646 slot and the guard key moved across into 647/601. Operators using 647 must shift the guard key to the slot marked "646/G" located at the extreme right-hand end of the Keyboard. This prevents a 646 key being inserted in the 646 slot whilst the Primary 645 key inserted into slot 645 prevents a 646 key being inserted into the 646/G slot. If either of these two slots are occupied then the second transmitter must not be turned on. The guard key can only be moved by the first transmitter operator to use either 645 or 646.
6. When the transmitter is switched off the Primary key must be removed from the transmitter immediately. The guard key stays in the keyboard at all times.

1. As delivered, the Silvertone Issue 7 Keyboard is set up correctly for immediate use with Method 2.
2. In use the operator simply checks to see that one or both of the slots required are free and if so inserts the key(s) into the appropriate slot(s). The Primary key is inserted into the appropriate slot marked in Black. The Lockout Key if required, is inserted into the slot marked with the appropriate Red number at the opposite end of the Keyboard. Thus if 601 is the transmitter operating frequency then the Primary key is inserted into the slot marked 601 in Black at the left hand end of the Keyboard. The Lockout Key is inserted into the slot marked 601 in Red at the right hand end of the Keyboard. Likewise, an operator on 659 would insert the key into the slot marked 659 in Black at the right hand end of the board and the lockout key if required would be inserted into the slot marked with the Red prompt 659, located at the left hand end of the Keyboard. A Red key may be used as the lockout key if required.
3. When the transmitter is switched off the key(s) must be removed from the Keyboard immediately.

The Issue 7 Keyboard is the latest in a long line of Keyboards and will provide clubs with the most cost effective, sophisticated yet easy to use frequency control system available anywhere in the world. It is the only system that can cope with variable bandwidth and intermodulation considerations and over the past 30 years has proven extremely flexible and adaptable. When carried through to the logical conclusion, which entails plugging the unused key into the transmitter to cut off all power to the transmitter (Frequency Interlock), it provides a level of flying field safety unmatched in the international marketplace.

Frequency Interlock may be summed up quite simply: There are only two places for the key; If the key is in the board it is safe to switch on - if the key is in the transmitter it is impossible to switch on.