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Ch69 revealed
Many users are well aware that the current plan for Ch69 has its limitations. But perhaps less known are the extent of these limitation and their origins.
Before we could design a channel
plan for Ch38 that was comparable or better than Ch69's we needed to
unpick the key technical parameters of Ch69 as our benchmark.
This page describes what we have found!
What did Channel 69 use look like when it first
became available for wireless microphones?
Note that the original plan provided access to only 6.75MHz for wireless microphones. Of this, 3.65MHz was restricted to indoor use and so only 3.10MHz was available outdoors!
Clearly, the designers of the current plan had to work around these constraints. Channel plans were designed for free-roaming (uncoordinated) and for specified locations (coordinated). The free-roaming set consists of the 14 channels that are available on a UK Wireless Microphone Licence.
Over time, some of the restrictions were removed and the band was extended to allow access to the full range from 854.0 to 862.0MHz as other users moved out of the band. We consulted with industry back in the late ‘90s to see if a new plan made sense but everyone agreed that it was best left alone as use was so well established.
How usable is Channel 69's 14 channel plan?
There are 14 channels designated for uncoordinated (or shared) use on our UK Wireless Microphone (UHF) licence but just how usable as a set are they?
Although manufacturers recommend a minimum spacing that is appropriate to their individual equipment, a typical figure is no less than 350kHz.
This table shows the spacing of each channel to its neighbour and indicates which channels are potentially too close to each other to be used safely at the same location.
| Channel 69 14 channel licence set | |
|---|---|
| Frequency list | Minimum spacing (kHz) |
| 854.900 |
0.375 |
| 855.275 |
0.375 |
| 855.900 |
0.275 |
| 856.175 |
0.275 |
| 856.575 |
0.400 |
| 857.625 |
0.325 |
| 857.950 |
0.250 |
| 858.200 |
0.250 |
| 858.650 |
0.450 |
| 860.400 |
0.500 |
| 860.900 |
0.300 |
| 861.200 |
0.300 |
| 861.550 |
0.200 |
| 861.750 |
0.200 |
If we remove some of the 14 channels we can identify a set that meets a minimum spacing of 350kHz. We are now left with 10 channels:
| Channel 69 10 channel licence set | |
|---|---|
| Frequency list | Minimum spacing (kHz) |
| 854.900 |
0.375 |
| 855.275 |
0.375 |
| 855.900 |
0.625 |
| 856.575 |
0.675 |
| 857.625 |
0.450 |
| 858.200 |
0.450 |
| 858.650 |
0.450 |
| 860.400 |
0.500 |
| 860.900 |
0.500 |
| 861.550 |
0.650 |
But frequency spacing is not the only important parameter. We also need to consider interaction between channels even if they are safely spaced apart. This unwanted interference can occur when equipment is operated close together in the same vicinity. It is known as intermodulation or “mixing”.
In the real world these unwanted signals appear on or close to the frequencies being used. To help make sense of this and create a benchmark we have devised a figure of merit that we have called the “Risk Index”. This is calculated according to the quantity of unwanted signals and their spacing from the frequencies we are trying to use. Closer spacings have a higher risk of causing interference so are given a higher weighting in the sum.
This table shows the performance of the 10 channel set we found above:
| Channel 69 ratings | |||||
|---|---|---|---|---|---|
| 2Tx IM(3) and 3Tx IM(3) | 10 channel: UK wide indoor/outdoor | ||||
| 25k0 | 50k0 | 100k0 | Risk Index | ||
| 854.900 | 2 | 4 | 2 |
 4.50 |
|
| 855.275 | 2 | 2 | 2 |
3.50 |
|
| 855.900 | 4 | 4 | 2 |
6.50 |
|
| 856.575 | 2 | 3 | 3 |
4.25 |
|
| 857.625 | 4 | 3 | 5 |
6.75 |
|
| 858.200 | 2 | 5 | 5 |
5.75 |
|
| 858.650 | 4 | 2 | 3 |
5.75 |
|
| 860.400 | 1 | 2 | 4 |
3.00 |
|
| 860.900 | 3 | 2 | 3 |
4.75 |
|
| 861.550 | 3 | 1 | 2 |
4.00 |
|
| Score total | 48.75 | ||||
| Average |
4.88 |
||||
Clearly, this does not give us great confidence in safely using all 10 channels!
It is clear from studying manufacturers frequency plans from popular equipment that they also share this concern. So finally, here is a typical plan from widely used wireless microphone equipment put under the same microscope as above. As you can see this 8 channel set now looks much more robust:
| Channel 69 - 8 channel ratings | |||||
|---|---|---|---|---|---|
| 2Tx IM(3) and 3Tx IM(3) | |||||
| 25k0 | 50k0 | 100k0 | Risk Index | ||
| 856.575 | 0 | 0 | 2 |
0.50 |
|
| 857.625 | 0 | 0 | 3 |
0.75 |
|
| 860.400 | 0 | 0 | 3 |
0.75 |
|
| 861.550 | 0 | 0 | 2 |
0.50 |
|
| 855.275 | 0 | 0 | 0 |
0.00 |
|
| 856.175 | 0 | 0 | 1 |
0.25 |
|
| 858.200 | 0 | 0 | 2 |
0.50 |
|
| 860.900 | 0 | 0 | 3 |
0.75 |
|
| Score total | 4.00 | ||||
| Average |
0.33 |
||||
Summary
The existing 14 channel plan is far from ideal and was designed around the constraints of the time. In practice, only perhaps 8 or 10 channels can be reliably used at the same time without straying off the designated (and licensed) channels!
We believe that the move to Channel 38 provides us with a unique opportunity to improve the number of genuinely compatible channels for users. Take a look at our Moving in to Ch38 page for more details.
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