AKG Acoustics WMS 400 manual Far-near difference, Here is a proven way to add clean frequencies

AKG WMS MULTICHANNEL TECHNOLOGY

The far-near difference

Unlike a hardwire microphone, even the best wireless system is susceptible to dropouts because the relative positions of persons and objects within the coverage area will change constantly during an event. At any moment, shadow loss and signal cancellation may coincide with intermodulation and sideband noise in such a way as to cause a dropout. The specified dropout probability under such conditions for the WMS 4000 is less than 0.1%.

This is equivalent to a downtime of one third of a second per hour. Noticing such short interruptions is difficult even for an experienced tonmeister.

However, this low dropout probability can only be achieved inside a Faraday cage in which no other RF or digital equipment is used along with the WMS 4000.

As mentioned above, the dropout probability or immunity to interference depends primarily on the antenna positions. Finding the best antenna locations is always a balancing act. If the antenna is too far away from the stage the re- ceived signal will be too weak and drowned out by receiver self-noise and other unwanted disturbances during a deep fade. If you install the antenna too close to or even on the stage, however, the dreaded intermodulation whistles

generated by the receivers and transmitters may become audible.

Here is where the far-near difference comes in. It is the dif- ference between the receiving antenna’s distance from the rearmost point on the stage where a transmitter will be used and the receiving antenna's distance from the front edge of the stage.

The Applet http://www.akgfrequency.at/antennaposition/ allows you to compute optimum antenna positions from known far-near differences.

To ensure intermodulation-free multichan- nel operation, AKG programs sets of fre- quency presets into each WMS 4000 system. Each preset contains groups of fre- quencies that do not disturb one another by intermodulation.

Depending on local frequency plans, up to 18 channels can be used simultaneously within each 30 MHz subband. "Enhanced- security presets" with 14 channels or less within the respective 30 MHz subbands are available for some countries.

Although all the frequencies of any preset are approved for use in the respective coun- try, you are still required by law to obtain a permit from the local authorities before you can use the system.

Adding channels by duplicating an existing frequency structure

Here is a proven way to add clean frequencies:

1.Refer to the Theater Frequencies table below and start with a frequency group you are using on stage A, for instance, Band I with 18 subchannels.

2.Add to each frequency half the minimum channel spa- cing. (In this example, the minimum channel spacing for a group of 18 frequencies is 300 kHz, one half of that being 150 kHz.) The resulting group for use on stage B (Band I + 0.15 in the table below) has the same pro- perties as the original group.

Provided the two rooms are separated by a brick wall at least 8 inches (20 cm) thick, you can use the new frequen- cy group you derived by adding 150 kHz without risking intermodulation distortion. By repeating this procedure for the other two frequency groups used on stage A, you will obtain the table shown below with 108 subchannels in Bands I, II, and V.

Theater Frequencies

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	Stage A			Stage B
	AT 1 preset frequencies		AT 1 preset frequencies increased by 150 kHz (0.15 MHz)
Band I	Band III	Band V	Band I + 0,15	Band III + 0,15	Band V + 0,15
650.850	720.700	790.900	651	720.85	791.05
651.350	721.200	791.400	651.5	721.35	791.55
654.850	724.700	794.900	655	724.85	795.05
656.350	726.200	796.400	656.5	726.35	796.55
667.950	737.800	808.000	668.1	737.95	808.15
673.250	743.100	813.300	673.4	743.25	813.45
650.150	720.000	790.200	650.3	720.15	790.35
665.050	734.900	805.100	665.2	735.05	805.25
666.050	735.900	806.100	666.2	736.05	806.25
675.450	745.300	815.500	675.6	745.45	815.65
672.450	742.300	812.500	672.6	742.45	812.65
676.050	745.900	816.100	676.2	746.05	816.25
650.450	720.300	790.500	650.6	720.45	790.65
652.450	722.300	792.500	652.6	722.45	792.65
658.150	728.000	798.200	658.3	728.15	798.35
661.950	731.800	802.000	662.1	731.95	802.15
663.350	733.200	803.400	663.5	733.35	803.55
671.150	741.000	811.200	671.3	741.15	811.35

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