The 195 and 200 series receivers had a dynamic noise reduction circuit that looked at RF level, audio level and audio frequency. Those three variables went through an analog multiplier and the result moved a variable high frequency filter up the frequency spectrum. Higher RF, higher audio level and higher audio frequency moved the high pass filter roll off, higher in frequency. The trick was to get the filter out of the way before the listerner could detect the roll off, so the attack time of the filter is less than 0.5 ms. In addition, for any significant high frequencies, the filter is 2 octaves above those high frequencies. There is a little different description in all the manuals for the receivers and is described as a trimode filter.
The filter helped remove compander breathing and "halo" effects around the audio in weak audio conditions. Some designs accomplish this with substantial transmitter pre-emphasis and receiver de-emphasis, such as our 187, 190 and IFB series. This can lead to other problems at high audio levels with high frequencies, most easily shown with the dreaded key test. The down side to our filter method, is that at very low audio levels with very little high frequency content, the 200 system's high frequencies are rolled off. These audio levels are generally so low that they typically show up only when listening to room noise or microphone self noise. The 200 systems also made lavalier microphones sound quieter than they really were. The other drawback, was that it was possible to hear the filter working if the transmitter gain was turned way up in a moderately quiet room, so the self noise of the lavalier and the room noise was high enough to cause the filter to move up and down in response to the random high frequency noise. This was audible as a moderate roughness to the noise.
Just for the record, all the 195 and 200 series have had a basic flat response to 20 kHz or more. The system roll off at 20kHz is not what testers are hearing. ( I base this statement on the fact that some systems with highly regarded audio, roll off before 20 kHz and in one case before 16 kHz and I'm not aware of complaints that these systems are muffled.) At high audio frequencies, the tri-mode filter itself is out past 40 kHz. The reason for the roll off above 20 kHz in the basic system is to prevent supersonics from messing up the compander circuits.
The 400 series does not use a compander or pre-emphasis and so the tri-mode filter was left out. We figured this would help is in competitive comparisons at very low audio levels by making the system sound more open and capable of accurately reproducing small sounds and room noises. However, when some of the beta testers compared 200 and 400 series systems side by side, using a common microphone, they were bothered by the fact that the 200 sounded quieter. (Heart attacks at Lectrosonics.) Further trials by the testers, suggested by a very worried engineer, convinced them that the noise was indeed self noise in the microphone and not noise in the 400 system. This is not to imply that the 400's are as quiet as a wire.
Here came the suprise to the recovering engineers; fully half the beta testers wanted the noise reduction left in, in order to reduce the self noise of their favorite lavalier. Since we don't want to lose low level competitive comparisons because of of lack of "air" or transparency, we have compromised with a menu selectable tri-mode filter. Since users have been "hearing" this noise reduction system for 15 years in all our wideband systems, we have decided to implement it with the same parameters in the 400. The noise reduction is menu selectable at three "strengths".