[TenTec] Orion Noise Reduction

Thu May 4 14:35:40 EDT 2006

Grant (NQ5T) did some careful measurement of the performance of the noise reduction in the current V2 (2032?) implementation in the Orion II. Although noise reduction is not the number 1 capability of the Orion DSP for me, I have gathered some information and data on the noise reduction in the Orion - running V1.373b5. With the many changes in the DSP firmware it is important to remember that operation of things like noise reduction can, and often do, change from release to release. When I say "the Orion" - I mean the Orion running 1.373b5. 

The Orion uses a Least-Mean-Squared algorithm to drive an adaptive filter structure in the IF chain. One write up about the NR in the Orion said that the "Multi Knob" selected one of nine different filters - I think that is basically correct. It also said that once the filter was "formed," even if you changed the Multi-Knob, the filter did not change. I think that was correct if the signal in the bandpass does not substantially change. The Orion noise reduction early documentation indicated that the "aggressiveness" of the filter adaptation was changed with the Multi Knob. I relate that to the "time to complete adaptation of the filter." With the multi knob in the 1 position, the filter is formed quickly. With the Multi Knob at 9 it takes longer for the filter to be formed. You can see the impact of the knob setting on the filter design by using SPECTRAN to look at the audio output signal while turning the NR on and off with different settings on the Multi Knob and looking at different kinds of signals in the IF bandpass. With the filter in the 1 position it appears that the Orion NR creates either a lowpass or highpass filter - based on where the significant signal power is. Lowpass if the signal energy is lower in frequency and highpass if the energy is higher in the frequency spectrum. When the Multi Knob is set to 2 or above, it appears that the filter created changes to a bandpass filter - which takes longer (But still very fast) and uses more DSP cycles. The higher the Multi-Knob setting the tighter the filter appears to get (Narrower and steeper skirts) when looking at a steady single frequency signal. When looking at an SSB signal you can see increased noise reduction in the color (Depicting the level of energy by frequency) of the waterfall display. The higher the Multi Knob number, the longer it takes to create the filter. Therefore on rapidly changing signals - frequency and amplitude - the harder it is for the LMS algorithm to keep up with changing the filter as the error signal changes. In many - most? - cases the multi-knob set to 1, or low numbers, should allow the noise reduction filter to adapt to SSB signals better with less distortion being created while the filter adapts (I really hear no distortion with low numbers on the NR setting in the Orion). Slower changing signals, like CW, can benefit from higher settings - which create tighter filters around the signal and reduce the noise to a greater degree. Using SPECTRAN you can see the filter get tighter around a CW signal as you increase the Multi Knob to 9. 

The Orion NR apparently uses a technique which causes the filter  to collapse when the signal is completely removed. This causes the background noise to be eliminated completely, such as between CW elements. 

The Orion noise reduction is within the AGC loop. Therefore when NR is engaged the noise signals are taken out of the IF bandpass and they do not affect the AGC level(S Meter drops). If you use a weak signal in the band pass you can actually see the level of the weak signal increased when the NR is turned on due to the AGC increasing the gain of the receiver due to the drop in the noise power. Thus the phrase - the signal popped up out of the noise - noise goes down and the gain goes up - note the gain being increased doesn't help the S+N/N - only the noise going down - more then the signal. 

Just a couple of observations about NR in the Orion with V2.056. If you look at the performance of the noise reduction with the new DSP code you will observe the following things:
1. It appears that only a lowpass filter is ever formed - regardless of the setting of the Multi Knob. This is true even if the signal energy is in the upper end of the frequency spectrum. So if the signal of interest has significant high frequency components, they are attenuated by the filter.
2. It appears that as the Multi-Knob is increased in number, moving towards 9, the lowpass filter skirt becomes steeper.
3. The NR function is no longer in the AGC loop. Therefore when the NR is engaged there is no change in the S-Meter/AGC level and if the signal and noise are reduced the AGC does not increase the gain of the receiver.
4. Engaging NR in V2.056 creates significant attenuation in the total signal and noise.

Let me say that I know that manually reducing the bandpass for CW operation can be more effective in improving the S+N/N then the NR of the V1 or V2 code. And, adjusting the bandpass for the speech frequency of the person you are talking to on sideband can also be effective in improving the S+N/N (But pretty awkward). However, I do find the NR on the Orion with V1.373b5 to be helpful in some operating situations - both on sideband and CW. I also find  the performance of the NR depends a lot on the noise and the signal  -  you can also see these effects by looking at SPECTRAN while tuning different signals on different bands with different background noise. NR is also dependent on other bandpass adjustments in the Orion. If you narrow the bandpass of the Orion manually (Reducing the noise bandwidth) the NR algorithm will "starve for noise" and the filter that is built will be ineffective due to the LMS error signal going to zero very fast. 

Observing signal power levels using the SPECTRAN waterfall display can be very helpful in seeing the effect of the NR algorithms. Depending on where the starting bandwidth is, I am confident that the NR algorithm in the Orion (V1.373b5) can improve the S+N/N for CW signals.
I also believe, from looking at the SPECTRAN data, that the S+N/N can be improved for SSB signals by using the Orion NR.

I think Grant's (NQ5T) data tells a story that is similar to my observations with V2.056.

Finally, TenTec is probably the only place where the real data about the Orion NR is known. I am aware that the same observation can come from different causes. The data above is from observations. The conclusions could have errors.

73,
Merle - W0EWM

Disclaimer: I like my Orion running V1.373b5, even though it has some stability issues. I don't plan to sell it. I plan to continue to work with TenTec to continue to improve the performance and functionality of the Orion V2 DSP code.