Idle channel audio distortions, a comparison of the IC-7800, 756Pro III, & 7000

Idle channel audio distortions, a comparison of the IC-7800, 756Pro III, & 7000.

By Matt Erickson, KK5DR.

I discovered this “idle channel noise” when comparing the IC-781 to the 7800. I then wanted to check the other radios I have for the same noise. I set up all three radios to the same settings, on the same band, without NB, NR, or any other features turned on, to make the test as equal as possible.

Here is what I found:

The 756ProIII seems to have the least amount of this distortion noise, the 7000 is the next up in noise level, and the 7800 appears to have the strongest level of this noise.

I need to explain this “noise”, and attempt to describe it here. The noise/distortion is best described as a sound like a flowing stream, or babbling brook. It is low level just above the base-line background noise. This noise does not become apparent until the user reduces the DSP filter bandwidth to below 500Hz or less. It is present in all the DSP based radios I tested to some degree. The noise can be heard only when no signal is present due to the action of AGC when a signal is present within the pass-band.

When I tested the IC-781, which is an analog receiver with crystal filters, no such noise was present, but there was audio distortions generated by the audio amplifier itself, these appear as a high frequency hiss, this is a well-known issue in the 781 and other models of that period. This high frequency hiss is also present in all of the current DSP radios, but is much less noticeable since the audio amplifier has been much improved over time. The DSP audio distortions overlay the hiss.

To some operators, the noise is not an issue, but to others it is fatiguing and annoying. Operators that use SSB most of the time would not likely hear this, since it does not appear until narrow filters are used in modes like CW.

I was not really aware of this noise until I heard the comparison of both analog and DSP, then it became clear, so I went in search of it in my other radios. I did find it in various levels.

The root cause of this distortion could be that the DSP algorithm is primarily optimized for SSB reception and not for CW. Another possibility is that noise power at narrow bandwidths “appears” to be more intense because of the narrow bandwidth alone, the same noise would be present at wider filter widths but has lower density or noise power level per hertz. When the filter is narrowed the noise power must go up in a DSP system, unlike an analog system. Yet another possible source could be what is known as “apperture jitter”, which is generated in the ADC of the DSP system, and or “aliasing”, which is a result of any DSP process in current production. The source could also be a combination of all these items.

It appears that all the current models of DSP based radio have this distortion to some degree, but as DSP improves, it will fade away, just like CW as a primary design concern for RF engineers. Like it or not, that is just the way it is...