Sunday, December 20, 2009

A lot of noise about noise

A very long discussion has been raging on the Elecraft reflector in the last few days over whether the K3 receiver is noisy. This discussion has remained surprisingly well-mannered although there have been one or two instances of someone complaining about someone else making criticisms of the K3 that they can't personally hear or see.

Paul, WW4PT has suggested that the problem is with people who won't use the RF Gain control. While I dare say that may be the answer for some of the people who complained that their K3s were noisy, I found it hard to say if that was the only issue being experienced. Some complained of the K3 being tiring or fatiguing to listen to for a long period. Others suggested it may be due to high frequency artifacts produced by the DSP. (Incidentally, why do people use the word "artifacts" in this way? I thought artifacts were objects made by ancient cultures.)

One benefit of the discussion is that it got a lot of people to experiment to try to put a finger on the problem. I always found the K3 audio to be a bit lacking in bass compared to other receivers, and actually preferred listening to SSB on my FT-817ND. After experimentation I found that the problem was partly due to the external speakers and headphones I have been using.

Early K3s used too small values of coupling capacitors in the audio output stages resulting in the loss of a lot of bass. Modifications have been published for the line output and speaker output, and I have carried out those mods on my K3, but the headphone output requires a modification to the DSP board that is deemed too difficult for DIY. The only solution is to return the DSP board for exchange, which Elecraft offers for $69, but by the time you add in international shipping, taxes and tax collection fees for those of us in the UK that will come to more like £69, which is quite a lot to spend to remedy what was arguably a fault in the original K3 design.

Fortunately there is a cheaper solution: use high impedance headphones. And it just so happened that I had a suitable pair of Sennheiser headphones already, which I bought to use with a Sony Walkman CD player. Even with these headphones I still prefer a "warmer" sounding audio, but the K3's receive equalization settings provide all the audio tailoring I need.

During the course of my experiments I used Spectran to analyze the audio output of the K3. What I found was interesting. The K3 has a text-book filter response with steep sides and a flat top, and virtually no noise (more than 100dB down) outside the filter passband. (See the screen grab above.) By contrast, the FT-817 frequency response falls away more gradually, containing considerable energy even at frequencies of 10KHz or more. Don W3FPR did some similar tests with his FT-817, his K2 and some Yaesu radios and got similar results. The absence of unwanted high frequency energy in the K3 output makes the receiver sound clearer, but it also allows you to hear noise more clearly, where other receivers produce a soothing mush. This might also be a reason why some people claim the K3 sounds "noisy".

After making some equalization changes, swapping to the high impedance headphones and getting rid of the external speakers my K3 sounds great. I just need to find some small external speakers that have a good bass response when used with the K3.

4 comments:

Steve GW7AAV said...

Artefact - Commonwealth English spelling of artifact

Artefact - In natural science and signal processing, an artefact is any perceived distortion or other data error caused by the instrument of observation.

Artefact - An object made or shaped by human hand; An object, such as a tool, weapon or ornament, of archaeological or historical interest.

Artifact is a Norwegian industrial metal band.

An artefact in the Unified Modelling Language (UML) is the specification of a physical piece of information that is used or produced by a software development process, or by deployment and operation of a system.

An artefact is the error or misrepresentation introduced by a technique and/or technology. For example a poor pseudo-random number generator would introduce artefacts into statistical research models.

Artefact - In the Dungeons & Dragons fantasy role-playing game, a magic item is any object that has magical powers inherent in it.

Artefact - A flaw or defect in a digital image created by errors in the video processing or noise/interference in analogue circuits

Artifacts (1994) is a tribal ambient album by the American artist Steve Roach.

Artifacts are a now-defunct hip hop duo consisting of El Da Sensei and Tame One.

Artefacts are misrepresentations of tissue structures seen in medical images produced by modalities such as Ultrasonography, X-ray Computed Tomography, and Magnetic Resonance Imaging.

You did ask! ;0)

g4ilo said...

That's a lot of facts about artefacts! Thanks, Steve.

But in ham radio usage the term appears to be used to describe "any perceived distortion" without any suggestion that it is "caused by the instrument of observation" (i.e. the operator's ears.)

Paul said...

"Paul, WW4PT has suggested that the problem is with people who won't use the RF Gain control."

Actually what WW2PT said was, there might very well be a real issue with noise. My comments about RF Gain were more a reaction to suggestions that it's either a bother to use RF Gain or a relic of the past. It might not be the only issue at play here, but if you never back off on the RF Gain you will experience higher background noise! I'm just sayin'... :-)

I've been using a pair of Roland MA-8 powered speakers that have a fairly wide frequency response. For giggles, I swapped them out with a pair of JRC NVA-515 speakers which have a much lower roll-off at the high end, and found the perceived background noise to be much less apparent. An even more drastic improvement when I swapped my usual pair of Audio-Technica stereo headphones (32 ohms, 20 Hz-21 kHz) for a set of JRC ST-3 communications phones (600 ohms, 20 Hz-18 kHz). Unfortunately the ST-3 phones are mono so AFX is rendered useless until I can re-wire them for stereo.

I think sometimes we forget this is HF radio; noise is part of the game. Using speakers and phones designed for hi-fidelity and accurate reproduction of a very wide frequency range is not always advisable.

73,
Paul WW2PT

g4ilo said...

Thanks for the comment, Paul. Sorry about getting your call wrong. Actually I have a suspicion that using the K3's so-called RF gain control which is actually (apparently) an IF gain control will not make much difference as it does not appear to be independent of the AGC Threshold. I was making some observations using the XG1 and Spectran software, to observe the effect of different AGC settings. But I have spent a lot of time on this already and am a bit tired and likely to jump to the wrong conclusions so I'm going to leave further investigation of this for another time.

You are probably right that speakers and phones designed for hi-fidelity aren't ideal for this application. But they are much more readily available and better value for money, and hi-fi headphones are much more comfortable. I guess that's why we have RX EQ.