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write your own review of the Michels Engineering Speech Extractor DX-21.
Oct 8, 2005 11:04
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Time owned: 0 to 3 months
I was introduced to the DX-21 by a local ham who was demonstrating one at the local radio club. Being belessed with exceptional hearing and the ability to pull stations out of the mud by ear, I had been previously disappointed by several DSP offerings in the past, which did reduce the noise, but turned the signal into unintelligible mush as far as I was concerned. I has begun to wonder if there was such a thing as DSP that worked for me, by the time this demo was happening. Anyway, to my surprise, not only did the DX-21 reduce noise, it actually improved the signal readability slightly. I was sold, and bought my own unit.
The DX-21 was a breeze to setup with my IC-745. Most radios should be as simple to setup with. Only a single connection from the radio's audio output to the computer, and an external set of speakers was needed, as well as a source of 5-8V DC (I used a 6V plugpack). One caveat on setup is that the polarity of the DC input is not marked (it's centre positive).
Once connected, the DSP-21 is dead simple to operate. There are two controls - a switch to enable or disable noise filteing, and a rotary level control, which adjusts the degree of noise reduction from zero to almost totality. In operation, you simply enable the unit with the switch and then rotate the level control until the desired level of noise reduction is achieved.
On air, the DX-21 was simply awesome. I tested mostly on 40 metres on a weak DX station. I am cursed by very high noise levels, so this was a good test of the unit's capabilities. With the noise reduction enabled, I was able to pull the DX station right out of the mud. Unlike with some other DSP units, using high levels of processing with the DX-21 does not overly degrade the audio, and it's easy to find a good balance between noise reduction and readability. Many stations are actually very readable (though robotic sounding) with the processing wound up full.
Most types of noise seem to be well suppressed. Also, tones and heterodynes are also kept well under control. During my testing, a number of stations tuned up. Their carriers were heard as a brief loud "pip", before being pushed down with the rest of the noise. As a more extreme test, I tuned into a local AM station which was playing music. On switching the DX-21 inline, the noise floor dropped to nothing, but also most of the instrumentals disappeared as well - the majority of the audio was just the vocals! Not a bad demonstration of the unit's ability to pick out the human voice.
I also conducted some quick tests on the local 10m FM repeater. Again, the background noise dropped to nothing. The audio was hardly affected, and even the courtesy tones, being short pips, came through. I also found I was able to operate with open squelch, as the background noise without signal was pushed down very low by the unit.
The only weakness I can pick so far is that the DX-21 is susceptible to pulling in off frequency signals that are in the upper edge (lower if using LSB) of the IF passband, so it doesn't handle adjacent channel QRM well. However, that's not a big issue for most HF operators, as the IF shift and bandpass tuning take care of this problem neatly.
If you're sick of hearing hash while trying to work DX, or get constant complaints from the XYL about "that bloody racket" when the HF rig is running, then the DX-21 might be the answer to your prayers.
1. 13.8V DC supply capability (saves hunting around for a wall wart)
2. Polarity markinds on the unit's case!
3. Ability to reject unintelligible high pitched off frequency signals.
It's now in my list of "must have" shack accessories.
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