Hi John, As has been mentioned, the limiter isn't designed to remove band and/or power-line noise.
Some noise blankers (which isn't what the 2B has) can be adjusted to reduce some impulse-type A/C line noise - things that are produced by poorly filtered light dimmers, and/or flourescent or gas-discharge lighting - think mercury-vapor or sodium-vapor lighting with damaged/defective bulbs or ballasts. There are digital signal processing filters that can be added between the radio's audio output connector and the speaker or phones you'd be using to listen to the radio. The general concept is that these analyze the audio content, and work to reduce what's constant and non-coherent or semi-coherent noise. They typically take a couple of seconds worth of audio to work on, before you hear the most significant effect. One that was originally sold as the "Clear Speech" speaker was pretty good, but because it was a very simple add-on, took a cautious approach to what it filtered out. It also didn't know when the radio system was in transmit, presenting no audio to the speaker, the so while you were transmitting, the filter was re-learning the currently available noise characteristics, so each time you unkeyed, the filter would again be presented with audio, and have to retrain, taking 3-4 seconds. I've gotten myself a high-end Timewave filter, which does have a PTT monitoring input, so when it detects you're transmitting, it freezes the retraining aspects of the filter - all the same conditions that were present when you keyed up are assumed to exist when you unkey, so any retraining is minimal. That filter also has selections for different modes, so you get different features and behaviors depending on that selection. Some DSP audio filters also have an automatic notch filter, such that if you end up with someone tuning up in the passband, they will perceive the steady audio tone and mathmatically suppress it. The most general purpose of these take several seconds to start eliminating the carrier, the thought being that the filter doesn't know, and can't be told, that you might be receiving a CW signal - so the filter is slow enough to not trash the signal. The high-end filter I mentioned above assumes that if you're in Phone mode, that any carrier IS something to be filtered, and does so very quickly - enough such that CW sounds sort of like "pings" - up to at least 25 wpm or so. Setting the filter to Morse or Data modes changes that behavior. The high-end filter has an audio CW filter that can be set as narrow as 10 Hz - which is too narrow for 15-20 WPM Morse, because the baud rate of the signal is faster than the bandwidth of the filter can process - but the bandwidth is adjustable from 10 Hz up to about 1000 Hz, as I recall. So unless other stations are really really close in frequency, you can usually find a compromise bandwidth and center frequency to knock down the interferer and copy the station you want. Since the filter is at audio frequencies, it won't directly deal with the station that is within the receiver's RF bandwidth, and is pumping up the AGC (and S-Meter), so the signal you're trying to copy may appear to fade when the other station is transmitting, but other than trying to shift the filter passband (which you fortunately can do with the 2B) to reduce the signal strength of the interferer, there's little to be done about that problem. In short, the audio DSP can achieve the effect you hoped for, in some circumstances, but to get one of the better ones may cost more than you've spent on the receiver. And the better external ones generally are more capable than the built-in audio DSP's present in many of the lower-end transceivers where DSP noise reduction is after the RF filtering. Ones I've used that are not up to the external audio DSP standards include the one in the Icom IC-706 MkIIG and the FT-847. Most of my operating with the external audio DSP has been with a Kenwood TS-440, and it can make a big difference on a marginal signal with that rig. After about 25 years away from being a regular Drake user, I'm coming back to the fold. The audio DSP is also a nice improvement for my TR-7, and I plan to try it on my most recently acquired Drake, an R-4C, too. 73, Bob, KD7NM -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of John King Sent: Wednesday, July 22, 2009 12:18 PM To: [email protected] Subject: [Drakelist] Noise Limiter in Drake 2B I bought my first Drake 2B in 1961, but like a dummy, sold it in about 1968 and regretted it until I was able to purchase a very nice replacement a couple of weeks ago. The replacement works well, but I am trying to determine how effective the NOISE LIMITER should be. I notice no difference in normal band static noise when I switch the Noise Limiter into the circuit. I would like your comments based on actual experience with a Drake 2B with a NOISE LIMITER. My experience with numerous Vintage Hammarlund, Hallicrafters, National and other brand receivers with a noise limiter has been that it is next to ineffective on noise and if the limiting is turned up on an adjustable noise limiter, it degrades gain and audio to the extent that listening to the noise is a better choice. Is that the same performance that I should expect on the Drake 2B noise limiter? Maybe I should look into noise BLANKERS for all my Drakes if that is the case. I know they are not common, but I seem to remember someone developing a blanker circuit for the Drake 4 Line. Anyone have info about that? 73, John, K5PGW _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
