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Creator
| | Posted on Friday, January 30, 2004 - 1:05 am: | 
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I appreciate the help. No matter where I research I get vague answers. Can someone spill the beans so crystal clear that I can say " Oh THATS what there for, Ah HAA!"..
1. What will happen If I turn ALC up or down?
2. What will happen if I turn APC up or down?
Dont forget I want to say..." Oh THATS what there for, Ah HAA!" after I read your response.
Thanks,
Rob |
Bruce
| | Posted on Friday, January 30, 2004 - 7:34 am: |
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ALC controls the total transmitter gain so that a swing in audio level does not distort the output .... it is the transmitt ver of AGC. Hummmmm APC Im missing something or know it by another name. |
Tech8541
| | Posted on Friday, January 30, 2004 - 9:43 pm: |
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APC = automatic power control. if this is for the 2510/lincoln (the only export radio i know that uses the term apc), it is used to control cw output power. in these radios the apc circuit is active in am mode. i have a swing mod for these radios that modifies this circuit to allow greater modulation.
ALC = automatic level control. the only thing that i can add to what bruce said is that the term alc is related to ssb. |
Creator
| | Posted on Sunday, February 01, 2004 - 12:03 am: |
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I found something interesting on ALC..
An ALC (automatic level control) is incorporated on sideband radios to limit the maximum PEP output. A radio with a miss adjusted or defeated ALC can clip in a manner similar to the illustration of over modulation on an AM transmitter. When the signal clips, information is being lost. It may look good on the meter, but it doesn't sound right at the receiver's end. Sideband is a different animal from AM. It doesn't take as much audio from a mic to carry a great distance. Therefore sideband is more susceptible to background noise and an overpowering mic gain adjustment is counter productive.
In understanding how to adjust your audio on sideband you must first know what the receiver AGC (automatic gain circuit ) does as it receives a sideband signal. The AGC has the job of varying the receivers gain to prevent overloading from strong signals. Without it, signals from around S-5 and above would start or completely overload the receiver rendering the signal unintelligible. In controlling the gain the AGC keeps the audio compressed to a somewhat constant maximum level. The AGC has a recovery delay time that varies from radio to radio. This delay keeps the gain according to last signal received. If a weak signal comes in after a strong one, the first word or two may be very low or completely missed.
Now, understanding that to increase a signal by 3db the power must double and a calibrated "S" meter reads I- S unit for every 6db. We can use the reverse for an example. If we cut the output power in half we loose 3db of signal. Receiving an 18 watt signal at S-9 we would receive a S-8 at 4.5 watts, a S-7 at 1.125 watts, a S-6 at .14 watts, a S-5 at .07 watts and a S-4 at .018 watts. That's 18 milliwatts to give a S-4 signal and 1000 times that (18 watts) to give a S-9 on the meter. Background noise that wouldn't be close to registering on your RF meter would be received with a decent signal strength. Imagine if the background was flicking the meter at around I watt, approximately 20% of full output. Your background noise would be giving a S-7 and your peak signal only 2 "S" units more. The background is desensitizing the receive with the AGC kicking in, making your peaks have less impact or punch. The background would get mixed in with the voice and make it difficult to understand what is intended to be heard. On sideband the name of the game is signal to noise ratio, the less noise the less distraction from the intended transmission. There's enough noise out there to get through without creating your own to overcome.
In setting the ALC it is good practice, with a loud whistle, to open the ALC up all the way and then back it down until you see a reduction of about 2 watts. This will give you peak performance without clipping in most cases. Some radios won't distort with the ALC wide open while others will distort terribly once the mic is cranked up. If you control the mic gain carefully you can leave the ALC open.
For true reproduction of your voice a good quality mic is essential. Using only the amount of gain needed to amplify your voice to a respectable level, holding the hand mic or desk mic close to your mouth and talking into the side or talking across the screen will give your transmission the most natural sound .
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Tech8541
| | Posted on Sunday, February 01, 2004 - 3:53 pm: |
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while this -is- interesting, it is false to some extent. the ssb peaks (which come from your voice, not background noise) is what will set the level of the agc, not the background noise. while the background noise in the audio may cause some problems in readability it isn't 'desensing' the rxer that is rxing this signal.
could you email a link to where you found this so i could read more (if there is a link, and if there is more)? |
Creator
| | Posted on Monday, February 02, 2004 - 8:11 pm: |
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Hi,
Here is the link for you and anyone else who wants to learn some good stuff...
http://cbworldinformer.com/cb_world_960901.htm#1
Rob
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