[digitalradio] Re: Digital on 2M FM: Audio settings ?
Hi Andy, I suppose this would be confusing to someone not used to FM operation on a multi-mode rig. In all modes, ALC is a function of transmitter output power. In SSB modes we keep our transmitted digital signals clean by running our transceivers below the power level that causes ALC to be generated. In FM, however, the transmitted signal is a constant level carrier. The audio modulation from the sound card causes the frequency of that carrier to 'deviate' above and below its unmodulated frequency. What we care about on digital FM is how clean the audio at the receive end is. The ALC reading reflects only how much RF we are putting out, not what the modulation looks like at the receiving end. Remember that essentially all voice FM transmitters use some sort of peak audio clipping to limit how wide our modulation 'deviates' to keep it inside the pass band of the receive IF filters. When we run digital FM, we need to keep the audio drive low enough that we are not driving the transmitter audio section into clipping. Setting the audio drive level for digital operation with an FM transmitter is not quite as simple as for SSB. We really must monitor the actual transmitted signal while we make that adjustment. That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. That said, though, a little extra distortion is not quite the problem on FM it is on SSB. On FM, audio distortion will generally not interfere with other stations. It may look bad on the water fall and may not decode quite as easily as if the level was correct but it will likely still work OK. That is why we can get away with just holding a handy talkie up to the computer speaker to operate digital modes on VHF FM. That make sense? Gary - N0GW --- In digitalradio@yahoogroups.com, Andy obrien k3uka...@... wrote: Hmmm. Well I thought it would be simple enough to transmit digital modes on 2M FM but one issue I just ran in to is the ALC is very high and my usual method of lowering it has no effect. I also lowered the mic gain but that had no impact. Something simple I am not taking in to account when using FM ? Andy K3UK
Re: [digitalradio] Re: Digital on 2M FM: Audio settings ?
That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. Comparing your own digital modulation... How do you do that? Please explain. K5WGM --- On Mon, 11/16/09, Gary grwes...@yahoo.com wrote: From: Gary grwes...@yahoo.com Subject: [digitalradio] Re: Digital on 2M FM: Audio settings ? To: digitalradio@yahoogroups.com Date: Monday, November 16, 2009, 8:51 AM Hi Andy, I suppose this would be confusing to someone not used to FM operation on a multi-mode rig. In all modes, ALC is a function of transmitter output power. In SSB modes we keep our transmitted digital signals clean by running our transceivers below the power level that causes ALC to be generated. In FM, however, the transmitted signal is a constant level carrier. The audio modulation from the sound card causes the frequency of that carrier to 'deviate' above and below its unmodulated frequency. What we care about on digital FM is how clean the audio at the receive end is. The ALC reading reflects only how much RF we are putting out, not what the modulation looks like at the receiving end. Remember that essentially all voice FM transmitters use some sort of peak audio clipping to limit how wide our modulation 'deviates' to keep it inside the pass band of the receive IF filters. When we run digital FM, we need to keep the audio drive low enough that we are not driving the transmitter audio section into clipping. Setting the audio drive level for digital operation with an FM transmitter is not quite as simple as for SSB. We really must monitor the actual transmitted signal while we make that adjustment. That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. That said, though, a little extra distortion is not quite the problem on FM it is on SSB. On FM, audio distortion will generally not interfere with other stations. It may look bad on the water fall and may not decode quite as easily as if the level was correct but it will likely still work OK. That is why we can get away with just holding a handy talkie up to the computer speaker to operate digital modes on VHF FM. That make sense? Gary - N0GW --- In digitalradio@ yahoogroups. com, Andy obrien k3uka...@.. . wrote: Hmmm. Well I thought it would be simple enough to transmit digital modes on 2M FM but one issue I just ran in to is the ALC is very high and my usual method of lowering it has no effect. I also lowered the mic gain but that had no impact. Something simple I am not taking in to account when using FM ? Andy K3UK
Re: [digitalradio] Re: Digital on 2M FM: Audio settings ?
Well for a week I sat on 145.000 Mhz 2M FM and call CQ and what I did was use my scaner to listen to my self and try to see if I was not over driving the FM, and if it sounded clean I was happy, I had no takers for the hold week and the rig was a Icom 706mk2g. GL Russell NC5O 1- Whoever said nothing is impossible never tried slamming a revolving door! 2- A government big enough to give you everything you want, is strong enough to take everything you have. - Thomas Jefferson IN GOD WE TRUST Russell Blair (NC5O) Skype-Russell.Blair Hell Field #300 DRCC #55 30m Dig-group #693 From: Warren Moxley k5...@yahoo.com To: digitalradio@yahoogroups.com Sent: Mon, November 16, 2009 9:37:23 AM Subject: Re: [digitalradio] Re: Digital on 2M FM: Audio settings ? That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. Comparing your own digital modulation.. . How do you do that? Please explain. K5WGM --- On Mon, 11/16/09, Gary grwes...@yahoo. com wrote: From: Gary grwes...@yahoo. com Subject: [digitalradio] Re: Digital on 2M FM: Audio settings ? To: digitalradio@ yahoogroups. com Date: Monday, November 16, 2009, 8:51 AM Hi Andy, I suppose this would be confusing to someone not used to FM operation on a multi-mode rig. In all modes, ALC is a function of transmitter output power. In SSB modes we keep our transmitted digital signals clean by running our transceivers below the power level that causes ALC to be generated. In FM, however, the transmitted signal is a constant level carrier. The audio modulation from the sound card causes the frequency of that carrier to 'deviate' above and below its unmodulated frequency. What we care about on digital FM is how clean the audio at the receive end is. The ALC reading reflects only how much RF we are putting out, not what the modulation looks like at the receiving end. Remember that essentially all voice FM transmitters use some sort of peak audio clipping to limit how wide our modulation 'deviates' to keep it inside the pass band of the receive IF filters. When we run digital FM, we need to keep the audio drive low enough that we are not driving the transmitter audio section into clipping. Setting the audio drive level for digital operation with an FM transmitter is not quite as simple as for SSB. We really must monitor the actual transmitted signal while we make that adjustment. That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. That said, though, a little extra distortion is not quite the problem on FM it is on SSB. On FM, audio distortion will generally not interfere with other stations. It may look bad on the water fall and may not decode quite as easily as if the level was correct but it will likely still work OK. That is why we can get away with just holding a handy talkie up to the computer speaker to operate digital modes on VHF FM. That make sense? Gary - N0GW --- In digitalradio@ yahoogroups. com, Andy obrien k3uka...@.. . wrote: Hmmm. Well I thought it would be simple enough to transmit digital modes on 2M FM but one issue I just ran in to is the ALC is very high and my usual method of lowering it has no effect. I also lowered the mic gain but that had no impact. Something simple I am not taking in to account when using FM ? Andy K3UK
[digitalradio] Re: Digital on 2M FM: Audio settings ?
Hi Warren, The cheap and dirty way we did it a decade or two ago to set up packet stations was to hook the speaker output of a handy-talky to a VOM and monitor the audio level coming from another packet station that was working well. You then adjusted the local transmit audio level to produce the same voltage on the VOM on the handy-talky. If you have no other stations to check against, my guess is you would just listen to your signal on the handy-talky and set digital modulation level to be a little below peak voice level when talking into the microphone. As long as the digital signal sounds fairly clean to your ear, it will probably be OK. If there is someone nearby who can monitor your signal with a digital program to display your signal on a waterfall, you could coordinate with them to find a level that is below the clipping distortion level but high enough to give good copy under noisy conditions. That is about the best I can come up with off the top of my head. I did buy one of the MFJ meters a while back and find it works OK when use with a scope. It's not what you would call a precision instrument but it has gotten the job done adequately. Gary - N0GW --- In digitalradio@yahoogroups.com, Warren Moxley k5...@... wrote: That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. Comparing your own digital modulation... How do you do that? Please explain. K5WGM
Re: [digitalradio] Re: Digital on 2M FM: Audio settings ?
Thanks, K5WGM --- On Mon, 11/16/09, Gary grwes...@yahoo.com wrote: From: Gary grwes...@yahoo.com Subject: [digitalradio] Re: Digital on 2M FM: Audio settings ? To: digitalradio@yahoogroups.com Date: Monday, November 16, 2009, 2:28 PM Hi Warren, The cheap and dirty way we did it a decade or two ago to set up packet stations was to hook the speaker output of a handy-talky to a VOM and monitor the audio level coming from another packet station that was working well. You then adjusted the local transmit audio level to produce the same voltage on the VOM on the handy-talky. If you have no other stations to check against, my guess is you would just listen to your signal on the handy-talky and set digital modulation level to be a little below peak voice level when talking into the microphone. As long as the digital signal sounds fairly clean to your ear, it will probably be OK. If there is someone nearby who can monitor your signal with a digital program to display your signal on a waterfall, you could coordinate with them to find a level that is below the clipping distortion level but high enough to give good copy under noisy conditions. That is about the best I can come up with off the top of my head. I did buy one of the MFJ meters a while back and find it works OK when use with a scope. It's not what you would call a precision instrument but it has gotten the job done adequately. Gary - N0GW --- In digitalradio@ yahoogroups. com, Warren Moxley k5...@... wrote: That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. Comparing your own digital modulation.. . How do you do that? Please explain. K5WGM
