Re: OT: "Line level"

Posted by Don Brown-4 on Jul 02, 2004; 9:37pm
URL: http://elecraft.85.s1.nabble.com/OT-Line-level-tp368401p368408.html

Hi

First of all I am not an expert on telephone systems or "pro audio" although I have worked with both a little from time to time. With that said it seems to me I read somewhere that 600 ohm in the telephone system came about because that was the internal resistance of the old carbon microphones used in all telephones up through the early 60's. This resistance was also needed in conjunction with the off hook current to activate the holding relay in the central office when the phone was off hook. Later phones with amplified microphones used transformers or IC's matched to 600 ohm impedance so the central office equipment was compatible. Perhaps this is where the 600 ohm standard in audio originated. I have a HP 200C audio generator and it has a 600 ohm output impedance and I have seen many other audio generators that also have 600 ohm outputs.

In consumer and pro audio the line levels that I remember used on most amplifiers was 47K input impedance and about .5 to 1 volt RMS for full rated output. Likely 0 dbm. Of course I have not messed with this for many years back when many amplifiers had 6L6's and big output transformers so the memory may be failing.

I also checked the specs on a high end sound card that I have and the line levels are +4 dbm or -10dbm jumper selectable.

So to answer the original question that started this thread, I assume to know what level to send to a sound card for PSK31 and such, it looks like 0 dbm to +4 dbm is about right when using the line inputs. If you are using a meter about .78 volts RMS or looking at it with a scope 2.2 volts pk to pk would be 0 dbm. The main thing is to not over drive the sound card and cause clipping but supply enough drive so the signal to noise is minimum.

If any of this wrong any of you experts feel free to correct it <G>


Don Brown

KD5NDB
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