Posted by
Jim Brown-10 on
Jul 02, 2004; 10:16pm
URL: http://elecraft.85.s1.nabble.com/OT-Line-level-tp368401p368409.html
On Fri, 2 Jul 2004 15:37:33 -0500, Don Brown wrote:
>I read somewhere that 600 ohm in the telephone system came about
>because that was the internal resistance of the old carbon microphones
The more likely scenario is that the carbon mics were made to whatever spec Ma Bell
needed to make their phone work well. The carbon mic in a Ma Bell phone did NOT
hang directly across the line. Rather there was a network called a hybrid (it included a
transformer) that hung across the line and combined the mic with the earphone to do
what became known as "sidetone cancellation." On a phone line, the local signal is far
stronger (on the order of 20 dB) than the distant signal because the distant signal is
attenuated by the line. The sidetone network reduces the level of the local signal in the
earphone so that the user hears him/herself, but not at a lot higher level than the caller .
600 ohms originally comes from the old intercity transmission lines.
>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.
Yes, and they really mean dBu, whether they say it or not. Most spec sheets are written
by marketing types, not engineers, and many are wrong.
>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.
Most computer inputs are much closer to consumer levels than pro levels. You're lucky
to have a pro sound card with switchable level matching. I have one on one of my
machines, but the others are the plain consumer cards.
>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.
You got it right, but you really need to look at it with a scope to see the peaks. That's
because what really counts is keeping it out of clip.
I recently hooked up my computer to generate PSK31. I was running the sound card
close to full output, and padding it down before going into the rig's mic input. I got
good level matching, but some guys on the Top Band list suggested that the sound
card might be generating some IM near its max output, even though it wasn't clipping.
Always trying to learn new stuff, I hooked up some fancy instrumentation and learned
that they were right. I dropped the output level of the rig about 6 dB and increased the
input gain of the radio, and the IM produced by the sound card dropped from roughly -
30 dB re carrier to about -50 dB re: carrier. If you spend any time around the PSK31
part of the band, you'll see some occasional signals that are producing significant
distortion sidebands.
Those same guys observed that one of the good reasons for running a rig well below
max output on PSK31 is to reduce the IM that is produced. I haven't seen any data for
a rig's IM vs. power output, and I haven't measured my rig, but I suspect they are
correct. Many rigs, including the K2 and K2/100, are rated for 3 dB less power on
100% duty cycle modes (RTTY, PSK, etc.) to prevent excessive dissipation.
Jim Brown K9YC
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