Help me measure line levels

[dano12]

I'm really into building DIY audio stuff. I'm fairly adept at building and hacking things together. But my focus on portable media players (iPods) etc. has been driving me crazy with regards to measurements.

I know from experimenting that the line level output of portable devices is lower than typical consumer gear. I've chalked that up to limitations of available power on portable devices and designers working to increase battery life.

But at this point I really want to know some exact numbers. I have a scope, loads of parts, test sources, tone generators and the like. I've read everything I can find on the web, but have yet to find a good straight-forward approach to measuring audio signals.

In a nutshell, how do I accurately measure the line-out level of various devices whether they be portable, consumer audio, or pro audio?

[DF96]

Connect the scope. Start the music. See how big the trace is.

[dano12]

Originally Posted by DF96

Connect the scope. Start the music. See how big the trace is.

Yep, have done that.

I guess I should provide a bit more detail on my question....

I'm connecting my scope to an iPod. Turn on some music, and watch the numbers. After about a minute of audio, I pick the highest value I've seen:

Headphone out
---------------------
Peak to Peak: 2.6 v max
Cyclic RMS: 206mv

Line Out
---------------------
Peak to Peak: 1.536 v max
Cyc RMS: 181 mv

Pk to Pk and Cyc RMS are the only measurement types that I've found on my scope that make sense.

Now if consumer line level is defined at 0.316v, why are my numbers so radically different?

[milsman2]

I'm not sure what the reason is, but I've read numerous forum posts and blogs that post those numbers as well. I don't think you're wrong. IMHO iPods and the like seem to have a much higher than usual line level.

[dano12]

Originally Posted by milsman2

I'm not sure what the reason is, but I've read numerous forum posts and blogs that post those numbers as well. I don't think you're wrong. IMHO iPods and the like seem to have a much higher than usual line level.

Here's the really strange thing. I take the exact same scope setup and hook it up to the line out of a commercial CD player. The highest number I get, averaged across a minute or so of audio:

Peak to Peak: 3.74v max
Cyc RMS: 447 mv

So, when compared to the iPod values, the iPod is significantly lower in output signal (which is intuitive for a battery powered device).

I think I'm missing something big picture here, regarding how to correctly measure line levels with a scope.

[DF96]

The actual voltage you measure will depend on the music, even for modern highly compressed music. Peak-to-peak will be approximately 2.8 times the RMS value for a sine wave, but it may be that you are comparing a maximum pk-pk with an average RMS. For music the ratio will be much greater.

I didn't know there was a definition of 'consumer line level'. Your numbers are not radically different.

Just seen your new post. CD standard max output level is 2V RMS, which is 5.6V pk-pk. No CD should get too close to this level. Good CDs stay well below it. Modern ones get close. Most other devices have a lower output level. I'm not sure why you seem confused. As you say, there is clearly something in the big picture which you have missed.

[dano12]

Originally Posted by DF96

The actual voltage you measure will depend on the music, even for modern highly compressed music. Peak-to-peak will be approximately 2.8 times the RMS value for a sine wave, but it may be that you are comparing a maximum pk-pk with an average RMS. For music the ratio will be much greater.

I didn't know there was a definition of 'consumer line level'. Your numbers are not radically different.

Just seen your new post. CD standard max output level is 2V RMS, which is 5.6V pk-pk. No CD should get too close to this level. Good CDs stay well below it. Modern ones get close. Most other devices have a lower output level. I'm not sure why you seem confused. As you say, there is clearly something in the big picture which you have missed.

Much less confused now thanks to your explanation of some of the numbers.

Cheers!