Line out definition, Curious about what people expect from line out Options

[saratoga]

Rockbox runs on various portable devices that have both a headphone jack and a line out. I'm an electrical engineer, so my bias is to think of the difference in terms of impedance and level: line out should be fixed at line level and have a high output impedance. Headphone should be variable and have a low output impedance. In terms of digital signal, I expect them both to take identical output from the DAC and differ only in the analog gain stage.

However, its been suggested to me that things like DSP effects, balance or EQ should be limited to headphone output because there is a general expectation that line out feed essentially unprocessed audio to the DAC.

What do people expect line out to actually do? Looking on google there is surprisingly little in the way of definitions for it so its unclear to me what people expect.

[mixminus1]

This:

...things like DSP effects, balance or EQ should be limited to headphone output because there is a general expectation that line out feed essentially unprocessed audio to the DAC.

I think of a line out from a portable media player in the same way as a line output from a Blu-ray player, DVR, or any other stand-alone device: as one audio source, of which there may be others, and therefore any processing to that audio should be the responsibility of the device (pre-amp, receiver, etc.) driving the playback transducers, whether those be speakers or headphones.

Using the headphone output on a portable device indicates that I expect that device to provide those processing functions as it is now driving the transducers (headphones) directly, and exclusively.

[hellokeith]

Since you are talking about portable digital audio players, then consider metadata, such as ReplayGain. If not applied to line out, then it will be lost in the DAC.

[Sebastian Mares]

For me, line out is the "unprocessed" analog version of the source without DSP or ReplayGain.

[edwardar]

I expect that line out should be a raw unprocessed signal, as I would be plugging it into another device which may further process the audio.

In the case of rockbox I always use crossfeed when listening on headphones, but I use the line out when plugging into my home stereo, where I'd never want crossfeed. I think a similar principle applies when people use EQ to suit their headphones - they probably don't want the same settings when listening through line out.

[googlebot]

I expect the different output couplings you have described, nothing else.

[Roseval]

If we talk conventional boxes (CD, pre-amp, etc) I expect the line out to be unprocessed audio.
In case of a “soundcard” so computer style audio I expect all kind of DSP affecting all outputs be it digital or analog

[punkrockdude]

Doesn't line out say more about it's electrical properties, i.e. signal volume? http://en.wikipedia.org/wiki/Line_level

[johnb]

When people use line out of a portable device (in conjunction with Rockbox), they probably feed it into a headphone amp. In such a case replaygain, EQ and crossfeed would be beneficial.

If you feed it into a home stereo, no such things are required, but since we are talking about Rockbox, I think the first statement applies.

Cheers
johnb

[Fedot L]

line out should be fixed at line level and have a high output impedance.

A LOW output impedance. Well explained in “Impedances” chapter of the above-cited link
http://en.wikipedia.org/wiki/Line_level

[Soap]

What I want out of a line out depends on the component in question, and more so depends on what I commonly have downstream.

Replaygain can't be done (to the best of my knowledge) on anything but the device playing the file. Therefore I want replaygain before the line-out.

While crossfeed can be done after the fact, I don't plug my DAP into anything which can, so I'd like that done before line-out. Likewise with EQ. I'd rather treat everything I plug my DAP into (regardless of attachment method) as a dumb amp and let all the audio processing be done on the DAP. Very few systems can do as much processing as well as Rockbox anyhoo.

[saratoga]

line out should be fixed at line level and have a high output impedance.

A LOW output impedance. Well explained in “Impedances” chapter of the above-cited link
http://en.wikipedia.org/wiki/Line_level

No, high impedance. As your link notes, 100 ohms is common. Compare that to ~1 ohm typical for a good headphone output. The point of line out is to drive a low power signal into an amplifier, hence the impedance must be much higher then headphone out where one is concerned about power transfer.

[pdq]

line out should be fixed at line level and have a high output impedance.

A LOW output impedance. Well explained in “Impedances” chapter of the above-cited link
http://en.wikipedia.org/wiki/Line_level

High relative to speakers and headphones. Typically a line out has an output impedance of a few hundred ohms to protect it from damage, but this is low relative to the input impedance of a line in, which is typically tens of kOhms.

[Brod]

Line out should be unprocessed, and also at a constant voltage output. If it has volume control it's a preamp out, not a line out.

This post has been edited by Brod: Dec 29 2011, 02:09

[saratoga]

Line out should be unprocessed, and also at a constant voltage output. If it has volume control it's a preamp out, not a line out.

In practice, most portable devices have volume control on the line out. There are some exceptions, but they're mostly older, obsolete parts. Its somewhat impractical to make the line out truly fixed, since theres usually a variable gain stage before the output mux that selects between headphone, line out, speaker out, etc. So when a device detects something on the line out, they have the software raise the level to the maximum that doesn't clip and then terminate it with ~100 ohms.

[Brod]

That's fine, but in that case I don't think portables should be calling it a line out.

[saratoga]

That's fine, but in that case I don't think portables should be calling it a line out.

Why is that?

[Brod]

Because line level nominal voltages are strictly defined and preamp outs do not meet that definition.

[Maurits]

Line out should be unprocessed, and also at a constant voltage output. If it has volume control it's a preamp out, not a line out.

In practice, most portable devices have volume control on the line out. There are some exceptions, but they're mostly older, obsolete parts.

Are you sure? I never noticed volume control on the line-outs of any of my iPods for instance. Never tested my iPhone but it's probably similar.

[Sebastian Mares]

It's the same with the iPhone and iPod Touch. The volume control even disappears from the UI when playing via line out and the volume buttons have no effect.

[Takla]

Both my iRiver H340 and H140 have a separate analogue line out; the level isn't fixed and any eq or crossfeed effects are also present. The iRiver manuals don't contain any specifications referring to voltage or impedance, the only references in the manual are to tell the reader that the line out can be used to connect external devices. I have no idea how typical (or not) this might be of older, well specified players.

As to what I'd expected when I first bought one: that was a fixed level line out that bypasses all volume/eq settings, as found on home audio equipment, so I was a little disappointed. In practice when I've used the analogue line out (the H140 also has optical out) I've disabled any eq and put the volume at maximum and this works well, giving similar volume levels to other sources. I assume this is how these and similar players' are line outs are intended to be used.

If Apple devices have a fixed level line out that bypasses eq and volume then imo they are doing it right.

[Brod]

In practice when I've used the analogue line out (the H140 also has optical out) I've disabled any eq and put the volume at maximum and this works well, giving similar volume levels to other sources.

If it's a digital volume control then maximum volume is fine, but if it's an analog volume control (ie. voltage control) then you should set it at around 90% to prevent clipping.

[saratoga]

Because line level nominal voltages are strictly defined and preamp outs do not meet that definition.

Why don't they meet that definition exactly? I don't understand what you're trying to say.

Line out should be unprocessed, and also at a constant voltage output. If it has volume control it's a preamp out, not a line out.

In practice, most portable devices have volume control on the line out. There are some exceptions, but they're mostly older, obsolete parts.

Are you sure? I never noticed volume control on the line-outs of any of my iPods for instance. Never tested my iPhone but it's probably similar.

Yes I'm sure. See what I said before:

So when a device detects something on the line out, they have the software raise the level to the maximum that doesn't clip and then terminate it with ~100 ohms.

You don't notice volume control in the Apple firmware because when you insert an accessory, it triggers an interrupt which automatically maxes out the ipod's volume and leaves it there until you unplug the line out connector. When you unplug, a second interrupt fires that restores the volume to the original setting.

[Axon]

AFAIK, and also speaking as an EE here, the primary difference between a "line out" and a "headphone out" is that a good headphone out must have a low (0.1-10 ohm) impedance, and a volume control of some sort. Line outs use impedance bridging to a de facto standard of 2-10 kohm, so they may have a similarly low impedance, but don't require it. They also do not need a volume control. I also think that the distinction between a line out and a headphone out has been getting progressively blurrier, as headphone outs have improved in quality. There is no inherent reason why a headphone out cannot also be a perfectly adequate line out: in the past, the noise/distortion required to drive headphones was sufficiently high that you did not want that in your line-out signal path, but that may not be the case anymore.

For instance, IIRC, the iPod 5G's noise levels do go down if you rip out its headphone driver stage and send the DAC output directly to an outboard headphone amp -- this was obviously audible to me when listening to such a setup with Etys in 2005, when cranking the volume up to a thoroughly ridiculous setting -- but at normal listening levels, the noise generated by the iPod's headphone stage was completely inaudible to me.

I do not believe there is any clear consensus on whether or not a line out will include DSP or not. I think there are good justifications for either choice.

[andy o]

FWIW, I found it really annoying with the iPod 3rd gen (one before clickwheel) that line out didn't have Sound Check (like RG) enabled. Then I got an iPod Photo (first with color screen) and they "fixed it", and everything was OK for me. Then, the Classic and iPod Touch 4 behaved the same.

But then they came out with this "USB Direct" ability to transfer the audio digitally to docks and receivers, and again, it was annoying that my car and home receivers, with the simplicity this connection provides, could not do Sound Check over this connection. THEN, Apple fixed it in iOS 5, and at least now with the Touch I can do SC (with RG values) in my receiver.

With AirPlay, SC always worked, but it also behaves like a headphone output where you can manually control volume. I don't remember this detail about EQ going farther back than the Classic, but at least since then, EQ also has always worked in all these outputs mentioned.