SOUND ON SOUND

[Hugh Robjohns]

..., one has to come to the conclusion that one of my two Focusrite interfaces is not functioning correctly (since my measurements are off by ~ 6 dB). What I am trying to achieve is to figure out which one.

Okay. I'm clearer now about your concerns. However, if one preamp was not functioning correctly I'd expect the fault to be obvious and significant, not just an apparent small difference in overall gain.

From what you've said, Focusrite acknowledges that the two interfaces are designed to have different overall max gains, but there seems to be some confusion over which is which.

I would not understand what the point of analogue gain is in the first place (or even more so, why most people want a lot of it), because you can always increase the signal in the digital domain after recording it. Why would one need a preamp at all then?

The analogue preamp raises the very small microphone signal up to a line level suitable for the A-D converter to handle. Without the analogue gain stage, the mic signal would lie too close to the nose floor of the converter, and would have a very restricted dynamic range. Boosting the level in the digital domain would also boost the converter noise floor, and you'd still have a poor dynamic range.

In other words, it's about gain staging to maximise the dynamic range capability and minimise unwanted noise.... And James has said it in fewer words while I was typing! ;-)

H

[fladd]

..., one has to come to the conclusion that one of my two Focusrite interfaces is not functioning correctly (since my measurements are off by ~ 6 dB). What I am trying to achieve is to figure out which one.

Okay. I'm clearer now about your concerns. However, if one preamp was not functioning correctly I'd expect the fault to be obvious and significant, not just an apparent small difference in overall gain.

From what you've said, Focusrite acknowledges that the two interfaces are designed to have different overall max gains, but there seems to be some confusion over which is which.

I would not understand what the point of analogue gain is in the first place (or even more so, why most people want a lot of it), because you can always increase the signal in the digital domain after recording it. Why would one need a preamp at all then?

The analogue preamp raises the very small microphone signal up to a line level suitable for the A-D converter to handle. Without the analogue gain stage, the mic signal would lie too close to the nose floor of the converter, and would have a very restricted dynamic range. Boosting the level in the digital domain would also boost the converter noise floor, and you'd still have a poor dynamic range.

In other words, it's about gain staging to maximise the dynamic range capability and minimise unwanted noise.... And James has said it in fewer words while I was typing! ;-)

H

Mmh, but doesn't this imply that high analogue gain and a high conversion point is always better as it brings you further away from the converter noise floor and increases dynamic range? I think I'm still struggling with this concept somehow. While I can somehow imagine that one might decide to decrease the conversion point to prevent possibly hitting the ceiling in the digital domain even with the gain knob all down (with very loud signals maybe...not sure how reslistic that is), I still don't seem to understand why one would ever want to do that without adding more analogue gain to compensate (as Focusrite apparently did on the 18i8). It seems that this just shifts the problem from possibly hitting ceiling to possibly hitting noise floor.

Thanks for being so patient with me btw. This forum is one of the most helpful ones I've seen! :-)

[James Perrett]

Mmh, but doesn't this imply that high analogue gain and a high conversion point is always better as it brings you further away from the converter noise floor and increases dynamic range?

You are making the assumption that the convertor is noisier than the preamp - which isn't necessarily true. In budget interfaces like this it would be better to think of the whole system as a black box rather than a separate preamp and convertor. There could be all kinds of interactions between the convertor circuit and the preamp circuit that wouldn't happen with separate units.

[Hugh Robjohns]

... but doesn't this imply that high analogue gain and a high conversion point is always better as it brings you further away from the converter noise floor and increases dynamic range?

Nope. Different sources generate different sound pressure levels. Different microphones have different sensitivities. Different applications require different headroom margins. There is no one perfect set of parameters to suit all situations, so the gain needs to be user-variable and the digital alignment either adjustable or chosen carefully to suit the typical application.

It seems that this just shifts the problem from possibly hitting ceiling to possibly hitting noise floor.

Yep... the art and skill (of both the user and the equipment designer) is to optimise the signal level somewhere between the two without falling foul of either! ;-)

H

[ef37a]

Sorry to be so late back on parade! I no longer have the means to generate an accurate ~1mV balanced signal so had to build a box to do it by indirect means and I eff'ed up the first time and just recorded hum. Mkll is better.

I don't have 2i2 but thought a comparison between an 8i6 and a KA6 might be of interest?

This HP laptop played a 1kHz sine into 'The Box' at 51.2mV and that put, as best I can calculate, 1.1mV into the 8i6 set to max gain. That gave an indicated -14.5dBFS in Samplitude.

Same input for the KA6 gave -25.4dBFS a gain difference of almost 11dB which surprised me as 'operationally' there seems no great difference. However, running a RightMark plot for both exported.wavs showed the KA6 to be only 3dB noisier (ref the 1khZ signal, the KA having a ~9dB lower overall noise floor but lower gain). Both figures are more than good enough for most application?
Both plots show a lovely 50Hz hum spike some 60dB above the noise floor but no hum is present when my lashup is disconnected!

The 8i6 has, IMHO a bit to MUCH gain and the control is very hard to set at the last 10% of travel but I dare say the new ones are better? The 8i6 is after all no longer made.

If anyone wants details of The Box PM me. I can also donate a ( cheap and fairly nasty!) 1:1 traff to the cause.

Dave.

[blinddrew]

Sorry to be so late back on parade! I no longer have the means to generate an accurate ~1mV balanced signal...

I can't help but feel that most people would have stopped there! :D

[Hugh Robjohns]

Sorry to be so late back on parade! I no longer have the means to generate an accurate ~1mV balanced signal...

I can't help but feel that most people would have stopped there! :D

Indeed! :D I have the means too... but not the inclination! :lol:

...and I have to be careful about that restraining order....

[ef37a]

Right!
**************I'll get me coat.

Dave.

[Hugh Robjohns]

:lol: :D :bouncy:

[fladd]

I thought I would give some feedback on how this played out.

After Focusrite Support asked me to do several tests that included loopback recordings as well as an external device (Novation Circuit) and send them the results, they concluded that my new 18i8 is performing according to the specs, but that my older 2i2 is not. To be more precise, it appears that my 2i2 has much more gain than it should have, and this explains the large difference to the 18i8 that I noticed.