Front vs. Rear Input Question

[Sterling]

Sorry to be slightly off topic but I have Bartolini active pickups. Which input should i use for this. The back inputs don't seem to give me enough input level even with the gain of my Bart preamp maxed. I do not want to use an outboard solution so I have been using the front input. Are you saying that I should be using the back or that I will not get the benefit of the secret sauce because my pickups are active?

 

[sampleaccurate]

It appears that the level of the input signal going into the unit must be extremely low in order to avoid the high frequency limiting that Jay Mitchell first mentioned.

Yes, if by "extremely" low you mean less than nominal line level (0dBu) but greater than nominal mic level (roughly -50 dBu).

There are reasons that this will not affect the sound of a guitar, though. Read on.

Here's the thing - the level of the signal coming out of my guitar is strong enough to push the input into high frequency limiting.

No. The level vs. frequency content of the guitar's signal has been taken into account in the codec process.

[quote:21fi5i6m]So while the reason my measurements were not showing a flat response was that the input level was too high, the input level first used in the tests was lower than that coming out of my instrument.

No. Setting the level below 2kHz of a sweep lower than the level of the guitar signal is not enough. The frequencies that will cause self-limiting are all at or above 2kHz, and the amplitude of a guitar's signal has much lower content above 2kHz than below. The levels of frequencies above 2kHz in a sweep are identical to the levels at lower frequencies.

In addition, the limiting appears to occur prior to the input gain potentiometer

Yes.

There is probably a very good reason for this behavior that I don't understand, and I'm not going to ask.

It is called "maximizing signal to noise." The response of the front input to a signal that contains the frequencies and levels that passive quitar pickups produce will be flat. Because of the frequency-dependent gain of this input, care is needed when using a broadband signal to analyze its performance, as you have discovered.

The manual clearly states that the front input is especially conditioned for electric guitars, I assume passive ones since the front input impedance is so high.

The input impedance of the rear input is the same (1 megohm) as the front.

I guess this is the special conditioning at work.

No. Input impedance and filtering/limiting are independent of each other.

One effect it has is to brighten your tone when you turn the volume pot on your guitar down,

No. With passive guitar signals, the only audible effect of the conditioning is to increase S/N (by effectively lowering the noise floor), specifically at higher frequencies.

It looks worse on the curve than it sounds even when the input level is relatively high.

See above. Were to to prefilter a sweep so that its frequency content matches that of a guitar, then match levels, you would find that no self-limiting occurs.

I do believe however that people with good ears, hot pickups or active preamps, and who use bright clean tones could notice a difference in the sound of the front and rear inputs.

"Hot" passive pickups are never "hot" at high frequencies. The parameters that make them hotter - greater number of turns on the coils - also increase series inductance - and therefore HF output - by the same amount as signal output is increased. Active pickups would be another matter, and that is why the rear input is different. Active pickups will not benefit from the "secret sauce" in the same way that passive ones will.[/quote:21fi5i6m]

That post was written before I had much of a chance to reflect on what I'd done. While thinking about this late last night I came to some of the same conclusions and I agree with everything you said Jay - those are some excellent insights. For one I realized that the high frequencies coming out of my guitar are much lower in amplitude compared to the highs in the constant amplitude frequency sweep I was using, and that my statement about the highs being compressed on a guitar was incorrect because of that. They aren't high enough in amplitude above 1500HZ to push the unit into limiting. The frequency sweep was tremendously higher in the high frequency range with a completely different spectral density. My ERROR!

I fully understand that the HF pre-emphasis process is for noise reduction. I thought maybe there was another intended purpose. I didn't say the high input impedance was part of the "special sauce" - you misread that but I could have worded it better.

What I've learned here is WHY to: Use the front input for a better S/N ratio with a guitar or bass. Use the rear input with full range program material that contains a lot of high frequency information, or with a guitar or bass. Just don't use the front input for anything that is full range like program material with a full mix of instruments. And don't rely on the input meter to indicate front input signal levels for anything but a guitar or bass.

As far as the sound is concerned, with a guitar, the front input sounds essentially the same as the rear to my ears. I was able to pre-EQ a guitar to make it extremely bright (and noisy as hell) and hear a slight difference with a bright clean patch between the front and the rear, but it was not a realistic situation and even then the difference was small. If any limiting was going on it wasn't much (based on listening tests). With program material (full mix with drums and cymbals) I can notice some attenuation of the cymbals, but the unit isn't designed to do this. The front input is for a guitar. Now I understand why - I didn't before. I hate it when I'm told "this is conditioned for a guitar" without telling me HOW! Now I understand how.

I guess the only thing I don't quite understand is the logic behind the design of the metering. Why not configure the input meter to show when there's limiting going on regardless of the input source or frequency causing it (like just about every other device on the planet)? That's just a question not a criticism. It's actually moot now that I understand what's going on in the input stage. I've never seen an input stage with HF pre-emphasis AND a limiter before the input gain control.

Thanks for the insights. I feel a lot better today knowing why I was getting the results I was getting. The time and frustration were well worth the outcome.

Thanks again to everyone who made suggestions. I wouldn't have made it this far without the help. Sorry if I seem a little overly concerned with what others perceive to be trivial, but I'm making a huge investment in this device, not the purchase price, but mostly the time our guitar player and I will spend re-programming hundreds of tones.

And thanks to Cliff Chase and Fractal Audio for making an incredible product.

Time to rock on! Have a great weekend! I am.

Stephen Cole

 

[sampleaccurate]

"Active pickups will not benefit from the "secret sauce" in the same way that passive ones will."

I don't understand why. An active pickup will have a low output impedance and a little bit of noise from the electronics, and there will be negligible HF attenuation due to cable capacitance, so they will generally sound brighter, no? Why would the HF pre-emphasis noise reduction circuitry not work the same for low impedance active pickups? Is it the brighter sound, or noise added by the electronics, or something else I'm not getting?

Thanks.

Stephen Cole

 

[Scott Peterson]

"Active pickups will not benefit from the "secret sauce" in the same way that passive ones will."

I don't understand why. An active pickup will have a low output impedance and a little bit of noise from the electronics, and there will be negligible HF attenuation due to cable capacitance, so they will generally sound brighter, no? Why would the HF pre-emphasis noise reduction circuitry not work the same for low impedance active pickups? Is it the brighter sound, or noise added by the electronics, or something else I'm not getting?

Thanks.

Stephen Cole

I imagine I'll get smacked around by the braintrusts here, but *from what I understand* passive pickups and electronics need to be loaded correctly from whatever they are plugged into to sound and feel right. Active pickups/electronics don't care about loading whatsoever.

 

[Jay Mitchell]

I don't understand why.

Active pickups can potentially produce enough level to drive the front input into self-limiting. Additionally, the noise floor in active pickups is likely to have more HF content than passive pickups, due to input noise in the preamp. In that case, the encode/decode process is far less likely to improve S/N.

Why would the HF pre-emphasis noise reduction circuitry not work the same for low impedance active pickups?

The spectral content of the noise will be different.

 

[steadystate]

This high frequency emphasis/self-limiting on the front panel input is a very clever design. It seems to me this whole thread could have been two posts long if someone stated at the beginning that the frequency response of the front input is flat, but only under the condition that the spectral density of the input signal is similar to that of a passive guitar pickup.

Like the OP, I originally assumed that "flat" did not require this qualification, so I can't blame him for the confusion. Makes sense to me now too. Very clever.

 

[sampleaccurate]

"Active pickups will not benefit from the "secret sauce" in the same way that passive ones will."

I don't understand why. An active pickup will have a low output impedance and a little bit of noise from the electronics, and there will be negligible HF attenuation due to cable capacitance, so they will generally sound brighter, no? Why would the HF pre-emphasis noise reduction circuitry not work the same for low impedance active pickups? Is it the brighter sound, or noise added by the electronics, or something else I'm not getting?

Thanks.

Stephen Cole

I imagine I'll get smacked around by the braintrusts here, but *from what I understand* passive pickups and electronics need to be loaded correctly from whatever they are plugged into to sound and feel right. Active pickups/electronics don't care about loading whatsoever.

That's correct, passive pickups have a high output impedance so they must not be loaded with a low input impedance on a device. That's why the AXE has a 1 million ohm input impedance. In simple terms what that means is it doesn't draw a lot of current from the pickup to capture the signal. If a passive pickup is forced to deliver a lot of current into a low impedance device it alters the sound. The other problem with passive pickups is that your cable acts like a capacitor and creates a low pass filter, effectively draining away the highs in your sound.

QUESTION RETRACTED - answer is noisy active pickup (making noise reduction unneccessary and ineffective) and stronger brighter output of active pickup more likely to drive input into limiting.

My preamps are very low noise and unity gain, so I think it may still do something for me.

Thanks.

Stephen Cole

 

[sampleaccurate]

I don't understand why.

Active pickups can potentially produce enough level to drive the front input into self-limiting. Additionally, the noise floor in active pickups is likely to have more HF content than passive pickups, due to input noise in the preamp. In that case, the encode/decode process is far less likely to improve S/N.

Why would the HF pre-emphasis noise reduction circuitry not work the same for low impedance active pickups?

The spectral content of the noise will be different.

That's what I suggested may be the case in my post - added noise in active pickups, and brighter.

Makes sense to me. I use unity gain preamps (home brew, very quiet) without a stronger output, they only lower the output impedance (a buffer essentially), so I don't think I'll have a problem with noise or limiting.

Thanks.

Stephen Cole

 

[sampleaccurate]

How about giving users a little credit for understanding how to use (and not to use) devices of this nature and save everybody who really should know this stuff a lot of time and spare them some unnecessary mistakes by simply stating in the manual something to the effect:

"The front input is especially conditioned for guitars and basses, particularly guitars with passive pickups, and SHOULD NOT be used for ANY other purpose. The rear inputs may be used for full range program material or guitar without the added benefit of the noise reduction on the front input. The front input employs a high frequency emphasis circuit that significantly boosts the higher frequencies in the signal of the guitar pickup to improve the noise performance of the AXE. This emphasis is reversed in the digital domain to recover the original EQ by setting the input parameter to ANALOG FRONT. Additionally, there is a limiter placed after this high frequency boost. The input gain control is placed after the limiter in the signal chain and will have no effect on input limiting. Because of the HF pre-emphasis, the limiter is more sensitive to the higher frequency content of signals than it would normally be for an unequalized signal. Therefore, if this emphasis is applied to sound sources other than a guitar that have significant energy in the higher frequency ranges (something a guitar doesn't have), the result will be an overall signal level that exceeds the threshold of the limiter, and limiting of the high frequency content of the signal will occur. The input meter will not reliably indicate the threshhold of limiting or the input level of sources other than guitars and basses".

If I have something misstated, someone correct me, and then put something that describes the input circuitry in the manual. That's my suggestion anyway.

I'll submit it to the Wiki manual if Jay would be kind enough to correct any misstatements or inaccuracies. It would have saved me a hell of a lot of time, and it also seems like something that users should know. Enough mystery!

Stephen Cole

 

[Jay Mitchell]

"The front input is especially conditioned for guitars and basses,

Here's a quote from the manual:

"6. Instr - Plug your guitar into this jack. This input is conditioned especially for use with your guitar.
Plugging a line-level device into this input is not recommended as this may cause clipping of the
input amplifier. Be sure to set the INPUT SOURCE to ANALOG FRONT in the I/O menu. If a plug is
inserted into the INPUT1 LEFT/MONO jack on the rear panel this input is defeated."

This is more than enough information to make clear the intended use of the front input. "Line level" may be either pro (+4dBu=0VU) or consumer level (-10 dBu). If you do not want to overload the front input, you should apply an instrument level (roughly -18 dBu) signal.

Here is another quote:

"The LED’s indicate the signal level at the A/D converter. When the red LED lights the signal into
the A/D is 6 dB below full-scale (6 dB headroom)."

From the two passages above - both in the same section on the same page and simultaneously visible - two things are clear:

1. The LEDs are not analog clip indicators, they are dbFS indicators for the A/D section.

2. Only instrument-level signals sound be applied to the front input.

I can easily see why Cliff would not want to go more into detail than this. He has developed an input stage that does a very good job of matching the noise floor of a typical guitar to the digital noise floor, and AFAIK this input configuration is unique to the Axe-Fx. I do not claim to have discovered everything about it, but the HF encode/decode is easy to find: just use the rear input with "Analg Front" selected and you'll see the decode.

 

[MisterE]

This post has been most informative and I've learned a lot from it.
But I was quite happy with:

Instr - Plug your guitar into this jack. This input is conditioned especially for use with your guitar.
Plugging a line-level device into this input is not recommended as this may cause clipping of the
input amplifier. Be sure to set the INPUT SOURCE to ANALOG FRONT in the I/O menu. If a plug is
inserted into the INPUT1 LEFT/MONO jack on the rear panel this input is defeated.

I feel that for most users, the simple version would suffice.
However your more elaborate explanation would be a useful addition to the wiki.
So would a detailed analyses of each block be.

 

[sampleaccurate]

"The front input is especially conditioned for guitars and basses,

Here's a quote from the manual:

"6. Instr - Plug your guitar into this jack. This input is conditioned especially for use with your guitar.
Plugging a line-level device into this input is not recommended as this may cause clipping of the
input amplifier. Be sure to set the INPUT SOURCE to ANALOG FRONT in the I/O menu. If a plug is
inserted into the INPUT1 LEFT/MONO jack on the rear panel this input is defeated."

This is more than enough information to make clear the intended use of the front input. "Line level" may be either pro (+4dBu=0VU) or consumer level (-10 dBu). If you do not want to overload the front input, you should apply an instrument level (roughly -18 dBu) signal.

Here is another quote:

"The LED’s indicate the signal level at the A/D converter. When the red LED lights the signal into
the A/D is 6 dB below full-scale (6 dB headroom)."

From the two passages above - both in the same section on the same page and simultaneously visible - two things are clear:

1. The LEDs are not analog clip indicators, they are dbFS indicators for the A/D section.

2. Only instrument-level signals sound be applied to the front input.

I can easily see why Cliff would not want to go more into detail than this. He has developed an input stage that does a very good job of matching the noise floor of a typical guitar to the digital noise floor, and AFAIK this input configuration is unique to the Axe-Fx. I do not claim to have discovered everything about it, but the HF encode/decode is easy to find: just use the rear input with "Analg Front" selected and you'll see the decode.

I disagree. I think more detailed information would avert questions and keep people from making mistakes. "may cause clipping" isn't a strong enough warning. And I believe some of the 1970s Electo-Harmonix analog pedals used hi-freq emph/de-emph as do vinyl records for noise reduction, so this is nothing new. It's just tailored to a guitar, and well done. That doesn't mean it should be hidden away.

As far as the meters being indicators for the A/D input, that's precisely the point where most digital processors clip. The analog front end usually has more than sufficient headroom to clip the A/D, and the meters indicate when this clipping will occur.

What's funny is that you enticed me to ask you about the "special sauce" used on the front input so I'd ask a lot of questions, which I did, and now you're saying the information that's been discussed here shouldn't be in the manual.

I don't understand at all why anyone wouldn't want to go into more detail on this. Why cloak it in mystery?

If there is a LIMITER placed in between the input jack and the input gain control it should be explicitly stated, period. This generic "may cause clipping" is lame. Give the users more credit to be able to understand and use this knowledge.

That's just me.

Stephen Cole

 

[JKos]

> If a plug is inserted into the INPUT1 LEFT/MONO jack on the rear panel this input is defeated.

On a slight tangent to the original post... I think this behaviour is backwards. I'm use to gear that is exactly the opposite. The rear connectors are intended for more static setups (racks for example) whereas the front input is provided for an occasional need to plug right in and thus the front input overrides the rear input.

- John

 

[mworkman]

"The front input is especially conditioned for guitars and basses,

Here's a quote from the manual:

"6. Instr - Plug your guitar into this jack. This input is conditioned especially for use with your guitar.
Plugging a line-level device into this input is not recommended as this may cause clipping of the
input amplifier. Be sure to set the INPUT SOURCE to ANALOG FRONT in the I/O menu. If a plug is
inserted into the INPUT1 LEFT/MONO jack on the rear panel this input is defeated."

This is more than enough information to make clear the intended use of the front input. "Line level" may be either pro (+4dBu=0VU) or consumer level (-10 dBu). If you do not want to overload the front input, you should apply an instrument level (roughly -18 dBu) signal.

Here is another quote:

"The LED’s indicate the signal level at the A/D converter. When the red LED lights the signal into
the A/D is 6 dB below full-scale (6 dB headroom)."

From the two passages above - both in the same section on the same page and simultaneously visible - two things are clear:

1. The LEDs are not analog clip indicators, they are dbFS indicators for the A/D section.

2. Only instrument-level signals sound be applied to the front input.

I can easily see why Cliff would not want to go more into detail than this. He has developed an input stage that does a very good job of matching the noise floor of a typical guitar to the digital noise floor, and AFAIK this input configuration is unique to the Axe-Fx. I do not claim to have discovered everything about it, but the HF encode/decode is easy to find: just use the rear input with "Analg Front" selected and you'll see the decode.

I disagree. I think more detailed information would avert questions and keep people from making mistakes. "may cause clipping" isn't a strong enough warning. And I believe some of the 1970s Electo-Harmonix analog pedals used hi-freq emph/de-emph as do vinyl records for noise reduction, so this is nothing new. It's just tailored to a guitar, and well done. That doesn't mean it should be hidden away.

I don't understand at all why anyone wouldn't want to go into more detail on this. Why cloak it in mystery?

That's just me.

Stephen Cole

It is just you. Out of the 2+ years I have been on the forum you are the first person that has gone into this much detail to "validate" the front and rear inputs.

I think nearly all of us were happy to plug in a play and were happy with what we heard and didn't require empirical evidence to prove what we were hearing.

The manual told me what I needed to know with its present wording.

This has been an informative thread. I learned a lot. But at the end of the day it just confirmed that I was doing the right thing from the get go.

I am happy for you that you got to a "happy place."

 

[sampleaccurate]

> If a plug is inserted into the INPUT1 LEFT/MONO jack on the rear panel this input is defeated.

On a slight tangent to the original post... I think this behaviour is backwards. I'm use to gear that is exactly the opposite. The rear connectors are intended for more static setups (racks for example) whereas the front input is provided for an occasional need to plug right in and thus the front input overrides the rear input.

- John

I thought exactly the same thing when reading the manual.

 

[MisterE]

I don't think it was cloaked in mystery.
I just feel that what's in the manual would be sufficient info for 99% of the users.
And along comes someone who wants to know why it's that way.
And that person clears it up for everyone.
Thanks Stephen and everyone who contributed.

But it would be like explaining to someone how an engine and transmission works before they learn how to drive a car.
99% don't care how a car works, as long as it starts.
Well, I followed a 2 year course to learn how a car works - I wasn't happy with the simple explanation.
Same with computers. another 3 year course for that.
I guess I'll have to take up sound engineering and audio electronics as well. :shock:

It's just your curious nature and perhaps the fact that you weren't satisfied with what's in the manual that lead to this educating post.
Maybe, if Cliff finds the time and he feels we could use a bit more in depth info about the axe-fx , he might share it.
But I still don't feel he left valuable information out on purpose.
With the ongoing developments of his products, I think he's got a lot more on his mind and the manual was good, if somewhat lacking in certain areas.
But people like yourself have helped to clear up some matters and I for on am very grateful.

 

[sampleaccurate]

I don't think it was cloaked in mystery.
I just feel that what's in the manual would be sufficient info for 99% of the users.
And along comes someone who wants to know why it's that way.
And that person clears it up for everyone.
Thanks Stephen and everyone who contributed.

But it would be like explaining to someone how an engine and transmission works before they learn how to drive a car.
99% don't care how a car works, as long as it starts.
Well, I followed a 2 year course to learn how a car works - I wasn't happy with the simple explanation.
Same with computers. another 3 year course for that.
I guess I'll have to take up sound engineering and audio electronics as well. :shock:

It's just your curious nature and perhaps the fact that you weren't satisfied with what's in the manual that lead to this educating post.
Maybe, if Cliff finds the time and he feels we could use a bit more in depth info about the axe-fx , he might share it.
But I still don't feel he left valuable information out on purpose.
With the ongoing developments of his products, I think he's got a lot more on his mind and the manual was good, if somewhat lacking in certain areas.
But people like yourself have helped to clear up some matters and I for on am very grateful.

Thanks for understanding. It's good to know someone does. Actually I think it might be beyond the scope of the manual - there are so many other things - the manual could end up 500+ pages. But it should definitely be in the Wiki manual.

If anybody ever tries to measure the response of the front input they are in for a riddle and have my sympathy! With no mention of a limiter or HF emphasis between the front input jack and the input gain control it gets tricky.

I just have bad luck with defective electronics, and I was probably overly concerned that my AXE was defective because of past experience. That's why I tested it. Then all hell broke loose!

Have a great weekend.

Stephen Cole

 

[Jay Mitchell]

I disagree. I think more detailed information would avert questions and keep people from making mistakes. "may cause clipping" isn't a strong enough warning.

Well, IMO "smoking may cause cancer" is a "strong enough" warning. If you smoke, the probability you will get cancer is less than 1. If you apply a "line level" signal to the front input of the Axe-Fx, the probability you will get clipping is less than 1. If you disregard the very explicit instructions and experience an unfavorable result, you are in no position to blame the instructions.

I don't understand at all why anyone wouldn't want to go into more detail on this. Why cloak it in mystery?

Second question first: it is not "cloaked in mystery." First question: What you personally might want to see in the manual is not definitive. One can reasonably argue that instructions for proper and safe use - beyond those things that a reasonable person would consider "common knowledge" - should be provided in the manual. And they are. Anything beyond that is entirely at Cliff's discretion.

This generic "may cause clipping" is lame.

No, it is instructive as to the possible result of a failure to follow explicit instructions. If I tell you "don't do this," and you do it instead, you are in no position to blame me for the consequences.

 

[Jay Mitchell]

If anybody ever tries to measure the response of the front input they are in for a riddle and have my sympathy!

It's not rocket science. About six months ago, I did the same thing you just did. I concluded that there is an HF rolloff in the front input. Cliff corrected me by saying that the front input has flat response if you apply a signal of the appropriate level. I retested with an instrument-level signal - which, if you think about it, is perfectly logical for an instrument level input - and got the correct result.

When you began this whole saga, I gave you precisely the same information Cliff gave me. Had you carefully followed up on what I told you, you would have saved yourself some number of hours fooling with the Axe-Fx and thousands of typed words on this forum. At the end of it all, there is no new information. This is all more than sufficiently disclosed in the manual.

 

[sampleaccurate]

If anybody ever tries to measure the response of the front input they are in for a riddle and have my sympathy!

It's not rocket science. About six months ago, I did the same thing you just did. I concluded that there is an HF rolloff in the front input. Cliff corrected me by saying that the front input has flat response if you apply a signal of the appropriate level. I retested with an instrument-level signal - which, if you think about it, is perfectly logical for an instrument level input - and got the correct result.

When you began this whole saga, I gave you precisely the same information Cliff gave me. Had you carefully followed up on what I told you, you would have saved yourself some number of hours fooling with the Axe-Fx and thousands of typed words on this forum. At the end of it all, there is no new information. This is all more than sufficiently disclosed in the manual.

There would have been no "saga" (nor did I consider it a "saga") had this information been presented in detail in either manual. I'm not going to argue with you Jay except to say that most digital processors have analog front ends that have the ability to clip the input to the A/D without doing anything to the signal other than amplifying it, the input to the A/D is where the clipping occurs, and most meters reflect the signal level at the input of the A/D precisely because of that. So the part in the manual about what the meters monitor gives no information other than what most people would assume and gives no clue whatsoever about what is happening to the signal after it hits the front input before it hits the A/D.

I'd like to add that the noise reduction circuit works extremely well to my ears - it's a great feature IMO. But let me offer one example of why I think the circuitry between the input jack and the input gain control should be disclosed.

Our band uses a pre-programmed mix and pre-programmed effects processing. When we set up each time, we go through a methodical calibration procedure that involves inserting a test tone into the input used by each instrument and selecting a special patch that's flat with enough gain to cause the level of the tone to match a certain benchmark. We assume LINEARITY of the response, and we make sure things like tube preamps are in a zero distortion linear mode when we measure the total gain of each signal path. If I didn't know that the AXE uses a frequency dependent limiter between the input jack and the meter I could easily fail to take that into account. The entire procedure is dependent upon our assumption of the linearity of the response. Now that I know, I can modify the process to account for the emphasis and limiting, probably by avoiding it by running into the rear jack for calibration purposes.

Some of us do need to know stuff like this. There's absolutely no reason this shouldn't be in the Wiki manual other than information hoarding.

In any event, I think I've almost wrapped up what I needed to know on this and every other question I had about the AXE.

Thanks for your assistance. The last word is yours - I'm going to work on making some music.

Stephen Cole