Rockbox Technical Forums

Support and General Use => Audio Playback, Database and Playlists => Topic started by: aargl on July 22, 2011, 06:59:00 PM

Title: EQ tricks & tutorial
Post by: aargl on July 22, 2011, 06:59:00 PM
The examples that follow are based on the Sansa Clip+ with Rockbox, but the principles are perfectly valid for any EQ in the universe...  ;)

Rockbox Graphical EQ is great, but the reason I've decided to give some trick about EQ is that, being a musician and a sound technician, I've noticed that most of the people - included musicians and novice technicians - have misconceptions about how to use an EQ (not to talk about normal human beings).
Sound talibans will tell you things like: “always do this! Never do that!”
I prefer thinking about the following rules as guidelines that you should stick to, but that you can infringe a little bit if needed, without risking burning in hell...

Example 1: boosting bass on the Clip
You think the stock earphones suck? Try boosting the Low Shelf (written “LS” in the Graphical EQ) by +3dB at 80Hz: if you listen to some bass loaded trip-hop (say Massive Attack), you'll notice distortion, even at low level playing.

1) #1 rule of a sound taliban: don't ever turn the gain knob toward plus values!
If you look at a mixer and you see most of the gain knobs are turned clockwise, chances are that the guy in charge has still a lot to learn...
An EQ is basically meant to cut annoying frequencies, not to colour the sound in any way (especially when listening to a 2-track mix as we do through our earphones).
I would not strictly follow this rule only when dealing with poor devices that lack certain frequencies (mainly treble), and that I need to compensate a bit.
The typical reflex is to boost bass and treble like the old HiFi “loudness” switch did, but if you've got a poor equipment, you'll soon notice that “pumping up the bass” creates distortion sooner than it does with treble.
That's what happened to our Clip's EQ (though it's a software EQ, but I'm not here to hold forth about why it behaves that way...  ;D).
Try instead the following settings to achieve the bass boost:
Reset the Low Shelf to 0dB and run to the High Shelf (HS), on the other side of the Graphical EQ, set it to -3dB at 80Hz (you'll have to be patient while scaling it down that low, but luckily it allows us to do so, while it's not the case on classic EQs on an analog mixer...).
Listen: same result but no distortion even pushing the level a lot more than before! Waow! The sound taliban was right!
(well, actually, it's more a specific trick rather than a sound engineer's normal behaviour  :D 8))

2) Presets? What Presets?
I've seen here and there people asking for presets: if you've read me this far, you understand that there are no preset like “Rock”, “Jazz” as you find them on low quality amps that will satisfy the sound taliban: Vade Retro Presetas!
If you've got time to loose, you can try and build presets for every song you listen to, but I really don't see the point.
What I'd suggest instead, is to have one unique EQ setting to compensate your faulty earphones, but always trying to remain sober (some say about EQ: “the less is the best”).
 
Example 2: annoying frequencies on the Clip
In addition to the bass boost (which was indeed softening all the rest, as you might have understood by now) I've cut something at 340Hz (PK1: -6dB 340Hz Q=2.5) and optionally boosted a little bit at 9000Hz (PK3: +2dB 9000Hz Q=2 - actually the gain can vary from 0 to +4dB, depending on how your ears or your earphones suck  ::)); Oh, and by the way, the Q for the High Shelf is set to 3.

3) Use your ears
Of course, these settings can vary much upon your tastes or the quality of your earphones, which mp3 player you have, and so on...
I found the 340Hz to be disturbing, but you might find it's 400Hz or 280Hz instead: actually, the sound taliban says that if you take a typical SM58 stage microphone, you'll never find two that sound the same! Think about our cheap earphones!
Of course, if you spit money, go and buy all the best, but if you're interested in sound, the Rockbox Graphical EQ can give good results and, the most important, it gives you the opportunity to educate your ears!  ;D

I hope this helps, and if it prevents someone from throwing away his stock earphones...  :P
Title: Re: EQ tricks & tutorial
Post by: AlexP on July 23, 2011, 01:10:03 PM
Thanks, but this should be on the wiki.

It will just get lost here.
Title: Re: EQ tricks & tutorial
Post by: Llorean on July 23, 2011, 04:09:28 PM
Though adjusting the other shelf is less useful (and intuitive) than just properly adjusting the precut. That would give you more the effect you're looking for than using the high shelf to lower everything above a point (rather than raising everything below that point) without even adjusting the Q.
Title: Re: EQ tricks & tutorial
Post by: Strife89 on July 27, 2011, 12:03:07 AM
This is definitely interesting for a non-audiophile like myself. I'll give this a try later. :)

I have to agree with AlexP, though.
Title: Re: EQ tricks & tutorial
Post by: soap on July 29, 2011, 04:33:39 PM
Though adjusting the other shelf is less useful (and intuitive) than just properly adjusting the precut. That would give you more the effect you're looking for than using the high shelf to lower everything above a point (rather than raising everything below that point) without even adjusting the Q.

I came here to say this.  This tutorial, for all its positives has one large negative - it overlooks the precut and all the awesome it can bring to your experience.

To rephrase Llorean:

If in your example with Massive Attack you had set the precut to -3 dB that would have made the entire track 3 dB quieter while creating 3 dB of digital headroom.  If you had then set the low shelf to 3 dB of positive gain you would be ensured of having enough room to play in the digital domain without clipping.  You could sleep soundly knowing that clipping or other distortion (if any!) heard when properly using the precut is mastering flaws in the original material and not new artifacts created by Rockbox's EQ.
Title: Re: EQ tricks & tutorial
Post by: aargl on August 01, 2011, 09:41:58 AM
Great! Thanks for the precisions!  :)
I must confess that I haven't been testing all the features of Rockbox (they are so many, which is great, so many approaches are possible).
I understand the "precut" function to be like the Dim switch on a mixer, so indeed — even if the result will be quite the same — it's a simpler approach.
In the end, that's me learning something!  ;D :D
That's the beauty of forums, ain't it?  ;)

P.S.: of course, the "sound taliban" would say that removing 3dB from the overall level is dividing the dynamics by 2 (correct me if I'm wrong?) — but of course it's the same with both methods (except mine implies probably little less processing, but I agree this is quibbling...  ::) ;)).

P.P.S.: anyway, I just wanted to share some knowledge with people not at ease with EQ, and mainly to make them understand that they have to be careful with what they're doing, and hopefully give them the will to search for info on the web. Sound is a fascinating thing, you always learn!
Title: Re: EQ tricks & tutorial
Post by: soap on August 01, 2011, 06:37:13 PM
P.S.: of course, the "sound taliban" would say that removing 3dB from the overall level is dividing the dynamics by 2 (correct me if I'm wrong?) — but of course it's the same with both methods (except mine implies probably little less processing, but I agree this is quibbling...  ::) ;)).

No, it is raising the theoretical noise floor by 3 dB.  It is only cutting the dynamic range in half if the full dynamic range was being used.  And it ain't unless you have a very very special recording.  ;)

Title: Re: EQ tricks & tutorial
Post by: Bolec on August 02, 2011, 06:03:08 PM
Yeah it would be nice if someone could post something more on equalizers, precuts etc. As this actually is a dark magic for me and trying everything blind doesn't always work. If not write, then maybe point a link or something to a good prepared materials about this, so that I could read.
Title: Re: EQ tricks & tutorial
Post by: iWalrus on January 31, 2014, 12:23:19 PM
The examples that follow are based on the Sansa Clip+ with Rockbox, but the principles are perfectly valid for any EQ in the universe...  ;)

Rockbox Graphical EQ is great, but the reason I've decided to give some trick about EQ is that, being a musician and a sound technician, I've noticed that most of the people - included musicians and novice technicians - have misconceptions about how to use an EQ (not to talk about normal human beings).
Sound talibans will tell you things like: “always do this! Never do that!”
I prefer thinking about the following rules as guidelines that you should stick to, but that you can infringe a little bit if needed, without risking burning in hell...

Example 1: boosting bass on the Clip
You think the stock earphones suck? Try boosting the Low Shelf (written “LS” in the Graphical EQ) by +3dB at 80Hz: if you listen to some bass loaded trip-hop (say Massive Attack), you'll notice distortion, even at low level playing.

1) #1 rule of a sound taliban: don't ever turn the gain knob toward plus values!
If you look at a mixer and you see most of the gain knobs are turned clockwise, chances are that the guy in charge has still a lot to learn...
An EQ is basically meant to cut annoying frequencies, not to colour the sound in any way (especially when listening to a 2-track mix as we do through our earphones).
I would not strictly follow this rule only when dealing with poor devices that lack certain frequencies (mainly treble), and that I need to compensate a bit.
The typical reflex is to boost bass and treble like the old HiFi “loudness” switch did, but if you've got a poor equipment, you'll soon notice that “pumping up the bass” creates distortion sooner than it does with treble.
That's what happened to our Clip's EQ (though it's a software EQ, but I'm not here to hold forth about why it behaves that way...  ;D).
Try instead the following settings to achieve the bass boost:
Reset the Low Shelf to 0dB and run to the High Shelf (HS), on the other side of the Graphical EQ, set it to -3dB at 80Hz (you'll have to be patient while scaling it down that low, but luckily it allows us to do so, while it's not the case on classic EQs on an analog mixer...).
Listen: same result but no distortion even pushing the level a lot more than before! Waow! The sound taliban was right!
(well, actually, it's more a specific trick rather than a sound engineer's normal behaviour  :D 8))

2) Presets? What Presets?
I've seen here and there people asking for presets: if you've read me this far, you understand that there are no preset like “Rock”, “Jazz” as you find them on low quality amps that will satisfy the sound taliban: Vade Retro Presetas!
If you've got time to loose, you can try and build presets for every song you listen to, but I really don't see the point.
What I'd suggest instead, is to have one unique EQ setting to compensate your faulty earphones, but always trying to remain sober (some say about EQ: “the less is the best”).
 
Example 2: annoying frequencies on the Clip
In addition to the bass boost (which was indeed softening all the rest, as you might have understood by now) I've cut something at 340Hz (PK1: -6dB 340Hz Q=2.5) and optionally boosted a little bit at 9000Hz (PK3: +2dB 9000Hz Q=2 - actually the gain can vary from 0 to +4dB, depending on how your ears or your earphones suck  ::)); Oh, and by the way, the Q for the High Shelf is set to 3.

3) Use your ears
Of course, these settings can vary much upon your tastes or the quality of your earphones, which mp3 player you have, and so on...
I found the 340Hz to be disturbing, but you might find it's 400Hz or 280Hz instead: actually, the sound taliban says that if you take a typical SM58 stage microphone, you'll never find two that sound the same! Think about our cheap earphones!
Of course, if you spit money, go and buy all the best, but if you're interested in sound, the Rockbox Graphical EQ can give good results and, the most important, it gives you the opportunity to educate your ears!  ;D

I hope this helps, and if it prevents someone from throwing away his stock earphones...  :P



Hi, what about the other peaks available to adjust? Could you suggest anything about?