PFL

[johndenim]

Hi All.

 

I understand why we use the PFL and that the input gain (on VU's)should just peak at 0dB when the particular fader is set at 0dB,

but why exactly is this?

If a microphone (for example) is set correctly pre main bus does this mean less chance of feedback?

 

I know this was discussed (gain) recently but are we looking at PFL'ing playback as well?

 

Cheers,

John Denim.

[Rob_Beech]

It's all about gain structure.

 

You can have all the channel faders at 0dB, or at a set level I usually like mine at -5dB as it gives me more flexibility. Sometimes -10dB with vocals as it gives me stacks of headroom for if they talk quietly between songs.

 

You have a similar signal coming in to each channel, so you can work with your outputs easily.

 

Also, by driving certain sections of the desk harder you alter the sound, and thus phase and frequency response. If you drive a preamp hard and have the fader right near the bottom, you'll get a different response as you will by using a more reasonable gain structure. This results in a different sound, and a rather messy looking desk.

 

Smaller issues include the way the faders work.

 

At the bottom of their travel, a fraction of an inch is several dB where as you have a much more controlled range near 0dB with maybe half an inch being a few dB.

 

The only way you would get more or less chance of feedback is if the way the preamp is driven alters the frequency response. It will do, but it has to be somewhat extreme.

[Just Some Bloke]

What Rob said!

 

But also, on a purely practical and non-technical note, it also means that when you look at the faders you know exactly what's going on: the faders which are the highest will be outputing the most sound, the faders which are the lowest will be the quietest channels. Personally I can't bear it if I sit on a desk that someone else has set up without setting the gains properly and the channel with the lowest fader is making the most sound! :)

[Sound In Gloucestershire]

correct me if im wrong, but I could have sworn it was to do with voltages?

 

http://www.prosoundweb.com/studyhall/studyjump.php?pdf=gain

 

its a link I bookmarked a while back but never found the time to read properly hehe

[mervaka]

yes, but voltage is referenced at 1v when working in dBV and 0.775v when working in dBu.

 

EDIT: sorry for the hasty response above, but what is "it?" I was assuming we were talking about the dB scale here.

[Bobbsy]

A comprehensive reply from Rob as usual. I'll just grab the last part of John's question.

 

In an ideal world you wouldn't have to adjust anything for line level sources. Unlike microphones which can vary greatly in output levels, in theory there is a set level that all line level devices adhere to. (Well, to be precise there are two standards. Professional gear with balanced outputs should have a nominal level of +4dBu while consumer unbalanced stuff is normally -10dBu).

 

However, in the real world this doesn't always work out since different people have different ideas about how to adjust their recordings. Particularly in this digital age many people compress recordings to within an inch of their lives and have everything at more or less 0dB(FS). This would actually equate to something like +18 to +22dB on an analogue scale.

 

On the other hand, the last playback job I did involved classical music and their CDs had dynamic range of a bit over 50dB. If I adjusted for the peaks the quiet bits were inaudible; if I went for the quiet bits, the crescendos hit Metallica levels! So...a compromise was struck, a compressor was patched in and I still had to ride levels enough to keep me awake!

 

So, my rather rambling answer is that, yes, you have adjust you playback sources in much the way you would with a mic input.

 

Bob

[Matt Pengelly]

Agree 100% with everything Bobbsy and Rob have said thus far, although...

 

[pedant]

 

Unbalanced consumer stuff is usually -10dBV (i.e. nominal level is -10dB when 0dB=1V), rather than dBU (where 0dB=0.775V), giving a difference of around 12dB compared to the usual pro standard of +4dBU.

 

[/pedant]

[Bobbsy]

I'll put my hand up to what Matt says, but....

 

since we're being pedantic, I was always taught it was written dBu (as opposed to dBU) to avoid confusion with dBV.

 

Pedantry war over...I'm going back to my nicely chilled Chardonnay (which, here in Aus, cost me under a pound a bottle for wine that would be near a tenner back in the UK).

 

Bob

[Matt Pengelly]

Haha, you're absolutely right. Schoolboy error on my part there.

 

And as for the Chardonnay. Well. Touché, sir!

[paulears]

For beginners the difficulty is that there are a number of rotary and linear faders that seem to alter 'volume'. Everyone is giving really sensible advice on gain structure. If you look this up, you are likely to get very confused. With some simplification, this is what happens. Inside the mixer there are quite a few stages where gain is needed. The first comes straight after the XLR socket, and gets the very small voltage up to a level where it isn't distorted by being more than the circuitry can take, but not so low as to mean it has to be further amplified more later on and be hissy and noisy. This gain control is at the top of the channel strip. At the bottom is the main channel fader that reduces the level being passed to the 'mxing' part of the desk. So these two faders can be adjusted together to give a certain output. Most modern mixers have quite a bit of range, so you could set the channel gain low, which would require the channel fader to be high for normal operation. Equally you could set the gain at the top to a higher setting and then the channel fader would be hardly on and still be loud enough. As people have said - this works, but you might have just 10mm of fader travel between very loud and off - not good for gentle fades. Prodding the PFL allows you to set the gain properly - somewhere nearish to the 0dB is common - BUT like most on here, once you know what your mixer can do you have a bit of leeway. Maybe you have four vocal mics? One singer Belts it, another is a bit feeble - the gain setting will be different for both, and if you are careful you can arrange things so both faders are level with each other for the volume you want. This is a better way to handle multiple mics - It's very annoying when you have to remember ch 3 can only go up to -10 on the scale, but ch 2 can go almost to full! Each section of the mixer should be working as far as possible within it's optimum performance range. Getting this right is important for sound quality AND operation. Get it wrong and the sound will start to get rough, followed by distortion. At the other end, if it is too weak and has to be amplified later, your wanted signal plus the hiss in the background will be permanently 'glued' together, meaning that if the hiss is annoying, apart from removing the HF, there's not a lot you can do to make it better.

 

My pet hate is desks without grouping because on them, your drum kit, for example can have some wildly different fader positions, but you can boost or pull back the entire kit with just one or two faders. Without groups, you have to retail the relative balance between the parts of the kit, and if they are not in a straight line - this is really difficult.

[waster]

My pet hate is desks without grouping because on them, your drum kit, for example can have some wildly different fader positions, but you can boost or pull back the entire kit with just one or two faders. Without groups, you have to retail the relative balance between the parts of the kit, and if they are not in a straight line - this is really difficult.

 

I would disagree with this because... firstly it is unlikely the whole kit is to loud, more the case that the snare or hat etc is to loud in smaller venues prehaps. Also by adjusting the drum kit on a group you will loose you effects balance. So you might end up with to much or two little reverb. You may also loose the balance of your mix as in a smaller venue if you are pulling the groups down for a drum kit say you will probably go below the level of the acoustic drums sound fairly easily.

 

I would suggest that groups be used to group audio for the purpose of

 

- Monitoring your mix of a group of instruments (like the drum kit)

- Creating groups of audio to feed to your matrix

- Group Compression or Compressing things like vocals when you are doing monitors from FOH and forgot your y-splits.

 

VCA's solve some of these issues.

 

Just some thoughts at the end of a long week and beginning of a long weekend.

 

Cheers.

[paulears]

I suspect we'll have to disagree. With either analogue groups or VCAs I use groups for balancing all the time.

 

Often I'll have the drums panned L/R, BVs as a group if there are two or three, the brass section as one and keys if there are more than two as a group. Things like bringing snare forwards or back are done with the usual fader in the drum section, but balancing all the sources I find easier on groups. At the moment, I am using them as a matrix, as mentioned - but it depends on the production.

 

Groups are just handy for doing lots of things!

[berry120]

But also, on a purely practical and non-technical note, it also means that when you look at the faders you know exactly what's going on: the faders which are the highest will be outputing the most sound, the faders which are the lowest will be the quietest channels.

This is exactly how I try to do things - or at least relatively! Often, I'll also set the gain to give an indication of an important channel - so it might be adjusted so the lead vocal is a bit higher than everything around it, purely because it helps me find it more quickly if something was to go horribly wrong.

 

In terms of groups, I'd have to go with paulears on that one. I generally find that I get the balance within the kit pretty much on the mark with a decent soundcheck, and it's often the level of the whole kit I'll want to bring up or down. Doing that without groups is a nightmare. Half the time I'm micing drum kits as well is just for recordings anyway, so I don't have to worry about bringing it to below the accoustic level. I guess beyond a certain point it's all down to preference - there's not really a "right" way, as long as it sounds good, if you're the one operating it you do things the way you're happiest! At least, that's the way I've always used :)

[Bobbsy]

I'm also with Paulears on this one. Once I have a reasonable drum balance (or it could be backing vocals or sometimes even a mix between DI and Amp mic on a guitar for example) it's nice to be able to adjust the overall level without changing the balance. This is where Groups (or VCAs or DCAs) can come in.

 

However, to give equal time to the opposition, recently over in the wild west (otherwise known as the AAPLS newsgroup) there was recently a heated discussion about setting gain structure. Darn near 50% of the people there (including many people who mix name acts) did their gain structure a bit differently. They set the fader to zero but, instead of setting the channel gain so everything is about zero on the PFL, they use their ears and set the board so, with all faders at zero, the mix is roughly correct. During the show they still make minor tweaks on the faders but know that in an emergency, just pushing everything to zero will get an adequate (if not perfect) mix.

 

I stress that I personally don't like this method--I like having the visual cue of seeing the faders telling me which sources are set higher and which lower--and I find I can set the gain structure more accurately with a meter--but mention this to show that, even among pros, there's more than one way to skin a cat.

 

Bob

[johndenim]

However, in the real world this doesn't always work out since different people have different ideas about how to adjust their recordings. Particularly in this digital age many people compress recordings to within an inch of their lives and have everything at more or less 0dB(FS). This would actually equate to something like +18 to +22dB on an analogue scale.

 

Bob

 

Thanks to all so far, as we are drifting into recording I would like to ask the optimum level please?

I'm talking recording from a pc via a usb-rca audio interface (thanks for advice on this Bobbsy) onto a minidisc.

 

I generally just clip -2dB on the vu's, (on the minidisc recorder) would it be suggested that this is too high/low?

Sorry, what's the "FS" please?

 

John Denim.