R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2 3 4   Go Down

Author Topic: S/N in big consoles  (Read 13645 times)

bushwick

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 624
S/N in big consoles
« on: January 06, 2011, 02:29:26 pm »

Do any of y'all have S/N specs on large format boards with all channels summed to the L/R buss. It came up in conversation today and I am very curious. Something north of 40 channels and along the lines of a 72 ch SSL, or 72 ch of 8078, or a legacy.

Thanks,
josh
Logged
Joshua Kessler
bushwick  studio
brooklyn, ny
www.bushwickstudio.com

Dominick

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 580
Re: S/N in big consoles
« Reply #1 on: January 06, 2011, 07:56:57 pm »

Neve would include factory measured noise floor levels measured under those conditions in the service manual of every console built.
Commissioning Neve consoles in the mid 90's, I would measure, under those conditions, using an Audio Precision PortaOne, around -80 dBu (unweighted, 20hz-22kHz passband) on 60-72 input VR consoles in the NYC metro area with no mains filtering or exotic grounding schemes.
Logged
Dominick Costanzo

John Roberts {JR}

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 266
Re: S/N in big consoles
« Reply #2 on: January 07, 2011, 06:07:59 pm »

bushwick wrote on Thu, 06 January 2011 13:29

Do any of y'all have S/N specs on large format boards with all channels summed to the L/R buss. It came up in conversation today and I am very curious. Something north of 40 channels and along the lines of a 72 ch SSL, or 72 ch of 8078, or a legacy.

Thanks,
josh


It's been a while so I don't have any numbers off the top of my head (the most I ever sent to a L/R bus in a console design was well over 100 (36 inputs, and 3 sends to L/R bus from each of the 24 monitor strips for another 72, + a handful of efx returns with assignment capability 4 or 8 more pairs).

The question suggests to me some discussion might be in order about the sundry noise sources in a console and how they combine.

In a console summing bus we are dealing with coherent and incoherent noise contributions. The coherent sources generally follow the number of inputs linearly  (actually n+1 for the popular virtual earth sum bus). Incoherent noise sources like the mic preamps and input strip build as the square root of the sum of the signals squared.

So pulling some hypothetical numbers out of my butt, a 72 input virtual earth bus with a good quality low noise bus amp (say -120 dbu ein) will contribute 73x that noise, or roughly -83 dBu.... But that is ignoring the rest of the console.

Lets say the input strips (ignoring the mic preamp) have a -100 dbu noise floor with fader at unity and pans centered. 72 of them squared and then square rooted will add another -82.5 dBu of noise.  Since these are incoherent with each other they will sum to about -80dBu... So that's your noise floor before one mic preamp is cracked open.

The theoretical ein for a low z mic preamp input is around -130dBu  so one perfect hypothetical mic preamp running 50dB of gain will come up to another -80dBu all by itself... and this is ASSuming the room noise is lower than the theoretical mic preamp noise.  Of course you will likely have more than one input turned up if 72 channels are punched up to the 2 mix. Of course not every input will get 50dB of mic gain, and not every fader will mix in to the L/R bus at unity gain fader.

I don't doubt there are differences between different console spec sheets, and there may even be slight audible differences listening to WFO bus noise floor with all inputs muted, but in practice most competent master bus designs should be well below the noise floor of the sundry input sources being combined.

While this noise analysis suggests to me there is no real noise concern between competent designs another consequence of the high noise gain virtual earth topology (73= 37dB noise gain) is that the pole caused by the compensation cap to stabilize the sum amp can lead to significant phase error in the top octave if total loop gain margin is not adequately high.  This should be well known to (big) console designers so managed using one of several strategies.  

Distortion will also rise with falling loop gain margin just like the phase shift if the design is marginal.

My numbers are crude, but my point is that noise is not the only or even the dominant problem in a large bus structure. In my judgement.

I hope this makes some sense. In my experience people read too much into bus noise floors and that number is not completely representative of final mix sound quality.

JR

PS: This is the short simplified answer and I rounded off some of my numbers for a neat result.  
Logged

QUEEF BAG

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 82
Re: S/N in big consoles
« Reply #3 on: January 10, 2011, 01:23:04 am »

but to achieve that lowest available
noise floor from that large console,
your system has to have a very good
ground system, with nothing forgotten.

or a number of noises in addition to hum
may appear at low levels (or not so low)
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1105
Re: S/N in big consoles
« Reply #4 on: January 10, 2011, 12:56:51 pm »

It all depends on what your reference point is. If +4 dbu analog tape, not so great. If you use elevated levels as found from some DAC's, it improves quite a bit.

If your signal levels reside at +12 dbu or more, you can improve the S/N by the same amount, 8 db. Take 8+ db of buss noise off the mix and complaints go away.
Logged
Jim Williams
Audio Upgrades

John Roberts {JR}

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 266
Re: S/N in big consoles
« Reply #5 on: January 10, 2011, 02:30:50 pm »

Jim Williams wrote on Mon, 10 January 2011 11:56

It all depends on what your reference point is. If +4 dbu analog tape, not so great. If you use elevated levels as found from some DAC's, it improves quite a bit.

If your signal levels reside at +12 dbu or more, you can improve the S/N by the same amount, 8 db. Take 8+ db of buss noise off the mix and complaints go away.


One thing I don't miss about selling consoles is the customers who would listen to consoles WFO with inputs muted and complain about the merest hint of output noise.

In the real world we are dealing with acoustic sources that have a dynamic range limited by nature. It is good practice to deliver a suitably lower channel noise to not compromise that natural dynamic range, but at some point we need to be practical. That is one important leg of the engineering stool.

Mic preamps and well designed bus structures have been quite good for decades. Mic preamps have bumped against theoretical limits for decades. With 70 odd inputs on a bus I find it hard to ignore the combined source noise.

Pushing the mix level to +12dBu is IMO rearranging the deck chairs on the Titanic. Any real source material will get it's noise floor raised 8 dB also. Unless you are mixing 72 channels of digital zero, i'd like to know what your friends are using as input sources that lead to complaints?  

JR

Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1105
Re: S/N in big consoles
« Reply #6 on: January 11, 2011, 10:38:28 am »

I'm talking buss and console noise contributions, not source noise from the storage device. By elevating the signal levels throughout the console, the console's noise contribution is lowered. Signal levels throughout my analog console reside at around +14~16 dbu. It clips at +28 dbu. That still offers enough headroom to be able to boost EQ without overloading the system. The opamps I use also sound best at higher levels. The buss noise does degrade with common digital reverbs, exception being the Bricasti M7.

Noise problems were solved many years ago here. It's not something I ever hear anymore. With a console loaded with .9 nv/hz/sq noise opamps, it's below the 16 bit theoretical noise floor so noise is not encoded to CD's.
Logged
Jim Williams
Audio Upgrades

John Roberts {JR}

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 266
Re: S/N in big consoles
« Reply #7 on: January 11, 2011, 11:48:11 am »

Jim Williams wrote on Tue, 11 January 2011 09:38

I'm talking buss and console noise contributions, not source noise from the storage device. By elevating the signal levels throughout the console, the console's noise contribution is lowered. Signal levels throughout my analog console reside at around +14~16 dbu. It clips at +28 dbu. That still offers enough headroom to be able to boost EQ without overloading the system. The opamps I use also sound best at higher levels. The buss noise does degrade with common digital reverbs, exception being the Bricasti M7.

Noise problems were solved many years ago here. It's not something I ever hear anymore. With a console loaded with .9 nv/hz/sq noise opamps, it's below the 16 bit theoretical noise floor so noise is not encoded to CD's.


Agreed, not noise, but phase shift and linearity still count.

FWIW, in the last "big" console I did, I included a gain trim on the master L/R bus that allowed for up to -10dB pad right at the bus amp.  This is pretty similar to running the bus 10dB hotter without the headroom compromise. (in fact better, because reducing the bus noise gain 10dB improves phase shift and linearity performance too).  Of course this gets complicated for full differential/balanced buses. I suspect a switched bus pad could accommodate that without compromising differential balance.

I wrote a column back in the '80s and in one issue I compared some practical differences between nominal +4dBu and -10dBV mixing platforms.  Even back in the '80s you could get decent performance from a (good) -10dBV system. Today with even quieter off the shelf electronics, I suspect the differences could be even smaller. (Yes there are practical benefits to passing hotter signals between chassis, but inside the box, not as much.)

I find it useful perspective to realize that most IC A/D converters run from 5V or 3.3V supplies. At the end of the day it's all about raising the bridge or lowering the water... to get a good ratio.

JR

PS: I'm not trying to be argumentative, just adding some perspective to the topic at hand.
Logged

Geoff Emerick de Fake

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 348
Re: S/N in big consoles
« Reply #8 on: January 11, 2011, 04:43:25 pm »

Not trying to be argumentative either, but...
I think the unmoving nature of bus noise makes it appear more intrusive than it is in a complete mix.
Typically, one starts mixing with the master faders up and builds the mix by raising faders one after each other, so bus noise is there from the beginning.
I must say that I took a serious liking to digital mixers because of the lack of bus noise.
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1105
Re: S/N in big consoles
« Reply #9 on: January 12, 2011, 12:33:42 am »

Your buss noise should be pretty low. If you unassign all the mix buss resistors, it really goes down. That noise adds up from both the buss resistor's thermal and current noise plus the electronic hiss from the input channels. The resistors are fixed but the electronic hiss contribution can be lowered. There are some very low noise fet input opamps that do very well lowering noise in older designs that used larger value resistors.
Logged
Jim Williams
Audio Upgrades

Geoff_T

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 406
Re: S/N in big consoles
« Reply #10 on: January 12, 2011, 01:47:39 pm »

Hi

Vintage Neve consoles had signal to noise measured in three stages.

1. The output noise with the 4T fader at minimum... usually up around high 90's

2. The output fader up and a single channel assigned to the 4T bus

3. A channel at 80dB gain assigned to the same path with 20 to 20KHz filter on all these measurements and the input terminated 200 ohms. That last test gave figures better than -45dB = EIN better than 125.

But note, they did not assign all the channels to a bus... just one channel.

All these tests are clearly listed at the rear of the technical handbooks along with a polaroid photo of the square wave response.

Plus all vintage Neve tests were referenced to 0dBu, not +4dBu and the noise was not subtracted from the maximum output like digital measurements these days and power amps.

Hence their wonderful figures!
Logged
Geoff Tanner
http://www.auroraaudio.net/main.shtml
http://www.grandmasterrecorders.com
    http://www.amazon.com/gp/reader/0595093329/ref=sib_dp_pt/104 -6861899-0350336#reader-link

NB Please do not pm me if you want a fast response... please email me.

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1105
Re: S/N in big consoles
« Reply #11 on: January 13, 2011, 12:26:58 pm »

John Roberts  {JR} wrote on Tue, 11 January 2011 08:48

Jim Williams wrote on Tue, 11 January 2011 09:38

I'm talking buss and console noise contributions, not source noise from the storage device. By elevating the signal levels throughout the console, the console's noise contribution is lowered. Signal levels throughout my analog console reside at around +14~16 dbu. It clips at +28 dbu. That still offers enough headroom to be able to boost EQ without overloading the system. The opamps I use also sound best at higher levels. The buss noise does degrade with common digital reverbs, exception being the Bricasti M7.

Noise problems were solved many years ago here. It's not something I ever hear anymore. With a console loaded with .9 nv/hz/sq noise opamps, it's below the 16 bit theoretical noise floor so noise is not encoded to CD's.


Agreed, not noise, but phase shift and linearity still count.
JR



Yes they do, at least around here. I have the analog console down 1 db at 200k hz. It's also flat to 2 hz. Those points were chosen because phase shift begins a decade above/below the target frequency. A phase plot on the Audio Precision shows a flat phase response 20~20k hz = no phase shift. The reason it's so low in the bass is the entire console is direct coupled, only 2 capacitors are used in the hybrid balanced sum amps, the rest is coupled with wires. I wish I could explain the quality of sound without capacitors or transformers, it's heavenly. It's as if a huge weight of crap has been lifted revealing the sounds, not the electronics.

Linearity is also important to me. THD+noise is .00045% at +20 dbu. That is the residual of the AP test rig. A CCIF IMD sweep shows .0002%. That's one input to mix outputs. Specs just qualify what I'm hearing, unadulterated music.
Logged
Jim Williams
Audio Upgrades

bruno putzeys

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1078
Re: S/N in big consoles
« Reply #12 on: January 14, 2011, 04:12:58 pm »

The curious thing is that when one keeps perfecting the console and the converters in this manner, you start approaching the sound quality you can get by not leaving the digital domain at all.
Logged
Warp Drive. Tractor Beam. Room Correction. Whatever.

Affiliations: Hypex, Grimm Audio.

John Roberts {JR}

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 266
Re: S/N in big consoles
« Reply #13 on: January 19, 2011, 11:15:33 am »

Jim Williams wrote on Tue, 11 January 2011 23:33

Your buss noise should be pretty low. If you unassign all the mix buss resistors, it really goes down. That noise adds up from both the buss resistor's thermal and current noise plus the electronic hiss from the input channels. The resistors are fixed but the electronic hiss contribution can be lowered. There are some very low noise fet input opamps that do very well lowering noise in older designs that used larger value resistors.


I wish this really was the problem people say it is..  Back in 1980 I wrote an article on console design for RE/P where I described a way to bus numerous channels together without the N+1 noise gain of a virtual earth summing amp (replace the resistors with current sources).  

In practice the current sources have noise too, and the commercial executions I did using this approach were populated with TL07x in the current sources so no where near as good as modern opamps (I did this decades ago). Rather than target near infinite output impedance from my "real" current sources I was happy to attain 20dB reduction in the N+1 bus noise contribution.

Like I say, I wish this was a real problem because I have a solution, but like I say, at the end of the day one microphone is going to dominate the noise floor of any properly designed console, so this is mainly a mental exercise (for noise, not for the phase integrity and distortion that also degrade with N+1).

Then there is digital with near perfect summation...   Time to move on to other problems.

JR

PS: am I the only one here who thinks it is spelled "bus" with one 's'?
Logged

Jim Williams

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1105
Re: S/N in big consoles
« Reply #14 on: January 19, 2011, 11:49:35 am »

bruno putzeys wrote on Fri, 14 January 2011 13:12

The curious thing is that when one keeps perfecting the console and the converters in this manner, you start approaching the sound quality you can get by not leaving the digital domain at all.


You have to leave it if you want music. It's an analog world. Digital audio is just encoding, nothing magical about that.
It's the tank and train, a storage and delivery service, it's not music, it's data.

Like 8 track was between 4 track cartridges and audio cassettes, digital is just a carrier medium to be decoded into an analog signal, eventually.

The encoding can never approach the resolution and sonic quality of the analog signal, it's an approximation at best. As such it will always be behind in quality to the analog signal. Digital audio doesn't start or end with data, it starts and ends with an analog waveform.
Logged
Jim Williams
Audio Upgrades
Pages: [1] 2 3 4   Go Up