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Bouke Vahl
strange volume trouble
on May 14, 2010 at 5:20:22 pm

Using a M-audio Fasttrack pro USB sound input device.

Put my mic in front of a speaker sending out a testtone.

My input gain is a knob that rotates 270 degrees (or something)

Fully open i get sound. Testtone is set so i record at -3 dB

Turn down the knob by 15 degrees, and it's SILENT.


Am i right assuming that the potentiometer used in this unit
is logarithmic where it should be lineair, or vice versa?
This can't be good, i'm used to lineair faders....

Or am i missing something else?




Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Richard Crowley
Re: strange volume trouble
on May 15, 2010 at 12:32:42 am

If you can measure full-open as "-3dB", then how about giving us a number for the other case. "silent" isn't a very objective measurement. How far can you turn it to get to -12? -24?

Note also that input trim (gain) controls are not intended to be as "linear" as the mixing controls. Their primary purpose is to allow a very wide range of input amplitudes to be accomodated.

I wouldn't say categorically that your symptoms describe an actual problem. But there MAY be a problem, but not enough info.


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Bouke Vahl
Re: strange volume trouble
on May 15, 2010 at 8:05:09 am

Richard,
Silent in this case means silent. No sound, nothing, nada.
- infiniti DB, or digital sample values of 0.
IOW, as loud as no mic attached...

As for your statements that trim controls are not always ment to be lineair, that's indeed new to me. I haen't seen it on any mixer i've used, and i can't understand in what situation it could be handy.
Humans think/move close to linear, not to log.

Besides, i can't believe it was intentionally on this thing, sicne it's being sold as a mic pre-amp. And having just a few degrees making the difference between overmodulating and no recording at all ain't the way i can work...

So i was hoping for something else i've missed.


Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Richard Crowley
Re: strange volume trouble
on May 15, 2010 at 4:08:22 pm

Can you try the source into some other preamp that you are familiar with? It has the symptoms of a very low-level source. I have seen some preamps that pump in a whole lotta' gain in the last few degrees of rotation of the trim pot. Generally with those kinds of preamps you don't really want to be operating in that region of the adjustment.


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Bouke Vahl
Re: strange volume trouble
on May 15, 2010 at 4:53:59 pm

Very funny.
I've tried. Believe me.

Let me introduce myself. I run a high-end video post production facility, and i write (custom) software for the audio/visual/film industry.

It's not that i don't know what could happen, its just that i cannot understand that anyone is putting out a piece of crap this unit is.
M-audio is Avid. I have used Avid products over more than a decade, and i've never seen something as bad as this.

My guess was that i've overlooked something, but the more i dive into it, the more i'm convinced that this is either bad desing or bad manufacturing.

Going to a VAR wednesday to get a decent piece of gear.


(what i'm doing is making a software based BWF recorder with LTC out, and i liked the front knobs of the Fasttrack, so i bought one.)




Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Kerry Brown
Re: strange volume trouble
on May 16, 2010 at 5:01:59 pm


see below


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Kerry Brown
Re: strange volume trouble
on May 16, 2010 at 5:04:33 pm

[Bouke Vahl] "Humans think/move close to linear, not to log. "

Not true.


The 'log pot' is used as the volume control in audio amplifiers, where it is also called an "audio taper pot", because the amplitude response of the human ear is also logarithmic. It ensures that, on a volume control marked 0 to 10, for example, a setting of 5 sounds half as loud as a setting of 10.




KB


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Bouke Vahl
Re: strange volume trouble
on May 16, 2010 at 7:14:05 pm

Yes.
using log potmeters to make mathematical correct log values into lineair values that humans understand.
that is exactly what i was saying.

Now where does the 'not true'fit in?



Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Richard Crowley
Re: strange volume trouble
on May 16, 2010 at 11:09:49 pm

Mr. Brown claimed that Mr. Vahl's statement was "Not true." And then he proceeded to demonstrate exactly why Mr. Vahl IS correct. Log pots are used to take log phenomenon and "linearize" them so that they are easier to manage by humans.


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Kerry Brown
Re: strange volume trouble
on May 17, 2010 at 6:29:29 am

[Richard Crowley] "Mr. Brown claimed that Mr. Vahl's statement was "Not true." And then he proceeded to demonstrate exactly why Mr. Vahl IS correct."
Maybe I am reading this wrong but his statement of "Humans think/move close to linear, not to log."seems backwards. We hear in a logarithmic way not linear.



KB


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Richard Crowley
Re: strange volume trouble
on May 17, 2010 at 6:34:03 am

But when we turn a knob to control something logarithmic (sound level, light, pitch, etc.) having it behave in a logarithmic manner is very difficult to control. That is why log pots are used: to "linearize" the control function.


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cowcowcowcowcow
Bill Davis
Re: strange volume trouble
on May 17, 2010 at 8:09:08 am


Not to put to simple a point on this, but from the original post, it appears your problem might be something as simple as someone having skipped the all important step of setting up the mixer to a proper GAIN STRUCTURE.

For the new folks here - here are the steps. They are as REQUIRED for using an audio mixer, as setting a proper WHITE balance is to shooting proper video.

FIRST - and foremost - check that the channel strip INPUT PAD is set correctly - Mic level signals and Line level signals are VERY DIFFERENT in amplitude. If this initial strip control is set incorrectly, you WILL have a mess of a time setting up your gain structure. (Note: this COULD cause the specific symptoms in the original post in this thread -If the channel strip is expecting a LINE level signal but being fed a WAY too low Mic level - ONLY the last part of the log travel of the fader might provide enough boost to get the signal up to the correct range for output.

NEXT: turn the channel's Trim Control to ZERO - (fully blocking any signal prior to delivering it to ANY fader, EQ, or GAIN slider in the channel strip) and bypass, zero out, or otherwise remove any EQ from that channel strip.

Then, present a KNOWN signal to the channel input - tone at 100% amplitude is IDEAL but music will do.

Then go back to the channel strip you're setting and SET the gain of the channel slider PLUS, any subMasters and Master gain controls that you've assigned to the signal path - until they are in the OPTIMAL Signal passing position. (neither boosting or attenuating a signal) This is the U (unity gain) or *0* position - or perhaps the bold line in the middle of the shaded areas about 80% of the way up the slider travel path.

At this point, the board is set up to pass any signal presented to a channel perfectly - EXCEPT the signal is totally blocked at the channel strip entry point - the TRIM POT.

THEN and ONLY then, go back and slowly raise the trim pot on the channel strip until you have the signal registering at the proper level on the meters and you're hearing the sound correctly at the monitor stage.

The point is that the "TRIM CONTROL" is set LAST. If you treat the "trim control" as "just another volume control" you're screwing things up. Because it's NOT. it's the Initial signal GATEKEEPER of gain structure and unless each trim control is set properly by using the above step by step process - the entire board may not mix properly.

This is the BASIC step in setting a mixer up to maximize the signal to noise ratio or each channel strip and give you the best control over gain on all channels.

It means you aren't presenting an overly strong OR weak signal to any channel strip.

The ONLY reason the nature of the trim pots range of control (log, or linear, or anything else) MATTERS is so you can get the signal into the right range for the channel strip to control.

If TRIM is not set properly - than it's NOT the channel sliders fault if it reacts incorrectly in controlling gain.

AND just as anything but a calibrated monitor can FOOL YOU as to color balance in a video signal - an improperly calibrated mixing board can FOOL YOU into thinking that your signals are flakey.


For what it's worth.




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Bouke Vahl
Thanks all, problem solved. (another piece of gear)
on May 21, 2010 at 6:54:15 am

Thanks all.
I've tried two other similar units from other brands.
One (i already forgot the name) had the same issues.

I've now bought a Steinberg CI 2.
It has better output leves, and a gain know that behaves in a nice human linear way. (turn the knob half down and the volume is half :-)

80 bucks more expensive while it has only 2 channels instead of four, but it comes with Cubase, and does what i need it to do.


Most important lesson here:
Do NOT buy this kind of gear on the internet just based on specs and reviews.
Go to a VAR, toy with the actual products and then decide what to buy. And buy from the VAR to keep him alive!
(I could get the same unit 50 bucks cheaper on the internet, but my VAR helped me, we've toyed for over an hour with the stuff, well worth the price.)




Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Bouke Vahl
update...
on May 27, 2010 at 3:48:51 pm

Just for the records.

The (internet) compan i've bought the M-audio from (and got it returned) just called.
They tested it and came to the same findings as i did.
They have had contact with M-audio (something mortals have trouble to acomplish), and sending the unit back to them to get it fixed.

I'm really interested in the outcome!



Bouke

http://www.videotoolshed.com/
smart tools for video pros


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Kerry Brown
Re: strange volume trouble
on May 16, 2010 at 8:10:33 pm

[Bouke Vahl] "Humans think/move close to linear, not to log."



KB


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