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dBu/dBFS question

post #1 of 7
Thread Starter 
I'm not quite sure where to put this, but Sound Science makes the most sense to me. Here is my question. Is there an easy way to reference dBu to dbFS? Here is my situation. The Sennheiser microphone receiver that I am recording from is outputting a line level signal of +4dBu. If I were to plug this into one of the rear TRS inputs on an M-Audio Firewire 610, (which have no preamps), and simply pressed record, what would the amplitude of the signal in dBFS be? Is there a way to tell this without actually having to go and do it first?
post #2 of 7
There is no direct connection between dBU and dBFS. One is an electrical level and one is a numeric level. However, you can usually infer the relationship from some basic specifications.
In your example the mic receiver outputs a line-level signal, which is a 'professional' level and which has a nominal +4dBu electrical level. Pro gear will often have a +20dBu headroom, so that the peak output level (electrical) from your mic receiver should reach +24dBu. Your A/D converter must be able to digitise this peak level, and ideally you'd like the 0dBFS numeric peak value of the ADC to be reached when the electrical input reaches +24dBu. The M-Audio receiver needs to be set to receive a +4dBu nominal level, and hopefully it will also have a headroom of +20dBu, ie capable of receiving a +24dBu input signal. So you'd really need to check the specs of the M-Audio unit, and see what the max input signal it is capable of receiving (in dBu), and see how that compares to the max output level of the mic receiver (in dBu). The M-Aidio unit will simply convert it's max input level (whatever it is) to 0dBFS.
post #3 of 7
Quote:
Originally Posted by PiccoloNamek View Post
I'm not quite sure where to put this, but Sound Science makes the most sense to me. Here is my question. Is there an easy way to reference dBu to dbFS? Here is my situation. The Sennheiser microphone receiver that I am recording from is outputting a line level signal of +4dBu. If I were to plug this into one of the rear TRS inputs on an M-Audio Firewire 610, (which have no preamps), and simply pressed record, what would the amplitude of the signal in dBFS be? Is there a way to tell this without actually having to go and do it first?
The previous post was not quite correct as far as I understand it. dBu refers to volts rms, so peak levels are not really applicable.

Although not always the case, most digital units are referenced to:
0dBu = 0.775v rms = -18dBFS.

As you have used the term microphone receiver, I take it you are using a sennheiser radio mic? It's not really important, if the output is +4dBu then the mic signal has already been pre-amped to line level and you *should* just be able to hit record and get a decent signal. Remember, the term "+4dBu" in this sense is just a specification used to describe a professional balanced line, it does not mean that you will actually get a signal of +4dBu and without playing with the unit in person, it's impossible to say for sure whether you will get a hot enough signal straight into your converter to avoid SNR problems.

G
post #4 of 7
Thread Starter 
Quote:
As you have used the term microphone receiver, I take it you are using a sennheiser radio mic? It's not really important, if the output is +4dBu then the mic signal has already been pre-amped to line level and you *should* just be able to hit record and get a decent signal. Remember, the term "+4dBu" in this sense is just a specification used to describe a professional balanced line, it does not mean that you will actually get a signal of +4dBu and without playing with the unit in person, it's impossible to say for sure whether you will get a hot enough signal straight into your converter to avoid SNR problems.
Yeah, the person I am recording from is using a Sennheiser belt unit. Speaking of SNR problems, with my current setup I feel as though my SNR is not what it ought to be. Right now I am recording directly from the receiver's XLR output into an M-Audio Fast Track Pro USB box. It takes minimal gain to reach my desired recording level, but the noise floor just won't go below around -54dBFS or so. I suppose on someone's average home stereo or car setup this would not be a problem, but it does bother me.

I don't like using noise reduction tools, so is there any way to fix this, or is it just the limitations of the equipment we have?
post #5 of 7
Quote:
Originally Posted by PiccoloNamek View Post
Yeah, the person I am recording from is using a Sennheiser belt unit. Speaking of SNR problems, with my current setup I feel as though my SNR is not what it ought to be. Right now I am recording directly from the receiver's XLR output into an M-Audio Fast Track Pro USB box. It takes minimal gain to reach my desired recording level, but the noise floor just won't go below around -54dBFS or so. I suppose on someone's average home stereo or car setup this would not be a problem, but it does bother me.

I don't like using noise reduction tools, so is there any way to fix this, or is it just the limitations of the equipment we have?
A noise floor of -54dBFS is a bit higher than I would expect, but not entirely unreasonable, depending on the mic and the environment. I take it you've done the obvious and played around with the position of the mic? I really need a bit more information to give any useful advice: Is it a hand held mic or a lavalliere, what environment are you recording in, what is the bit depth you are using and how close to peak level (0dBFS) are you getting?

G
post #6 of 7
Thread Starter 
The microphone is an ear-worn mic that is pretty close to his mouth. We're recording in a large sanctuary in a church (My job is to record and edit sermons for broadcast on FM radio). It is extremely quiet in there as long as nobody in the audience coughs or something. I record at 44.1/16. The average level of the sound is around -4 to -6dBFS, and there are no peaks over -1dBFS.
post #7 of 7
Quote:
Originally Posted by PiccoloNamek View Post
The microphone is an ear-worn mic that is pretty close to his mouth. We're recording in a large sanctuary in a church (My job is to record and edit sermons for broadcast on FM radio). It is extremely quiet in there as long as nobody in the audience coughs or something. I record at 44.1/16. The average level of the sound is around -4 to -6dBFS, and there are no peaks over -1dBFS.
My first course of action would be the position of the mic capsule. Moving the capsule an eighth of an inch can make a big difference with those mics. Try to get it as close to the mouth as you can without touching and without being directly in the air flow. The second point is that churches are always difficult to mic properly, the usually large volume of air and highly reflective sufaces do cause quite a high noise floor, even if it appears silent. Brownian motion can have an influence in churches. Although, a good lavalliere should combat this reasonably well.

Is the receiver unit reaonably close to (say within 15ft) and in clear line of sight of the belt pack? Do you change the batteries in the belt pack after every service? Is there a high level of RF interference in the area or others using similar RF frequency devices? UHF are almost always better than VHF models. Diversity systems can provide a great improvement if the subject is moving around but if they are stationary and the receiver is well placed there shouldn't be much difference.

If after playing around with the mic position and eliminating the above you still can't get a much better SNR the only real alternative may be a tighter pattern or better quality mic and possibly a higher quality transmitter and/or receiver. A cheaper alternative might be to use noise reduction, although good noise reduction can be more expensive than a brand new high quality radio mic system. If you have to use noise reduction it's best to do two or three passes at light settings rather than a single pass with stronger settings.

This is probably the most advice I can give without actually being there to see and hear the problems for myself.

G
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