Re: [LAU] USB audio interface support?

On Thu, Nov 26, 2009 at 05:07:16PM +0800, Ng Oon-Ee wrote: The question to ask is not how much gain would be required. All these interfaces will provide enough gain to the point that increasing it will not improve S/N ratio. The important question is then: how much noise do we have in those conditions ? Two values are important for this : the self noise of the mic, and the equivalent input noise of the preamp. For the SM57 the self noise is easy to find: Sensitivity = 1.9 mV/Pa or -54 dBV/Pa The reference sound level is -94 dBPa, so the mic will produce -148 dBV at that level. The SM57 is a passive microphone, so its self noise is determined by its impedance which is 310 ohm. This will produce a noise voltage of around -130 dBV in the audio range. Comparing these two numbers, the result is that the mic will produce the equivalent of around 18 dB of acoustic noise. For example if you record a sound of 78 dB SPL, the S/N ratio will be 60 dB. This is a typical figure for a mic of this type, which is not really designed for low-noise use such as recording classical music or soft instruments. It's used mainly for solist voice, amplified instruments and drums. The best low-noise mics would be some 10 dB lower. The second parameter, equivalent input noise, is the noise of a preamp, measured in some operational condition, and assuming all that noise is generated by adding it at the input. For a good preamp this will be very close to reality. It will have the best EIN at max gain, and this level will remain almost constant over a large gain range. None of the four manufacturers provide this information, nor any other data from which it could be computed. This is no coincidence: for most 'consumer' type preamps the EIN is not very good, and manufacturers want to avoid that you compare their performance to the competition. For my Edirol UA-5 I just measured it. For a source (mic) impedance of 150 ohm it is around -120 dBV, at 310 ohm it would probably be around -117 dBV. Now if you compare this to the self noise of the mic, -130 dBV, the conclusion is that the preamp will produce 13 dB more noise than the mic. Not very good, but quite typical for any product in this price range. Combining all this, using the SM57 with the Edirol UA-5 would produce a noise level that is equivalent to 31 dB of acoustic noise. This is, in the end, the only figure that matters, as it determines what you can record with a good S/N ratio and what not. HTH, -- FA Io lo dico sempre: l'Italia è troppo stretta e lunga.