little confused about signal to noise ratio.

M

mrpink89

This link

http://www.steves-digicams.com/know...ftware/all-about-signal-to-noise-ratio.html#b

He does a good job making it simple, yet I feel I am having a mix communication with his messages.

In one paragraph he says:

"A larger number means a better specification. For example, an audio component that has a signal-to-noise ratio of 105 dB produces an audio signal that is 105 dB higher than the level of noise created by the component. Therefore, a device with a 105 dB signal-to-noise noise ratio specification would produce less signal noise than a component that is rated only 90 dB."

Then at the next paragraph he says

"However, it is important to keep the noise in the signal as low as possible in order to produce accurate and clear sound. In short, the lower the signal-to-noise ratio a component produces, the better the aural quality audio or music that you will hear."

Question:

Why does he say to keep the S/N ratio low as possible in paragraph 2 when before in paragraph 1 he was saying a large number means a better specification?
 
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In the first paragraph, he's referring to the S/N rating of a piece of audio equipment. In the second, he's referring to the S/N ratio of a recording created by the equipment. A larger number in the rating equals a lower ratio in the recording, ignoring other factors.

EDIT: I take it back, that guy is being confusing. A S/N ratio is typically calculated as the power of the signal divided by the power of the noise, so you would actually want a higher S/N ratio. He really means what he says in the sentence before, you want to keep the level of the noise in your recording as low as possible. In other words, the noise to signal ratio should be as low as possible.
 
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okay, cool thanks. I am just confused on the whole spectrum of the number some things are listed as.

For example my recorder I am getting (PMD 661) has a s/n ratio of 65dB Mic
and my Shotgun mic (NTG 2) has a S/N of 76dB

are theses db out of 100 or something? and is it better to get something that has a lower number or a higher number closer to 100?

I am noob as hell and needed an explanation dumbed down.
 
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dB - The Decibel scale catches out a lot of people, actually at some stage it catches out pretty much everybody! The dB scale is not out of 100, there are three things to understand about the dB scale:

1. It is not a single scale, there are infact about 20 different dB scales used for measuring the energy contained in audio signals. Frequently you will see other letters after the "dB" letters, indicating which scale is being measured. For example dBSPL is the scale used to measure the energy of sound waves travelling through the air, the SPL stands for Sound Pressure Level. dBFS is another scale and the one you'll come across most frequently, the FS stands for Full Scale and is the dB scale used to measure a digital audio signal. So dBFS is what you'll see in all the meters in your DAW or NLE and in your and PMD661.

2. The dB scale is not a linear scale, it's a logarithmic scale. In practise for the scales you'll use most frequently, this means that double say 50dB is not 100dB, it's actually 56dB. So, +6dB is double, +20dB is ten times and +60dB is one thousand times.

3. There is no easy reference point for the dB scale, it does not directly relate to the human perception of loudness for example. So while +6dB indicates double the amount of energy (input/output voltage in the case of the dBFS scale) in the audio signal, that does not necessarily mean it would sound twice as loud. This also makes the scale difficult to relate to for an audio newbie. Some rough examples of dBSPL are given near the bottom of this wiki page which will give you a start.

In the cases you have listed, for example the PMD 661 says it has an SNR of 65dB. This 65dB is the difference between the internal noise the unit generates and the signal it is recording, so the bigger the number the better. Your mic specs are showing an even greater SNR, which is better, according to the figures, significantly better (76dB vs 65dB).

I have to say, 65dB is NOT a good SNR for a recorder! However, audio equipment specs are notoriously difficult to interpret because they may not be measured the same way from manufacturer to manufacturer. In this case, it *could* be that the SNR quoted is a worse case scenario, IE., they *might* have measured the SNR from the mic input with the pre-amp gain set to maximum. Another recorder might quote it's SNR as measured with a line level input (rather than a mic input), with the pre-amp gain set at zero, which is where the pre-amp produces the least noise and would therefore show a much higher (better) SNR, even though in practise it might be the noisier unit. The devil's in the details!

I don't suppose the last paragraph has helped you much, it's likely just confused you more! Sorry about that, it's the nature of the beast unfortunately.

G
 
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