Gain Control And Decibels?

Hi: I don't exact understand the unit of db? Would someone advise what does it mean in detail ?

Why is it related to gain?


The decibel is a "ratio between two values of a physical quantity." Two common rule of thumbs are +3 db = doubling and +10 db = 10x increase.

So if you increased the gain level of a camera from 32dB to 35dB, you would be doubling the level of gain.

For purposes of work with surveillance cameras, I am not sure how much more you need to know about decibels. The key lesson is that higher dBs of gain results in brighter but noisier images.

Let's see how much of my high school physics I remember...

Although the term originates from the unit of a "bel", named for Alexander Graham Bell and used to measure sound pressure level, dB in this case isn't a unit, but a ratio, used because it's a logarithmic scale, and so easier to express large factors of change. When dealing with units like power, every 3dB represents a doubling or halving of power, and every 10dB represents a ten-fold change. A 6dB change would represent a four-fold change, 9dB is eight-fold; a 100-fold change would be 20dB, 1000-fold is 30dB, and so on. When talking about units like voltage and current, those figures are doubled (6dB = double/half, 20dB = 10x change). This works because power = voltage times current.

This is especially useful when dealing with human senses, because we don't perceive sensory input linearly. With sound, for example, a 3dB increase, or doubling, in volume is about the lowest change readily detectable by the average human hearing, whereas a 10dB increase is generally heard as "twice as loud".

When it comes to exposure gain, the same math applies: you can double the actual output signal, but it won't make the image twice as bright; instead, it requires a ten-fold boost in level to double the image brightness. Generally the "max" setting is designated 0dB, and attenuation levels below that are marked at negative dB levels... so you can either have -6, -12, -18, -24, -30... or 1/2, 1/4, 1/8, 1/16, 1/32, etc. Measuring this in dB is not only more efficient, it takes up less room on the label :)

There's more detail in this Wikipedia article.

I concur with John. If you want to read up on it I'd start here

Note: 3 dB equates to doubling power (of camera amplifier), not voltage signals. Also, it's true that increasing gain will increase amplifier noise, but this may not become resolvable in the image unless the light level drops quite low and/or the amp is really cranked up. Which leads one to scrutinize the sensitivity spec as well....

As a practical matter, I can hardly think of a camera where we tested in at 5 lux or lower, which we did not see visible noise. It's basically a fact of life for cameras. I am not criticizing the noise. Indeed, it's far better to have some noise at 5 lux, than the equivalent of a dark image where nothing can be made out (which would typically happen if gain was disabled).

I think this discussion is mixing decibels (in this case probably an audio level measurement) with gain as applied to a video camera. This is a bit confusing since the original question might have been related to audio gain. Perhaps the original poster could expand a bit on his/her questions.

Paul, while the decibel AS A UNIT OF MEASURE is based in audio, the fact that it uses a logarithmic scale makes it useful for other measurements that are better quanitfied logarithmically. In all cases, it's used to express a ratio based on an aribtrary level that's relevant to where it's being used - in power ratings, for example, you'll generally see positive values based on "zero" power output; for sound levels, it's positive values based on "zero" air movement. In most kinds of signal electronics, you'll see it expressed as a negative (attenuation) value counting down from a maximum signal level.

It's really only confusing if you think about it too much :)

To expand on this point, as it a primary feature demanded by the clients / consultants.

Is there any standard or certification to the WDR (db value) testing?

I ask that because manufacturers claim to have this or that db levels on their respective cameras, with the buzz word being 120db, but you never know what you get (like a box of chocolate) unless you test.

And if that is not confusing enough, Samsung have two WDR db ratting for the same camera series, the “WiseNetII”. Its 120db on the website, and 100db on the PDF specification sheet.

Nidal, unfortunately, no, there is no standard nor certification for WDR testing, so you can never know who is lying or using misleading figures. That Samsung example is funny.