CCD is analog?

[fjrabon]

Yep. Digital is a means of communication using two, and only two, predetermined states. It could be described as amplitude (voltage), phase, frequency, position or any other measurable quality that can be compared against some other quantity.

But it's not digital until the quality represents something.

yeah, in a manner of thinking all an A/D converter is a machine for applying the agreed upon choice to the signal and then turning it into another signal where all information except the 0 and 1 is lost.

That's more like a simple schmitt trigger threshold. While the ADC does eliminate some data due to aliasing, ADC's will encode an analog value, from high to low according to it's bit depth: a 4-bit ADC might encode black as 0000 and white as 1111, with middle grey being 1010.

Imagine a simple ADC which is an array of threshold triggers that are set in evenly spaced intervals, and are in turn fed to binary encoder. When the first trigger voltage is reached, it's output to the encoder goes high, if the second threshold voltage is met, then it's output goes high, and so on until the maximum voltage condition is met. The encoder then converts the input into digital data which can be used elsewhere to say something about the analog input.

This is not really how a camera ADC works, but is rather a simple example of how analog data can be converted into digital data.

well, right, but you're still turning a continuous signal into a discrete signal, which necessarily loses data (not whether the data is relevant or not is another matter). which is what i was referring to. Not necessarily how fine grained the discrete signal is.

 

[Garbz]

Well, for example, CCD react really nasty if the maximum voltage is reached and the pixel overloads. Thats why most CCDs not used for science or the military have overflow protection mechanisms in each pixel. These mechanisms already kick in before the pixel actually overloads, causing a somewhat different reaction to this situation than with CMOS fotosensors. This leads to an image somewhat more closer to what chemical film looks like.

Theres other things and I really only found very little sources to this kind of thing. All in all CCDs tend to create more "beautiful" colors. Thats one reason why Leica used CCDs, not CMOS, in their Leica M8 and M9 cameras, even if today CCDs are inferior to noise compared to CMOS.

CCDs are in no way inferior to noise, actually quite the opposite. CCD is a superior technology, it is more linear, has a far wider spectral sensitivity, can read out an entire sensor in one go (global shutter) and ... lower noise. HOWEVER, and this is a really big however, they are complicated. CCDs are a true analogue chip, CMOSs are a complete integrated circuit. They have all signal processing and conversion on the silicon, so while they are more expensive to purchase they require far less components to integrate, and the integration has much less impact on performance. For CCDs the opposite is true. While any grad engineer could buy a CMOS and make a basic camera that performs quite well, to get a CCD to perform really well requires incredible attention to detail and skill in designing low noise circuitry. CCDs do have a downside though. They are far more susceptible to thermal noise and (when I last looked at them) ran at higher voltages causing them to heat up more (hence more noise).

The above is why most scientific instruments are CCD, and also why most of them have bigarse heatsinks / cooling units slapped to the back of them. You think Leica's are expensive you should check some of the astrophotography cameras. $6000 can in some cases buy you .... a sensor. That's it. No shutter, no AF, no bells and whistles, just a sensor with a USB socket.

 

[Solarflare]

We talk about photography, not science, here. We dont talk about huge machines that are cooled with liquid nitrogen. We talk about transportable cameras. And there, without the cooling, CCDs have more noise than CMOS, right now.

DxOMark - Camera Sensor Ratings

All the top cameras on that list are CMOS, as is the Canon 1D X which soon will enter the top of that list.

 

[manaheim]

I was going to post some witty remark until I saw this. bhop wins the thread.

 

[unpopular]

We talk about photography, not science, here. We dont talk about huge machines that are cooled with liquid nitrogen. We talk about transportable cameras. And there, without the cooling, CCDs have more noise than CMOS, right now.

DxOMark - Camera Sensor Ratings

All the top cameras on that list are CMOS, as is the Canon 1D X which soon will enter the top of that list.

This may be a very good point. If within the practical limitations of digital photography CMOS is cleaner, then CCD is noisier, regardless the theoretical capabilities of the technology. In theory, you could have a PMT tube or Avalanche Photodiode attached to a high magnification lens, scanning the scene one pixel ata time with a 32-bit ADC on the other side, but this would be anything but practical for general photography.

Let's not forget, the M9 has relatively low maximum ISO and the HD40's lousy low light performance. Perhaps the CCD is superior technology, but once you implement one into a useable, mobile it will have some limitations.

 

[KmH]

Forgive me if this is common knowledge, but as I was looking into the difference between CCD and CMOS sensors I was surprised to find that CCD sensor is analog.

CMOS is analog too. CMOS is a type of transistor. Transistors are analog devices. Transistors can be arrayed as 'logic gates' to produce a digital output
The image sensor in every digital camera is an analog device.
CMOS - Wikipedia, the free encyclopedia
Active pixel sensor - Wikipedia, the free encyclopedia

At least the pixels and amplifier circuits are. After the pixels store an analog voltage value proportional to how many photons hit the pixel, the voltage has to be amplified.
How much the analog voltage gets amplified is determined by the camera ISO setting.
The analog voltage is then run through a Analog-to-Digital converter (A/D) and converted into a digital number. For DSLR cameras that is a 12-bit or 14-bit number.

An analog voltage can be any value from 0 up. A digital number can only have discrete base 2 values, so a 12-bit digital number is limited to 4096 'steps', while the photo sensor that is the pixel was able to record millions of discrete voltage values. consequently, some amount of rounding error occurs in the A/D conversion of the voltage each pixel recorded.
14-bit A/D conversion allows for 16,384 discrete 'steps', thus limiting the rounding error that occurs in the A/D converter.

Transistors can be arrayed as 'logic gates' to produce a digital output. That is what a A/D converter is.
http://en.wikipedia.org/wiki/Logic_gate

For digital cameras CMOS is somewhat less costly to produce, uses less power, can be read faster, and the amp and A/D circuits can be on the image sensor chip.

 

[fjrabon]

Forgive me if this is common knowledge, but as I was looking into the difference between CCD and CMOS sensors I was surprised to find that CCD sensor is analog.

CMOS is analog too. CMOS is a type of transistor. transistors are analog devices. Transistors can be arrayed as 'logic gates' to produce a digital output
The image sensor in every digital camera is an analog device.
CMOS - Wikipedia, the free encyclopedia
Active pixel sensor - Wikipedia, the free encyclopedia

At least the pixels and amplifier circuits are. After the pixels store an analog voltage value proportional to how many photons hit the pixel, the voltage has to be amplified.
How much the analog voltage gets amplified is determined by the camera ISO setting.
The analog voltage is then run through a Analog-to-Digital converter (A/D) and converted into a digital number. For DSLR cameras that is a 12-bit or 14-bit number.

An analog voltage can be any value from 0 up. A digital number can only have discrete base 2 values, so a 12-bit digital number is limited to 4096 'steps', while the photo sensor that is the pixel was able to record millions of discrete voltage values. consequently, some amount of rounding error occurs in the A/D conversion of the voltage each pixel recorded.
14-bit A/D conversion allows for 16,384 discrete 'steps', thus limiting the rounding error that occurs in the A/D converter.

Transistors can be arrayed as 'logic gates' to produce a digital output. That is what a A/D converter is.Logic gate - Wikipedia, the free encyclopedia

I think what the OP meant, without knowing enough to know what they meant, was that CCD doesn't typically have a built in A/D converter, while CMOS typically does. So, the output of the whole CCD unit is analog, while the output of the whole CMOS unit is digital. Or at least that was my understanding.

 

[timor]

Correction: photography measures the likelihood of light being in a particular place or not....

Do you mean : digital photography ? If so this is well said; likelihood... or probability... or just guessing. Well, it's a virtual world.
Film doesn't consider likelihood of light, reacts to it directly, molecule of silver halide is changing its state or not, 0 or 1, nothing in-between, pure digital system.

 

[fjrabon]

Correction: photography measures the likelihood of light being in a particular place or not....

Do you mean : digital photography ? If so this is well said; likelihood... or probability... or just guessing. Well, it's a virtual world.
Film doesn't consider likelihood of light, reacts to it directly, molecule of silver halide is changing its state or not, 0 or 1, nothing in-between, pure digital system.

I think you missed the quantum mechanics joke.

 

[timor]

I am joking to.

 

[table1349]

Pick the one you are.

 

[Garbz]

We talk about photography, not science, here. We dont talk about huge machines that are cooled with liquid nitrogen. We talk about transportable cameras. And there, without the cooling, CCDs have more noise than CMOS, right now.

DxOMark - Camera Sensor Ratings

All the top cameras on that list are CMOS, as is the Canon 1D X which soon will enter the top of that list.

No I think you missed the point. It's not a case of science vs photography. It's about the best technology for the job. The top cameras are CMOS for one very simple reason, economies of scale. By offloading a lot of the circuitry from the camera design straight onto the silicon wafer you end up with a smaller set of electronics that is standardised and with consistent performance. Also with the vast majority of the production lines running CMOS sensors buying CCDs is actually more expensive even before you start adding the additional circuitry needed to make use of it. This is very important when you have multiple cameras using identical sensor (Nikon) or if you're a company who buys sensors from others and want to do only minimal amount to get it working (Olympus), or if you're trying to make something very small (every small camera on the market), or if you're fighting on a cost basis (everyone except Leica ).

CCDs do in no way have more noise than CMOS sensor when designed properly, also I'm not sure where you got words like huge machine and liquid nitrogen from. Quite the opposite actually. I got some comments at the last astro fest asking me if my tracking mount copes well with having something as big and heavy as a DSLR attached to my telescope. Even basic astronomy cameras outperform DSLRs and they are a tiny fraction of the size and weight. They need to be to ensure the mounts stay well balanced.

 

[KmH]

No liquid nitrogen for these amateur astrophotography cooled CCD cameras - Parsec 8300M Monochrome Astronomical Imaging Camera | Orion Telescopes

StarShoot Pro V2.0 Deep Space Color CCD Imager | Orion Telescopes

The advances that have resulted in the improved ISO performance of DSLR cameras has more to do with software than with hardware, according to DXOMark.