Are digital flat panels really digital devices?

[sxr7171]

With digital flat panels, we now can keep the signal from video card to monitor in the digital domain. If this signal is to drive a CRT monitor or projector it is clear that the signal needs to undergo a D/A conversion in the monitor or projector. But is that still true when driving fixed pixel devices such as LCDs, DLP devices, Plasma Screens or D-ILA devices? Does the signal need to converted to analog before driving individual pixels, I would imagine so. So as far as I know then, there is no such thing as a digital display device. Right or wrong?

 

[Mark R]

Certainly for LCDs and plasma displays, a DAC is a critical component. Each individual sub-pixel is controlled by applying an analogue control voltage to it.

In fact, one of the advancements that has made TFT LCD displays more affordable has been the techinique of building the DAC onto the glass substrate (reducing the component count needed inside the monitor).

DLP, uses a PWM technique (which could be thought of as a DAC), relying on the phenomenon of persistance of vision to perform the filtering. However, the net result is that the output of each sub-pixel is digital 'on or off', although it has been converted from a PCM (pulse code modulation) to PWM (pulse width modulation).

There are fully digital displays, although they are less used on computers these days. A simple LCD like on a wrist watch or most mobile telephones. Or what about the TTL input CRT monitors from days of yore - fully digital control of grid voltage?

 

[dnoyeb]

Why does an LCD use a DAC? I thought the pixel was either on or off??

 

[Mark R]

An LCD such as is in your wrist watch is either on or off.

For a TFT LCD, such as is used in computer monitors, each sub-pixel can have 1 of many states - in modern screens the DAC is an 8 bit converter providing 256 states; with three subpixels per pixel (red, green and blue) this provides 16.7 million colours per pixel.

 

[sxr7171]

Thanks for the infor Mark R, I suspected that LCDs had DACs, I suppose they have a DAC for each pixel to control the voltage that goes to each pixel. I take it that each pixel is actually 3 pixels each for R, G, and B and that each one has its own DAC to produce the 256 levels for each color. It's also interesting how DLP is a true digital device it that it is controlled entirely in the digital domain. I suppose that there are techniques for digital sound at the loudspeaker level, even though I suppose it is possible that you could drive a loudspeaker with an amplified, filtered PWM signal.

 

[heliomphalodon]

This would seem to indicate that the digital outputs on a card like the Parhelia do not benefit from the 10-bit color accuracy of the card.

 

[CTho9305]

Originally posted by: heliomphalodon
This would seem to indicate that the digital outputs on a card like the Parhelia do not benefit from the 10-bit color accuracy of the card.

no, you could get benefit because of errors caused when rounding. when you are lighting, 1.54999 might go to 1 with 8 bit, but 2 with 10 bit. or the other way around. The benefit probably won't be great, but I think there will be at least a small difference.

 

[Mark R]

I suppose that there are techniques for digital sound at the loudspeaker level, even though I suppose it is possible that you could drive a loudspeaker with an amplified, filtered PWM signal.

This type of amplification is usually only used for sub-woofers, because it does tend to distort the high frequencies. It is used for high power amps, especially when input power needs to be kept low - e.g. in-car. If you get a 1000 W in-car amp, the chances are that it will operate in this mode (called Class D).

One Hi-Fi manufacturer took this idea one step further and made a 'digital' amplifier, that took a digital optical input from CD/MD/etc. and converted it internally to PWM, driving the speakers with the filtered PWM output.