Why is HDMI->DVI image sharper than VGA?

Question

I have a Dell U2312HM monitor connected to a Dell Latitude E7440 laptop. When I connect them via laptop -> HDMI cable -> HDMI-DVI adaptor -> monitor (the monitor doesn't have a HDMI socket), the image is much sharper than with laptop -> miniDisplayPort-VGA adaptor -> VGA cable -> monitor. The difference is difficult to capture with a camera, but see my attempt at it below. I tried playing with brightness, contrast and sharpness settings, but I can't get the same image quality. The resolution is 1920x1080, I'm using Ubuntu 14.04.

VGA:

HDMI:

Why is the quality different? Is it intrinsic to these standards or should I suspect a faulty VGA cable or mDP-VGA adaptor?

Answer 1

VGA is the only analog signal from the above mentioned ones so it's already an explanation for difference. Using the adapter can further worsen your situation.

some further reading: http://www.digitaltrends.com/computing/hdmi-vs-dvi-vs-displayport-vs-vga/

Answer 2

Assuming brightness,contract and sharpness are the same in both cases, there could be 2 other reasons why text is sharper with DVI/HDMI:

The first has already been stated, VGA is analog so will need to go through an analog to digital conversion inside the monitor, this will theoretically degrade image quality.

Secondly, assuming you are using Windows there is a technique called ClearType (developed by Microsoft) which improves the appearance of text by manipulating the sub pixels of an LCD monitor. VGA was developed with CRT monitors in mind and the notion of a sub pixel is not the same. Because of the requirement for ClearType to use an LCD screen and the fact that the VGA standard doesn't tell the host the specifications of the display ClearType would be disabled with a VGA connection.

Source: I remember hearing about ClearType from one its creators on a podcast for This().Developers().Life() IIRC, but http://en.wikipedia.org/wiki/ClearType also supports my theory. Also HDMI is backward compatible with DVI and DVI supports Electronic Display Identification (EDID)

Answer 3

The others make some good points, but the main reason is an obvious clock and phase mismatch. The VGA is analog and is subject to interference and mismatch of the analog sending and receiving sides. Normally one would use a pattern like this:

http://www.lagom.nl/lcd-test/clock_phase.php

And adjust the clock and phase of the monitor to get the best match and the sharpest picture. However, since it is analog, these adjustments may shift over time, and thus you ideally should just use a digital signal.

Answer 4

There are a few answers indicating a digital signal vs. analog which is correct. But that does not answer the why? A few mentioned translation layers, this is sorta true too, a mainstream A/D conversion can cause a loss in fidelity, but you'd have to measure this as it is hard to see the differences with the naked eye. A cheap conversion and all bets are off.

So why is digital better than analog?

An analog RGB signal (such as VGA) uses the amplitude of the signal (.7 Volts peak to peak in the case of VGA). This like all signals has noise which if large enough will cause the levels to be incorrectly translated.

Reflections in the cable (impedance mismatches) are actually the biggest downfall of an analog video signal. This introduces additional noise and gets worse with longer cables (or cheaper ones), the higher the resolution of the video also increases the signal to noise ratio. Interestingly, you should not be able to see any difference in a 800x600 signal unless the VGA cable is too long.

How does a digital signal avoid those pitfalls? Well for one the level is no longer relevant. Also DVI-D/HDMI uses a differential signal as well as error correction to assure the ones and zeros are faithfully transmitted correctly. There's also additional conditioning added to a digital signal that is not practical adding to an analog video signal.

Sorry for the soap box guys, but thems the facts.

Answer 5

Another issue is a lot of VGA cables are junk. If the VGA cable is less than 1/4" thick, you will probably notice ghosting on larger monitors (Higher the rez, more likely ghosting). I've even noticed ghosting on the attached VGA cable on some 19" CRT montitors. My VGA cables are about 1/3" thick and it really helps with the sharpness (Thicker wires, more shielding)

Answer 6

HDMI and DVI are very similar. Both are digital; the main differences are that HDMI supports audio and HDCP. When switching between HDMI and DVI, there is little actual conversion, but rather connecting the matching pins on the connectors. Display Port is a digital protocol, but VGA is analog, and the converter likely introduces small flaws and the reduction in sharpness into the signal. The monitor probably converts the signal back to digital, incurring another conversion that adversely affects the quality.

Over all, computer->HDMI->DVI->monitor has fewer actual conversions than computer->DisplayPort->VGA->monitor and never converts the signal to analog, giving a sharper image.

Answer 7

HDMI and DVI are actually one and the same (at least in your context). DVI connectors are really just HDMI connectors with a different pinout (the rows of pins), plus VGA connectors with a different pinout (the pins arranged around a cross shape). If you look at your HDMI-to-DVI converter, you'll probably notice that the cross-shaped part is missing.

So you are comparing an HDMI image with a VGA image, and, as @Josh pointed out, one that involves an actual conversion.

There actually is a difference between HDMI and DVI, but it's not relevant to you. HDMI can carry additional signals, including the encryption signal for content protection, sound, Ethernet, etc.