Personal computing discussed

Moderators: renee, Dposcorp, SpotTheCat

 
bryanl
Gerbil Team Leader
Topic Author
Posts: 251
Joined: Mon Dec 15, 2003 10:27 am
Location: Nevada
Contact:

Trying to grok digital video

Sun Jul 12, 2009 10:55 am

Is there something on a conceptual level to better understand modern video? Or is it just a mudpile as it sometimes seems to be? Here is some of how I conceptualize what is going on in the computer to put a display on my screen or in that new TV. Any comments about how I might better see what is to better understand what might be and where things might go would be welcome.

It seems digital would be just like digitizing VGA but then, that is just RGB channels presented as for a CRT complete with scan lines, blanking intervals and such. It seems digital video might use something like the camera sensors with a shift register type data pump but that doesn't seem to be the case.

As I understand it, DVI/HDMI uses 2 methods of data compression and maintains the timing characteristics that go back to CRT days, including the blanking intervals. One method of compression is that of differential color channel resolution (luminance, chrominance) harking back to old CRT color TV technology. The other is jpeg like where blocks of screen real estate are analyzed as a whole. How that jpeg like compression and the scan line model work together is a conundrum as it seems the display device would need to reassemble several scan lines before showing them, but I guess modern displays assemble an entire frame before putting it up for display. But then, if doing this, why the timing instead of some more direct pixel addressing scheme (like in image files?)?

The blanking intervals, as I understand it, are being used for audio and control signals. That is some testimony to bandwidth if something less than ten percent of the signal time is sufficient for 7.1 channels of audio plus some other stuff. It also brings up some thinking about reassembling the data for continuous output at a proper rate.

The old, analog mechanisms were highly synchronous. It seems the newer digital methods are keeping some of this as unnecessary artifacts and I wonder why. But that gets into the issues of streaming and the definition of codecs for live display and their issues. Then there is the modern codec model and its processing needs and how those are being met in hardware and the evolution of its placement in the video data reception and display string.

The reliance of modern digital video on concepts developed in the very early days of TV says something about the quality of that research but I wonder if there is an indication we'll get something other than incremental improvements on that?

Then there's video over firewire and USB - do they follow the DVI scheme or something different?

Is this view somewhat in line with reality? When might the future break with the past? Or if not, why not? If I'm lost at sea, which way should I point the life raft?
Bryan
 
just brew it!
Administrator
Posts: 54500
Joined: Tue Aug 20, 2002 10:51 pm
Location: Somewhere, having a beer

Re: Trying to grok digital video

Sun Jul 12, 2009 11:38 am

I think a big part of your confusion comes from the fact that your mixing up the signaling system used to send the raw bits down the wire to the display device (DVI/HDMI) with the codecs used to compress/decompress the video for storage and transmission (MPEG-2/MPEG-4). Aside from the fact that they are both involved when you watch digital video, they have very little to do with each other.

DVI/HDMI data is uncompressed. It requires a very high bandwidth connection (gigabits per second), and so is used only for short interconnects (like, say, from your PC to your monitor). The data may be encrypted for copy protection purposes, but there is no data compression -- pixels in the data stream still correspond one-to-one with displayed pixels (disregarding any image scaling which may be done in the display device). The timing of the data is derived from "old school" analog standards, with sync pulses (also digitally encoded), blanking intervals, etc. -- it is essentially just a "dumb" digital replacement for an analog component video cable. (Well, not quite... HDMI can carry audio as well... but the take-home message is that the video data is still essentially raw pixels, in digital form.)

The various MPEG codecs are used to reduce the number of bits by several orders of magnitude, making it practical to store digital video on a disc or hard drive, and transmit it over the air, via cable, or satellite. This is where the sophisticated image analysis comes into play. Redundant data is removed, and the data is stored/transmitted as a series of "key frames" at a low frame rate, with the intermediate frames between the key frames only requiring enough data to describe the differences between the current frame and the previous one. There is no "timing" in the normal video sense, it is just a stream of bits that gets encoded/decoded at each end, and the final playback decoder is responsible for sending the data out with timing that is compatible with the display device.

Most schemes that store or transmit video over other types of digital connections (USB, FireWire, Ethernet, etc.) use some from of MPEG (or similar) compression, to keep the data rates reasonable.

If you want to know more, Wikipedia is your friend. :wink:
Nostalgia isn't what it used to be.
 
Flying Fox
Gerbil God
Posts: 25690
Joined: Mon May 24, 2004 2:19 am
Contact:

Re: Trying to grok digital video

Sun Jul 12, 2009 12:06 pm

What JBI said, to me, essentially boils down to you can bitstream compressed+encoded audio signal over HDMI, but not video signal. There is signalling (or encoding if you want some confusion) involved in transmitting the digital data, but it is not the frame analysis stuff, since they are about the codecs which is a bit earlier in the playback pipeline.

Although I am not sure if there may be some compression involved in that "digital signalling".
The Model M is not for the faint of heart. You either like them or hate them.

Gerbils unite! Fold for UnitedGerbilNation, team 2630.
 
just brew it!
Administrator
Posts: 54500
Joined: Tue Aug 20, 2002 10:51 pm
Location: Somewhere, having a beer

Re: Trying to grok digital video

Sun Jul 12, 2009 12:20 pm

Flying Fox wrote:
What JBI said, to me, essentially boils down to you can bitstream compressed+encoded audio signal over HDMI, but not video signal. There is signalling (or encoding if you want some confusion) involved in transmitting the digital data, but it is not the frame analysis stuff, since they are about the codecs which is a bit earlier in the playback pipeline.

Although I am not sure if there may be some compression involved in that "digital signalling".

I deal with digital video interconnects at a very low level as part of my day job. It's just raw pixel data. I suppose you could consider YCbCr signaling (which can in fact be transmitted over DVI/HDMI) as a form of "compression", but it's rather crude, having been designed decades ago for implementation with analog (vacuum tube!) electronics. With YCbCr you're still transmitting data for every pixel of every frame; for an uncompressed digital interconnect like DVI/HDMI, AFAIK the only savings would be that you could slightly reduce the number of bits-per-pixel (allocate more to the luminance, and less to the color) without sacrificing perceived image quality.

Video compression in the modern sense generally means MPEG (or something similar).
Nostalgia isn't what it used to be.
 
bryanl
Gerbil Team Leader
Topic Author
Posts: 251
Joined: Mon Dec 15, 2003 10:27 am
Location: Nevada
Contact:

Re: Trying to grok digital video

Sun Jul 12, 2009 1:05 pm

see Extron on HD SDI etc for the 4:2:2 I was thinking about with HDMI jpeg like bit packing or audioholics HDMI guide on color spaces or this one on chroma subsampling. There are some interesting things going on to reduce bandwidth.
Bryan
 
just brew it!
Administrator
Posts: 54500
Joined: Tue Aug 20, 2002 10:51 pm
Location: Somewhere, having a beer

Re: Trying to grok digital video

Sun Jul 12, 2009 1:15 pm

bryanl wrote:
see Extron on HD SDI etc for the 4:2:2 I was thinking about with HDMI jpeg like bit packing or audioholics HDMI guide on color spaces or this one on chroma subsampling. There are some interesting things going on to reduce bandwidth.

Yes, but these simple techniques only reduce total bandwidth by a factor of 2-3 or so. You're still sending luminance (brightness) information for every single pixel of every frame. It is still effectively an uncompressed format (the Extron link even says this).

The compression used for digital TV transmission, DVDs, Blu-Ray, etc. have to take this to the next level (as in a factor of 20-30 compression), otherwise a full-length movie would require a stack of 25 discs that need to be swapped out every few minutes!
Nostalgia isn't what it used to be.

Who is online

Users browsing this forum: No registered users and 1 guest
GZIP: On