Front and back porch, etc

tryingtimes

Well-known Member
Hi there
A newbie question this, especially for a crt owner, but....

What role do Front and Back porches have in modern preogressive displays? And why do we use them in our VPs when it uses valuable pixel clock bandwidth?
1080x1920x75 = 155MHz
But once you add in all the Porches etc, you get way over the limit of most VPs and maybe even cables and other peripherals.

As I said - complete newbie question, so apologies.

Oh and I know that 75Hz isn't as necessary with progressive displays but it could be again with some of the new display technologies which draw the image line by line.

Cheers
tt
 

JohnWH

Active Member
Good question!

I may be wrong here, but I think its so that non fixed pixel displays could be driven directly by DAC's without a frame store, making it hystorical?

You also wouldn't have anywhere to hide audio without them.

John.
 

Welwynnick

Distinguished Member
Front and back porches are very necessary with scanning displays, so there is sufficient time for the line retrace, and the new scan can be stablised in time to display the line linearly.

Fixed pixel displays don't need these dead times, but the analogue interfaces have to be consistent with established standards that support CRT displays.

Digital interfaces are a different matter entirely, and porches and sync pulses are handled in a different way entirely. There are still frame and line sync periods, but these are signalled digitally rather than analogue.

I don't believe that audio comes into it; that's multiplexed at a different level.

Nick
 

JohnWH

Active Member
welwynnick said:
Front and back porches are very necessary with scanning displays, so there is sufficient time for the line retrace, and the new scan can be stablised in time to display the line linearly.

Fixed pixel displays don't need these dead times, but the analogue interfaces have to be consistent with established standards that support CRT displays.

Digital interfaces are a different matter entirely, and porches and sync pulses are handled in a different way entirely. There are still frame and line sync periods, but these are signalled digitally rather than analogue.

I don't believe that audio comes into it; that's multiplexed at a different level.

Nick
I'm pretty certain the audio is embedded in "data islands" that live in the non active periods of the video signal, at least thats what it looked like last time I looked at the spec!

Obviously if DVI had originally been designed to drive fixed pixel displays only there would have been no need for blanking periods, so they would have had to come up with another way of carrying the audio data in HDMI, in fact we may even of ended up with a CE standard interconnect based on something else...

John.
 

tryingtimes

Well-known Member
ok - I understand the need for them to exist for analogue connections.

Nick, you say that they
are handled in a different way entirely. There are still frame and line sync periods, but these are signalled digitally rather than analogue.
So does that mean that they don't actually take up processing bandwidth.
The only reason I ask is that Crystalio II has a 165MHz max bandwidth and I wondered about 1080p75, whether it's within spec because it's 1080x1920x75 = 155MHz or outside because of the additional porch info.

It's all theorectical - by the time I need 1080p I'll probably be on a digital display running 48, 50 and 60Hz anyway.
 

JohnWH

Active Member
Yes they increase the required BW, you migth be able to cut the size of the blanking down a bit dpending on your display device, but I'd be surprised if you could do it enough to bring the CII into spec (I think you need ~200MHz)

Note that I've also found "strange" issues when messing with the timings when the signal is HDCP encoded.

John.
 

Welwynnick

Distinguished Member
tryingtimes said:
Nick, you say that they
So does that mean that they don't actually take up processing bandwidth.
Yes, they do eat up some processing bandwidth, but IIRC it's not as much as the 2200x1125 total resolution that VGA uses for example. It's in the DVI and HDMI specs, but take a bit of time to find.

By the way, I thought that audio was multiplexed into different channels to video within the shared HDMI TDMS channels, rather than being saved up for the blanking periods (which WOULD be nice and efficient). But I'm not sure. Anyone got an HDMI spec and a spare half hour?

Nick :)
 

JohnWH

Active Member
welwynnick said:
By the way, I thought that audio was multiplexed into different channels to video within the shared HDMI TDMS channels, rather than being saved up for the blanking periods (which WOULD be nice and efficient). But I'm not sure. Anyone got an HDMI spec and a spare half hour?

Nick :)
Had a look at the spec again today, it definitely shows the data islands containing the audio in the blanking periods.

John.
 

bushwakko

Standard Member
Anyone have any idea where I can find info on this on specific models. I'm trying to find these settings for my Samsung LE40A786, so that I can generate a valid 1080/24p setting with 23.976hz on mu mac.
 

The latest video from AVForums

Podcast: Home AV, TV, Tech News & Reviews, Plus The Best of July 2020
Top Bottom