Gus

DTV time delay between channel changes
« on: May 03, 2009, 01:54:00 PM »
Watching "over the air" TV using a DTV converter box I noted a 2 to 5 or more second delay between channel changes.

I am guessing the delay is due to buffering of the frames before displaying the screen and the screen being blanked so not to see fragments of the images.  Is the slow channel change more of the converter box having a slower conversion(less powerful CPU etc) Do other converters change channels faster?  I am assuming the different formats 480I,480p,1080i,720p have different time delays because of the pixel count differences and the computation change due to that and frame rates.

So is the slow change a ATSC tuner converter issue that can be "fixed" (more money) with a "better" converter or is it something people are just going to have to live with? 

So far not a fan of the pixelation and just going blank and other crap like not being about to see HDTV channel13 over the air NY NJ because of low power. My tax dollars at work and then you have the patents



http://en.wikipedia.org/wiki/High-definition_television
http://en.wikipedia.org/wiki/ATSC_Standards
http://en.wikipedia.org/wiki/MPEG-2
http://en.wikipedia.org/wiki/MPEG_Licensing_Authority
 


rodabod

Re: DTV time delay between channel changes
« Reply #1 on: May 03, 2009, 03:40:54 PM »
The GOP structure decides how long the box will need to take to wait for another I-frame which if I remember correctly, it needs before displaying good video.

Some boxes in the UK buffer the mux before channel changes which makes the switch marginally quicker. Older boxes in the UK (yes, I know we don't use ATSC, but...) were slow like you describe, but newer boxes here are much better. I'd guess the same probably applies in the US, and you can probably find quicker boxes.
Quote from: tv
punchy fat bastard chip

Svart

Re: DTV time delay between channel changes
« Reply #2 on: May 03, 2009, 03:53:39 PM »
there is a digital video decode chip in the box.  It has to perform a "QAM lock" which is to tune to the frequency and then power range the signal for best MER(SNR) and then decode a certain number of symbols before it buffers the signal into a small amount of memory before stripping out the MPEG before sending that to another decode IC for further buffering.  It's just the price you pay for higher signal quality.

Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

AnalogPackrat

Re: DTV time delay between channel changes
« Reply #3 on: May 03, 2009, 06:14:18 PM »
Are you watching digital "over the air" TV or analog?  What output are you using from the converter box (coax, S-video, HDMI)?  If you're watching an analog channel it sounds like the thing is digitizing before outputting.  If you're watching digital channels then read on...

Digital video is complex stuff, but one of the basic ideas is to only periodically send a full frame of data (a.k.a. key frame or I frame).  Frames between these key frames are generated by modifications to the key frame--the stream only contains descriptions of things that have changed from frame to frame.  There is no set key frame rate.  Key frame rate can be used to throttle bit rate of th stream or it can be dynamically determined depending on content. 

One side effect of this is what you're seeing.  When you switch channels (data streams) the new stream cannot be displayed until it gets a new key frame.  Since your box only has enough processing power to decode one stream it can't be synced to the next channel you're going to watch.  So you wait.  Welcome to the Nirvana of the digital age.  Everything is so much better now, isn't it?

A P
If it is to be, it is up to me.

MartyMart

Re: DTV time delay between channel changes
« Reply #4 on: May 04, 2009, 06:20:26 AM »
My 10 year old 28inch "TV with a tube" still looks better than any Flat screen TV that I've ever looked at.
Progress is great isn't it ?

MM.
"Success is the ability to go from one failure to another with no loss of enthusiasm"

Greg

Re: DTV time delay between channel changes
« Reply #5 on: May 04, 2009, 09:16:32 AM »
My digital tuners experience the same lag... It kind of sucks but I've learned to deal with it. I've been watching high definition OTA (Over The Air) for several years now on a 42" rear projection. I don't have cable or satelite... they seem to really compress HD signals.

And at Marty, what do you mean by flat screen TVs (all high definitions of just flat LCDs)? I'm a fan of high def rear projections because the black is deeper and image "softer." And if you see no difference then I doubt you've ever seen a high definition signal from a network with minimal comrpession. The differences aren't sutble. The colors are undoubtedly richer and the edges are so much sharper. But granted the improvement might not be as noticable on a 28" TV. On a large TV, the edges on a 480i signal are very squirmy and the colors washed out.
Greg Stein
New Orleans, LA

Gus

Re: DTV time delay between channel changes
« Reply #6 on: May 04, 2009, 11:14:27 AM »
Thanks so basically the converter needs a I frame key frame to store and than to work off that with the changes on the frame until a new I key frame?
 
I use over the air

AnalogPackrat posted
"There is no set key frame rate"
Is the I/key frame sent at some MAX fixed time interval or only when needed,  Say the picture is a still of a painting or photograph or something like that is the time between I/Key frame longer then?

What do I need to read about?  MPEG2 and what else?

Can a channel switch faster with faster hardware decoding or is the limit the I/key Frame rate I guess is my next question

I don't like LCD TVs I still like CRTs over almost any new display I have seen.  I though DLP looked good with the led laser light source and not the spinning color wheel when I saw one at a store some time ago but DLP seems to be disappearing and then there is the light source cost and being bigger volume wise than a LCD. 



AnalogPackrat

Re: DTV time delay between channel changes
« Reply #7 on: May 04, 2009, 12:45:21 PM »
Thanks so basically the converter needs a I frame key frame to store and than to work off that with the changes on the frame until a new I key frame?

Right.
 
Quote
I use over the air

Digital only I assume?

Quote
Is the I/key frame sent at some MAX fixed time interval or only when needed,  Say the picture is a still of a painting or photograph or something like that is the time between I/Key frame longer then?

AFAIK the MPEG standard does not specify a minimum key frame rate.  Think about the different use cases for digital video.  A video stream stored on disk or transmitted over the internet has different tradeoffs than one that is broadcast via DTV, Satellite, etc.  In the non-broadcast cases the main tradeoff is quality vs. data size.  Here you may want highly dynamic key frame generation with dependence on the scene.  The classic case is what you illustrated--a static scene.  You don't really need to store anything beyond the initial frame until the scene changes.  So you can optimize total stream size.

But in the broadcast case you have an upper limit on bandwidth, not total size.  You've got to be able to squeeze all the data for a video stream through the pipe at frame rate all the time.  If the available bandwidth is low, quality must be compromised.  Keeping the per frame data size way below the available bandwidth saves you nothing since you're just wasting the available channel bandwidth--it's not allocated to anything but your broadcast.  I don't know what the key frame policy is for the DTV broadcasters, but I suspect "tuning lag" is one factor they consider.

The dirty secret about DTV is that the broadcaster gets to make more of the quality tradeoffs.

Quote
What do I need to read about?  MPEG2 and what else?

That's the place to start.  If you can grok the standards you can understand the fundamentals of the process (encoding and decoding).  You could also read product literature for some of the MPEG decoding ICs out there.

Quote
Can a channel switch faster with faster hardware decoding or is the limit the I/key Frame rate I guess is my next question

Most set top boxes, satellite receivers, DVD players, etc. are doing the decoding in HW (or HW assisted firmware) already.  The limiting factor is the key frames.  The only thing I can think that would help is if the box could "guess" which channel you're switching to and pre-emptively start decoding that stream as well as the one you are currently viewing.  That implies that the HW has to capability to decode two streams which is unlikely.  The exception being DVRs, but if the other decoder is busy recording "Top Gear" you're SOL.

Quote
I don't like LCD TVs I still like CRTs over almost any new display I have seen.  I though DLP looked good with the led laser light source and not the spinning color wheel when I saw one at a store some time ago but DLP seems to be disappearing and then there is the light source cost and being bigger volume wise than a LCD. 

I feel the same way.  My wife had been nagging me for months to get a new TV to replace our (perfectly good) 10 year old 27" Sony CRT.  I resisted until the new LED backlight LCD TVs came out.  I've worked in the LCD TV capital equipment industry for the past 3 years so I have access to a little extra info about what's coming down the pipe. 

We got a Samsung 950 series which uses white LEDs for backlighting instead of CCFLs.  The advantage being that the LEDs can be individually modulated.  So blacks can be really black and highlights can be really bright.  The problem is that the LED array is much coarser than the pixel array so there is some haloing, but you don't really notice unless you look for it.  And of course the things cost more than CCFL--more complex controllers are needed for computing the LED modulation, etc.  The main power sink in an LCD TV is the backlight--LEDs are more efficient than CCFL, so the power per screen area is much less as well.

These LED backlight models are the first LCD TVs I've seen that actually have acceptable image quality.  LG also has a couple of models out and Sony does, too, though Sony uses RGB LEDs for even better color control.  Of course the Sony costs about 2x the others.

A P
If it is to be, it is up to me.

Greg

Re: DTV time delay between channel changes
« Reply #8 on: May 04, 2009, 01:08:43 PM »
Nice posts. Good info.
Greg Stein
New Orleans, LA

Svart

Re: DTV time delay between channel changes
« Reply #9 on: May 04, 2009, 01:13:59 PM »
Gus,

You have to remember that what AnalogPackrat is talking about is only ONE layer of a multiple layer system.  As I mentioned before, you have to "range" a signal to find the optimum power level as well as adjusting the center frequency for QAM offset.  You then apply digital equalizers to level out the symbols before you can start to output a decoded data stream into data for framing.

And yes the tuners can decode two or more signals that are interleaved into a single QAM haystack.  

It's extremely complicated!

And this isn't even taking into consideration that digital channels can be compressed and/or "scrambled", so you might also have to uncompress a channel and/or descramble/unlock them too.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


AnalogPackrat

Re: DTV time delay between channel changes
« Reply #10 on: May 04, 2009, 01:19:59 PM »
Yeah, I don't know much of anything about the broadcast layer at all.  I'm sure there's more latency inducing processing overhead there as Svart says.  If you thought NTSC and PAL were complicated--digital video is really messy stuff.

A P
If it is to be, it is up to me.

hodad

Re: DTV time delay between channel changes
« Reply #11 on: May 04, 2009, 02:21:34 PM »
I'm a DTV/broadcast viewer as well, & while the channel changing lag doesn't bother me too much, the thing that drives me nuts is that the audio doesn't seem to sync to the picture very well.  I don't know if I'm just being hypercritical (I don't think I am), but it's driving me slightly nuts.  Is there a reason/remedy for this?

rodabod

Re: DTV time delay between channel changes
« Reply #12 on: May 04, 2009, 02:42:00 PM »
Thanks so basically the converter needs a I frame key frame to store and than to work off that with the changes on the frame until a new I key frame?

Yep, that's vasically it. Svart mentions re-tuning, but often you'll just be decoding a different service within the same multiplex so a re-tune is unnecessary. It's probably just a case of referencing a table to find which stream to switch to. That's the case for DVB anyway. I think ATSC might be a little more crude.

I'm not sure if you guys support variable bit-rate or not. And in this case, I don't know if this would cause more lag, but in the case of the UK, it causes no issues at all.

Audio should be time-stamped to sync with video; if it's not in sync, then try re-tuning. If that fails, then I'd suspect a hardware bug.

I'm not sure if you can even buy grade-1 LCD screens yet. We bought the latest JVC grade-2 HD screens which are so-so. The grade-1 CRT HD monitors are phenomenally better though in almost every respect. One trick I found that did make them better with SD material (often looks iffy due to re-scaling) was to use a Snell and Wilcox upconverter, but the fact that it was worth tens of thousands of pounds probably explained why.
Quote from: tv
punchy fat bastard chip


 

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