My apologies if this is an old topic but I don’t seem to be able to ‘search’.
We got our first LCD TVs. We don’t have cable or satellite and do have an old ‘through the air’ antenna system. We now receive up to 3 versions of some stations; ‘analog’, Standard Definition digital, and HDTV digital.
HOWEVER - when we switch back and forth between the analog and digital versions there is a noticeable delay of the digital channels. Both the SD and HD channels seem to be running several seconds behind the analog version.
I have seen the same effect with multiple stations, and on multiple tv’s from different vendors, on both 740 and 1080p sets.
Does this delay originate at the station or in my set? Is there something in the digital processing that results in such a delay? (Sounds like a lot of buffering would be required!)
It’s definitely not your TV set. Digital cable itself has a delay - my folks have a “straight from the wall” cable hookup in their kitchen that is at least 3 seconds behind their digital-cable-box-to-CRT-tv setup in the living room. I do believe the delay got even longer when they got HDTV but I am not sure.
I was watching the MLB playoffs this year on digital cable, with the radio to my ear. I counted the seconds between hearing the crack of a bat on the radio and on the tv - it was 10 seconds.
It’s partly because of the way compressed digital video works. A full frame is sent every few seconds, but in between only the differences from the previous full frame are sent. So when you change channel your receiver has to wait a while for the next full frame before it can display a picture.
[ETA] Of course, that doesn’t explain why digital TV runs behind analogue. I guess because they have to do all that MPEG encoding in real time before transmitting, it adds a delay.
You learn pretty soon that the time blips on DAB digital radio should NOT be used to set a clock if you want up to the second accuracy.
The delay is in the digital decoding - to prevent data stream errors interrupting the output, the decoder buffers several seconds before playing. Any stream errors are corrected before display by error correction routines that are considerably slower than the decoding process. The buffer can soak up the additional time.
The length of the buffer (and thus the delay) is implementation and data stream dependent, and thus cannot be reliably compensated for by broadcasters.
In addition to the decoding delay, there is also the encoding delay. Even though the MPEG encoders used are real-time there is still delay introduced as it’s encoded, wrapped and uploaded. Add maybe a half second if this is going through a satellite link due to the slow speed of light.
But the broadcaster can compensate for known encoding delays and synchronise the analog and digital feeds. Because there is no standard for decoding delays, they can’t fix the problem.
Digital television sets and flat panel displays can introduce a significant delay. This is apparent when playing video games, where any delay slows the player’s reaction time.
That has never happened with me or anyone else I know. I have used just about every console ever made on my flat panels with no problems whatsoever. I currently own a PS2, PS3, Gamecube and Wii and none of them have any delay on my TVs.
The delay is due to digital encoding, decoding and on DVR boxes, the write, read delay on the hard drive which buffers everything you watch.
Games that are highly dependent on precise timing, such as Guitar Hero and Rock Band, have configuration settings that let you choose your TV type to calibrate it for slight time delays. I, however, have never personally experienced it either.
One of the Chinese restaurants in my area has 2 TV sets in their bar, where I wait for our order to come out, and have a drink while I wait.
A while back, I happen to be there during a Red Sox game, and they had both TVs going, with the sound coming from the HDTV flat screen. I was watching the regular (standard) TV in front of me, and for a while I couldn’t figure out why I wasn’t able to correspond the action on the screen to the audio. There was more than 5 seconds delay on the HDTV set/audio. Both were coming from the same ‘broadcast channel’, but but one was the regular signal, and one was the HDTV set.
It was very confusing to me, and I still don’t exactly understand why the delay would be so long. 5 seconds in computer terms is a VERY long time.
“Can” and “does” are two different things. Some of the over-the-air HDTV stations I watch don’t even get the audio and video in synch, and people were complaining about it on message boards a couple of years before I even got an HDTV. Synchronising the analogue and digital broadcasts is going to be way down their priority list.
speed of light.