Recently upgraded our main TV from basic cable to digital HD, with a cable box. Another TV in the kitchen where usually watch the news while eating dinner is still on the regular cable. Boxless.
Last night when finished dinner, went to the main TV and turned it on while my wife was still in the kitchen. Tuned in the same news program, and as I could hear the other set, thought maybe we were on different stations.
No, same station, but the audio from the kitchen set was about five seconds ahead of what I was watching on the digital cable set. Assume the video was likewise delayed.
Why the heck is that? Does it take five seconds for the signal to travel through the receiver and then to the TV set?
Digital TV is compressed - pictures with larger resolution require more work to compress them, so although it is done in real time, the compression pipeline is longer.
HDTV goes through a completely different set of hardware at your TV station. If it’s a recorded program, it’s got to be played through an HD player, for example. So small errors in timing are common.
Also, some HDTV sets have a built-in delay because it takes them time to decode the picture. Newer video games often have a compensation mode built in so you can synch the picture up with the audio. But 5 seconds is far too long for that to be a factor. So chalk it up to your TV stating cueing up its HD programming slightly off time.
OK, first of all, understand that most TV stations now broadcast two separate signals. The old analog one an the new digital one.
Your cable provider (probably) takes both and puts them into your channel line up. For example, my local Channel 13 (CBS affiliate) is actually on Channel 13 on my box but the digital broadcast from the same station is on 212. The HD programming can be seen on 212.
So although I can’t give you the technical reason for the analog and digital signals not being in sync, they are not the same signal.
Also, I have both an OTA HD receiver and the one given to me by my cable provider. The digital signal on the OTA receiver is always ahead of the cable company since it is not going through the company’s system but straight from the antenna at the TV station to the antenna in my attic.
Is your HD cable box also a DVR? If so, there’s some delay there as well - even when you’re watching “live” TV, the box is actually writing the digital signal to the disk and reading it back - not just routing the digital signal directly to the decoder.
Why on earth would it do that? It might delay the display of the frame until it was compressed and written to disk, but decompressing it to provide live would be a waste of resources. It already have the YUV frame in hand for compression, why not just display that?
Because the HD signal (and all digital channels for that matter) comes in to the cable box as compressed MPEG2 alread, not YUV. It doesn’t have to compress anything to write the HD to disk. Cable boxes only have an MPEG encoder onboard for analog channels, and even that’s being phased out.
True, you could just write the frame to the hard drive and the decoder simultaneously, but it’s procedurally simpler to treat writing and decoding as unrelated actions. It also makes it easier to get trickplay to work right - if you’re already reading from the disk, things like pause/resume and rewind work fine at all times. If you’re watching a live stream, there’s some work involved in switching from a live stream to reading from the disk to get into a trickplay mode.
On a related note, my spouse asked me to inquire as to why it is possible to fast forward and rewind programs that we are watching? Being a macho man male type, I of course know perfectly well why it is so, but my wife, you see. . .
To simplify it as much as possible, at the same time you’re watching the program, it’s being recorded onto a hard disk, similar to the one in your computer. When you press the rewind button, you are replaying the information that was already recorded, while at the same time the current stream continues to record, so that you don’t miss out on what happened while you were rewinding.
You can only fast forward if you rewound or paused and are now watching the program a certain amount of time behind the actual broadcast. You cannot fastforward into information that was not yet broadcast for obvious reasons.
This happens on our non-HD, non-Tivo digital set top box as well. It’s very strange when I have the TV in the kitchen and the one in the living room on the same station. HD probably make sit more pronounced.
As groman said, you can’t FF past the current frame being written to disk. If you find that you can, send your wife to the convenience store 20 minutes before a lottery drawing, FF to the drawing, call her up with the winning numbers, and live the life of luxury. Don’t forget my 10% cut.
