Why the DTV lag?

August 7, 2008 8:04 AM

Why does a tv station's digital signal lag behind its analog signal?

We're one of those annoying families that refuses to pay for TV. We forked over the cash for an HDTV with a digital tuner, and absolutely love the results -- more channels, better quality, total freeness.

But we've noticed a weird thing. Our other two TVs are still on rabbit ears. One of them, in the kitchen, is well within earshot of the HDTV in the living room. Yet if we have the same show on both TV sets, there is a distinct lag, not more than maybe two or three seconds, between the two shows.

This happens on all networks, so I know it's not a channel-specific thing. I haven't timed it, but I'm positive the lag is precisely the same length regardless of the channel, too.

Is there a technical reason for this lag to occur? I know the DTV signal is actually being transmitted on a totally separate frequency (channel 2's digital channels, 2.1-2.4, are actually broadcasting on channel 5's frequency, for instance), so what could be the reason for the delay?

posted by middleclasstool to technology (17 comments total)

The delay is normal. They convert the analog signal to a digital format before transmitting it. The conversion induces a delay.
posted by Marky at 8:11 AM on August 7, 2008

There could be lag introduced by the digitization and compression itself (or by your TV), but a good part is probably the result of error correction. In order to have a robust signal, some amound of redundant data is transmitted that can be used to reconstruct the datastream if there is interference. That data gets spread out over time a bit. The TV buffers data so that it doesn't have to freeze when it gets some bad data and wait for the redundant data to correct things.
posted by Good Brain at 8:16 AM on August 7, 2008

It's the digital encoding delay.

If you have old-fashioned analog telephones and newer digital (e.g. cordless) telephones on the same, you can see the same effect. Carry the digital handset to where the analog one is, make a call, and the same sound will leave the digital handset later.

(Party trick: stand in front of an answering machine, call it on a cell phone, and leave yourself a message. The delay is a couple of seconds. Sing a round with yourself.)

If you've ever pointed a camcorder at a television, you've seen this delay as the any motion propagates down the nested images of the television.
posted by fantabulous timewaster at 8:23 AM on August 7, 2008 [1 favorite has favorites]

Good Brain is right that there may be delays at both ends, and it's tricky to separate them.
posted by fantabulous timewaster at 8:25 AM on August 7, 2008

Seconding Marky and Good Brain, it's conversion and buffering. Sometimes conversion is not an issue, since the show is recorded directly to digital, so that any analog signal cones from the digital feed, too. But there is always some buffering of the signal on the TVs side to allow for error correction. The effect is especially pronounced with over the air digital signals (like dvb-t in Germany) where error correction is even more important than with cable or satellite signals.

This is very annoying with sporting events, when the people in the sports bar on the other side of the road cheer way before you can see the goal yourself.
posted by mmkhd at 8:26 AM on August 7, 2008

There's several layers of processing that go on before the digital signal leave the building - in the TV master control of the TV/Radio station I used to work at, they had 4 LCD HDTVs hanging on the wall - I think the first was the network feed, and the next two were to monitor after a specific processing step, like color correction, or adding the local "bug." Each was a few frames behind the monitor above. The 4th was the off-air monitor, and it lagged significantly (maybe .5-.75 seconds) behind the others.

I'm sure someone with more TV Master Control knowledge will explain much more accurately, but that's what I remember.
posted by god hates math at 8:32 AM on August 7, 2008

This actually creates interesting issues for the DTV transition with live coverage, since the New Year countdown is skewed by the DTV broadcast delay. (It's also something that Roy & H.G. encountered with their triple j commentaries: if you're watching on DTV, you get better sync if you listen online rather than over the air.)
posted by holgate at 8:36 AM on August 7, 2008

Even if you ignore encoding delays, you'll find that HDTVs have their own delays as well. This delay can be pretty high, like in the hundreds of milliseconds. This is why many TVs have a 'game mode' so you can actually play videos designed for much quicker analog TVs. Especially if the TV has to re-size the signal for its native display resolution.
posted by damn dirty ape at 8:36 AM on August 7, 2008

That's interesting. I haven't noticed any lag on my Wii (heh, that sounds dirty). I haven't tried Guitar Hero, but I've played about half a dozen games so far with no ill effects.
posted by middleclasstool at 8:43 AM on August 7, 2008

I've seen this when watching games in bars when they have a mix of TV types. Half the bar watching analog will erupt in cheers a few seconds before the other patrons watching the digital TV will see the play that they are cheering about.
posted by octothorpe at 8:50 AM on August 7, 2008

The TV buffers data so that it doesn't have to freeze when it gets some bad data and wait for the redundant data to correct things.

Incidentally, this is one reason changing channels tends to be slower on digital sets.
posted by smackfu at 8:54 AM on August 7, 2008

Is that a lag joke octothorpe? Or did you really not read mmkhd's comment?

In the old days, when watching English films in foreign countries, I noticed the audience laughed before the jokes were told. Oh, wait...that's subtitles.
posted by weapons-grade pandemonium at 9:06 AM on August 7, 2008 [1 favorite has favorites]

The TV buffers data so that it doesn't have to freeze when it gets some bad data and wait for the redundant data to correct things.

That's interesting. I haven't noticed any lag on my Wii...

That's because it's not the TV itself that is doing the buffering, it's the digital tuner. The Wii picture goes straight to the screen.

Additionally, I've noticed that cable often suffers an even bigger lag (this is Virgin Media in the UK, may be different elsewhere), which I assume is due to the fact that they use satellites to get the video to a local repeater station first... so it's got that extra hop to make. I've seen as much as a 10 second lag between an analogue signal and our digital cable TV signal on the same thing.
posted by jon4009 at 9:18 AM on August 7, 2008

As damn dirty ape notes, I have a lag between an HD broadcast and a non HD broadcast of the same show on my digital cable service.
posted by spicynuts at 9:28 AM on August 7, 2008

That's interesting. I haven't noticed any lag on my Wii...

On my Wii, guitar hero is unplayable without doing the calibration thingy. That's because my TV takes the 480p picture and then has to resize it to 1080i. On my specific model that operation adds like 100-200ms of lag.
posted by damn dirty ape at 9:49 AM on August 7, 2008

weapons-grade pandemonium: "Is that a lag joke octothorpe? Or did you really not read mmkhd's comment?

In the old days, when watching English films in foreign countries, I noticed the audience laughed before the jokes were told. Oh, wait...that's subtitles."

Hah, not intentional.
posted by octothorpe at 10:35 AM on August 7, 2008

The digital encoding/decoding processes introduce delay because much of the compression involved is based on interpolating differences between frames, and storing only those differences. There are several different schemes, but as a simplified example: if frames 2 through 9 are coded as interpolated differences between fully coded frames 1 and 10, then you can't start assembling frame 2 until frame 10 has been received into local memory (since frame 2 is dependent on info in frame 10). This happens every time a video stream is converted between digital and analog, in both directions, which may happen more often than you would think in the entire path between the source and your TV.
posted by rocket88 at 11:47 AM on August 7, 2008