Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Digital TV signal delay



John, you didn't use the word I used (buffering), and you gave a different explanation about the need for buffering, but when you said...

They leave out most
of the detail on some frames, and interpolate. But they can't
do that until they've seen what comes after the affected frame.

you are essentially describing buffering... the need to receive and store "future data" (data beyond the frame you're about ready to display) in order to display the current frame correctly.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu


--------------------------------------------------
From: "John Denker" <jsd@av8n.com>
Sent: Tuesday, September 23, 2008 12:41 AM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] Digital TV signal delay

On 09/22/2008 09:14 PM, Steve Highland wrote:
Tonight one of the fellows on my bowling team asked me why the digital TV
signal he receives is substantially delayed relative to the radio broadcast
of the same live football game.

He says he used to be able to watch live TV and listen to the radio together
and they were nearly simultaneous. Now he says the digital TV is about two
seconds behind the radio.

Why would this be the case?

One word: Compression.

All digital TV is compressed. Otherwise it wouldn't fit into the
available channel. For HDTV, the compression ratio is on the order
of 80 to 1, sometimes higher. The algorithms used to achieve this
compression are extremely complex and non-obvious. Think of the
hardest thing you can imagine: It's about that hard.

The goal is to have
-- high compression ratio
-- with good video quality
-- with little delay
-- without spending a gazillion dollars on compression hardware.

Pick any three of the four. You can't have all four.

Even with an unlimited hardware budget, there will be some delays
due to the workings of current algorithms. They leave out most
of the detail on some frames, and interpolate. But they can't
do that until they've seen what comes after the affected frame.

Presumably a few years from now delays will come down, due to
faster cheaper hardware and cleverer algorithms.
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l