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From: grege@gold.gvg.tek.com (Greg Ebert)
Newsgroups: sci.electronics
Subject: Re: HDTV and aspect ratio
Message-ID: <2193@gold.gvg.tek.com>
Date: 17 Apr 91 19:11:43 GMT
References: <16642@chaph.usc.edu>
Organization: Grass Valley Group, Grass Valley, CA
Lines: 57

In article <16642@chaph.usc.edu> rongchen@nunki.usc.edu (DragonSlayer) writes:
>This is probably the closest group I can find for this question I want
>to ask. HDTV is going digital. And once in a while the 16:9 aspect ratio
>is mentioned to compared with the 4:3 ratio to hint the advantage of
>the 16:9 mature. What really is the advantage of the 16:9 aspect ratio in
>HDTV? I know what this ratio is, but I don't see the big deal behind it.
>Can someone help on this one? Thanks.
>

The film industry uses 35mm and 70mm media which have a 16:9 aspect ratio. 
Current TV uses 4:3, so when movies are televised, part of the screen is
truncated (except when credits are shown, then people look like string beans).
Having matching aspect ratios will improve home viewing considerably,
especially with a projection TV. A 16:9 tube seems quite wide compared to 
a standard TV tube.

I saw some HDTV demonstrations at NAB this week, and they are visually
impressive. Although there were demos by NHK (Japan), General Instruments,
ATT/Zenith, and David Sarnoff Labs, I only had enough time to inspect the
system by ATT/Zenith in great detail. The GI booth also gave out a
pamphlet describing the American TV Alliance, which is a joint venture
between GI and MIT.

The ATT/Zenith method  uses digitzed images which are compressed
from a 1.2 Gbit/sec stream into 20-25 Mbits/sec. No typos; that's a
50:1 compression. I don't know how the data is compressed or transmitted,
but power spectrum graphs show that it does fall within the 6Mhz band. An
interesting feature is that power is spread more evenly over the channel,
whereas NTSC has peaks at the carrier, color sub-carrier, and the sound
carrier. The encryption algorithm focuses on change-information over
multiple frames. Even with rapidly moving images, resolution is maintained.
Also demo'd were the effects of signal degradation, which cause localized
glitches in fast-moving portions of images. The effect of channel-changing
was also shown, and you could see details fill-in around objects for a
few frames (a fraction of a second). Note, though, this is NOT the same as
keying the source video at the studio, because that occurs prior to
encoding.

GI's DigiCipher technique also has a 50:1 compression from 1.2Gbits/sec.
I overheard 'QPSK' being said by someone at a GI demo, so I assume they
use quadrature phase-shift-keying to encode the compressed data stream. 
That makes some sense to me because the ~24 Mbit/sec stream when QPSK 
encoded with a 16-point constellation (I think that's the term), produces 
6Mega-thingeys per second. I saw a vectorscope with 16 spots on it, which 
seems to confirm the QPSK theory.

An NHK demo showed considerable horizontal artifacts for moving objects,
but I don't know if it was MUSE or some other format. Another vendor
(Sony ?) showed another system which transmits 20Mhz luminance / 7Mhz
chroma. Though unacceptable for general broadcast because of it's high
bandwidth, it was targeted for sports bars, etc on special cable or
satellite channels; it had excellent clarity even on a projection system.

- - - -

Designing ASICs for 10-Bit D2 (140 mbits/sec) is challenging enough. I'm
going to have a lot of sleepless nights when HDTV arrives ;-] .