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Professional HF Work?
On Wed, 9 May 2007 15:23:30 EDT, AF6AY wrote:
On a more consumer-oriented basis, the broadcasting industry is generally wondering what will become of all those old analog TV transmitters after the transition to HDTV in the USA. There's no easy answer for that, either. Do not confuse digital television (DTV) with high definition TV (HDTV) which is a subset of DTV. All TV stations will be required to use DTV but the use of HDTV is optional. As far as the transmitters go, in general they are being junked because most of them are near the end of their useful lives and are held together by duct tape and baling wire in anticipation of the transition. 73 de K2ASP - Phil Kane Life Member - Society of Broadcast Engineers |
#2
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Professional HF Work?
Phil Kane wrote on Sun, 27 May 2007
23:00:15 EDT: On Wed, 9 May 2007 15:23:30 EDT, AF6AY wrote: On a more consumer-oriented basis, the broadcasting industry is generally wondering what will become of all those old analog TV transmitters after the transition to HDTV in the USA. There's no easy answer for that, either. Do not confuse digital television (DTV) with high definition TV (HDTV) which is a subset of DTV. All TV stations will be required to use DTV but the use of HDTV is optional. I made an "aside" for comparison and haven't confused anything. Firstly, DTV isn't about using HF spectrum...I am very aware of that. The transmitters for television represent a sizeable capital investment by broadcasters. I am aware of the complaints of the broadcast industry (pro and con) during the entire time of the "Grand Alliance" testing that led to the eventual DTV broadcasting format that went into broadcasting regulations. It's been years since I "rode gain" at a control console or "took" a particular camera or tape unit, but I do know something of the MPEG video transport format even though it generally gives me considerable confusion every time I've studied it. :-) The MPEG video transport format can accommodate four different pixel arrangements and DTV receivers are supposed to be built to decode all of them automatically. That video format includes the old-style analog format (converted from analog to digital) on up to the High- Definition TV which is considered by most consumers as "wide screen TV." The DTV transmitters themselves handle the entirety of the video format sent up from the studios,including the quadraphonic sound, text for hearing impaired, and whatever else the studio central control sticks in there. In the old TV transmitter arrangements of any appreciable power, they were almost always TWO, one for video (AM sorta SSB called "vestigal sideband"), one for audio (FM) with a Diplexer (passive filter) to connect both to the same wideband antenna. The DTV transmitter is a single one since ALL of the modulation information is conveyed by it, no "extra" one for sound, seldom any need for an external filter, let alone a diplexer. The internal design of the DTV transmitter HAS to be different than either the AM video or FM aural transmitters by nature of the modulation mode. But, that DTV transmitter can handle ANY of the video transport formats automatically. Whether the TV station central engineering sends old NTSC video converted to "low grade" digital format or has gone all-out to run everything in "high definition" doesn't bother the DTV transmitter. The end result for conversion to DTV was a replacement of the transmitters (old) by one new one. Draconian for the broadcasters but visual and audible pleasure for millions of consumers receiving the DTV. As far as the transmitters go, in general they are being junked because most of them are near the end of their useful lives and are held together by duct tape and baling wire in anticipation of the transition. I disagree with that "junk" figurative phrasing. Here in Los Angeles, there is one central TV (and most FM) broadcast transmitter site, Mount Wilson. It's been a few years since I was up there but all the stations (7 on VHF, 4 on UHF) had good, long-lasting transmitters. At the time KTLA (ch. 5) was beginning to convert to DTV; they were a pioneer broadcaster in L.A. and this year is their 60th anniversary here. Both studio and transmitter sites have excellent equipment and their DTV signal is absolutely HDTV. Their morning show content is, in my view, JUNK, but that is CONTENT, having nothing to do with equipment or signal quality...a personal critique. What used to be a TV "leader" in quality of equipment, NBC (ch. 4) under the 'general's" eye of RCA, hasn't fully converted to HDTV in their studios. NBC evening news is HDTV but local news is still narrow TV. NBC is owned by General Electric and "RCA" exists solely as a brand name now. I think...haven't kept up on the mergers and acquisitions of large corporations lately. :-) TV is above 30 MHz, definitely not in the HF spectrum. However the type and kind of modulation carried by any transmitter will determine whether or not it will be "junked" for a new replacement, NOT it's supposed "held together by tape and bailing wire." If hams could run 4 KW PEP SSB on HF (not in the USA) then the ages-old Western Electric LD-T2 SSB transmitter would be a great surplus bargain for them...Class A stages up to the PA which is AB_2, push-button QSY to any one of 10 pre-tuned frequencies. Problem is, the SSB format is 12 KHz wide and has internal frequency multiplexing for four 3 KHz audio inputs combined for the output SSB modulation. A no-no for amateur radio use, even if well-designed after WW2 by WE. Too much modification required to fit the "tranditional" and technical specs even if a superb design for its intended use. That was the general point I was making, not something about the video format of DTV. 73 de K2ASP - Phil Kane Life Member - Society of Broadcast Engineers 73, Len AF6AY Life Member - Institute of Electrical and Electronic Engineers No pixels were harmed in the generation of this message but billions of electrons were rudely shoved around. |
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