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OFDM vs 8VSB

D

DanStrassberg

Some of you must be far better versed in this subject than I am. Comments, please!

If the problem with over-the-air DTV reception really is multipath, and if the inventors of OFDM (orthogonal frequency-division multiplexing) are correct about their system, OFDM _should_ be the solution to the multipath problems. As I understand it, the whole idea behind OFDM is to make possible the transmission of high bit-rate data streams at low symbol rates. OFDM accomplishes this feat by packing a large number of bits into each symbol. Thus, a modest number of symbols can transmit a large number of bits. Multipath echoes appear to the decoder as delayed repetitions of the transmitted symbols. Because the OFDM symbol duration is relatively long, the effect of multipath is supposed to be minimized, I guess because the echoed symbols appear within the primary symbol time and, during the symbol time, each bit in the symbol is decoded as only a single value. The long symbol time thus gives the decoder a good shot at determining the correct value of each of the many bits in the symbol.

OFDM is economically practical in consumer applications only because advances in IC (integrated circuit) technology have made it possible for IC designers to pack enormous amounts of DSP (digital signal processing) computational capability into low-cost IC chips. I believe that, on paper, there can be no doubt that OFDM (the DTV transmission standard in most of the world outside of the US) is more immune to multipath than 8VSB (eight-level vestigial sideband), the US DTV-transmission standard. Whether this theoretical advantage exists in practice may be a different matter, however. For certain, each new generation of 8VSB-decoder ICs seems to improve upon the multipath immunity of previous generations.
 
Yup. Once again the US picked the worst of the two systems. OFDM would have been a better thought but I guess there were bandwidth channelization issues or something that caused the US industry to pick the 8VSB system. Now the industry is talking aout changing out exiciters and using up some their channel bandwidth to do the mobile bitrate reduced system for cell phones and portables. Hopefully we can correct from our stupidity. Keep in mind the FCC here in our fine country originally selected the spinning mechanical wheel system for color before everyone complained enough that they changed their minds. They also selected the very flawed Magnavox AM stereo standard then just threw up their hands when everyone squacked. Now we have IBUZ for AM and FM. The only reason we do is because broadcasters are afraid to publically call it a bunk system in fear that the FCC will turn it into the AM stereo mess. I'm guessing that's basically the same reason we have the 8VSB system in a lot of ways. Yes, it's inferior but at least they stuck with something that sort of works.
 
OKCRadioGuy said:
The only reason we do is because broadcasters are afraid to publically call it a bunk system in fear that the FCC will turn it into the AM stereo mess.
Click to expand...
I know any number of broadcasters who have publicly cried foul on HD/IBOC...for all the good it did. Nothing at all is better than HD/IBOC.

On HDTV, I was told be a TV CE that it takes a whopping 26db of signal superiority to capture an HD TV receiver. If that is factual, the group that designed it should be run out of town on a rail for designing it and the FCC should be right behind them for placing stations on the same channel less than 250 miles apart. There are full power VHF's at 111 miles on the same channel. Do the math...no wonder people can't watch this stuff reliably at any distance.
 
Yup. When I referred to people saying it's bunk, I wasn't really talking about engineers. Corporate management and their mouthpieces (including some corp. engineers that drink the coolaid with a smile) are what I was mainly referring to.
 
Whereas its true that COFDM has been around longer and well proven, 8VSB modulation is better adaptable for future applications and compression algorithms, mainly because more spectrum is available for data not repetitive information for Bit Error Correction as COFDM uses.

I was involved in some of the bake-off tests between COFDM and 8VSB back in the 90's and at the time COFDM was more easily received than 8VSB, but the new generation of receiver chips has pretty well leveled the playing field.
 
The answer is simple: cODFM works BETTER in multipath (lower data errors) while 8-VSB works WORSE (many more data errors).

This is why TV stations use COFDM for all their STLs, Remote pickup links, Intercities, etc,
 
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