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Cool Member
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Discussion Starter · #1 ·
After a recent storm, my 5lnb dish got "out of whack" and had a service call to fix. With the old settings, I was getting satellite strength signals in the low to mid
80s. Following the service call, everything is 95+.

Is my mind playing tricks on me, or is it possible that my HD picture quality seems to have improved with the stronger signals? Picture looks much "smoother".

Any thoughts or am i just crazy?
 

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Jhorwitz said:
Just what I thought, I'm crazy.
Digital is digital. If it's there, it's standard. If it's not, the picture will pixelate. However, lately, D* has been switching from mpeg2 to mpeg4, and everyone agrees the the latter is better HD -- mpeg2 being characterized as "HD-lite". So, maybe, your timing coincides with the better compression technique.

Other than that, please refer to your quote above! :)

Stan
 

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Stanley Kritzik said:
Digital is digital.
That's a very common misperception.

You've got the 'basic' concept, but are missing the finer points. If the reception is good enough for the ERROR-CORRECTION to recover the bits 'perfectly' (known as the digital threshold), yes, the bit stream will be bit-for-bit identical to the original transmitted bitstream.

BUT (there is always a but!)....

The more work the system has to do to recover those bits in reception, the more jitter is introduced in the recovery process. If one looked at it on an oscilloscope, bit bits would not be perfect blocks of ones and zero's, but be 'fuzzy'. The fuzzier the bits are, the harder it is for the timing recovery to properly 'decide' exactly where the bits are, resulting in more jitter into the recovery process.

This is why there is a LIMIT to first, how many times a digital signal (say, either with satellite or with fiber transmission), the signal can be recovered, re-timed, re-generated, and re-transmitted. Satellite digital transmission in particular has a host of problems, kinda like digital terrestrial microwave on steroids, that require things be VERY tight in the design stage.

A good consumer example is CD's. Many of them, particularly in the early days (early 1980's) sounded VERY harsh, almost unlistenable. This was mostly a result of poor Analog-to-Digital encoders the studios were using, and in addition the poor Digital-to-Analog chipsets that CD player manufacturers were installing in their players. Eventually, they figured out such techniques as oversampling, and things slowly got better.

From a satellite microwave transmission standpoint, the various things that happen to the ANALOG RF transmission (the carrier is ANALOG, only the transmitted signal is 'digital'), cause all sorts of wacky things to happen. We all know about atmospheric absorption, and depolarization. But the receiver gets the signal, and the decoder has to decode it.

Although the signal strength can be enough for the circuit to fully 'lock' and decode the bitstream, the jitter can cause circuits further down the chain to become 'confused' on a bit-by-bit analysis of that stream. Well before such huge effects such as macro-blocking and such become apparent (due to the forward error correction being unable to fully recover the bitstream), small errors can introduce both audio and video 'effects' (basically 'noise' or 'distortion') that can be seen or heard.

The more 'packed' a digital steam is by the encoding process (and MPeg4 is a much more 'dense' encoding scheme than MPeg2 by it's efficiency), those streams have much more potential to be affected by jitter.

Exactly how it can be affected is a good question. I did a lot of the original work with MPeg2 satellite video transmission in the late 80's and early 90's, and the level of the encode/decode process back then were boxes about the size of a small refrigerator; it wasn't until 92-93 that things got squeezed down to a handful of chips, to be used in consumer receivers like the ones designed and built for the first generation of DirecTV transmissions.

Broadcom does most, if not all, of the MPeg receiver chipsets today, and I'm sure that somewhere in their specs are the figures on how the circuits are able to deal with all types of errors, and give us at least most of the time something that doesn't look like mush.

But, just saying 'digital is digital' is far from the real truth. It would be interesting to see if lower than optimal (yet above digital threshold) reception would result in visual (and/or audio) effects that people could pick out, even with modern equipment. I would say VERY YES.
 

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1948GG, I have been "jumped on" in forums when I have questioned the "digital is digital" statement, and I had no first-hand knowledge to back it up. It just seemed to me that something similar to what you have posted must be true.

However, even though I have "felt" this to be true and you say it is, I have not been able to find a suitable reference for this info on the web. At least, not one that I could understand!
 

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paulman182 said:
1948GG, I have been "jumped on" in forums when I have questioned the "digital is digital" statement, and I had no first-hand knowledge to back it up. It just seemed to me that something similar to what you have posted must be true.

However, even though I have "felt" this to be true and you say it is, I have not been able to find a suitable reference for this info on the web. At least, not one that I could understand!
A good place to 'start', is The Mathematical Theory of Communication by Claude Shannon, 1948. It is, quite literally, the 'bible' of digital communications. As an undergrad I had to re-prove all the mathematical theorems both in it and Pierce's Symbols, Signals, and Noise.

Those volumes are still a part of my library, some 35 years later; I can swivel my head about 45 deg from the screen, and there they are. Every other reference book traces it's roots back to these. Makes my brain hurt just thinking about the math involved. Remember, these were 'slide rule' days; decent hand held calculators didn't exist until I went to graduate school, cost hundreds, and can be run circles around with a $10 Wal-Mart model today.

Anyway, the general lack of what I consider really basic knowledge as to satellite communications is usually woefully 'less than zero' on these public message boards. I usually keep out of things unless the basic lack of ANY knowledge is truly profound, like it was in this case.
 
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