Periodicity of Digital TV Signal

everything is variable bit rate and it causes other choices to get made dependent upon which bit rate it is encoded at.

Don't know if this helps...

You have to piece the string back together.

On a sunny day (Tue, 30 Dec 2014 12:19:56 -0800) it happened Ken Morrow wrote in :

Not 100% sure what you mean, dunno about GSM and your definition of 'periodicy'. As I seem to be the only one in this universe who released C source code for DVB-S encoding, (S for satellite), I can make some comments. First download specification ets_300421e01p.pdf (google finds it, it is free). This shows you how the signal is encoded, explains the randomizer etc (periodicy begins there). The document is a bit in well, not always clear for the innocent bystander so to speak language. So if you want to see what I made of it, look at the C source:

I left some of that pdf in the source, to explain what happens, what 'things' are done to the signal. Note the copyright and the beheading if violated part on the FSF website. Once you find the periodicy (and your definition of it) please let me know too ;-) For DVB-T (not aliens) I really have not bothered to study it.

Using your analogy, there realy is no periodicity that you can hear as there is in a gsm signal Mark

GSM is TDMA (time division multiplex). The 217 Hz is the multiplex rate for the phone:

DTV (DVB-T in Europe and ATSC in USA) do not use TDMA multiplexing and therefore have no comparable "tone".

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Yeah, it is called H.264 and now H.265.

You have PC clock issues. Posting to Usenet 'from the future' is a bad thing. Just ask George Orr.

"8VSB carries a symbol rate of 10.76 megabaud, a gross bit rate of 32 Mbit/s, and a net bit rate of 19.39 Mbit/s of usable data"

It's interesting... the carrier is amplitude-modulated with a signal that changes 10.76 million times per second, and the resulting signal is bandpass-filtered down to 6 MHz in width.

I would presume that there's a higher-level (but lower-frequency) ordering to the signal, due to the fact that what's being modulated is an MPEG transport stream.

says that ATSC uses a transport stream packet size of 208 bytes (including 20 bytes of R-S error correction coding). If I've done my math right (quite possibly not) this would be a rate of 3233 MPEG transport packets per second.

Whether you'd actually detect either of these periodicies in question by looking at the RF signal is a good question. The fact that the MPEG data is highly compressed would make it look rather random, prior to trellis encoding.

The ATSC signals I've looked at over the air with a spectrum analyzer, looked pretty "white" (energy spread out rather evenly) across their 6 MHz bandwidth. There's a small spur of carrier peeking out above the broadband spread (I gather it's there to give receivers a stable frequency reference to lock onto) but other than that I don't recall seeing any particular energy spikes.

That's fine. (Nyquist or Shannon?) says BW = baud/2, give or take.

Although, that's for baseband signals. 6MHz around DC is actually

+/-6MHz, or a total 12MHz width. Modulating that at frequency needs 12MHz total BW. You can't cut off one of the sidebands because it's PAM...right? I forget.

But that wouldn't make sense, because a signal around DC still has to be real, not analytic, so the LSB has to be hermitian to the USB. And thus you can lop off a sideband as long as you recover it in kind. So, 6MHz would suffice for VSB or SSB.

Though there's also a scrambler, isn't there? So that'll randomize the spectrum pretty well.

Yeah, even without scrambling or trellis, it should be pretty flat to begin with.

Had an interesting conversation with a friend the other night. We were remarking on SIGSALY (the first digital PCM encrypted communications system, from WWII) and the frequency band detection and modulation scheme. It occurs to me, such a process should be very similar to a typical MPEG scheme, but one-dimensional. Video could be encoded in the same way -- of course, NTSC is a whole hell of a lot easier to decode than MPEG, so even if they went to the trouble to implement it for video, back in the day, it wouldn've never gone anywhere -- but nowadays, as a curiosity rather than a standard, it would be interesting to play with. The video could be sampled into pixels, for instance, and the DFT performed in hardware with muxes, gain and accumulators.

Tim

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Which DTV system ?

8VSB, DVB-S, DVB-C are single carrier systems, while DVB-T/T2 and DVB-C2 are multicarrier systems, e.g. with more than 6000 subcarriers.

All use some kind of scrambling to remove any DC components and discrete signals and the signal would look "white".

All use interleaving to spread a single baseband packet over a longer time span. If there is a burst error, taking out multiple consecutive bits from the actual transport stream, the deinterleaver in the receiver will convert these burst errors into single bit errors in multiple base band packets. The error correction code can then correct these individual bit errors.

Thus, you would still not see any periodicity in the transmitted signal.

What I seem to hear from the replies so far is that a DTV signal has no repetitive component (pulse, interval, etc.) in its signal structure.

is that correct?

All the literature I have found so far online presumes a level of expertise and professional involvement.

I am just looking for an overview of how the system actually works. Has anybody seen one?

Ken Morrow