Re: OT: Technical Question for Jeff Liebermann

On 10/13/2011 7:56 PM, GreenXenon wrote:
On Oct 13, 7:44 pm, miso<m...@xxxxxxxxx> wrote:


First of all, I don't think you have a
handle on what AM means. For instance, do you mean OOK (on off keying)?
Are you taking bytes of data and PCMing? Just strong enough to be
clearly recognized by the DAC? What does that mean? You feed a DAC
digital signals. There is nothing ambiguous in the data.

Intrinsic in any communications scheme is data framing and clock
recovery, not to mention having to whiten the data (scrambler).

1. Why does the data have be scrambled?

2. No, I'm talking about OOK.

Since we haven't nailed down the modulation, I really can't go into the scrambler much, but basically data could be a bazillion zeroes or one. That can lead to modulation that doesn't really "flog" the system (use all possible symbols). For instance, if you had a simple 4 point constellation, it would only use two out of the 4 locations. You run the data through a scrambler to whiten it, i.e. make it look more random. It is generally hard to do clock recovery on a signal that isn't scrambled. Note this is not encryption. I'm having a hard time describing this because whitening the data is generally job 1 so I have to think hard about the bad things that happen if you don't whiten it.
http://en.wikipedia.org/wiki/Scrambler
Actually this wiki looks good.

Let's take the case of simple FSK with no scrambler and worse yet, no framing. (Note FSK doesn't really need clock recovery, though in practice, you dejitter the recovered data.) If you sent a long string of zeros, it would correspond to one frequency of the FSK pair. Your data recovery would have a hard time locking on the data. Now if yo framed it, say with start and stop bits, you could now lock on the data. But for analysis purposes, say estimating a bit error rate, you rather have both frequencies used. Better yet, most of the comm schemes have already been analyzed, so you can plug and chug, but the analysis generally assumes white noise and randomized data.

OOK doesn't need clock recovery either. You could look at how an aircraft transponder works if you need an example. You do need apriori knowledge of the data rate to recover the data, and usually it is oversampled to aid in recovery.
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