Convolutional byte interleaving screws this up. Intermittent short sequences of known data don't provide adaptive equalizers that rely on Kalman or Wiener auto-regression technique much help to open the data-slicing eye. Long repetitive PN sequences allow instant channel-impulse-response computations that can pop open the eye. Recomputation of the CIR using long-signature analysis on a sliding window of a few thousand symbols of ordinary data can the support equalizer tracking even during fast fades. Auto-regression techniques are too slow. Al ----- Original Message ----- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> To: <opendtv@xxxxxxxxxxxxx> Sent: Tuesday, November 07, 2006 3:41 PM Subject: [opendtv] Re: Backward Compatibility of Robust DTV Transmissions > John Shutt wrote: > > > Once a receiver is built, how much can be done ex post facto to > > improve the performance? With so many receivers designed around > > "blind equalization" exclusively, I don't understand what sort > > of "training wheels" can be added to ATSC to help them. > > > > I can see how one could create pseudo-training sequences ala > > A-VSB that could help new receivers improve their performance > > without upsetting existing hardware, but I think the best one > > can hope for with legacy receivers is similar to the Hippocratic > > oath, "first do no harm." > > Hmmm. Since blind equalization works best with a strong signal, why > wouldn't it help these legacy receivers if the 8-VSB transmissions > included some variable number of "bogus" segments, each of which is, for > example, always modulated at 100 percent (i.e. all large amplitude > symbols)? > > -------Quote from A/54 p. 96--------- > Tracking dynamic echoes requires tap adjustments more often than the > training sequence is transmitted. Therefore, the prototype Grand > Alliance receiver was designed so that once equalization is achieved, > the equalizer switches to a decision-directed equalization mode that > bases adaptation on data symbols throughout the frame. In this > decision-directed equalization mode, reception errors are estimated by > slicing the data with an 8-level slicer and subtracting it from the > equalizer response. > > For fast dynamic echoes (e.g., airplane flutter) the prototype receiver > used a blind equalization mode to aid in acquisition of the signal. > Blind equalization models the multi-level signal as binary data signal > plus noise, and the equalizer produces the error estimate by detecting > the sign of the output signal and subtracting a (scaled) binary signal > from the output to generate the error estimate. > > To perform the LMS algorithm, the error estimate (produced using the > training sequence, 8-level slicer, or the binary slicer) is multiplied > by delayed copies of the signal. The delay depends upon which tap of the > filter is being updated. This multiplication produces a > cross-correlation between the error signal and the data signal. The size > of the correlation corresponds to the amplitude of the residual echo > present at the output of the equalizer and indicates how to adjust the > tap to reduce the error at the output. > ------------------------------------- > > I've noticed, for example, that in certain cases, DTT channels with a > lot of echo take longer to show up on the screen. It seems to me that > there must be some particular sequence of data symbols that makes the > blind equalization process easier, i.e. more robust. And once synched > up, the receiver tends to stay locked in, so this happy sequence of > symbols wouldn't need to be repeated extremely frequently. > > Of course, it takes up some bandwidth. > > Bert > > > ---------------------------------------------------------------------- > You can UNSUBSCRIBE from the OpenDTV list in two ways: > > - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org > > - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line. > ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.