[PATCH] Re: [linux-dvb] [PATCH] Multi protocol support (stage #1)

Johannes Stezenbach js at linuxtv.org
Fri Jun 2 15:53:18 CEST 2006


On Fri, Jun 02, 2006, Manu Abraham wrote:
> Johannes Stezenbach wrote:
> >
> >So, it seems that for hierarchical BC mode PLHEADER does
> >not use pi/2-BPSK. Now, I don't know if and how the
> >receiver could figure out that hierarchical modulation
> >is used, however, once it knows that, it can get the
> >other transmission parameters from the PLHEADER.
> 
> Well, in BC-BS mode we have the S2 satellite delivery system descriptor
> I will try to explain what i understood.
> 
> 
> According to the patent
> 
> United States Patent Application: 20050089068

[snip]

> An approach is provided for supporting signal acquisition and frame 
> synchronization in a digital broadcast system utilizing Low Density 
> Parity Check (LDPC) codes. Hierarchical modulation is utilized to 
> provide backward compatibility, whereby the lower layer signal is 
> encoded using LDPC coding. A signal is received, whereby the signal is 
> modulated according to the hierarchical modulation scheme including an 
> upper layer and a lower layer. The signal includes a data pattern and a 
> coded frame. The dependency of the received signal on the upper layer 
> modulation is removed. The modulation removed signal is correlated with 
> multiple predetermined data patterns to determine the data pattern of 
> the signal. The code rate of the coded frame is derived based on the 
> determined data pattern. The above arrangement is particularly suited to 
> a digital satellite broadcast system.

[snip]

> >While STB0899 does not seem to have a register for
> >enabling hierarchical BC mode, it still has a (read-only)
> >"low priority stream detection" bit in register TSULSTKM.)
> >Maybe this is a hint that it can detect hierarchical
> >BC mode by itself.

Well, the question is not how hierarchical modulation works,
the question is if and how the demod can detect this
automatically. The text from the patent you quoted doesn't
shed any light on this, it says (if I understand it right)
that one has to know that hierarchical modulation is used,
then remove the HP part and then "correlate with multiple
predetermined data patterns" (the ones given in EN 302 307
annex F) to find the code rate.

This would match my understanding as decribed in my previous
mail, that the PLHEADER is not pi/2-BPSK modulated but part
of the LP bitsream in hierarchical mode.
(This is consistent with annex F, the input to the
hierarchical mapper is a *bitstream*, not a pre-modulated
signal. Also, I guess pi/2-BPSK PLHEADER would break
DVB-S HP receiption.)

> For the STB0899 the information might not be needed, but could be needed 
> by another demod some time later.

possible

Currently I think the demod could just try to find SOF pattern
in pi/2-BPSK demodulated bitstream, and if that fails try
to find one of the BC patterns in hierarchical demodulated
LP-bitstream. If a particular demod cannot do this in firmware, but
has a register to explicitely choose hierarchical BC mode, we
could emulate it in dvb-core (similar to INVERSION_AUTO emulation).


> I think for the time being we can go with the minimalist approach as it 
> is in multiproto7

I would do it that way, yes.


Johannes



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