Re: [Discuss-gnuradio] question about dvb_bch_bb_impl.cc

[Ron Economos]

The gr-dtv BCH encoder has been heavily tested with both DVB-S2 and 
DVB-T2 receivers and is known to be correct. I did verify that your 
suggested change breaks the BCH encoder.

You can see a nice demo of the DVB-S2 transmitter here:

https://myriadrf.org/blog/digital-video-transmission-using-limesdr-gnu-radio/

Ron

On 06/13/2016 07:22 AM, Tiankun Hu wrote:
> I found dvb_bch_bb_impl.cc use shift register to implement BCH encode, 
> but it seems different with my knowledge.
> If I remember right, after shift the register, we should add the info 
> bit to register's LSB
>
> dvb_bch_bb_impl.cc:526~535
>               b = (temp ^ (shift[5] & 1));
>               reg_6_shift(shift);
>               if (b) {
>                 shift[0] ^= m_poly_n_12[0];
>                 shift[1] ^= m_poly_n_12[1];
>                 shift[2] ^= m_poly_n_12[2];
>                 shift[3] ^= m_poly_n_12[3];
>                 shift[4] ^= m_poly_n_12[4];
>                 shift[5] ^= m_poly_n_12[5];
>               }
>
> should be like this ===========>
>               b = shift[5] & 1; // use register's MSB to decide 
> whether need to XOR poly
>               reg_6_shift(shift);
>           shift[0] = shift[0] ^ (temp<<31);//  add the info bit to 
> register's LSB
>               if (b) {
>                 shift[0] ^= m_poly_n_12[0];
>                 shift[1] ^= m_poly_n_12[1];
>                 shift[2] ^= m_poly_n_12[2];
>                 shift[3] ^= m_poly_n_12[3];
>                 shift[4] ^= m_poly_n_12[4];
>                 shift[5] ^= m_poly_n_12[5];
>               }