Sigmund Augdal Helberg wrote: > Two's complement format is the common format used nearly everywhere for > storing signed numbers. Hello, First of all, thanks for your anwser. Ok got it but ... does the statement "X is in two's complement format" imply that X is used to represent a signed value ? :-) Moreover I do not understand the need of a signed value in our case (AGC accumulator). A negative value should be nonsense and the precision of the positive value is /2 compared to an unsigned one. >>So, I think the following line should be corrected (patch attached). >> >>- *signal_strength = cx24123_readreg(state, 0x3b) << 8; /* larger = better */ >>+ *signal_strength = ~(cx24123_readreg(state, 0x3b) - 1) << 8; /* larger = better */ > > This piece of code would be the proper way to change sign of a number > Does the following line give meaningful values for you? > *signal_strength = (cx24123_readreg(state, 0x3b)+0x80) << 8; /* larger = > better */ > This would turn the number into an unsigned. Wow. Are you sure of this ? By doing this you add 1000000 to the return value (say 0x23 so 00100011). So now we have 10100011. Look like a negative value in 2's complement format ? I'm lost here :-) Anyway by doing this, I have: using '/dev/dvb/adapter0/frontend0' FE: Hauppauge Nova-S-Plus CX24123/CX24109 (SAT) signal%: 63| snr%: 00 status 1f| signal a300| snr 0000| ber 003fffff| unc 003fffff| FE_HAS_LOCK The signal value (63%) is still very different from what I should have (~70%). > Warning: My memory of these things is a bit shaken, I could be very > wrong here. I wasn't aware of such things (1's and 2's complement format) until yesterday so anyway I learnt something here :-) Regards, -- Romain DEGEZ, System and Network Administrator SMARTJOG S.A. 16 Place de la R?publique, 75010 Paris, FRANCE http://www.smartjog.com Phone : +33.1.49.96.63.19 Fax : +33.1.49.96.64.05 Mobile : +33.6.84.24.65.21 romain.degez@xxxxxxxxxxxx -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 189 bytes Desc: OpenPGP digital signature Url : http://www.linuxtv.org/pipermail/linux-dvb/attachments/20060112/a52409ee/signature.pgp
