On 11.11.2013 16:19, Andy Walls wrote:
On Sun, 2013-11-10 at 19:16 +0200, Antti Palosaari wrote:
Convert unsigned 8 to float 32 [-1 to +1], which is commonly
used format for baseband signals.
Hi Annti,
I don't think this a good idea. Floating point representations are
inherently non-portable. Even though most everything now uses IEEE-754
representation, things like denormaliazed numbers may be treated
differently by different machines. If someone saves the data to a file,
endianess issues aside, there are no guarantees that a different machine
reading is going to interpret all the floating point data from that file
properly.
I really would recommend staying with scaled integer representations or
explicit integer mantissa, exponent representations.
Do you mean scaled presentation like a sample is always 32 signed
integer and what ever ADC resolution is, it is scaled to 32-bit signed
int and returned?
What I would like to implement is 8-bit int, 16-bit int and maybe 32-bit
int (if there comes ADC outputting more than 16-bit). These all
conversions are done inside Kernel, which actually has price about
nothing as it is simple integer math with bit shifting (scaling == bit
shifting). If you do that kind of conversion on USB URB interrupt at the
same time as memory copy from URB to videobuf2 is needed, it is
basically free.
Then it is up to caller to select int8, int16, int32 and driver does the
rest, selects actual ADC resolution using info like sampling rate.
Also returning SDR floats directly from Kernel driver could be very
handy, but as floats are not allowed in Kernel...
But now all conversions are in the libv4l. However, it is possible to
add new formats to driver later - removing existing formats from driver
is about impossible.
regards
Antti
--
http://palosaari.fi/
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