Re: [PATCH RFC] libv4lconvert: SDR conversion from U8 to FLOAT

[Hans Verkuil <hverkuil@xs4all.nl>]

On 11/14/13 14:45, Antti Palosaari wrote:
> On 11.11.2013 16:38, Hans Verkuil wrote:
>> On 11/11/2013 03:19 PM, 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.
>>
>> For what it's worth: ALSA does support float format as well (both LE and BE).
> 
> I want use existing data formats and that [-1 to +1] scaled 32-bit IEEE-754 floating point is de facto format for SDR application (actually pair of floats as a complex).
> 
> Doing conversion inside libv4lconvert makes it very easy for write application. Currently I have implemented GNU Radio and SDRsharp plugins that feeds data from device via libv4l2 using mmap and conversion.
> 
> Thanks to pointing endianess issue, I didn't though it all. I suspect those apps just relies to local endianess. So do I have to implement float format conversion with both endianess?

I would say that it should be written with endianness conversion in mind. It's not
likely to be used on a big-endian system, but it should be possible to support it
without too much work.

Regards,

	Hans