Re: [REVIEW PATCH 10/13] DocBook: Software Defined Radio Interface

[Antti Palosaari <crope@iki.fi>]

On 25.01.2014 10:26, Hans Verkuil wrote:
> A few comments below...
>
> On 01/23/2014 10:08 PM, Antti Palosaari wrote:
>> Document V4L2 SDR interface.
>>
>> Cc: Hans Verkuil <hverkuil@xs4all.nl>
>> Signed-off-by: Antti Palosaari <crope@iki.fi>

>> +    <section>
>> +      <title>V4L2 in Linux 3.14</title>
>
> This should be 3.15.

OK. The goal was that 3.14 but fixing that documentation has taken over 
month.


>> +
>> +    <para>
>> +The SDR capture device uses the <link linkend="format">format</link> ioctls to
>> +select the capture format. Both the sampling resolution and the data streaming
>
> I understand why the data streaming format is bound to the format, but why is
> the sampling resolution bound by it as well?

How can I explain that... it is not always bind to format nor it could 
be known 100% from sure from format. But resolution has some deep 
relation to format. Data is usually packed to smallest reasonable size 
in order to minimize needed transmission bandwidth. If you change 
sampling resolution then format likely changes too, as greater 
resolution needs more bits per sample and format carries samples. Lets 
take a some simple example:

Lets take an examples:
A is 8-bit sample, number from range 0-255.
B is 16-bit sample, number from range 0-65536.

Then your formats are defined, lets say V4L_FMT_SDR_U8 and V4L_FMT_SDR_U16.

Streams are sequence of those samples, use 10 samples here as example:
A0A1A2A3A4A5A6A7A8A9 = 80bits, 10 bytes
B0B1B2B3B4B5B6B7B8B9 = 160bits, 20 bytes

But you still don't know surely what is sampling resolution, only how it 
is represented. It is always more or less than that nominal value, die 
to many reasons. ADC datasheets usually define ENOB (effective number of 
bits) value. It is fairly common having 12bit resolution but ENOB is 
only around 10bit.

Here is example from Mirics, which shows different formats and resolutions:

format,resolution,sample rate (~max)
252    14         8613281
336    12        11484375
384    10+2      13125000 (packed, 2 bits dropped using some formula)
504    8         17226562

All in all, the idea was to tell user that the sampling resolution is 
selected according to dataformat he uses.


>> --- a/Documentation/DocBook/media/v4l/vidioc-g-fmt.xml
>> +++ b/Documentation/DocBook/media/v4l/vidioc-g-fmt.xml
>> @@ -172,6 +172,13 @@ capture and output devices.</entry>
>>   	  </row>
>>   	  <row>
>>   	    <entry></entry>
>> +	    <entry>&v4l2-format-sdr;</entry>
>> +	    <entry><structfield>sdr</structfield></entry>
>> +	    <entry>Definition of an data format, see
>
> s/an data/a data/

OK


regards
Antti

-- 
http://palosaari.fi/