On 01/25/2014 02:01 PM, Antti Palosaari wrote:
> 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 <hverk...@xs4all.nl>
>>> Signed-off-by: Antti Palosaari <cr...@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.
I'm sorry for having you work so hard on explaining this when the problem was
with
my brain that confused 'resolution' with 'rate' :-) Well, they both start with
'r'...
Your text is fine.
>
>
>>> --- 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
>
Regards,
Hans
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