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@xxxxxxxxx>
Signed-off-by: Antti Palosaari <crope@xxxxxx>
+ <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/
--
To unsubscribe from this list: send the line "unsubscribe linux-media" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
