Re: [PATCH RFCv10 00/15] DVB QoS statistics API

[Antti Palosaari <crope@iki.fi>]

On 01/17/2013 05:40 AM, Manu Abraham wrote:
> On Thu, Jan 17, 2013 at 3:31 AM, Mauro Carvalho Chehab
> <mchehab@redhat.com> wrote:
>> Em Wed, 16 Jan 2013 19:29:28 +0000
>> Simon Farnsworth <simon.farnsworth@onelan.com> escreveu:
>>
>>> On Wednesday 16 January 2013 23:56:48 Manu Abraham wrote:
>>>> On Wed, Jan 16, 2013 at 10:51 PM, Mauro Carvalho Chehab
>>> <snip>
>>>>>
>>>>> It is a common sense that the existing API is broken. If my proposal
>>>>> requires adjustments, please comment on each specific patchset, instead
>>>>> of filling this thread of destructive and useless complains.
>>>>
>>>>
>>>> No, the concept of such a generalization is broken, as each new device will
>>>> be different and trying to make more generalization is a waste of developer
>>>> time and effort. The simplest approach would be to do a coarse approach,
>>>> which is not a perfect world, but it will do some good results for all the
>>>> people who use Linux-DVB. Still, repeating myself we are not dealing with
>>>> high end professional devices. If we have such devices, then it makes sense
>>>> to start such a discussion. Anyway professional devices will need a lot of
>>>> other API extensions, so your arguments on the need for professional
>>>> devices that do not exist are pointless and not agreeable to.
>>>>
>>> Let's step back a bit. As a sophisticated API user, I want to be able to give
>>> my end-users the following information:
>>>
>>>   * Signal strength in dBm
>>>   * Signal quality as "poor", "OK" and "good".
>>>   * Ideally, "increase signal strength to improve things" or "attenuate signal
>>> to improve things"
>>>
>>> In a DVBv3 world, "poor" equates to UNC != 0, "OK" is UNC == 0, BER != 0,
>>> and "good" is UNC == BER == 0. The idea is that a user seeing "poor" knows
>>> that they will see glitches in the output; a user seeing "OK" knows that
>>> there's no glitching right now, but that the setup is marginal and may
>>> struggle if anything changes, and a user seeing "good" knows that they've got
>>> high quality signal.
>>>
>>> VDR wants even simpler - it just wants strength and quality on a 0 to 100
>>> scale, where 100 is perfect, and 0 is nothing present.
>>>
>>> In both cases, we want per-layer quality for ISDB-T, for the reasons you've
>>> already outlined.
>>>
>>> So, how do you provide such information? Is it enough to simply provide
>>> strength in dBm, and quality as 0 to 100, where anything under 33 indicates
>>> uncorrected errors, and anything under 66 indicates that quality is marginal?
>>
>> Unfortunately, not all devices can provide strength in dBm.
>
> MB86A20 is not the only demodulator driver with the Linux DVB.
> And not all devices can output in dB scale proposed by you, But any device
> output can be scaled in a relative way. So I don't see any reason why
> userspace has to deal with cumbersome controls to deal with redundant
> statistics, which is nonsense.

What goes to these units in general, dB conversion is done by the driver 
about always. It is quite hard or even impossible to find out that 
formula unless you has adjustable test signal generator.

Also we could not offer always dBm as signal strength. This comes to 
fact that only recent silicon RF-tuners are able to provide RF strength. 
More traditionally that estimation is done by demod from IF/RF AGC, 
which leads very, very, rough estimation.

So at least for the signal strength it is impossible to require dBm. dB 
for SNR is possible, but it is very hard due to lack of developers 
knowledge and test equipment. SNR could be still forced to look like it 
is in given dB scale. I think it is not big loss even though SNR values 
reported are a little bit wrong.


About half year ago I looked how SNR was measured every demod we has:

http://palosaari.fi/linux/v4l-dvb/snr_2012-05-21.txt

as we can see there is currently only two style used:
1) 0.1 dB (very common in new drivers)
2) unknown (== mostly just raw register values)


regards
Antti

-- 
http://palosaari.fi/