On Tuesday 15 September 2009, Andy Walls wrote: >On Tue, 2009-09-15 at 08:26 +0200, Hans Verkuil wrote: >> On Tuesday 15 September 2009 06:18:55 Michael Krufky wrote: >> > On Tue, Sep 15, 2009 at 12:07 AM, Dmitri Belimov <d.belimov@xxxxxxxxx> >> > wrote: >> > >> > Personally, I don't quite understand why we would need to add such >> > controls NOW, while we've supported this video standard for years >> > already. I am not arguing against this in any way, but I dont feel >> > like I'm qualified to accept this addition without hearing the >> > opinions of others first. >> > >> > Can somebody help to shed some light? >> >> It's the first time I've heard about SECAM and AGC-TOP problems. I do >> know that the TOP value is standard-dependent, although the datasheets >> recommend different SECAM-L values only. So I can imagine that in some >> cases you would like to adjust the TOP value a bit. >> >> The problem is that end-users will have no idea what to do with a control >> like that. It falls into the category of 'advanced controls' that might >> be nice to add but is only for very advanced users or applications. > >The AGC Take Over Point (TOP) is the signal level at which the 2nd stage >of the amplifier chain (after the IF filter) takes over gain control >from the 1st stage in the amplifier chain. The idea is to maximize >overall noise figure by boosting small signals as needed, but avoiding >hittng amplifer non-linearities that generate intermodulation products >(i.e. spectral "splatter"). > >The TOP setting depends on the TV standard *and* the attenuation through >the IF filter. I'm fairly sure, it is something that one probably >should not change to a value different from the manufacturer's >recommendation for a particular TV standard, unless you are dealing with >input signals in a very controlled, known range aor you have taken >measurments inside the tuner and precisely know the loss through the IF >filter. If the user doesn't understand how the AGC-TOP setting will >affect his overall system noise figure, for all inoming signal >strengths, then the user shouldn't change it. (IMO) As a retired broadcast engineer, I can say that generally speaking, this is a knob that shouldn't be enabled. It may in some cases be able to get a db's worth of improvement, but the potential for worsening it by many db, by someone who doesn't understand the interactions, is much too high. Given a knob, it will be tweaked, usually detrimentally. >> The proposed media controller actually gives you a way of implementing >> that as tuner-specific controls that do not show up in the regular >> /dev/videoX control list. I have no problems adding an AGC-TOP control as >> a tuner-specific control, but adding it as a generic control is a bad >> idea IMHO. > >If needed, it should be an advanced control or, dare I say, a tunable >via sysfs. Setting the TOP wrong will simply degrade reception for the >the general case of an unknown incoming signal level. > >The tuner-sumple code has initialization values for TOP. Also there are >some module options for the user to fix TOP to a value, IIRC. Both are >rather inflexible for someone who does want to manipulate the TOP in an >environment where the incoming RF signal levels are controlled. > >Regards, >Andy > >> Regards, >> >> Hans > >-- >video4linux-list mailing list >Unsubscribe mailto:video4linux-list-request@xxxxxxxxxx?subject=unsubscribe >https://www.redhat.com/mailman/listinfo/video4linux-list > -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> We've run out of licenses -- video4linux-list mailing list Unsubscribe mailto:video4linux-list-request@xxxxxxxxxx?subject=unsubscribe https://www.redhat.com/mailman/listinfo/video4linux-list