> From: [email protected]
> To: [email protected]
> Date: Wed, 17 Feb 2010 19:35:57 -0500
> Subject: Re: [ivtv-users] hvr-1600, frame CRC mismatch - incomplete frame
>
> On Wed, 2010-02-17 at 08:46 -0500, Kyle Lil wrote:
> >
> >
> > > From: [email protected]
> > > To: [email protected]
> > > Date: Mon, 15 Feb 2010 19:14:00 -0500
> > > Subject: Re: [ivtv-users] hvr-1600, frame CRC mismatch - incomplete
> > frame
> > >
> > > On Mon, 2010-02-15 at 16:22 -0500, Kyle Lil wrote:
> > >
> > > > Previously I'd had the tuner connected at the end of additional
> > > > splitters and devices, but I shortened the chain in
> > troubleshooting.
> > > > I'll try attenuating the signal again to see if that helps. I
> > hadn't
> > > > consider the possibility that the cable signal might exceed the
> > > > devices input limits.
>
> > > I think you'll just need to attenuate your cable signal before it
> > hits
> > > the HVR-1600.
> > >
>
> >
> >
> > I built a -10dB attenuator as you described in your link (thanks for
> > that) and still saw the glitches. In fact, I see them whether I
> > connect the cable line straight from the wall into the HVR-1600 or
> > through two splitters and the attenuator (and every other
> > combination). In trying to get a better sense of what's happening to
> > the signal,
>
> My hypothesis is the HVR-1600 digital tuner and demodulator AGC circuits
> cannot attenuate the signal enough to avoid occasional clipping of the
> analog signal. The clips show up as uncorrectable blocks.
>
>
>
> > I went into the service menu of my cable box, a Motorola DCH-3200,
> > which lists various signal statistics. One of the parameters it lists
> > is "AGC", which I'm guessing is some sort of automatic gain control
> > that puts the signal into the optimal range for the tuner.
>
> Yes.
>
>
> > If the cable box is connected after the second splitter, AGC is 18%.
> > If it's connected to the first splitter, it's a few percent lower.
>
> 18% is on the low side for an AGC to be operating. That indicates that
> you have a strong signal.
>
>
> > If I add the attenuator, the AGC goes up another 5%. It makes sense
> > to me that the attenuator would cause the AGC value to go up, but I'm
> > sure why it only went up 5% with a -10dB attenuator.
>
> Well, without knowing 5% of "what" (AGC full scale voltage?), it doesn't
> really matter as long as your attenuator appears to be attenuating the
> signal.
>
> >
> > Does the HVR-1600 have similar AGC built in?
>
> Yes, there should be two: an AGC in the MXL5005s and an AGC in the
> CX24227. The MXL5005s will be used for AGC until the AGC Take Over
> Point (TOP) setting in the CX24227 is reached, and then the CX24227 will
> take over with it's AGC.
>
> The idea is to amplify very weak signals as best you can while getting
> the best possible Noise Figure with the MXL5005s; or if the signal is
> strong enough, use the CX24227 AGC to get maximum signal while avoiding
> clipping.
>
>
> In my hypothesis, the "glitches" are caused by occasional clipping of
> the analog signal in either the MXL5005s or the CX24227.
>
>
> > If the cable box cuts the signal to less than 1/5th of its amplitude
> > (even after two splitters),
>
> No, that's likely 5% of AGC full scale. You don't have enough details
> to know what AGC percentage level corresponds to what gain and if it is
> expressed as a percantage for linear units or logarithmic (dB) units.
>
> FYI: -14 dB is 1/5 of signal amplitude, -7 dB is 1/5 of signal power.
> A 1-to-2 splitter will drop your signal power by more than 3 dB.
>
>
> > maybe this is an indication that I need to build an even stronger
> > attenuator? Do you have any insight into what value attenuator I
> > should be shooting for?
>
> Attenuate by 20, 30, or 40 dB I guess. You'll find one or more of:
>
> a. The glitches stop. (Good attenuation value to use.)
> b. The signal is so weak you can't lock the stations anymore. (Too much
> attenuation)
> c. My hypothesis is wrong. :P
>
>
> As an experiment, I can look at trying to adjust the CX24227's AGC TOP
> to kick in earlier, at a much lower threshold, in an attempt to prevent
> the MXL5005s from trying to do any amplification at all. However, a
> series of tests with external (cheap home-made) attenuators would be
> more deterministic in terms of experimental results. The experimental
> trials are bounded by an attenuator value so large that the signal
> cannot be tuned anymore.
>
> Regards,
> Andy
>
>
>
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OK, so I built an attenuator with a potentiometer in position Z2 for the
circuit diagram you sent me. I'm sure there are probably a whole slew of
problems this could introduce (especially since I did not put a pot in position
Z1 also), but I wanted to be able to easily look at the various attenuation
levels (and I don't have a soldering iron at home, just at work). What I found
was that the glitches are still apparent even when I turn the potentiometer to
a level that make my cable box OOB AGC go to >30% (which it now rates as "fair"
instead of "good"). The cable box and HVR-1600 are at the output of the same
splitter, which is after (downstream of) the adjustable attenuator I built. The
way I constructed the circuit, I could not provide enough attenuation to cause
the tuner to lose station lock, but with the maximum attenuation (~-14dB from
attenuator + -6dB from the two splitters between the HVR-1600 and the wall) I
could achieve, the glitch rate was the same (or maybe slightly higher).
I will try to build an attenuator today that is able to reduce the signal all
the way down to an un-lockable level. Is it possible that the HVR-1600 is so
sensitive that a signal level my cable box rates as only fair is still too
large for it?
Many thanks,
Kyle
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