[mythtv-users] OT Re: HDMI_SPDIF directly to reciever

Johan johan.vanderkolk at dommel.be
Sun Jul 6 06:44:21 UTC 2008


On Sat, 2008-07-05 at 05:30 -0700, Alen Edwards wrote:
> 
> 
> > > > > > >  
> > > > > > > If your voltmeter is on a DC range and the output is bipolar, +0.5V
> > > > > > > and -0.5V then that will "average" out to 0V. This is irrespective of
> > > > > > > the data travelling over the spdif as it is Manchester Coded.
> > > > > > > 
> > > > > > > Try an AC range on the voltmeter if you have one.
> > > > > > >   
> > > > > > >     
> > > > > > >         
> > > > > > >             
> > > > > > AC is 3 something.
> > > > > > So if my receiver expects 5V and it is getting less, am I hurting it?
> > > > > > 
> > > > > >   
> > > > > >       
> > > > > >           
> > > > > > > HTH,
> > > > > > > 
> > > > > > > Roo.
> > > > > > > ___________________
> > > > > > >         
> > > > > > >             
> > > > > If you are getting 0 DC and 3AC, I am surprised.  I would expect it to
> > > > > be the other way around (0AC and 2-3DC).  No matter, less it good and
> > > > > your receiver is probably saying it can take anything up to 5 volts
> > > > > and it can probably take much less, like maybe .1 volts.  Some people
> > > > > on this list have said their equipment works all the way to .025
> > > > > volts.  I would double check your numbers and readings then try it.
> > > > > 
> > > > > 
> > > > > Allen
> > > > > 
> > > > > _
> > > > >     
> > > > >         
> > > > Summarizing:
> > > > 
> > > > the signal is digital manchester coded and probably bipolar, so will not
> > > > contain a DC component.
> > > > It's sampling rate is up to 96kHz, to high for most multimeters. 
> > > > Multimeters do not measure peak values, but only average (they are even
> > > > "calibrated" in RMS (root MEAN Square). So you will never know what peak
> > > > voltages are actually present. 
> > > > Only way to be sure is to use an oscilloscope.
> > > > 
> > > > know it does not help, but clarifies a shortfall in the measuring
> > > > technique..
> > > > 
> > > > Johan
> > > > 
> > > >       
> > > That is why I was surprised he measured 3 volts on AC and 0 on DC.
> > > When I measured the output on my MB, it was between 2 and 3 on DC and
> > > 0 on AC for the reasons you state and the fact that the signal levels
> > > were TTL and not spdif.  I AC coupled and attenuated it so now I
> > > measure 0 and 0. 
> > > 
> > > btw, you can measure the peak voltage if you use a diode and a small
> > > capacitor in front of the DC voltmeter, but that is another discussion
> > > and is not without its issues.
> > > 
> > >     
> > You'll have to use a diode bridge, and a capacitor type and value which
> > is matching the frequency. Too slow..won't charge/decharge. To
> > small...will decharge too fast...(depends on the multimeter used) too
> > big...is a short cct for AC, and will not charge fully.
> > 
> > Johan
> >   
> No, actually, you can use a schottky diode from the dut to the VM
> input and a small capacitor across the voltmeter input. The capacitor
> will charge up to the peak voltage and discharge only through
> parasitics.  You would not want to use a bridge as it drops too much
> voltage.  You can measure RF signals this way. Here, some guy built
> one that works to 450MHz:
> http://www.ad5x.com/images/Articles/LoadDetectors.pdf
> Really OT but ...
> 
> Allen

I grew up in the time that VM's were not ideal.....(kohms/volt) now they
are close to perfect and don't discharge the cap that dramatically. And
yes, for low voltages and high freq's schottky's are better. Forgot
about that, its 25 years ago since I worked with them.


Johan 
> 



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