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

Johan johan.vanderkolk at dommel.be
Sat Jul 5 11:59:09 UTC 2008


On Sat, 2008-07-05 at 02:38 -0700, Alen Edwards wrote:
> 
> 
> Johan wrote: 
> > On Fri, 2008-07-04 at 22:16 -0700, Alen Edwards wrote:
> >   
> > > James wrote: 
> > >     
> > > > Roo wrote:
> > > >   
> > > >       
> > > > > 2008/7/5 James Lockie <bjlockie at lockie.ca>:
> > > > > 
> > > > >   
> > > > >     
> > > > >         
> > > > > > There is 0 volts on the RCA.
> > > > > > 
> > > > > > Why do I get sound if there is a zero volt signal?
> > > > > > I tested my voltmeter on a battery to verify it works.
> > > > > >     
> > > > > >       
> > > > > >           
> > > > > James,
> > > > > 
> > > > > 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
> 



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