[mythtv-users] HDMI_SPDIF directly to reciever

Alen Edwards allen.edwards at oldpaloalto.com
Sat Jul 5 09:38:50 UTC 2008



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.

Allen

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