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

Alen Edwards allen.edwards at oldpaloalto.com
Sat Jul 5 12:30:42 UTC 2008



Johan wrote:
> 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
>   
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

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