[NBLUG/talk] Raspberry Pi post-talk tech update

ac at sonic.net ac at sonic.net
Sat Jun 16 17:47:53 PDT 2012


Greetings,

I wanted to post an update on the technical difficulties from my recent
Raspberry Pi talk (which I've been posting about far too much lately - I
promise to only send one more E-Mail about slides after I give the talk at
BALUG this Tuesday :).  In summary, there were two main issues encountered
during the live demo.  First, we could not get a reliable image on my TV
through HDMI and second, when we switched to composite video we
experienced lockups every time we tried to play anything with audio.  As
you might imagine, this made the live demo far more eventful than we were
hoping.  During the talk both Kyle and I unsuccessfully tried combinations
of our respective Raspberry Pi's and power supplies as well as different
HDMI cables (one of which was delivered mid-talk by the wife of someone in
attendance - thanks, by the way! :).

The composite video problem was fairly straightforward after doing some
quick research - when video is going through the composite jack and no
HDMI device is connected, the audio signal currently needs to be manually
set to output through the analog jack instead of HDMI to avoid a lockup. 
I've confirmed I can play HD video through XBMC with the video going out
the composite / RCA jack after making this change.

The HDMI issue was far more interesting and additional testing at home
produced some interesting results that I wanted to share.  I have an Onkyo
model TX-SR506 sound system which has an HDMI switch in it.  If the
Raspberry Pi is connected through this receiver as it usually is in my
house, the image on my Vizio TV model VOJ320F1A is just fine.  However, if
the Raspberry Pi is connected directly to the TV, the display shows "No
Signal!", "Not support!" [sic], or has a very poor image with flashing
lines.  I could not find any information on this so I started doing some
independent research.

The power supply I bought for my Raspberry Pi is rated at 5V, 1.0A; when
connected to the Raspberry Pi under normal load, the test points read a
voltage of 4.65 which is less than the recommended minimum of 4.75V.  If I
use my N900 charger which is rated at 5V 1.2A the measurement is 4.78V.  I
observed that using the better N900 charger did not improve the quality of
the signal on the TV so I did some measurements on the HDMI output voltage
while using the N900 power supply.  With the flat part of the end of the
HDMI cable pointed up I measured between the outside casing and the +5V
pin on the lower-right; coming directly from the Raspberry Pi, I get 4.75
volts, but when the signal is routed through my receiver the measurement
is 5.01 volts.  The inferred conclusion is that my specific TV cannot
handle this reduced voltage and that my receiver's HDMI switch is boosting
the voltage.  I've tested two other models of Vizio TV's with my Raspberry
Pi and have had no trouble.

I hope this information is of use.  Please feel free to contact me if you
have any other questions or want me to try any other tests.  Thanks!

A.C.
******




More information about the talk mailing list