Use 4-line earphones to connect Banana Pro's AV port can hear the video signal?

hard

Recently , I used the 4-line earphones to connect Banana Pro's AV port, then heard a burst of other sounds, after some discussion, let us find the reason, come and go!

It is well known that the Banana Pro's AV port can output audio signal, but in fact the AV port also output video signal, which is a composite audio and video signal output interface, there are four level interface, please see below.

So if we use the TRS earphones, we won't have to worry about the question. The definition of the TRS terminal, which includes the left channel and the right channel and the ground from top to bottom, that is to say if you put the 3-line TRS terminal interface to Banana Pro's AV port, that is absolutely no noise (I have tried), because the video signal interface and the grounding interface are combined together in 3-line TRS earphone. SO, you will suddenly understand why 3-line TRS headset compatible with any intelligent mobile phone, and sometimes you use the 4-line TRRS earphones may not be able to use it on your mobile phone!

In fact, there are two kinds of interface definition standards of the 4-line TRRS earphones,OMTP and CTIA, OMTP is commonly known as the national standard, CTIA is the international standard.

Theoretically speaking, if we use the OMTP standard earphones to connect the Pro's AV port,we should not hear the noise, because the video signal is connected to the microphone interface of the earphone, so the circuit should not be affected.

But if you use the CTIA standard earphones, because the video signal is directly connected to the grounding end of earphones, the circuit signals will be affected, so this time you will hear a burst of sound in earphones.

The main difference between these two standards is that different definitions of microphone interfaces and ground interfaces. There are many adapter cable for conversion between the two standards.
In short words, Pro's AV port is designed to output a composite video signal, allowing users easy to use RCA interface display, so you should use a 3-line TRS earphone or AV cable(as follows) to connect to it.

Banana Pro Video Plug cable 2 in 1

And this

edfardos

Thanks for the details!   I just thought I'd share my observations using two different3.5mm cables with male on both ends.  One is TRRS the other is TRS:


TRS cable to Bose portable speaker = ticking unless you pull out the jack 3mm, then it's fine
TRRS cable to Bose portable speaker = ticking unless you pull out the jack 3mm, then it's fine
TRRS cable to Jawbone Jambox portable speaker = good
TRS cable to Jawbone Jambox portable speaker = good
TRRS cable to Altec Lansing H2O Jacket portable speaker = ticking unless you pull out cable 3mm
TRS cable to Altec Lansing H2O Jacket portable speaker = ticking unless you pull out cable 3mm
TRRS earphones (with mic, works on samsung phone) = ticking with audio on both sides, but no stereo separation


So given those observations, I'm still not sure what's going on.  The sleeve pin on the bpro jack isn't a real ground, it has a 1.5V DC offset from the earth ground.

Additional experiments:

tip to second ring = reduced volume, ticking
tip to sleeve = full volume no ticking

Ohms between outputs and case mounting screws (ground)
(T,R,R,S)
(1.5M, 1.5M, 14, 1.5M)

The last test suggests it's not OMTP, or the ground is isolated.

Now, if only I could find a TRRS to TRS(audio) cable with 3.5mm male on both ends.  That's the hard part.  I'll probably have to make one.  If somebody figures this out, please let me know.

thanks again!
-edfardos

edfardos

I just created a wire like this and it works.  It works perfectly other than it makes a horrible buzzing noise when the system is halted.

Bpro  Speaker
T   -      T
R   -      R
R        
S   -      S


best,
-edfardos

hard

edfardos replied at Jan 28, 2016 08:37
Thanks for the details!   I just thought I'd share my observations using two different3.5mm cables w ...

When did you buy this board? I recently bought. The sleeve pin on the bpro jack has access to the real ground  in the current version. I have consulted them and they said  their products have been updated relative last year. Perhaps their product previously had such a problem, you can look at this http://forum.lemaker.org/forum.php?mod=viewthread&tid=11412&extra=page%3D1

edfardos

The board I tested was purchased a week ago.  There is 1.5M ohms between the 3.5mm sleeve and the mounting holes (assuming they're ground).   Also, when the unit is on, there are  1.5Volts between the tip/ring/sleeve and ground, so you can't just ground it out without inducing a current, and a DC-offset in the audio signal.

I think it works great except for the fact that there's a loud hum over the speakers when the unit is powered down (no real ground).

Thanks for that thread info -- I was on that earlier.

-edfardos

edfardos

I did some more measurements between the ground pin on the AV jack and actual ground.  The reason it's confusing is because it changes.

Voltage between ground pin on AV jack and actual ground:

Unplugged:  0.00V
Powerup:  0.20V
GreenLed Blink:  1.50V
Short ground pin to actual ground:  0.00V
Unshort ground pin to actual ground: 0.00V
Power down (red led goes dim):  0.20V


So It's 1.5V until you short the ground pin to ground (ie plug something in that's grounded), then it becomes ground, and stays ground, even after you stop shorting it.

lawkmlaw

thanks for all the details

adolfalexander

Hypothetically, on the off chance that we utilize the OMTP standard headphones to associate the Pro's AV port,we ought not hear the commotion, Dissertation Writing in light of the fact that the video flag is associated with the mouthpiece interface of the headphone, so the circuit ought not be influenced.

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