jjh76
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3B+ trrs jack audio voltage level

Mon Aug 06, 2018 12:41 am

I'm using a TIPI with my TI-99/4a. The TI-99/4a has an audio input line on it's expansion port. Here is a link to the data sheet for the sound chip it uses:

http://www.unige.ch/medecine/nouspikel/ti99/tms9919.htm

How much power is sent through the trrs jack on the 3b+? Going by this data sheet, would it be safe to connect it directly to the audio input line on the TI, or would I need to put some kind of level shifter between them?

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Burngate
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Re: 3B+ trrs jack audio voltage level

Mon Aug 06, 2018 8:51 am

Have a look at https://www.raspberrypi.org/documentati ... educed.pdf
Almost dead centre is the audio output.

The signals are approx. 1v p-p centred on ground, source impedance ~70R
If your TI chip can handle that, you're ok.

klricks
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Re: 3B+ trrs jack audio voltage level

Mon Aug 06, 2018 1:33 pm

jjh76 wrote:
Mon Aug 06, 2018 12:41 am
I'm using a TIPI with my TI-99/4a. The TI-99/4a has an audio input line on it's expansion port. Here is a link to the data sheet for the sound chip it uses:

http://www.unige.ch/medecine/nouspikel/ti99/tms9919.htm

How much power is sent through the trrs jack on the 3b+? Going by this data sheet, would it be safe to connect it directly to the audio input line on the TI, or would I need to put some kind of level shifter between them?
The RPi uses standard line level audio as found in TV or other audio equipment. Red and white RCA jacks.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Stretch w/ Desktop OS.

drgeoff
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Re: 3B+ trrs jack audio voltage level

Mon Aug 06, 2018 5:47 pm

klricks wrote:
Mon Aug 06, 2018 1:33 pm
The RPi uses standard line level audio as found in TV or other audio equipment. Red and white RCA jacks.
There isn't any such thing as a standard line level for audio on consumer equipment. But most are somewhere between 200 mV and 2 volts.

hippy
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Re: 3B+ trrs jack audio voltage level

Mon Aug 06, 2018 8:26 pm

I believe line-level is something like 2V peak-to-peak. The Pi appears to be somewhat half that.

Plugging my Pi analogue audio straight into the connections of my HiFi the CD player usually uses; music felt less than half as loud. I could comfortably turn the volume 'up to 11' when I couldn't do that with the CD player.

drgeoff
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Re: 3B+ trrs jack audio voltage level

Mon Aug 06, 2018 10:52 pm

hippy wrote:
Mon Aug 06, 2018 8:26 pm
I believe line-level is something like 2V peak-to-peak.
Professional audio equipment typically defines "Line Level" as 0 dBm into 600 ohms balanced. 0 dBm is 1 milliwatt. From Power = V*V/R that corresponds to 775 milliVolts RMS into 600 ohms. Multiplying by '2 root 2' yields the peak-to-peak figure 2.19 volts.

Consumer gear does not use 600 ohm balanced inputs and outputs. They are unbalanced. Input impedance is usually at least several thousand ohms. Output impedance is generally much lower than input impedance. Voltage level is not standardised even between products from the same brand. Manual controls or auto level controls usually have enough range to cover the differences.

jjh76
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Re: 3B+ trrs jack audio voltage level

Tue Aug 07, 2018 5:30 am

The data sheet for the chip says the audio pin has a max input of 0.9v/1.8mA, with a 330 Ohm resistor on the line between it and the pin on the expansion port. If the PI only puts out 1v then should it be ok?

Looking at the schematics, the tip is video, the two center rings are audio, and the last ring is ground?

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Burngate
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Re: 3B+ trrs jack audio voltage level

Tue Aug 07, 2018 7:49 am

No. Tip = left, ring = right, ring = ground, sleeve (you called last ring) = video.

Raspberrypi-spy has a good description (as with many things)
https://www.raspberrypi-spy.co.uk/2014/ ... ideo-jack/

fruitoftheloom
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Re: 3B+ trrs jack audio voltage level

Tue Aug 07, 2018 8:39 am

Burngate wrote:
Tue Aug 07, 2018 7:49 am
No. Tip = left, ring = right, ring = ground, sleeve (you called last ring) = video.

Raspberrypi-spy has a good description (as with many things)
https://www.raspberrypi-spy.co.uk/2014/ ... ideo-jack/

Model-B-Plus-Audio-Video-Jack-Diagram.png
Model-B-Plus-Audio-Video-Jack-Diagram.png (22.68 KiB) Viewed 382 times
Adieu

jjh76
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Re: 3B+ trrs jack audio voltage level

Wed Aug 08, 2018 6:42 am

So, If I were to use a standard stereo or mono audio cable the video would just go to ground?

Also, if I were to combine the stereo audio outputs into mono would that double up the power level on the mono line?

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rpdom
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Re: 3B+ trrs jack audio voltage level

Wed Aug 08, 2018 7:47 am

jjh76 wrote:
Wed Aug 08, 2018 6:42 am
So, If I were to use a standard stereo or mono audio cable the video would just go to ground?
Yes. The video circuit is designed to cope with that.

I'm not sure that a mono cable will be ok. Probably, but I'd have to check the output circuit to make sure.

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Burngate
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Re: 3B+ trrs jack audio voltage level

Wed Aug 08, 2018 7:58 am

jjh76 wrote:
Wed Aug 08, 2018 6:42 am
So, If I were to use a standard stereo or mono audio cable the video would just go to ground?
Yes. It's designed to be used that way - with standard earbuds. The video circuit can happily be grounded without harm
Also, if I were to combine the stereo audio outputs into mono would that double up the power level on the mono line?
No. The voltage level will stay about the same, and so will the power.
You'd get the "sum" of left and right - dead centre of the stereo stage, and lose the "difference" - what's happening in the wings.

Putting a mono plug in the socket would short the right signal, but again it would be quite happy.

Shadow_7
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Re: 3B+ trrs jack audio voltage level

Thu Aug 09, 2018 4:27 am

When I was a kid, I would audio out from various devices to a line in. The trick was to put volume at about 10%. Not that I would do that to anything expensive. And I somehow burned out a midi channel on an old soundblaster awe32 by doing similar things. Which freaked windows out a lot, but linux was fine with the 7 of 8 remaining midi channels.

jjh76
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Re: 3B+ trrs jack audio voltage level

Thu Aug 09, 2018 6:09 pm

Thanks guys, this helps. I'll try to hack something together and do some tests this weekend.

jjh76
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Re: 3B+ trrs jack audio voltage level

Sun Aug 12, 2018 5:35 am

OK, connected a mono audio cable to the audio in on the TI. Playing an mp3 on the PI with omxplayer the sound chip on the TI passes it through to the TI's audio out with no apparent problems so far.

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