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Thread: What's the right way to make a TV (audio-video) audio-out circuit for the Prope

  1. #1

    Default What's the right way to make a TV (audio-video) audio-out circuit for the Prope

    Isn't there some specification like you're supposed to stay within 0 to 1 volt? Or can I just use the same circuit I'd drive a speaker with but with the volume down? Do I need a particular size resistor somewhere?

    I thought I'd just use the same setup the demo board uses to connect to the max4411 headphone amp, but run it through a 1K pot instead. Will this work?
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  2. #2

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    A resistor from an output pin of a value in the 200Ω range (220 should be ok), the other end of the resistor is attached to a 0.1F capacitor whose other end is attached to ground. A 10F capacitor tied to the node the resistor and other capacitor are on with it's other end tied to the tip of the RCA plug. Finally the outer portion of the RCA plug is tied to ground.

    So the circuit makes a T:

    Code:
    Pin --- 200Ω ---- 10F --- RCA tip
                   |
                   |           RCA outer
                  0.1F           |
                   |              |
                   |             GND
                  GND

    Its not the optimal circuit but it will work well enough.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  3. #3

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    Would you be able to connect the audio amp to a TV also?
    RoboGeek

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    Adam Savage, Mythbusters
    www.parallax.com
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    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  4. #4

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    Paul,

    1. Is it appropriate, do you think, for the 10uF cap to be polarized, with the + end "facing" the Propeller? In other words, can one always assume that a line-level audio input will be ground-referenced?

    2. Or do you recommend a non-polar cap?

    3. What about just leaving the series cap off altogether? Most line-level audio inputs are AC-coupled anyway -- um, right?

    The reason for these questions is that I designed the MoBoProp with such an audio output, but left off the series cap (tight real estate). Feeding the signal into a TV seems to work fine. But maybe I just got lucky.

    Thanks,
    Phil
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  5. #5

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    I would use a non-polarized tantalum, but I cant say for sure a polarized cap wouldn't work. I typically only use such caps for regulation though.

    Because the 200Ω is there it should be ok, if you tried to make the cutoff frequency higher for higher bandwidth audio output (circuit shown has a 3dB cutoff of 1/(2π R C) or 7957 Hz), I would definitely include the AC coupler cap, because you never know when someone will attempt to connect it directly to a speaker.

    Another thing to consider about connecting a speaker without the AC coupler cap, you are making a tank circuit, which can cause strange behaviour if the 1/2π sqrt(LC) frequency is in the audible range. The coupling cap keeps this from occuring.

    Its just one of those accounting for inexperienced users things.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.

    Post Edited (Paul Baker (Parallax)) : 9/22/2006 3:58:49 AM GMT
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  6. #6

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    Thanks, Paul. One final question: If one uses an NP tantalum, say, what voltage rating is appropriate for an app like this? Might an inductive load, such as a speaker, cause a higher than 3.3V drop across the cap?

    -Phil
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  7. #7

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    Hmm good question, since there's no DC component, I would think the amount of back emf would be managable. IOW I would begin with a factor of 2, using a 6V cap, a headphone as load and scope the waveform just to be sure before finalizing it on a PCB.

    Something interesting to note, Chip has connected high end speakers directly to the Propeller pins using the differential DUTY cycle counter mode. The cable lead and speaker windings act as a current integrator, and it sounds pretty good. Not something I would design into a system, but it's a testament to how well the Propeller was designed.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  8. #8

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    Thanks Paul! I wired up the circuit, it works great! I attached a picture of it and my keyboard and video out on my breadboard.

    I used a 220 ohm resistor, and an electrolytic with the + facing the Propeller. If it burns out, I can always just put a new one in the breadboard. I don't think it will be a problem though. In my experience, the smaller the capacitance, the less susceptible the electrolytic is to reverse voltage. When I was a kid, I actually tried to find creative ways to blow up capacitors, and I could never get the small value ones to explode. I've watched a 47 uF withstand 22 volts reverse bias indefinitely. On the other hand, wire a 2200 uF cap in reverse in a 12 volt power supply and you will get a hell of a bang. (I once had one go off next to my head while I was peering into the power supply.)

    When I went to test my audio out setup, I couldn't find much in the way of sound demos, so I wrote my own, which is attached to this post. Just some silly arcade-machine style sounds, but good for testing.
    Attached Thumbnails Attached Thumbnails Click image for larger version

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    Attached Files Attached Files
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  9. #9

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    What kind of caps are connected (on demo board)to INR, INL and between C1P & C1N?

    Can I use polarized tantalum caps? I tried to find non-polarized tantalum caps at digikey.com

    but I couldn't locate them.

    PVJohn
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  10. #10
    PJ Allen's Avatar
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    Polarized caps can be used to make an equivalent non-polarized cap (see attached.)

    ** Post Edit ** Yes, the Cap-meter agrees. [ Hey, would I lie to you? ]

    Post Edited (PJ Allen) : 12/10/2006 8:13:23 PM GMT
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    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  11. #11

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    This may be a silly question, but now that I have my propeller setup all built on a Protoboard what demo programs can I expect to hear sound from? While on the topic of sound are there any demos that use the Microphone?

    My propeller circuit is wired like the demo board with a couple exceptions. First i'm using a MAX3232 to connect it to the host which works fine now. The second is that instead of the MAX4411 headphone amp I was going to use the simple circuit that Paul Baker suggested above with the 200ohm resistor, .1uf cap, and 10uf cap going to the RCA jack. I'm assuming that the output from that should be able to go to the audio in on a TV of amplified speakers. Are there any other suggested ways to wire it up?

    I just need to test the MIC and Audio outputs and then i'm good to go for a bit until I start expanding it.

    Robert
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  12. #12

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    RobotWorkshop said...
    This may be a silly question, but now that I have my propeller setup all built on a Protoboard what demo programs can I expect to hear sound from? While on the topic of sound are there any demos that use the Microphone?

    My propeller circuit is wired like the demo board with a couple exceptions. First i'm using a MAX3232 to connect it to the host which works fine now. The second is that instead of the MAX4411 headphone amp I was going to use the simple circuit that Paul Baker suggested above with the 200ohm resistor, .1uf cap, and 10uf cap going to the RCA jack. I'm assuming that the output from that should be able to go to the audio in on a TV of amplified speakers. Are there any other suggested ways to wire it up?

    I just need to test the MIC and Audio outputs and then i'm good to go for a bit until I start expanding it.

    Robert
    Try my sound engine if you want sound.
    www.andrewarsenault.com/hss/

    Thanks!
    --Andrew Arsenault.
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  13. #13

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    Hello Andrew,

    That looks really cool! I'll have to try it out this weekend.

    I've tried all the canned demo's that display on the VGA and composite screens and was amazed to see what the chip can do! I just didn't know if I should expect to hear any sound from any of them or if any used the Mic input.

    I suppose what would be a nifty little utility (if one isn't written yet) is to alter a couple of the DEMO's to also use the Mic, output some sound, and perhaps display the status of P0-P7. This would make an excellent diagnostic tool for checking out the propeller DEMO board and also for anyone building up a similar circuit on a breadboard/protoboard to verify that all is well and functional.

    Robert
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

  14. #14

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    Here check this post out.
    http://forums.parallax.com/showthread.php?p=587520

    There is a test program that takes samples from the Microphone and outputs them to an audio pin.
    ADC and DAC test.

    This could help test out your boards microphone.

    --Andrew Arsenault
    Last edited by ForumTools; 10-01-2010 at 10:58 AM. Reason: Forum Migration

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