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TVP5150AM1 losing lock

Drew Rainwater
Intellectual 440 points
Other Parts Discussed in Thread: TVP5150AM1, TVP5150

Hello, I'm using a TVP5150AM1 to decode a composite NTSC video signal that is being transmitted/received over an FM RF link.  Occasionally, I see the Lost Lock Detect bit in Status Register #1 go high, along with a mixture of losing the color subcarrier, vertical sync, or horizontal sync lock (one or more of their status bits go low).

Things work perfectly fine if a direct-wired camera is used instead of the FM RF link.  I realize this may indicate the real problem lies in the RF link, but I'm looking for any possible ideas that I can try in getting the TVP5150AM1 to deal with the video from the RF link, since it's required in the application.

I've tried forcing both TV and VCR modes, without change.  I've also tried forcing NTSC-M mode, again without change.

I've also tried an anti-aliasing filter *close* to what's described in the app note, with 100pF caps and a 10uH inductor (the app note suggests 180pF caps and a 2.7uH inductor).  These were the closest values I had nearby--I'm ordering the exact parts but don't have them yet.

Can anyone suggest any other settings to look at and/or modify to see if the decoder can make better use of the "noisy" video?

(I should add that this testing is happening well within the range of the RF link; in other words, I'm not experiencing the lock problems due to the RF link being out of range.)

  • 0
    TI__Genius 15310 points

    Hello Drew,

    I think there may be a disruption in the TVP5150 input and that cause the problem.  Have you try to verify the input by connect the input to a TV to see its behavior?  The TVP5150AM1 can lock to weak signal but if there is no sync or non-standard, it may lose lock.

    Regards,

    Viet

  • 0 in reply to Viet Dinh
    Intellectual 440 points

    Hi Viet,

    Sorry, I should've been more clear in my original post.  If I run the output of the RF link to an NTSC monitor, I don't see the loss of lock.

    Drew

  • 0 in reply to Drew Rainwater
    Intellectual 440 points

    Also, if I continuously poll the status registers, I occasionally see it report changing from TV mode to VCR mode, or vice versa.  Also, I occasionally see that the AGC and offset frozen status switches back and forth, although I'm not sure if/when this should happen.

    Drew

  • 0 in reply to Drew Rainwater
    TI__Genius 15310 points

    Hi Drew,

    Do you use any patch for the TVP5150AM1?  If you do, what version are you using?  If you don't, can you download and use the latest patch v048CAA from this link:

    http://software-dl.ti.com/dsps/dsps_public_sw/dsps_swops_houston/ANALOG_VIDEO/Analog_Video_Decoder_Versions.htm

    Regards,

    Viet

  • 0 in reply to Viet Dinh
    Intellectual 440 points

    Thanks for the suggestion, Viet.

    I haven't applied any patches.  Reading the Major/Minor Version registers, I get 0x04 and 0x00.  I assume that means I need the patch?

    Drew

  • 0 in reply to Viet Dinh
    TI__Genius 16810 points

     

    This could also be related to the tuner being used and its source impedance and output drive strength.   The TVP5150AM1 internal clamp circuit is designed for a 75-ohm source impedance and 75-ohm end termination.  If the source impedance is too high, the TVP clamp and lock performance will degrade.  Some designs have used a buffer between the tuner and TVP to resolve this.

  • 0 in reply to Larry Taylor
    Intellectual 440 points

    Hi Larry,

    Thanks for the suggestion.  I'm trying Viet's suggestion of patching the firmware, but I'd also like to follow-up on yours.

    Is there something indicative I can look for to see if the RF module's output impedance is too high?  I'm using the recommended two 37.5-ohm resistors in a divider at the input of the TVP5150.  When I look at the video signal after the DC-blocking cap at the input of the TVP5150, the sync tip is very close to ground DC-wise.  Does that sound correct?

    Drew

  • 0 in reply to Drew Rainwater
    TI__Genius 16810 points

    Drew,

    If you are terminated with 75-ohms and the video level after divider is near 500mV, you should be OK.  If the levels are severely attenuated, then you may have a source impedance/drive issue.  Yes, the sync tip should be clamped to near ground.

    Larry

  • 0 in reply to Larry Taylor
    Intellectual 440 points

    Thanks, Larry.  Yes, I'm seeing ~0.5-0.6V peak-to-peak at the input.  I saw something in the release notes for the newest patch that said something about sync tips that were clipped, so I wanted to make sure the sync tip should be that close to ground anyway.

    Thanks for the help.

  • 0 in reply to Drew Rainwater
    Intellectual 440 points

    Hi Viet,

    OK, I've updated the TVP with the patch and verified that I see 0x048C returned as the version number, but I have a quick question on the explanation of "finalizing" the patch in the Guidelines document.

    Do I always (after each hardware reset) need to do Step 5 (Unlock, write 0x05 to address 0x10, Lock) to get the TVP to run the patched firmware?

    Or should I only have to do this step once, and then it should always run the patched firmware automatically on power-up?

    Thanks.

    Drew

  • 0 in reply to Drew Rainwater
    TI__Genius 15310 points

    Hello Drew,

    every time you do system reset, you must redo all the steps which is download patch, etc...

    If you do a soft reset (active low pulse to pin-8), download patch again is not required but you have to do Step 5 to run patch.

    Regards,

    Viet

  • 0 in reply to Larry Taylor
    Intellectual 440 points

    Hi Larry,

    I've been looking more at the circuit between the RF module and the TVP.  Originally, I had a 0.1uF cap feeding into the TVP's video input (AIP1A), like the Application Example in Section 6.1 of the datasheet.  Substituting this cap for a much larger value, like 470uF or even 1000uF helps things out a great deal.  Can you elaborate on this or give me an idea why the Application Example suggests a much smaller value?  Is that cap serving another purpose other than DC-blocking (and the DC-blocking cap is assumed to be off the page)?

    Thanks in advance.

    Drew