Broadcast TV output

Mike Green

I've been trying to get the VHF broadcast video output to work and, basically, the signal appears to be very weak. I'm using the TV driver v1.1 and tv_text v1.0 that I've modified to allow setting the DAT tv_params before starting the TV cog. I've tried both the default tv_mode and suppressing color (mode %11110). Since channel 3 is unused here, I'm using 61_250_000 for tv_broadcast. I'm not sure if that's correct, but the B&W TV I'm using has an analog tuner (a Sony Watchman). I've got a 4-5 inch wire antenna plugged into the TV output of the Propellor Demo Board. My program (a BOE-BOT /PING navigator) displays fine with video output, but with the broadcast output, there's a lot of noise on the signal (I'm not surprised at this), the Sony's antenna has to be essentially touching the TV output (I am surprised), and the sync levels don't seem to be adequate. The location of the signal (on the dial) seems to be off (although the dial is not particularly accurate anyway). Anyone have suggestions? Any prior successful experience with using broadcast output?

Ryan Clarke

When we had the VIC training at Parallax in one of the sessions we had a TV about 15 feet away pick up a signal without problem.

Ryan

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Ryan Clarke
Parallax Tech Support

RClarke@Parallax.com

Mike Green

Yeh, I know it worked in the training sessions. I did try changing the tv_pins value as well ... didn't seem to make any difference. I'm sure there are some caveats in using the broadcast mode of the TV object, but there's not too much information documented yet. Again, any suggestions would be appreciated. Fortunately, the video baseband mode works great, but I would like to play with the broadcast mode as well.

Beau Schwabe

Mike,

Perhaps you could post your code?

I will test this tomorrow, but a simple series LC should turn the 15ft Ryan is refering to into at least 150ft
without any problems.

                              ANT.   
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Propeller Video Out >---5pF----o----1.35uH---> GND

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

cgracey

The biggest problem you are seeing is the fact that 61.250MHz does not synthesize cleanly from an 80MHz NCO. Use 60MHz, instead, and the snow will go away. 60 is 3/4 of 80. Simple fractions like these synthesize well because the phase noise pattern repeats every few cycles, allowing the PLL to effectively filter it out.

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Chip Gracey
Parallax, Inc.

Mike Green

Thanks Chip. I had copied the frequency from some demo. The 60MHz works much better.

Beau, thanks also. I have a short loaded whip antenna for the 6M ham band that I'll use when I want to get some range. For now, a foot or two is more than enough and I can get that with the 5 inch "paper clip" antenna.

I'll post my code when I get a little further along. I want to get the HM55B calibration routine working ... may get to it this weekend.

Have a nice 4th of July weekend!

rokicki

The utility of the onboard frequency generator is really hampered by the poor quality waveforms it
generates when trying to generate "odd" frequencies such as 62.5 MHz.

Perhaps in the next version, we could get the choice between a quickly-stabilizing PLL, that
is good for things like FM synthesis and the like, and a slowly-stabilizing PLL, that is good for
carrier generation? For many applications, a PLL that takes a second to stabilize, but
generates a pretty clean spectrum, would be far more useful than a PLL that stabilizes
in a few dozen clocks but generates very jittery output.

Of course this may not be possible given the technological constraints and the amount
of switching going on inside the chip.

Another option is to somehow use multiple bits of the counter to help generate the
PLL error signal more carefully, but this quickly gets complicated.