Broadcast video quality with and without audio.

Phil Pilgrim (PhiPi)

I've been experimenting with broadcast video from the Propeller and just made an interesting discovery: The boradcast picture quality improves considerably if you add the 4.5MHz audio subcarrier. Initially I did this to mute the annoying audio white noise that accompanies broadcast video without the subcarrier. But then I noticed that the displayed video exhibited much cleaner vertical edges. I'll soon be posting an updated version of tv_wtext that allows for broadcast audio and adds the mute subcarrier if no audio cog is specified and if CTRA is free.

-Phil

VIRAND

I think that most 21st century TV tuners are very picky about the quality of the signal,
for example, many will show a blue screen in cases when
an older TV would show a speckly picture or black "Vertical Hold" bars.
Maybe your TV doesn't lock on frequency until it finds an audio subcarrier.

Phil Pilgrim (PhiPi)

As it happens, things are more complicated than my original post indicates. Although the display is always pretty crappy without the sound subcarrier, adding the subcarrier doesn't always improve things by a lot. I have a demo program in EEPROM that broadcasts with a mute subcarrier. I have only to press reset, restarting it, to see variations in picture quality. It may be some sort of phasing relationship between the PLL that generates the video carrier frequency and the one that generates the audio subcarrier.

-Phil

Bob Lawrence (VE1RLL)

Phil,

I tried Mike Green's mod last week and although it was a great start, I had a problem with locking in the signal(I believe that it was channel freq related). At least I could see the"Hello World" text scroll across the screen. I didn't have much time to play with it but I did try changing channel frequency's, using a longer ant ... etc. With your demo all I had to adjust is the vertical hold on the portable 5", B&W TV that I use for testing. I'll try it on a color TV later. I tested it on a Proto board with a 3" wire antenna. I only tried it with a distance of a little over 3 feet so far but the screen has great text quality and it's steady. Thanks! for the demo. Your timing was perfect.

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Aka: CosmicBob

VIRAND

Phil said...
It may be some sort of phasing relationship between the PLL that generates the video carrier frequency and the one that generates the audio subcarrier.

The aural subcarrier is normally FM so it shouldn't or can't have a phase relationship with anything else.

Perhaps the color subcarrier frequency is off a little and can't stay in phase with HSYNC.

Phil Pilgrim (PhiPi)

Right about the FM. In my experiments, though, the subcarrier was unmodulated. Also, the differences noted also occur without the color subcarrier. Nonetheless, I seriously doubt the audio subcarrier phase theory, myself, but can't think of another explanation for the divergent behaviors.

-Phil

VIRAND

You don't have a NTSC+PAL compatible TV by any chance do you? I don't know exactly how they work but
I just remembered that PAL color subcarrier frequency is almost the same as NTSC sound subcarrier frequency.

Phil Pilgrim (PhiPi)

No, it's NTSC only.

-Phil

VIRAND

Ok, how about this:
The Propeller is radiating RFI with the help of the antenna in addition to the video signal.
Processors generally do that because data pulse edges have lots of radio frequency noise.

By adding the sound carrier you are increasing the strength of the broadcast signal,
and the TV responds by lowering it's sensitivity to get the same signal input level
(Automatic Gain Control)
and that makes the RFI either half as significant or one fourth as significant to the TV.

If that's what's happening, maybe loading the antenna with a tv balun or a resistor to ground
(probably between 75 and 300 ohms) might kill most of the RFI, hopefully not your signal.
A tv balun is also known as a "coax to antenna wire adapter" or a "75 ohm to 300 ohm transformer".
The simplest thing to do would be try connecting just one side or the other of the balun first,
although it's designed to have the antenna (such as rabbit ears) connected to the opposite side
which should be the 300 ohms side. Although they're relics from the 1970's, perhaps you do have a balun
and an extra pair of rabbit ears lying around.
If not, it's much more likely you have a resistor to try loading the antenna with.
(It would be a waste to go buy a balun and rabbit ears if this won't help. Radioshack probably still sells them.)

Phil Pilgrim (PhiPi)

I've been looking for the extra rabbit ears. I know they're around somewhere...

It's interesting that rabbit ears are provided with 300-ohm twinlead and baluns, since the characteristic impedance of a dipole antenna is 75 ohms to begin with. Folded dipoles, like the old rooftop units from the '50s, are 300-ohm antennas.

I tried Beau's 5pf-cap/1.35uH-inductor antenna booster (from this thread), which is supposed to increase signal strength. But I wound my own coil, guessing at the number of turns. That never works, and neither did the antenna. (See photo below.)

While it would be nice if Parallax or someone else offered such an antenna prebuilt, I don't expect it will happen. That would be giving their imprimatur to video broadcasting, which would run counter to FCC regs (unless it could be accommodated under Part 15). After next February, there won't be any new analog TVs anyway. So this whole endeavor to broadcast video with the Prop will soon be mooted. (The same does not apply to baseband analog video, however. The CCTV market is alive and kicking.)

Another thing I've noticed with broadcast video is that the color saturation — even with a strong signal inches from the receiving antenna — is poor compared with baseband.

-Phil

bambino

Starting out with TV's and the propeller was a futile thing for me , because nothing I did got me anywhere. I gave up thinking it was just the cheap TV's I had. For some reason I dug it back up and tried broadcasting and it worked. Thats the only way I can get a half decent screen on these B/W TV's.

I later aquired a monitor, with no antenna. Some sort of camera monitor I assume, and it did do baseband.