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Tandy CoCo3 vs CoCo1 RF and Composite output comparison

voidstar78

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May 25, 2021
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258
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I've made a brief video showing some differences between the RF output of the CoCo3 versus CoCo1.

Indeed, the RF output of the CoCo3 is "cleaner" but the audio on the CoCo3 isn't quite the same as the prior CoCo's. Yes, I realize most CoCo3 users would use the Composite output -- but I've been working on a Daggorath complete play-thru, and I noticed the CoCo3 isn't playing that cartridge exactly how I remembered it (whether using Composite or its RF output). So, I dug into why that is.

Here is the link:
or search for the following title:
Tandy Color Computer RF Comparison CoCo1 vs CoCo3 using Daggorath Demo


Nothing Earth shattering here, just an interesting comparison. Being a 1982 cartridge, obviously they coded to the hardware available at the time (the CoCo1, since the CoCo2 wasn't released until 1983). It's very subtle, but to me the original CoCo1 "experience" of this particular title is more "creepy" (a good thing) than how it plays on the CoCo3 or any emulator.
 
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voidstar78

Experienced Member
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May 25, 2021
Messages
258
Location
Texas
For reference, here is my complete Daggorath playthrough using the original CoCo1 RF output:


No commentary, this was inline recorded to capture purely the CoCo RF output signal only (just for future prosperity). I'll try to add CC commentary next month as I get more time, because I think a few aspects need some clarification (conserving torch time, the drop-inventory strategy, why some walls are passable, order of using flasks, incantation and limited usage of rings, etc.).

NOTE: Also I was hoping the topic of RF would gain some interest in understanding how that worked (Motorola VDG), and why that caused issue for the TRS-80 Model 1 (re: jamming or causing interference), and its legacy related to the VideoTex "Green Thumb" terminal. Couldn't someone make a "wireless audio/video" CoCo? (making it effectively a small broadcast station -- not recommended since CH3 ~60MHz is probably now regulated, and for the same reason FPV racing drones are required to have limited transmitter power). I'm curious how the TRS-80 Model 3 video works -- it's not an internal RF/TV tube, correct? After the PET and TRS-80 Model 3, it seems like a "step backwards" to go to "internally converted RF." But I understand it was a strategy to reduce cost by adapting to existing home television tubes. It seems like wireless transmission of video on analog signals is a sort of lost art (except I've flown a DJI Mavic and how they've achieved such pristine wireless video quality is very amazing).
 
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voidstar78

Experienced Member
Joined
May 25, 2021
Messages
258
Location
Texas
Also, I noticed during ZSAVEs, I could hear the save-game audio on the RF Output signal. Back in the 1980s I originally played Daggorath on a CoCo2, and I don't recall hearing the tape-audio during ZSAVEs -- so is this "bleed through" from the cassette output port to the RF an aspect of the CoCo1? Or maybe I'm remember it wrong and that's just how it always was. But I noticed that audio is sometimes loud and sometimes faint, which makes me think something else is going on to cause this audio bleed into the RF signal.

I guess as an experiment I can try SAVE on a long BASIC program (on the CoCo1), see if the same thing happens there. But looking at the Daggorath assembly for SaveToTape section, nothing stands out on why a signal sent to the cassette output pin would bleed over to the RF Output signal.
 

wmcbrine

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Joined
Nov 27, 2019
Messages
41
Location
Maryland
Couldn't someone make a "wireless audio/video" CoCo? (making it effectively a small broadcast station -- not recommended since CH3 ~60MHz is probably now regulated, and for the same reason FPV racing drones are required to have limited transmitter power).
"Now"? It's always been regulated. The RF modulators of the late 20th century were allowed because they were very low power, and were (supposed to be) connected by cables that didn't leak RF. But yes, you can trivially turn any of them into short-range broadcast stations -- just run the output through an amplifier, and then to an antenna (connected to the amp's "out" rather than "in"). Voila. It's illegal, but no one is likely to care, unless you get carried away.

I'm curious how the TRS-80 Model 3 video works -- it's not an internal RF/TV tube, correct?
No, it's essentially an internal composite monitor. That is the same as a TV tube, but there's no RF involved.

An RF modulator (as built into home computers, VCRs, or as a standalone device) takes a composite video signal and an audio signal, and puts them together onto a TV broadcast frequency, mainly for purposes of connecting to old TVs with only an antenna input. More modern displays have direct composite and audio inputs, so they can skip the lossy modulate/demodulate step.
 

wmcbrine

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Nov 27, 2019
Messages
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Location
Maryland
P.S. The Model I doesn't use RF to interface with its monitor, either (it uses composite, plus a separate 5V line). It just happens to be sensitive to / generative of RF interference, unintentionally. Among the TRS-80s, only the CoCos have actual RF outputs, AFAIK.
 

Eudimorphodon

Veteran Member
Joined
May 9, 2011
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4,403
Location
Upper Triassic
No, it's essentially an internal composite monitor. That is the same as a TV tube, but there's no RF involved.

It's actually not composite, it has separate H/Vsync and video lines.

(* Actually they may be direct H/V *drive* lines now that I think about it, like on the IBM 5151, with no independent sync circuitry on the monitor drive board, but I'm not 100% sure of that.)

P.S. The Model I doesn't use RF to interface with its monitor, either (it uses composite, plus a separate 5V line). It just happens to be sensitive to / generative of RF interference, unintentionally. Among the TRS-80s, only the CoCos have actual RF outputs, AFAIK.

Indeed. Just to expound on the Model I, it did use a literal TV chassis for its monitor (if you pry off the plastic piece with the TRS-80 logo on the front of the US model's monitor you'll actually see the two holes where the tuner knobs went), but Radio Shack deleted the tuner (which was about the most expensive part of an old-school B&W TV) and chucked together a really primitive interface buffer board to go in its place. There's no way Radio Shack could have supported 64 column text on the Model I if they hadn't pulled the tuners out.

You do see in old magazines people griping about Radio Shack using "TV Quality" tubes in the Model III, but when they say that they're referring to how they used inexpensive P4 white phosphor tubes (which were indeed the same used in consumer B&W TVs) instead of the long-persistence green that was becoming standard on "business" computers. RF modulation has nothing to do with the tube.
 
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voidstar78

Experienced Member
Joined
May 25, 2021
Messages
258
Location
Texas
Just a quick update: added CC commentary to the playthrough video described above - to give some context of what's going on, for those who may not be familiar with the game.
 
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