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I've been doing more reading about the early 8-bit home computers, a fascinating era, in which the cost of the hardware was of course a constant, dominating constraint, and trying to understand exactly how various costs drove design decisions.

To that end, and given that many home computers of the era connected to a TV set (to save on the cost of a separate monitor), and looking at the very cheapest monochrome-only machines like the Sinclair ZX81, versus the slightly more sophisticated color machines like the Spectrum and MC-10,

How much extra manufacturing cost did it add, in the late 70s and early 80s, to use a color RF modulator, as opposed to a black-and-white one? Now, I am not talking here about the digital side of things at all, not the cost of the RAM to hold color data, or a video chip capable of doing interesting things with it (even though this could certainly be significant). Just the cost of the RF modulator itself.

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    BTW, the cheapest color RF modulator at that time (at least where I live, which is not the US) was the VCR you already owned anyway. There were also TVs that didn't require an RF modulator.
    – dirkt
    Commented Jun 6 at 6:03
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    IIRC having the ability to modulate sound was an extra-cost option, requiring a physically larger modulator. I never studied in detail what was inside (to my shame).
    – Frog
    Commented Jun 6 at 8:15
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    " the cost of the hardware was of course a constant, dominating constraint" ==[ It still is, or I wouldn't be on this crappy laptop Commented Jun 6 at 12:37
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    This can get confusing because (at least for NTSC) the signal which would ultimately get modulated up to RF is composed of a monochromatic raster luminance signal combined with a color component; that color component consisted of hue and saturation values which respectively phase and amplitude modulated a 3.58MHz carrier. BTW, the entire video baseband signal could be output on a single "RCA-style" connector for input to any display device having a corresponding input.
    – Anthony X
    Commented Jun 6 at 18:03
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    From what I remember of the era, by the time consumer-friendly computers like the VIC-20, C64, etc. arrived on the market, the cost concerns focused more on other factors like processing speed, color depth, display resolution, working memory and disk storage; when it came to TV-compatible display output, color was a given and any opportunity to save a few pennies to go monochrome-only was irrelevant.
    – Anthony X
    Commented Jun 6 at 18:14

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An RF modulator is an RF modulator. It just modulates whatever signal you feed to it. There should be no difference between RF modulators that either can or cannot do colour, as it does not care if the incoming video signal has colour or not.

Based on that, there is no extra cost.

It's really about the video chipset what kind of signal it outputs to the modulator.

To clarify a bit, the modulators generally don't take in analogue RGB signals, but the same composite video signal that is already generated for the TV, and the composite video is either colour or grayscale depending on how the composite video is made.

If video chip generates RGB colour component, it can be encoded into greyscale luma Y only or YUV colour component.

The video chip can also generate greyscale Y or colour YUV component video directly

The greyscale Y can be combined with syncs and thats greyscale composite video out.

The device can use a colour encoder chip which takes in either RGB or YUV, and encodes the YUV signals into colour composite signal.

However just to mention an exception how things can be made differently, in C64, the VIC-II chip does not generate RGB or YUV, but directly the luma signal Y and the chroma signal C, which already modulates all the colour info, i.e. UV component signals into single signal. And the RF modulator simply takes in the S-Video-alike YC signals, buffers them for the Y and C output on the connector, but internally sums them together to form a single colour composite signal that is then modulated and sent out as RF.

The upside is that if you have a monochrome video monitor or monochrome TV with composite video input, you can simply use the monochrome luma output which provides sharper picture than displaying colour composite video on a monochrome device.

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    I guess it's feasible that some modulators might not have the requisite bandwidth to carry the chroma subcarrier (which at least in PAL, sits out as a vestigial sideband above the luminance signal). I'd be surprised if they didn't simply disappear when colour became widespread, though. Commented Jun 6 at 15:35
  • @TobySpeight But same applies to broadcast signal too, so problem is not unique to RF modulators in home computers. And it's the modulated hroma signal whose bandwidth is so large that part of the upper sideband is cut - not the subcarrier itself.
    – Justme
    Commented Jun 6 at 16:27
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    Not sure what you mean. The modulators chosen by broadcasters clearly have the bandwidth for the signals they intend to transmit. And yes, I did mean the entire chroma signal - I just used the word "subcarrier" as a shorthand for its contribution to the composite signal. In any case, the path through the baseband stages of the modulator needs a wider range to carry a composite signal without distortion than it does for a monochrome signal. Commented Jun 6 at 16:36
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    @TobySpeight You might have a point. In order to have fast enough edges for the sync signals, you need high enough bandwidth to make fast enough edges. For PAL, there's about 230 subcarrier sinewave cycles per visible scanline, or roughly 460 lines (pixels) of horizontal resolution. The sync signals need slew rate of 200ns, equaling about 1.75 MHz of bandwidth, which is hit at 224 pixels per line, of which less than 182 visible, would have high enough bandwith for video but not color.
    – Justme
    Commented Jun 6 at 17:08
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A bit of googling finds this for the Apple II:

Dealers would sell an Apple II with a “Sup’R’Mod” (costing about $30) if the buyer wanted to see the graphics on their color TV.

Also, I don't think there would be a difference between "black and white RF modulators" and "color RF modulators", basically all they had to do was to RF modulate the composite signal.

And as mentioned in the comment above, if you already had a VCR at home, those usually had a composite input, so then you didn't need an additional RF modulator, except for convenience. And in particular after VCRs became common, many TVs started to have a composite input.

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    That's more about price than cost, I think. What consumers there (where?) were willing to pay (what kind of $, and when?). Commented Jun 6 at 15:37
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    In the early days of the Apple II (1978/79), most people used the Sup’R’Mod with a color TV. You could mount it inside the Apple II case. Yes, I remember it being about $30. Home users rarely used B&W monitors. Color monitors were too expensive for most people. Around 1980, I bought a crappy $400 color monitor, basically a cheap TV without a tuner. These were not popular.
    – Mattman944
    Commented Jun 7 at 1:59
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    cf. en.wikipedia.org/wiki/Sup%27R%27Mod, which lists the price as $29.95 in 1978.
    – fadden
    Commented Jun 7 at 4:14
  • Just use the one that came with your Atari.
    – Mazura
    Commented Jun 8 at 16:25
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I bought an RF modulator for my compaq deskpro, which had composite out. I was delighted when the computer that had only an amber text monitor suddenly had a color tv that I could play games on. the first thing I did was add a statistics panel to lunar lander.

It was the coolest computer I had seen since the 1108 mainframe.

It cost $20 at radio shack, so I imagine adding it to a new PC would only cost about 10.

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    Can you add some context to that "$20"? At what time, and which currency? (there are several that use $ as the symbol) That would help make it more comparable. Commented Jun 6 at 15:33
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    @JessFuckett: many visitors of this site (me included) use, maintain and invest into old technology, even though it's obsolete. We don't have any means to know when you spent these 20 (apparently) US dollars, and I had to google what "radio shack" meant. I realize that it's obvious to you, because it was your $20 invested into your Deskpro during the course of your life, but it's not obvious to me and other users. If you specified the year and the country (and, if possible, the model of the Deskpro), it would help me understand your answer better. Could you please specify them? Thank you.
    – Quassnoi
    Commented Jun 6 at 18:31
  • It's certainly not the case that all $ are "about the same." I don't have historical rates to hand, but given there's well over two orders of magnitude between the first pair I looked up today (JMD and USD), your "$20" is likely just as meaningless in the 1975-1990 range we're considering, too. Commented Jun 7 at 10:04
  • If it's "manifestly irrelevant", why even bother answering a question that asks exactly that? I'm interested. Commented Jun 7 at 10:07
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    Someone deleted all my comments but left the replies to my comments. See, this kind of inexplicable, mysterious, negative crap happens to autistic people. It's why I walked off my nuc job and lived in the woods for 3 years. Commented Jun 8 at 10:12
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From what I recall of the time, color vs monochrome was never a consumer-facing option when buying any of those systems. If we consider only home/hobby computing, devices offered RF output on either 75Ohm coax connector or 300Ohm twin lead for connection to the corresponding antenna input of a TV and/or unmodulated video output on an "RCA" style connector for connection to a monitor (although TVs came on the market with compatible inputs). Consumer-level computers like the C64 would have had "composite" output (one baseband video signal which for NTSC consisted of a luminance signal mixed with a 3.58MHz chroma carrier amplitude and phase modulated with saturation and hue respectively). Since devices like the VIC-20 and C64 based their appeal on color graphics, it would not have made sense to offer a version with monochrome-only because the cost saving for that sacrifice would have been far too small (at most a few dollars, perhaps only pennies).

I think a good way to look at the early evolution of these devices is that when they first appeared, most homes already had color TVs. Most of the cost of these devices was in memory, so monochrome video was more about economizing on memory than anything else. When the first color-capable computers appeared, they were more capable in many respects, largely rendering all their monochrome predecessors obsolete, so a side-by-side color vs monochrome "all-else-equal" was never a possible comparison.

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The machines of that period generally used composite video, not RGB.

The core of the composite signal was the intensity, and with "blacker than black" being used to signal horizontal and vertical retrace.

There were also two subcarriers for video and audio.

The 3.58mhz (ok, 3,579,545 hz) subcarrier was for color. It shifted slightly in frequency to indicate color, while audio was on a 4.5mhz subcarrier, which was essentially FM radio.

Merging those carriers into the signal happened before going to any RF modulator.

Conceivably a modulator could have been built that couldn't handle the bandwidth for those, but I never met one. So any modulator that could pass sound could also pass color.

The issue wasn't so much the modulator being able to handle color, but whether the television could: the surplus electronics store I worked with fed the signal from the apple ][ into its multiplexer in the room full of new televisions. Only one, a 13" quasar, handled the color hires graphics without dropping into black and white!

Apple kind of cheated to get color--there were seven intensity bits to the word, with the eighth used to shift timing by about half a bit, tickling the color trap of the television/monitor into producing 6 colors!

This was possible because the apple ][ clock was formed by dividing the frequency of a crystal of an exact multiple of the colorburst (~14mhz, iirc), which left it passing exactly two pixels per colorburst cycle, allowing the tickling.

In short, no, no separate modulators for color & B&W, so no price difference.

The Supermod modulator, the most common (and iirc, best) at the time was originally part of the apple ][ design, but had it been included, the machine wouldn't have passed FCC requirements.

So apple handed it over (M&R enterprises? it's been a while).

$30 back then, which was much higher than it could have been with mass production.

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