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Video Output Frequency of Macintosh LC - 25hz??


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I don't know where this information comes from, but some people have been quoting that the video output frequency of a Macintosh LC is 25hz.  And they quote this whenever someone's LC doesn't work with a particular monitor.  That perhaps the monitor they're using doesn't support 25hz.

 

However, Apple used several RGB monitors that were fixed at 60hz and 66hz.  Where does this tidbit of (mis?)information come from ?

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Perhaps they’re confusing the vertical scan frequency and the horizontal scan frequency. The 12” 512×384 display that was introduced with the LC has a vertical scan frequency of 60.15 Hz and a horizontal scan frequency of 24.48 kHz. Modern monitors can handle 60.15 Hz vertical, but likely not 24.48 kHz horizontal, though it’s more that 384 lines is too narrow and 24.48 kHz reflects that narrowness.

 

It’s not really a concern either way, since an LC can support 640×480 in 16 colors.

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8 hours ago, sfiera said:

Perhaps they’re confusing the vertical scan frequency and the horizontal scan frequency. The 12” 512×384 display that was introduced with the LC has a vertical scan frequency of 60.15 Hz and a horizontal scan frequency of 24.48 kHz. Modern monitors can handle 60.15 Hz vertical, but likely not 24.48 kHz horizontal, though it’s more that 384 lines is too narrow and 24.48 kHz reflects that narrowness.

 

It’s not really a concern either way, since an LC can support 640×480 in 16 colors.

So at 640x480, with a VGA monitor adapter that forces 640x480, pretty much any VGA monitor should work, shouldn’t it?

 

I have many, many LCs from the LC up until the 630. I’ve never had an issue with my monitors not picking up a signal. I’ve had people tell me theirs isn’t working because their monitor doesn’t support 25hz. Which I don’t understand. All of mine run at 60hz no problem. 
 

I am of the opinion those with video issues on boot likely have a need for recapping more than their monitor isn’t syncing. Or am I wrong here?  My experience so far is 100% of machines that work display on my monitors. 

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Yes, anyone who says "25hz" is on crack. They're probably confusing it with the (roughly) 25khz horizontal scan rate of the 512x384 mode. I'm actually somewhat skeptical that this is usually a root cause of their issue because they're obviously going to be trying to use a generic monitor with a DB15-to-HD15 adapter, and I wouldn't *think* there'd be many in circulation that have the sense lines set up for that mode because there were never very many monitors that supported it. (Pretty much be restricted to early generation "Multisyncs"; monitors that will happily sync with sub-31.5khz sources mostly died out by the early-1990s, and they were generally "premium" models.) Most of the monitor adapters I've seen that don't have mode selection switches are either "multisync" or locked at 640x480. That said...
 

8 hours ago, sfiera said:

Yeah, if the LC is set to output VGA (640×480@60Hz), then it should work. Just a question of setting up the DA15-to-VGA adapter right.

 

(There’s also a separate, Mac-specific 640×480@66.7Hz mode, which I think would work on modern displays, but not old ones)


So, regarding "setting up the adapter right", if you have one of the switchless ones you kind of need to figure out *what* it is. The "multisync" ones were targeted for later Macs (last-gen Quadras? and Power Macs) that have "Extended Sense Line" support, and I'm not positive LCs grok those properly; I'm not sure *what* video mode they'll default to with one attached. Re: the 640x480 ones, I think those come in both "real VGA" (640x480x60hz) and "Mac" (66.7hz) flavors; it was the LC that introduced the sense line combination for 60hz. Most CRT monitors will work fine with either unless you have a positively ancient fixed-frequency VGA monitor, like one that came with an IBM PS/2. Older LCD monitors will also generally work, but you *may* run into trouble with newer LCD monitors if the Mac is spitting out the 66.7hz refresh rate; in the mid-2000's they started getting lazier with the scaler boards and expecting the attached computer to honor DDC, rendering the monitors incapable of syncing with anything but a limited set of VESA modes. For these you'll pretty much have to have an adapter that *specifically* gives you the "real VGA" mode.

Here's another thing I'll just toss out there: LCs, in my experience, are one of the Macs that might give you a black screen (not initialize video at all) if their PRAM battery is dead.

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9 hours ago, Gorgonops said:

Yes, anyone who says "25hz" is on crack. They're probably confusing it with the (roughly) 25khz horizontal scan rate of the 512x384 mode. I'm actually somewhat skeptical that this is usually a root cause of their issue because they're obviously going to be trying to use a generic monitor with a DB15-to-HD15 adapter, and I wouldn't *think* there'd be many in circulation that have the sense lines set up for that mode because there were never very many monitors that supported it. (Pretty much be restricted to early generation "Multisyncs"; monitors that will happily sync with sub-31.5khz sources mostly died out by the early-1990s, and they were generally "premium" models.) Most of the monitor adapters I've seen that don't have mode selection switches are either "multisync" or locked at 640x480. That said...
 


So, regarding "setting up the adapter right", if you have one of the switchless ones you kind of need to figure out *what* it is. The "multisync" ones were targeted for later Macs (last-gen Quadras? and Power Macs) that have "Extended Sense Line" support, and I'm not positive LCs grok those properly; I'm not sure *what* video mode they'll default to with one attached. Re: the 640x480 ones, I think those come in both "real VGA" (640x480x60hz) and "Mac" (66.7hz) flavors; it was the LC that introduced the sense line combination for 60hz. Most CRT monitors will work fine with either unless you have a positively ancient fixed-frequency VGA monitor, like one that came with an IBM PS/2. Older LCD monitors will also generally work, but you *may* run into trouble with newer LCD monitors if the Mac is spitting out the 66.7hz refresh rate; in the mid-2000's they started getting lazier with the scaler boards and expecting the attached computer to honor DDC, rendering the monitors incapable of syncing with anything but a limited set of VESA modes. For these you'll pretty much have to have an adapter that *specifically* gives you the "real VGA" mode.

Here's another thing I'll just toss out there: LCs, in my experience, are one of the Macs that might give you a black screen (not initialize video at all) if their PRAM battery is dead.

I have a box full of the NEC Multisync RGB to VGA adapters.  They are all 640x480, 832x624, 1024x786 specific.  When I plug one into a vintage Mac like the LC475 and older, I get 640x480 default, 60hz or 66.7hz.  I've never had a problem displaying a picture from an old Mac with one of those specific adapters.

 

Like you're saying, a Mac with a dead PRAM (or I've also found with the need for a complete recap) will result in a black screen.

 

I've had several discussions with people who say I don't know what I'm talking about, the LC is 25hz yadda yadda.  I've never encountered any of my Macs to display at 25hz.  Ever.

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“25hz” on a conventional phosphor CRT would blink like a strobe light, it wouldn’t be remotely usable. Ask anyone who had an old Commodore Amiga how much fun the interlaced 400 line mode was; the lines flickering were visible to most humans at 30hz and could give you a roaring headache, but this would be worse not just because of the even slower update speed but because a progressive scan that slow would mean that there would be a perceptible “rolling” effect across the entire screen. (An interlaced scan at least keeps the total brightness across the screen more consistent because it’s getting scanned top to bottom twice as often than the full line update rate.)

 

And yeah, like I said, I’d be *really* surprised if people keep tripping over 512x384-wired adapters in the wild. People should check their PRAM batteries first, and then, if they don’t have an adapter with switches, investigate if it’s set for a resolution *higher* than an LC can handle, or try a different monitor and rule out their LCD being super picky about non-VESA refresh rates.

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19 hours ago, Gorgonops said:

“25hz” on a conventional phosphor CRT would blink like a strobe light, it wouldn’t be remotely usable. Ask anyone who had an old Commodore Amiga how much fun the interlaced 400 line mode was; the lines flickering were visible to most humans at 30hz and could give you a roaring headache, but this would be worse not just because of the even slower update speed but because a progressive scan that slow would mean that there would be a perceptible “rolling” effect across the entire screen. (An interlaced scan at least keeps the total brightness across the screen more consistent because it’s getting scanned top to bottom twice as often than the full line update rate.)

 

And yeah, like I said, I’d be *really* surprised if people keep tripping over 512x384-wired adapters in the wild. People should check their PRAM batteries first, and then, if they don’t have an adapter with switches, investigate if it’s set for a resolution *higher* than an LC can handle, or try a different monitor and rule out their LCD being super picky about non-VESA refresh rates.

So I’ve pretty much confirmed that this is a myth and not true. So then comes the question why people have such a hard time getting video from an LC:  I think the answer is already above, which is either the board needs a recap, a battery, or the adapter they’re using isn’t setup right. 

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On 9/14/2020 at 8:10 PM, MrFahrenheit said:

So then comes the question why people have such a hard time getting video from an LC:  I think the answer is already above, which is either the board needs a recap, a battery, or the adapter they’re using isn’t setup right. 

I'm not sure about this. My experiences with a IIsi are similar: adapter and monitor work with a different computer, adapter works also with the IIsi and a different monitor. Perhaps there is a sync signal that somehow differs.

 

Anyway, I'll be getting an LC soon (condition unknown) and will run a few tests.

 

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12 hours ago, Oberlehrer said:

LC arrived. Dead power supply, logic board probably needs a recap too. But it boots in general. Tests will have to wait after I got everything in order.

I have several LC computers and they seem to output without issue like any other Mac I have. I have several different multi sync adapters as well as fixed 640x480 adapters and one with dip switches. All combinations work on all of my Macs with several different monitors that I have (modern LCD). 
 

However the reason always given for a Mac LC not working with a monitor/monitor adapter is the false fact that they only output 25hz signal and monitors don’t like it. This can’t be true. I have had LC computers not output video to my monitor. It’s only been when it needs a recap and a fresh new battery (or both). 

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As I said I will be able to make detailed tests only after I got it recapped. It might well be that you are correct about this.

My experiences come mostly from a (recapped) IIsi (same timeframe albeit different video subsystem) which is behaving somewhat strange in the video department.

 

I'm using dip switch adapters for the IIsi and I have made the following observations:

- adapter set to VGA/SVGA: none of my monitors work

- adapter set to 640x480: two monitors work (a Panasonic PanaFlat LC50S and a NEC Multisync), the third one doesn't get any signal (or isn't able to process the incoming signal)

 

There are no other apparent issues with the video system.

 

Interestingly on my Performa 475 both adapter settings and all three monitors work fine...

 

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