How can TTL sync be converted to analog with just resistor?

[nes.og]

I understand that TTL sync voltage but be attenuated to 75ohm voltage level with a resistor which acts as a voltage divider.

What I don’t understand is how TTL sync can be converted from digital to analog with just a resistor since a resistor isn’t a DAC.

I’ve also heard people refer to arcade RGB outputs as TTL but is this just because of the higher voltage level? I think of digital RGB as CGA, EGA, which are digital and can not be converted to analog just with a resistor.

If someone can provide a good explanation than I will provide some money to buy a pint!

Thanks!

[matt]

Video sync is not very complex. It's just a series of pulses, the timing of which tells the monitor when to start a new line or frame. This is the same for digital or analog RGB. Adding a resistor lowers the voltage of the sync signal, but it doesn't affect the timing so it's interpreted the same.

TTL, in this case, refers to the level of the sync pulses (0 to 5 volts), since that's the voltage level at which TTL chips operate.

Arcade boards typically output TTL level sync and analog RGB, although the RGB signal is amplified to a higher level than computers or game consoles.

You can also reduce the voltage of an analog video or audio signal using resistors as well. This is necessary for arcade boards if you want to display them on a standard RGB monitor.

[VEGETA]

what resistor value is needed? series or parallel?

[nes.og]

Video sync is not very complex. It's just a series…

Thanks for your response but I still don’t understand how digital information (1’s & 0’s) can be converted to analog just by lowering the voltage.

[nes.og]

what resistor value is needed? series or parallel?

Typically a 330 or 470 ohm resistor based on the source voltage.

You put it in series which creates a voltage divider. Putting in parallel creates a current divider.

[Sirotaca]

Thanks for your response but I still don’t understand how digital information (1’s & 0’s) can be converted to analog just by lowering the voltage.

Don't think of it as 1s and 0s, think of it as high and low voltage thresholds, because that's what it actually is. There's not really any difference between "digital sync" and "analog sync"; it's effectively a digital signal regardless of whether it's on its own or combined with an analog video signal. So all you really need to do is divide it down to a voltage level that's safe for your sink and you're good to go.

Incidentally, digital RGB formats like EGA can quite easily be converted to analog RGB using nothing but resistors. Look up R-2R DACs. CGA is a bit more complicated because there's some added logic to deal with brown, but if you don't care about that, you can certainly use R-2R DACs for that as well (the intensity bit is used as the LSB for all three channels).

[VEGETA]

what resistor value is needed? series or parallel?

Typically a 330 or 470 ohm resistor based on the source voltage.

You put it in series which creates a voltage divider. Putting in parallel creates a current divider.

actually, voltage divider is a bit different but this is not the topic here.

I guess a series resistor is ok only if there is a termination resistor on the sink which I think is the case. isn't the source voltage of ttl videos is just 5v or what?

[nes.og]

Thanks Sirotaca, I’m trying to wrap my head around it lol.

what resistor value is needed? series or parallel?

Typically a 330 or 470 ohm resistor based on the source voltage.

You put it in series which creates a voltage divider. Putting in parallel creates a current divider.

actually, voltage divider is a bit different but this is not the topic here.

I guess a series resistor is ok only if there is a termination resistor on the sink which I think is the case. isn't the source voltage of ttl videos is just 5v or what?

The resistor you add is absolutely a voltage divider which forms in conjunction with the resistance of the output device device.

[VEGETA]

The resistor you add is absolutely a voltage divider which forms in conjunction with the resistance of the output device device

this is literally what i said xD. my "sink" is your "output" device. assume it is 75 ohms termination resistor and 5v input with 470 ohm... output voltage will be 0.688v which is totally ok.

but will crts and monitors accept this low sync? which ones who only accepts ttl level?

[nes.og]

First you ask if the resistor should be in series or parallel and I answered your question. You then said it isn’t a voltage divider and I explained that it was. You have thanked me for nothing. I’m not trying to answer any more of your questions lol.

[VEGETA]

First you ask if the resistor should be in series or parallel and I answered your question. You then said it isn’t a voltage divider and I explained that it was. You have thanked me for nothing. I’m not trying to answer any more of your questions lol.

I was trying to respond to the voltage divider thing which is not always related to sync topic. since most people here are not electronics engineers, therefore they should know that voltage divider needs 2 resistors not just one. that is why i said such stuff to make sure there is another resistor somewhere because I never tore down a scart cable or any sink device to verify the existence of the 2nd resistor.

currently my design uses 324 ohms resistor... but still gonna try to find the best sync combiner method to use since people claim xnor is not always good.

[Sirotaca]

but will crts and monitors accept this low sync? which ones who only accepts ttl level?

It should be safe to assume that anything with a 75 ohm termination resistor will be expecting video level sync, i.e. 300 mVp-p or so. If it doesn't have a termination resistor, or if it has a large value one like 2.2k, it's probably expecting TTL level sync, i.e. 2-5 Vp-p. If your source device outputs sync at TTL level and you put a series resistor on its output, it should work fine either way: if there's no termination resistor at the sink it will remain at TTL level, and if there is one it will drop down to video level.

[matt]

I've yet to find a monitor or scaler that didn't accept attenuated sync.

[maxtherabbit]

Something needs to be addressed here:

THERE IS NO SUCH THING AS ANALOG SYNC

Video sync, by nature, is a digital signal. It consists of a stream of pulses between two distinct values (1/0, high/low, on/off, etc.)

This remains true regardless of the amplitude of the sync signal. 5Vpp/3.3Vpp aka "TTL Sync" and 1Vpp/300mVpp aka "75ohm Sync" aka "Video Level Sync" are all the same, digital signal. The only difference between them is magnitude of the "on" state.


Don't conflate Digital RGB (aka TTL RGB aka CGA/EGA) vs Analog RGB (aka VGA) with TTL vs "75ohm" sync. Both types of RGB still use digital sync pulses.

[nes.og]

Confused since analog video and sync predate digital.

[maxtherabbit]

Confused since analog video and sync predates digital.

I think your issue is that you seem to regard digital and analog as different universes.

Circuitry that is "analog" can still produce and utilize "digital" signals. Reality is grey

[nes.og]

I now see that they both operate in either a “high” or “low” state in regards to sync pulse with 0 or 0.3 volts for analog (75 ohm) and 0 or 5 volts for TTL (digital logic states).

They should both share the same AC waveform pattern.

Someone please correct me if I’m wrong.

[matt]

Yes, that's what we've been trying to tell you. It's the same signal with different amplitudes.

[nes.og]

Yes, that's what we've been trying to tell you. It's the same signal with different amplitudes.

Thanks to you and the others, I appreciate it. It was just taking awhile for me to wrap my head around it. I now see how the signal can be interpreted as digital with the high/low pulses becoming 1’s and 0’s even know the sync was designed without binary in mind.