Analogue vid output & order of termination resistor & DC offset removal cap

[Constant Cluelessness]

Hi, I have a question about analogue video output circuits.

Nintendo, in their usual friendly nature towards PAL regions decided to remove the termination resistors and dc offset removal capacitors from the N64 in PAL regions.
I am attempting to build an s-video / composite out adaptor and wondered which way round is correct for the components.
The capacitors are 220μF for composite video and luma and 68nF for chroma. The resistors are all 75Ω.

https://github.com/dmkfasi/nus-cpu/blob/master/reference/Schematics/N64%20NUS-CPU-03-04.pdf
Here are output schematics for the NTSC N64 showing the components removed from PAL consoles that I believe to be r5,6 and 7 and c14, 15 and 16. As you can see, from the console side towards the television, the capacitors are first, followed by the resistors to ground.
However, if I take apart my official PAL gamecube composite cable (they did the same thing on this console), it has the resistor to ground before the capacitor.

So I was wondering which is the correct way round and what difference will it make, or does it not matter at all?


https://archive.org/details/scph-5500-5501-5502-5503_202109/SCPH_9000_3RD_ED/page/11/mode/1up
If we look at a similar output circuit for the Playstation 1, they also do a similar thing where the resistors are before the capacitors, however the resistors are in series with the capacitors and not to ground. Interestingly the Playstation 1 appears to keep all components for composite and s-video in all regions, just removing the capacitors on the RGB lines, much how the Gamecube does the same (the N64 does not output RGB).

[Nominal Animal]

Here are output schematics for the NTSC N64 showing the components removed from PAL consoles that I believe to be r5,6 and 7 and c14, 15 and 16.

75Ω source/series termination resistors R5, R6, and R7 are in series with the AC-coupling capacitors C14, C15, C16, and not to ground.  The capacitors C11, C12, C13, if at all populated, are tiny filter capacitors smoothing out any switching spikes in the output video signals.

In the analog video circuits I've seen, including TI THS7374 video amplifier datasheet (figure 62 on page 25), the source/series termination resistor with AC coupling is before the capacitor.  If you see an additional resistor to ground after the source/series termination resistor, that can be used to attenuate the signal peak-to-peak voltage.

I'm only a hobbyist, and do not actually know whether the order of the source/series termination resistor and AC-coupling capacitor matters or not, though.

[Constant Cluelessness]

Thanks for your input,
I had assumed that the resistors were effectively to ground through the tiny filter capacitors c11, 12 and 13 but I guess that was my mistake. In the cable the resistor is to ground at least. So I am still unclear 

Thanks

[Nominal Animal]

If I've understood you correctly, you want to duplicate this or this?

Whether the series/source termination resistor is before or after the series DC-blocking capacitor, does not seem to affect anything based on simulations.  (I don't have suitable test equipment to test it in practice.)  These signals are transmission lines  with the characteristic impedance of 75Ω, and a termination resistance of 75Ω to ground in each receiver device.  However, me being a hobbyist, I don't have the experience to tell whether there is some transmission-line magick where the order of the resistor and capacitor matters.

As you can see in the THS7374 datasheet I linked to earlier, many (most?) video signal sources produce 2× voltage amplitude swing, so 2×0.700Vpp = 1.4Vpp for PAL S-Video luma, and 2×0.885Vpp = 1.77Vpp for PAL S-Video 100% color bar chroma, according to this Analog article.  The source has 75Ω in series, and the sink has 75Ω to ground.  This minimizes reflections and makes for a very good transmission line.  The two resistors do form a voltage divider by half, which is why the source voltage amplitude is typically twice that seen on the receiver.

So, what we expect to see in analog video signals, is this:

where the characteristic impedance of the cable is also 75Ω.  (Its DC resistance is less than 1Ω; "characteristic impedance" is a transmission line property for varying/non-DC signals.)

Rs is the source/series termination, Cc is the AC coupling (DC blocking) capacitor (220µF to 330µF is common, exact value is not that important), and RL is the load termination.  While Rs and RL do mean that the voltage seen at the video ADC is half that at the video DAC output, it gives excellent signal transmission properties, and is the reason this topology is used.

Physical connectors don't affect this picture much.

Adding a capacitor to ground between Rs and Cc will attenuate the AC-coupled signal.  Just 47pF will reduce a 24 MHz signal by 3% in voltage amplitude, and 1µF drops it to one-thousandth.  This means the C11-C13 must either have tiny capacitances, or be omitted altogether; but they can be used as a low-pass filter, removing thin spikes and sharp edges.  Because of their frequency dependent behaviour, they cannot be used to attenuate the entire voltage signal; a resistor is used for that.

A resistor to ground, basically anywhere right of Rs, can be used to attenuate the voltage.  In the case we have here, with Rs = RL = 75Ω, the amplification factor caused by that added resistor Rg is calculated as Rg / (Rg + 37.5), as it is a simple voltage divider (with 2× source voltage by default).   For example, Rg = 75Ω scales the voltage amplitude at the video ADC in the receiver by 2/3, and Rg = 37.5Ω halves it.

Let's say that your device has omitted Rs and Cc, replacing them with simple shorts.  (If they're simply omitted, there is no continuity to the back panel connector.)
Then, the cable would need to have the Rs and Cc in it.  Again, for smoothing the signal (forming a low-pass filter), there could be a small few-picofarad filter capacitor to ground after the series resistor.

If you ask me, I would bet that the existing PAL N64 S-Video cables have a THS7374 in them, with a 3.3V linear regulator from the 5V available (to get a steady, low-noise supply), and have 75Ω in series and 75Ω to ground on luma and chroma inputs; and on output, a 75Ω and 220µF/330µF in series with the actual cable.  Even the connector shape matches!  The reason is that the THS7374 acts like a buffer, and makes sure that the video signals on its output have the correct levels and drive.

If so, it would be very easy and cheap to make such a board at e.g. JLCPCB.  THS7374IPWR (TSSOP14 footprint, 5mm×4.5mm body, 0.65mm leg pitch; hand-solderable using the dragging method like 0.65mm pitch TQFPs) itself is just 0.48€ in singles at Mouser, 0.45€ at LCSC, and $0.50 at JLCPCB for assembly.  The connectors will cost much more than the electronics parts.  Some of the component values may need slight tuning, observing the signal with an oscilloscope, for best results; in particular if the low-pass filtering picofarad capacitors are used.  Which brings me back to the existing cables: 50€ or so for a known, tested one sounds like a good buy in my opinion; you'll certainly end up spending that much in parts and shipping if you go the DIY way.

[Terry Bites]

There is no defacto standard for the RC coupling of video signals.

What matters is that the 75R terminations and total series capacitance create a high pass corner frequency much lower than the fame rate.
If the capacitance is too low, the highpass corner will be raised and you can suffer loss of sync.
See attached on Sag and droop.

[Nominal Animal]

To be precise, the capacitors are optional, but the 75Ω resistors are not.

[Constant Cluelessness]

Thanks for your replies, my understanding of electronics is too low to fully understand at the moment but I appreciate the technical responses
From my understanding, the capacitors bring down the overall signal level by removing so that it falls within range and white for example doesn’t overshoot (I.E if you don’t have them, the picture will be too bright). The termination resistors I am not sure exactly because they seem to affect signal level too but in a different way.

Anyway, thanks for confirming (I think, unless I have misunderstood) that it (probably) makes no difference the order of components in this instance.

Looking at your picture, I imagine the resistor to ground is in the display device (tv) itself? What makes me confused is that it seems you can either choose to put a resistor in series, like in your picture or a resistor to ground, and it seems like they are both valid ways of signal attenuation (this may be the wrong word). At least, in my searching I saw something about the pros of putting the resistor in series or to ground. It was a bit beyond my level.


On your question about official PAL S-video cables, there were none ever produced. However, there were composite ones. The n-64 ones just contain a resistor (i can’t remember if to ground or in series, I can check later) but the gamecube (they took the components out here too) had it done properly and included the capacitor too, with a resistor to ground and then a capacitor.

None of the cables use THS7374 in them but interestingly, if you have an early ntsc or early french pal n64, you can mod it to output rgb using a THS7374 from what I understand. I haven’t looked into it because I have neither and so S-video will suffice for me (you can get RGB with a more expensive mod but in this case I think S-video will probably suit me fine).

The reason I am making my own adaptor is that I already have shielded S-video / scart cables and you can purchase nintendo AV-out connectors extremely cheaply on aliexpress, along with S-video and RCA connectors and some resistors and capacitors, it is far cheaper this way and I already have shielded cables. I only need to plug in one console at a time so it makes more sense to me just to make simple adaptors for consoles this way.

Sorry for my lack of knowledge, I really do need to learn electronics from the beginning but not sure where to begin :$

Thanks!

[Nominal Animal]

Thanks for your replies, my understanding of electronics is too low to fully understand at the moment but I appreciate the technical responses
From my understanding, the capacitors bring down the overall signal level by removing so that it falls within range

No, the two resistors do that.  They do double duty: on one hand, they ensure the signal (transmission line) is as good as it can be without reflections et cetera when using 75Ω characteristic impedance cables (including S-Video cables).  On the other hand, they form a voltage divider that halves the voltage at the receiver compared to the source video amplifier (THS7374 or the built-in one in N64) outputs; which is why they have 3 dB = 2 V/V amplification in the first place.

The 220µF or 330µF capacitors have a different purpose: they block any DC voltage, passing through only changes in the signal.  This is called AC coupling.  With S-Video, chroma signal is not a problem and could use a much smaller-valued capacitor, but luma (brightness) may suffer, depending on the receiver circuitry.  The datasheet for e.g. the THS7374 video amplifier shows both ways.  It can help or hinder, but you need a proper EE to explain why and when.  Above, I just followed the known working existing products.

Sorry for my lack of knowledge, I really do need to learn electronics from the beginning but not sure where to begin :$

Nope, no need to be sorry.  We all started from zero, remember?