Will an RF modulator provide NTSC System M spec video from a DVD player?

Question

I've been attempting to recreate the effect of an old NTSC television broadcast lately with the use of DVDs. The DVD-Video spec allows for up to 720 x 480 pixels at a display rate of 29.97 frames per second, interlaced.

My DVD player is outputting composite video to an RF modulator which is then connected to my TV and the signal is demodulated by the receiver, effectively simulating the NTSC transmission model.

What I'm wondering is if RF modulators output NTSC System M spec video for everything you feed it, allowing for a maximum video bandwidth of 4.2 MHz or about 330 lines of horizontal resolution (440 x 486 from a digital perspective), or if it's converting the composite video signal to RF however it sees fit and the resulting image will look more like 540 line DVD?

Any help would be greatly appreciated.

Answer 1

That would depend on the specific modulator, but RF signal output is just basically the same composite video it gets but modulated to RF.

So the RF output will be NTSC-M if the baseband CVBS that is sent to modulator is also NTSC-M compliant, and that the modulator modulates it in NTSC-M compliant way.

Some sources can/may output non-broadcast compliant CVBS baseband signals, because the output is meant to be connected to a TV or other equipment, and not meant to be transmitted over RF in a compliant fashion. The modulator can and will most likely limit the bandwidth to fit the signal to the RF channel, but it will still modulate whatever compliant or non-compliant signal it gets in.

Answer 2

The DVD player composite encoder will low-pass filter the 720x480 YCbCr digital picture prior to encoding in NTSC or PAL. This is to comply with the spec for baseband composite, allowing the video signal to fit in the allotted RF bandwidth prior to modulation.

If the composite video exceeds that allotted 4.2MHz bandwidth, bad things happen to the sound carrier when they get modulated to RF, so that's why it's not supported or available on a DVD player.

Y/C (S-video) is the step-up alternative that provides better luma bandwidth (although chroma is still limited to that provided by composite).

So the number of lines on baseband composite is only as good as RF composite format allows - about 330 lines, as you state.

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So why did (and do) DVD players limit composite video bandwidth, even though it's technically possible to send higher resolution signal if RF isn't in the signal chain?

It's not a technical issue, but a marketing one.

In 1997, when DVD was introduced, TV coax-in was the norm. Sets with A/V inputs were still a premium item. High end cinephiles adopted DVD component for home theater, migrating from (composite) laserdisc. They upgraded their whole signal chains because, well, they could.

Meanwhile, low-end consumers delayed their TV upgrade for HDTV (1999) and/or flatscreen+HDMI (2002-ish). In those several in-between years they soldiered along with coax-in RF or, if they were lucky, A/V-in until their TV upgrade cycle came, and with it, new-TV choices sporting a slew of better inputs including Y/C (S-video), component, and slightly later, HDMI.

We can divide the DVD player era into two distinct time periods:

• Pre-HDMI (1997-2002): Component for high-end, RF for cheapskates and Grandma
• Post-HDMI (2002 onward): Whoo-hoo! A new big ol' flatscreen from Costco! Forget RF!

In either pre- or post-HDMI era, there was no incentive for 'better', non-RF-compatible composite in a DVD player. Pre-HDMI it would have been an annoyance for the many, many RF users. Post-HDMI, buyers couldn't care less about juiced-up composite because there were so many better connection choices on the new flatscreen TVs whether they wanted HDMI or not.

And let's not forget about the content providers who were freaking out at the idea of any higher-quality format enhancing the ability to capture analog out to re-encode for 'piracy' (nevermind DeCSS rendering that point moot). The last thing they wanted was better composite (let alone component); they worked really hard to make it worse with Macrovision. That rights-freakout hellbeast carried over to HDMI, with its own fun set of problems and hassles.

Anyway. And now, storytime. My first trip to Costco, January 2005. Standing in the next checkout line, a younger, Santa Cruz Mountains looking dude with exactly two items on a flat cart:

• 40" Vizio TV
• handle of Tanqueray

And I thought to myself, "that cowboy is ready to par-tay!"

Answer 3

Welcome to Electrical Engineering.

Your video quality is likely to be limited by the fact that you are using composite video rather than anything the modulator is doing. The colour difference signal needs to be restricted in bandwidth before being added to the composite signal. The TV has various options for separating the luminance (black + white) and colour difference signals out of the composite signal. The cheaper options will restrict the luminance bandwidth.

I played around with the various connection options when I first got a DVD player (to use with a wide-screen CRT TV). I had a baseband composite video connection, without needing to use a modulator. It was still noticeably worst than the next best quality connection - S-Video.

My best connection was component video but this was only superior to S-Video for menus generated in the DVD player itself. The DVD format records a colour difference signal so there's no extra information for a component video connection to carry in addition to that available via S-Video.

(I'm in the UK so my TV offered RGB for component video rather than YPbPr. The DVD player had both.)