2

Using coaxial cable, there was a really bad channel bleed - as shown in this video, putting a NES on channel 3 caused distortion to the static on channels 2 and 4-7.

This distortion was so bad, you could see glimpses of the game (Days of Thunder) through the static. It also made the screen shake like crazy (on a CRT).

What was the cause of this distortion? There was no wireless transmitting involved with the linked video (the NES was directly connected through coaxial, no antennas installed).

(A similar issue happens with radio)

  • You're the author. check priveledges and alike. Also you may want to descibe the setup, and why you assume that the cable is the reason. At least if you want a serious answer. – Raffzahn Jul 23 '18 at 22:10
  • @Raffzahn - plays OK for me (Chrome/Mac). You may need to be logged in to a Google account for it to work? – Jules Jul 23 '18 at 22:31
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    Doesn't play for me either. Perhaps it relies on codecs specific to Chrome? Using Firefox over here and I got nothing, even though I can load the page and click the play button. – mnem Jul 23 '18 at 23:06
  • Plays for me in both Firefox and Safari on a Mac. This is an hour later though; possibly Google does some transcoding after upload? – Tommy Jul 23 '18 at 23:39
  • @Zackary Could you put the video somewhere more permanent and accessible please? Think of the future people. (Also, I can't play the video either, but that might be because my computer's /usr and /lib folders are AWOL.) – wizzwizz4 Jul 25 '18 at 15:10
7

The problem is most likely the TV itself. Typical consumer grade equipment suffers from a rather poor ability to reject strong signals on nearby frequencies. Channel 2 and 4 through 7 are all pretty close to channel 3.

The coax is merely delivering a lot of signal. More than the simple tuning circuits can handle.

This is one reason that modulators often have isolation switches in them to protect the downstream receiver.

  • 1
    Right. There's no such thing as an exact bandpass filter. But likely the Nintendo is putting out some sideband as well? – Tommy Jul 23 '18 at 23:40
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    If the receiver has poor dynamic range, the more powerful signal even without any sidebands would distract the weak one -- in the receiver itself. – lvd Jul 24 '18 at 5:18
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    Very, very few receivers can cleanly receive adjacent signals in the wake of a strong signal. In fact, in ham radio we have to use duplex chambers on our VHF and UHF repeaters even though the signal we are transmitting is anywhere from 600 kHz to 5 MHz away from what we are receiving, else we can't hear the received signals at all - and this is on professional-quality radio receiver gear. A typical consumer TV, with a transmitted signal injected directly into its antenna jack, has no hope of not having that signal stomp on adjacent signals without some frequency isolation. – Jim MacKenzie Jul 24 '18 at 14:51
  • @JimMacKenzie any idea what the usual broadcast distances were for analogue NTSC? That might be persuasive information re: the receiving end being the most likely culprit. – Tommy Jul 24 '18 at 18:15
  • @Tommy That depended a lot on market size and terrain, and the type of antenna being used at the receiver site, Where I live, our stations are VHF, and three transmitters are ~10 km away and the fourth, about 60 km. When they transmitted an analog NTSC signal, three of the four used 100,000 watts. (Now with ATSC they are between 23,000 and 30,000 watts, depending on the station.) The signal from an RF modulator is much weaker of course, but it's inserted directly into the antenna jack. – Jim MacKenzie Jul 24 '18 at 19:01
1

A quality RF modulator will only produce signals within the frequency range of the selected channel. On the other hand, an RF modulator which kinda-sorta works, but produces a fair mount of junk all over the spectrum (including on the channel of interest) can be built using less than a handful of transistors (if memory serves, Pong-style consoles used a single-transistor design, and the Atari 2600 used a two-transistor design). Such a design would not be even remotely acceptable for broadcast use, but for feeding a signal directly into a television set via coaxial cable, even nasty horrible designs may be sufficient.

For most vintage devices which include both RF and composite outputs, feeding a composite output through a quality modern RF modulator will yield a much better picture than using the RF modulator built into the vintage device.

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