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The Atari 800 had a nice range of output sockets: four joystick ports at the front:

Atari 800 joystick ports

Along the right hand side, a serial interface (SIO) port, an S-video type output, and a power jack socket:

Atari 800 side ports

However, at the rear there was a (rather daft) flying RF lead for connection to a TV.

Atari 800 flying RF lead

The case included a heavy duty cable relief design for this cable, comprised of four sturdy lugs, built into the heavy cast aluminium RF shield (image from Troubleshooting the beautiful Atari 800 at 23:58):

RF cable strain relief

Note that none of the RF circuitry on the motherboard is actually contained within the RF shield, with the RF modulator box and the cable going to the onboard RF socket placed to the right of the RF shield, as this photo shows (taken from the same video at 22:30):

RF cable and RF modulator shown to be external to the RF shield

Why was a standard RF socket not used instead? Why wasn't it obvious to the designers that this ugly "tail" rather ruined the aesthetic of the whole machine. It surely wasn't a cost cutting exercise as the Atari 800 was one of the more pricey machines at that time.

It might be worth noting that there was a "hidden" edge connector at the rear of the motherboard - that wasn't provided an opening in the case - which is only revealed once the RF shield is removed. This edge connector is in roughly the same location as the flying RF lead egress, so maybe it had something to do with that? From the video at 27:30:

Atari 800 rear edge connector

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    Pure guess: RF leakage due to connector caused failure to get FCC certification?
    – dave
    Commented Dec 30, 2022 at 18:53
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    @another-dave: Another guess: the FCC would have required the use of F-type coaxial connectors, rather than an RCA plugs and jacks, to discourage the use of inappropriate audio cables that would have radiated horribly.
    – supercat
    Commented Nov 27, 2023 at 21:00

2 Answers 2

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The internals of the Atari 800 were enclosed in an earthed aluminium chassis that screened the internal electronics and reduced the radiated electro-magnetic interference (EMI) it produced. Radiated EMI is the unwanted transmission of frequencies in specific and wide frequency range that includes RF. Clocked digital logic circuits like this computer are very good at generating radiated EMI.

Radiated EMI is like water: give it the tiniest of gaps and it'll find a way to get something out. Drilling a hole in the aluminium chassis for the RF connector would provide a hole for the radiated EMI to escape through. The designers either found, calculated or felt that the larger connector hole would pass EMI levels above what the FCC regulations allow. So no approvals and no Atari 800 sales allowed.

Hence the trailing cable, through a smaller hole with an earthed shield. The unscreened gap is then only the thickness of the cable's outer insulation. It would appear that it reduced EMI sufficiently for the FCC approval to be gained.

There's plenty of text online covering the Atari 800's internal chassis for FCC approval. The following text is from the 'FCC Issues' section of the Wikipedia covering the Atari 800:

The introduction of many game consoles during this era had led to situations where poorly designed modulators would generate so much signal as to cause interference with other nearby televisions, even in neighboring houses. In response to complaints, the Federal Communications Commission (FCC) introduced new testing standards which are extremely exacting and difficult to meet.

...

To meet the off-the-shelf requirement while including internal TV circuitry, the new machines needed to be heavily shielded. Both were built around very strong cast aluminum shields forming a partial Faraday cage, with the various components screwed down onto this internal framework. This resulted in an extremely sturdy computer, at the disadvantage of added manufacturing expense and complexity.

The FCC ruling also made it difficult to have any sizable holes in the case, which would allow RF leakage. This eliminated expansion slots or cards that communicated with the outside world via their own connectors. Instead, Atari designed the Serial Input/Output (SIO) computer bus, a system for daisy-chaining multiple, auto-configuring devices to the computer through a single shielded connector.

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    Many thanks for your answer. However, the RF circuitry was external to the RF shield as this photo shows, so there wasn't actually a hole in the RF shield - even though the photo of the strain relief appears to show a hole near the top of the rear of the shield. I have updated my question to reflect this additional info.
    – Greenonline
    Commented Dec 31, 2022 at 2:40
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    "Radiated EMI is like water: give it the tiniest of gaps" - EMI in the VHF and UHF range can be measured on a yardstick and the holes needed to leak have to be on the same order of magnitude. This is why you can see your food cook in a microwave without cooking your brain. Drilling a small hole and then filling that with a cable would produce zero leakage. The "holes in the case" (my words) refer to much larger ones needed to insert and remove cards and their large ribbon cables. A hole for this cable would be perfectly usable.
    – Maury Markowitz
    Commented Dec 31, 2022 at 14:46
  • @Greenonline I believe the actual RF modulator circuitry is shielded. The idea here is to limit the RF trace length so that it won't act as an antenna and efficiently radiate at regulated frequencies, as Maury Markowitz describes. What it looks like is the RF trace leaves the shield and immediately transitions into shielded coax for the rest of the way, the unshielded trace is too short to cause problems. A further issue could be coupling into other signals on the PCB (power supply, I/O) which then radiate, a short trace avoids this.
    – user71659
    Commented Jan 1, 2023 at 19:23
  • @MauryMarkowitz, actually it's "Radiated EMI is like water: give it the tiniest of gaps and it'll find a way to get something out". Note the 'something', not on about any 'frying brain' stuff, or freqs just in FCC range. It's a general introductory statement for the subject, one we've used for decades in design for EMC and EMC compliance testing. As an aside, most recently when seeing measurements inside/out of EMI range on loose gaskets of small cable holes in solid chassis housing logic boards. Had our own EMC test equipment so could experiment before going to the accredited EMC test house.
    – TonyM
    Commented Jan 4, 2023 at 16:07
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If one feed the RF output of a late 1970s personal computer through some random RCA-to-RCA cable that happened to be on hand, such a cable would likely act as an an antenna, radiating the output signal somewhat effectively. If the machine had a jack, then even if the unit said to only use the supplied cable, many users would likely plug in something else (either because they thought the supplied cable wasn't long enough, or because they lost it). Having an end of the supplied cable held captive by the case was an effective way of preventing such substitution.

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  • Are you saying that the supplied (fixed) flying lead had more shielding than a random RCA-to-RCA? I guess cheap RCA cables don't have any shielding at all. It's a shame that I can't find a photo of a stripped flying lead, showing the shielding (assuming that it was shielded)...
    – Greenonline
    Commented May 17 at 21:30
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    RCA plugs were used a lot for low-impedance audio applications which didn't require shielding, and the most common type of device with RCA phono plugs that would require shielding (a phonograph turntable) would also often have flying leads.
    – supercat
    Commented May 18 at 4:18
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    @Greenonline: The supplied lead is almost certainly 75-ohm coax, which has a signal conductor inside a braided ground. The switchbox is not just a switchbox, but also contains a 75/300-ohm balun transformer which converts the unbalaunced 75-ohm signal to a balanced 300-ohm twin-lead signal expected by television sets of that era.
    – supercat
    Commented May 18 at 18:38

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