Saturday, November 22, 2014

The Purity of RF Output

Before the NES and Sega Master System, all video game consoles only supported RF output.  RF combines video and audio output and modulates those signals into frequencies suitable for an analog TV.

Analog Television Broadcasting

Until the mid 1980s, there were typically only one type of input on an US TV, RF terminals.  There were a pair of screw terminals to attach the cable for a VHF antenna and a pair of screw terminals to attach a UHF antenna.  The VHF antenna mounted on top of the TV and comprised of two telescopic metal rods that could bemoved, a.k.a. "rabbit ears".  People would have to adjust these rods to get the best signal for each channel or invest in an externally mounted antenna.  A typical UHF antenna was just a big, round loop that was not really capable of adjustment.  VHF used channels 2-13 and UHF 14-83.  Typical TVs would have one dial for each.

With Digital Television, the same VHF and UHF channels are used, but the ATSC signals are digitally modulated and are vastly different from the analog NTSC signals.  In the advent of digital TV, analog TVs cannot understand the encoding of the digital signal and these dials are useless as of September 1, 2015 without a digital converter box.

RF Switchboxes

A typical pre-crash (1st and 2nd generation) console came with an RF switchbox with a pair of prongs that screwed into the VHF terminals with the wire for the VHF antenna.  The VHF antenna would have a wire with two prongs on it which would screw into a pair of terminals on the switchbox.  The old-style switchboxes would have a sliding switch with the label TV/GAME.  When the switch was slid to the TV, the TV would receive regular broadcast reception.  When set to GAME mode, the switchbox would block the over the air signals and let the console's signal go through.

The 4-port Atari 5200 and the RCA Studio II use unique RF Switchboxes which also provide power to the console.  Other US consoles use generic switchboxes.

RF Modulators and Channel Select

All home console systems of the first and second generation of video games connected via this method.  Inside each console is an RF modulator that takes video and audio information and modulates them into a signal a TV would recognize as sufficiently similar to a broadcast signal that it would demodulate it into picture (hue, saturation and brightness/luminescence and sync) and sound.  The original Fairchild Channel F used and internal speaker like many of the Pong home consoles, and the RCA Studio II does not output color.  The difference between the TV station and a console was that the TV station typically broadcast over the air and the console sent its information directly through a wire.

Consoles in the US "broadcast" on channels 2 (54-60MHz), 3 (60-66MHz) or 4 (66-72MHz).  Early consoles may not have had a channel selector switch.  The Fairchild Channel F and some heavy-sixer Atari 2600s do not have a channel selector switch and broadcast only on channel 3.  Channel 3 was not typically used by TV stations.  Channel 2 was usually the local PBS station and Channel 4 often belonged to one of the big three network's local affiliate.  Other heavy-sixers and all later Atari 2600s have a channel selector switch, channel 2 or 3 can be selected.  The RCA Studio II, Atari 5200 and 7800 also use channel 2 and 3.  The Arcadia, Odyssey 2, Astrocade, Intellivision and Colecovision allows channel 3 or 4 to be selected.  This would become standard in post-crash systems.

RF Connectors

Even in the early 1980s, some TVs were coming with a screw-threaded coaxial connector instead of the two screws for VHF.  In this case, early TV switchboxes would require a 300 Ohm to 75 Ohm connector.  Later switchboxes, like for the NES, SMS and every console thereafter, would only use coaxial wires.  To connect to an older TV, reverse connector, the 75 Ohm to 300 Ohm connector, would have to be screwed into the VHF screw terminals of an older set.  Even Atari eventually released a coaxial version of its manual switchbox.

Today, unless you rely on OTA transmissions, typically the only thing a TV coax screw will be used for is cable input.  Newer high definition capable cable boxes use HDMI and Component video for the Hi-Def signals. If you are not using this coax for cable, you can typically connect a simple RCA to coax adapter and toss the switchbox.  Some switchboxes, like the ones you could buy from Radio Shack, degrade the signal.  Other switchboxes, like the most recent Atari switchboxes with coaxial wires, do not, so I have read.

Exceptions to the General Rule of RF Availability

No pre-crash console allowed for composite video output.  Mods were not generally known during the pre-crash era or for many years thereafter.  Composite video monitors were seldom found outside of studios and computer monitors until the mid-1980s when VHS players became ubiquitous in US households.  However, composite video and audio may be appropriate for two systems, the Atari 5200 and Colecovision, due to their computer based roots.  The Atari 5200 uses the same chips and behaves similarly to the Atari 400 and 800 computers.  The Atari 800 had a jack that provided composite and separated chroma/luma output. Composite video was found on computer monitors at this time.  Separated chroma/luma is essentially S-Video.  I would strongly advise against an S-video mod because it eliminates the composite artifacting effect that games like Choplifter on the 5200 use.

The Colecovision uses the TMS9928A Video Display Processor, which natively outputs a form of component video which is converted to RF in the system.  The Colecovision only offers RF, but the Coleco Adam, which can play Colecovision carts, offers composite video and audio.  Coleco composite, S-video and even component video mods exist.  Also, the Colecovision is a very close hardware cousin to the MSX home computers, which could support RGB video.  Many MSX games have been ported to the Coleco in recent years.

3rd Generation & Beyond

The NES and SMS switchboxes did not have a sliding switch. Instead, the signal from the console would switch the signals automatically when the console was turned on.  Earlier consoles do not provide the power to cause the switch automatically.  Both also allowed for composite video and audio connections and came with AV cables.  All post-crash systems could provide at least composite video (in their first iteration at least and some require special cables or an adapter).  For this article, the Atari 7800 is a pre-crash system, it was designed and test marketed before the crash.

Of course, I must make an exception to the exception for the Japanese Famicom, released in 1983 and which would become the NES when released in the US in late 1985.  By its date, the Famicom is in the 2nd Generation, but its graphics and sound capabilities are substantially superior to any second generation console.  I recommend that the Famicom be modded to support composite video like the NES, the Famicom AV, the Twin Famicom and the Sharp Famicom and NES TVs.  However, the best mods may require lifting the PPU out of its socket and placing a copper sheath around it, so that is not for everybody.

The Famicom only supports RF output, but the Japanese TV broadcasting used different frequencies than US broadcasting.  The Famicom broadcasts on Japanese channel 1 (90-96MHz) or 2 (96-102Mhz).  In the United States, these broadcast frequencies were and are assigned to FM radio (87.9-107.9MHz).  Japan's FM radio frequencies (76MHz-90MHz) are mainly taken up in the US by TV channels 5 (76MHz-82MHz) and 6 (82MHz-88MHz).

On higher end CRTs sold in the US, channels 95 and 96 tune to the Japanese channel 1 and 2 frequencies, respectively.  No TV manufacturer ever intended that these channels would be used to receive over the air broadcasts.  These channels were a late addition to the US channel lineup and placed in higher end TVs that were intended to be marked in NTSC countries like the US and Japan with only minor changes to the TV's firmware necessary to assign the appropriate channels to the appropriate countries.

The Famicom came with a white RF adapter with some screw terminals.  A (S)NES or Sega Master System/Genesis RF adapter can be used, but you will need to use the switches on the back of the Famicom to switch the input and the channel.

Home Computers

When the Apple II was first released, it had an RCA composite video output jack and a video pin header on its motherboard.  However, most buyers of the device only had a TV set with antenna leads.  Third parties marketed RF modulators that plugged into the header on the motherboard and connected to an external RF switch.  The most popular one was the Sup'R"Mod. II, which came out in a version that broadcast on Channel 3 and another version that broadcast on Channel 33.  This is the only example of a popularly available RF modulator broadcasting on a UHF channel.

When IBM released the PCjr., it believed that many buyers would want to hook it up to a standard TV set.  It designed and released an RF adapter with a special 2x3 BERG connector that was keyed for the Television port on the back of the PCjr.  This adapter took a standard video and audio signal from the PCjr.  The adapter was in a long, silver box and contained both the modulation circuitry and the switchbox circuitry.  It had a "TV/Computer" switch and a Channel Select (3-4) switch.  Unfortunately it only has screw terminal input and output, so you will need both types of converters to use the box on a modern TV and with cable TV input.  This photo shows a fully modernized version of this beast :


IBM had previously released the IBM Color/Graphics Adapter.  This adapter had an RGB monitor output, a composite video RCA jack and a header for an RF adapter.  IBM never released its own RF adapter for the PC.  The RF is 4 pins and is identical to the Apple II header except it does not have the pin or connection for the -5v line.

Matching Transformers

Twin lead connectors require 300 ohm impedance connections.  Coaxial connectors require 75 impedance connectors.  Therefore, in addition to using completely different physical connectors, electrically these signals are not quite compatible.  If you are trying to use one type of connector with another type of connector, you need an appropriate matching transformer, or Balun.  A 300-to-75 ohm transformer, shown in the top of the above photograph, will allow you to connect to a coaxial connector on a new TV.  A 75-to-300 ohm transformer, shown on the left of the above photo, will allow you to connect a twin lead connector on an old TV.  These can be purchased cheaply at Radio Shack.

Conclusion

Prior to the 3rd generation, home video game consoles were often treated as toys and marketed and designed as such.  High end video connections had barely reached into the home.  Higher end-audio was somewhat more common, but there is not a lot of sophisticated music to be heard in the pre-crash era.  The B&W switch on the Atari 2600s was not placed there simply for cosmetic purposes, many game consoles connected to B&W TVs.  Consumers were generally satisfied when they got the system to work.  RF provided a known quantity for programmers, they had to make their graphics look clear with it.  Tiny text and high resolutions lost much of the detail on the modulated signal.  RF was the way these consoles were meant to be seen.  Except where indicated, it is the way these consoles should be played.  Anything better is not true to the classic console experience.

2 comments:

Aybe said...

I have a Famiclone that has wireless RF, never been able to get a good picture at all from it!

It seems like it is not willing to die, the X360 still offers an RF adapter, I guess it's for the RF fanatics (UK) :D

Great Hierophant said...

Wireless RF? As in controllers? That has nothing to do with RF video/audio output.

Any modern console can output RF by plugging in the composite video and audio cables into an RF modulator, Radio Shack may still sell them.