Turbo EverDrive 2.x and the PC Engine - The Affordable NEC Experience

 Over two years ago, a friend of mine kindly let me borrow a Turbo Duo and a Turbo EverDrive.  I wrote about the experience here : http://nerdlypleasures.blogspot.com/2015/04/turbo-duo-issues-and-solutions.html  I had to give it back (and the Framemeister which came with it), not without a good deal of sadness.  I vowed that I would find a way to play NEC Turbo games again on real hardware/  Now, I have acquired an affordable, upgradeable solution.  Let me talk about it and about plans for future upgrades.

More HDMI-ifying your Consoles, the UperGrafx for the NEC Turbo/PC Engine Systems

Today I found out about an upcoming Japanese product called the UperGrafx. The UperGrafx, as its name suggests, is intended to be used with a NEC PC Engine. What it does is it upscales the native PC Engine graphics (typically 256, 320 or 512 horizontal pixels by 224 or 239 lines) to the 720p resolution (1280x720). It plugs into the back of the PC Engine, Core Grafx or Core Grafx II, where there is a 69-pin expansion connector. It has also been confirmed to work with the US TurboGrafx-16 expansion connector. It won't work with a PC Engine Shuttle or any of the handhelds, Duos or the SuperGrafx.

Although the Expansion Connector supplies the analog RGB signals, it also supplies a lot more video information. The UperGrafx takes this information and builds a digital picture, then upscales it for the 720p resolution. The UperGrafx works similarly to the NESRGB and HiDef NES Mod.  

The PC Engine has sixteen sixteen color palette entries available for backgrounds and sixteen sixteen color palette entries for sprites. The first color entry of all background palettes is set to the universal background color and the first color entry of all sprite palettes is set to transparency. This gives a maximum of 482 colors on the screen at once. The Expansion Connector is constantly outputting these palette indexes on a separate video data bus that usually goes to the Color Encoder chip. Unlike the NES it also has a special signal to distinguish sprite palette indexes from background palette indexes, so you can still view the native video signal. The PC Engine displays 9-bit RGB for 512 colors maximum. The UperGrafx must be able to snoop on the color values stored for each palette index like the NES mods. The full CPU address and data bus is available on the Expansion Connector, so the color values for the palettes have been available. The result, when combined with the dot clock and sync signals, can give a truly digital representation of the screen image. Even better than the NES is the fact that there is no need to guestimate a composite to RGB palette.

There is a minijack next to the DVI port, which would suggest that audio is passed through from the Expansion Connector. The Expansion Connector supports stereo audio. The audio is output through the DVI connector, which is something of a pseudo-standard. Essentially the audio must be converted from analog to digital inside the UperGrafx.

Finally, the unit acts similarly to a Hudson Tennokoe 2 Backup Unit or a NEC Backup Booster. It allows you to save games to battery backed RAM instead of using passwords for those games that supported it. Unlike the original devices, there will be a USB port which you can use to transfer saves to and from the UperGrafx. The original devices came with 2KB of RAM, I do not know how much RAM will be available for the UperGrafx, but it is likely to have more RAM than the old devices.

The greatest benefit to the UperGrafx is that you are getting a pure digital video signal. There is no analog to digital conversion or degradation. There is nothing like jailbars induced by analog noise. When I had an RGB-modded PC Engine Duo last year, I could observe alternating bands of light and dark areas in the green background of Bonk's Adventure through RGB but not through composite. This cannot happen on the UperGrafx.

The second greatest benefit to the UperGrafx is that you do not need to mod your system to get perfect quality video out of it. The original PC Engine and TurboGrafx 16 only support RF output and the Core Grafx only handle composite video. No NEC console handles RGB without a mod, and there are more than one school of thought about what the perfect RGB mod should be. The unit I was using last year had a mod from doujindance, who has an excellent reputation for modding within the PC Engine/Turbo community. However, given the banding in his RGB mod, there is room for improvement.

There are some downsides. First, there is no passthrough for the expansion connector, so you cannot connect a CD-ROM unit. Second, you need a DVI to HDMI cable if your TV or monitor does not have a DVI input, but they are pretty cheap ($5-10). Third, it only upscales to 720p, not the more common 1080p of higher end displays. Fourth, it attaches to the back of the console like an "L", sticking straight up into the air, and there is nothing but friction keeping it attached to the console (not unlike attachments for the ZX Spectrum). Finally, it always displays "UperGrafx" in the borders of the frame, which some people may not appreciate.

So, for the suggested retail price of ¥40,000/$368, why should anyone buy this unit over a Framemeister? The Framemeister costs just as much and can work with almost any system. Also, if you buy this modern version of the Turbo Booster http://db-electronics.ca/product/dbgrafx-booster-ttp/ for $65, you can get the highest quality analog signals (RGB + CSync, S-Video and Composite) at a fraction of the cost.

You can see Jason of game-tech.us test the device and give his initial thoughts here : https://www.youtube.com/watch?v=b4baUgr0Ym0

A pure video of the DVI output for the device is available here : https://www.youtube.com/watch?v=LN3L1mLRWhs

Here is more information and pictures about the device, translated from Japanese : https://translate.google.com/translate?hl=en&sl=ja&u=http://www.gdm.or.jp/crew/2016/0423/159899&prev=search

The guy who supplied Jason with the device says that the lag on the device is between 0 and 1 frame. By comparison, kevtris' Hi-Def NES gives lag in the 2-4ms range. One frame is 16ms. The Framemeister gives lag in the 16-24ms range, depending on the settings used. But both the Hi-Def NES and the Framemeister support 1080p. The UperGrafx and the AVS only support 720p. What processing speed advantage given by the pure digital signal may be taken away by the upscaling done by non-true 720p displays (I am not even sure if there is a such thing as a true 1280x720p display). Lag comes from many sources and is quite insidious. Given the large number of shmups for the Turbo/PC Engine where timing is critical, maintaining a latency of well under one frame per second is essential if you want to buy this device over a Framemeister.

Turbo Duo - Issues and Solutions

The Japanese PC Engine console was released as the TurboGrafx-16 in the US in 1989.  Conceived as a competitor to the NES and the Sega Genesis, it flourished in Japan, floundered in the US and barely had a presence in Europe.  However, it has many great arcade ports, fun platformers and lots of Shoot-em-ups.

Using HuCards

The TurboGrafx-16 can be a very expensive system to collect games for.  Japanese systems use HuCards for the games on ROM.  HuCards are slightly thicker than a credit card and have exposed contacts which get inserted into the card slot on a Turbo system.  Some US material called them TurboChips.  Japanese and US HuCard games are functionally identical but not pin compatible, requiring a region mod or a converter PCB to use cards for the other system.

However, it  is surprisingly easy to play games on any NEC system these days if you don't want to engage in the pricey journey to collect HuCards.  A Turbo Everdrive from Krikzz will play any official, licensed game (with one exception, see below). The Everdrive has a switch to select the console's region and an SD slot to load ROMs. The current card is flash based and loading a new game requires rewriting the old game.  The currently written game can be selected instantly by pressing the select button.  Writing a large 1MB game like Bonk's Big Adventure only takes about 11 seconds. There will be a newer, RAM based version released in the near future that will make loading faster and end worries about reaching the write limit of the flash media. However, the media should be rated for at least 10,000 write cycles, so if you flashed a new game every day it would take you at least 27 years to exhaust the memory.

The Everdrive can also be used as a CD System Card 2.0 replacement by loading the ROM.  It does not have the extra RAM needed for Super CD System 3.0 Card support or an Arcade Card.  It will work with the game Populous only in the Turbo/PC Engine Duo console line because that game came with extra RAM that is duplicated in the CD systems.  However, you can include the Super CD System ROMs so you can play CD games with an Everdrive installed in your Turbo Duo.  You can also load the US ROM in case you forget what the save RAM options are if you have a Japanese system.

One other irksome issue with regions is that the PC Engine uses a different gamepad connector from the TurboGrafx-16.  All Japanese consoles uses a mini-DIN-8 connector while TurboGrafx 16s use a DIN-8 connector. The US Turbo Duo also uses the min-DIN connector.  The controllers and multitaps themselves are otherwise compatible and there are converters available.  Japanese controllers and multitaps are often easier to find than their US equivalents.

A final issue is that the original PC Engine and TurboGrafx-16 were RF only.  The TurboGrafx-16 required a Turbo Booster or the TurboGrafx CD add-on for stereo composite AV output .  The PC Engine usually did the same through the PC Engine CD.  As I mentioned in my RF blog entry, Japanese RF channels are not the same as US RF channels.  NEC later released the Core Grafx and Core Grafx II consoles which replaced RF with stereo AV output.  There is also the PC Engine Shuttle which supports stereo AV but has no expansion port for a CD add-on.

Using CD Backups

The TurboGrafx CD was the first CD-based console add-on.  It came with the CD unit, a Docking station and a CD System Card (2.0).  All three pieces are required to make it work with the TurboGrafx-16, and it makes the system look like a large, black inverted-T.  The system could be upgraded to play Super CD-ROM games with a Super System Card (3.0).  These are hard to come by for the US console.

In 1992, the TurboGrafx-16 and TurboGrafx CD were released in a combined unit called the Turbo Duo. Japan's version is called the PC Engine Duo.  The Turbo Duo plays US HuCard games but uses the controller ports from the Japanese consoles.  It also uses a 5-pin DIN AV output connector which supplies composite video only and stereo audio.  Finally it has a stereo mini-jack for headphone output.  The Japanese version is identical except for the color of the buttons and the support for Japanese HuCards.

NEC later released the white PC Engine Duo-R which removed the headphone jack and the lock switch for the CD cover.  Finally, the PC Engine Duo-RX was slightly cosmetically different from the Duo-R but included a six-button gamepad.  Only a few Japanese games like Street Fighter II - Championship Edition supported the 6-button pad.

Any of the Duo consoles are expensive ($250-$350) to acquire, but playing backups of the CD-ROM games are cheap.  The original Duos will almost certainly need to have the capacitors replaced. The Duo R and RX have a reputation for being more reliable. The lens assembly may also need to be replaced.  However, you are really getting the full NEC console experience with one of these systems and it can play a ton of great games.

In burning CD backups, however, you have a challenge.  Turbo CD games rarely look like standard CD-ROMs.  Except for a very few CDs with only a data track, all CD games are mixed mode games with data and audio tracks.  For these games, the first track is always an audio track intended to warn the user not to play the CD in a CD player.  The second track is always a data track (and the reason for the warning), and for some of the simpler CDs, they only have one data track.  CDs with more complex mastering also have a second data track as the last track on the CD.

There is nothing inherently non-standard about mixed mode CDs.  Many, many MS-DOS CD games used a data track and one or more CD audio tracks.  Most Sega CD, Neo Geo CD, Jaguar CD, 3D0 and some Sega Saturn and Sony Playstation games used mixed mode CDs.  However, for the MS-DOS CDs, Sega CDs and Sony games, their CDs always have just one data track and it is always the first track.  One or more audio tracks follow.  Turbo CDs, even those with only a data track, are not readable in a PC except as an audio CD.

A good backup uses high quality media.  I have read good things about Taiyo Yuden CDs, and I have also used Sony SUPREMAS CD-Rs with some success.  Ideally these CDs should be burnt as slowly as your burner can burn them. However there are several caveats to this.  First, modern CD-Rs were designed to be burnt at 16x speeds or better, as are modern burners.  Second, if you use a really old burner it may not support the ideal settings for burning Turbo CDs.  The ideal setting is to use a CUE sheet (either with a BIN or ISO and WAV) in the Disk at Once mode.  Really old burners may not support Disk at Once or may balk at the nutty mixture of audio and data tracks some of these images have.

Compared to playing with original pressed discs, the CD motor may exhibit more noise and the load times may be longer.  Load times for pressed media are often very quick.  It takes virtually no time for even a 1x CD-ROM unit like the one in a Turbo console to switch and audio track.  Additionally, there is only so much RAM to fill in a Turbo system (8KB CPU RAM + 64KB Video RAM + 64KB ADPCM RAM + 64KB CD-ROM RAM + 192KB Super System Card RAM).  A 1x CD-ROM transfers at 150KB per second, so load times should be fairly reasonable.  When you get to Arcade Card games, which add another 2MB of RAM, things might take longer.

Here is a list of all US released CD games by their type :

Two Data Tracks US CD Games
Buster Bros.
Fighting Street
J. B. Harold Murder Club
Jack Nicklaus Turbo Golf
Last Alert

Two Data Tracks US Super CD Games
Bonk III - Bonk's Big Adventure
Cotton - Fantastic Night Dreams
Dragon Slayer - The Legend of Heroes
Dungeon Explorer II
Dungeon Master - Theron's Quest
Fantasy Star Soldier
Forgotten Worlds
Godzilla
John Madden Duo CD Football
Loom
Lords of Thunder
Might and Magic III - Isles of Terra
Prince of Persia
Riot Zone
Shadow of the Beast
Sim Earth - The Living Planet
Super Air Zonk

Interleaved Data and Audio Tracks US CD Games
Cosmic Fantasy II
Magical Dinosaur Tour
Valis III

Two Data Tracks US Super CD Games
3 in 1 DUO Demo CD
4 in 1 Super CD

Single Data Track US CD Games
Exile
Final Zone II
It Came from the Desert
Lords of the Rising Sun
Splash Lake
Valis II
Vasteel
Ys Book I & II
Ys III - Wanderers from Ys

Single Data Track US CD/Super CD Hybrid Games
Sherlock Holmes Consulting Detective Volume II
Syd Mead's Terra Forming

Single Data Track US Super CD Games
Beyond Shadowgate
Camp California
Dynastic Hero, The
Exile II - Wicked Phenomenon
Implode
Meteor Blaster DX
Shape Shifter (98 Tracks!)

Data Only US CD Games
Addams Family, The
Bikini Girls
Hawiian Island Girls
Local Girls of Hawaii, The
Sherlock Holmes Consulting Detective

http://www.necstasy.net/ can inform you of the proper CUE sheets for every CD game, US or Japanese.

Even if you have a Turbo Duo, you should still keep the BIOS images for the Super System Card on your Turbo Everdrive.  You can load the BIOS image into the Turbo Duo to play CD games without having to remove the Everdrive.

Getting into a PC Engine Duo requires using a T10 Torx Security bit, a Turbo Duo requires a 4.5mm gamebit, which is also used with the SNES and N64 consoles and Genesis cartridges.  Inside the CD systems are five potentiometers marked VR101-105.  You can use a small screwdriver to adjust these if the drive is not spinning or CD audio is not playing.

Video Output

Unfortunately, no Turbo console outputs anything better than composite video.  Despite their limited 512-color palette, the Turbo consoles could put out some very colorful images and do not generally look their best with composite.  Fortunately, Hu6260 GPU outputs all the signals needed for RGB and S-Video.  There are many mods, and all require obtaining these signals from the chip itself or the expansion connector.  The PC Engine Duo I have been testing has an RGB mod from a Japanese seller called doujindance.  His mod is passive and very small.  The 5-pin AV DIN is replaced with an 8-pin DIN, with the extra lines being wired to R, G and B.  Sync is taken from the composite video pin.  This mod retains compatibility with existing composite video cables.  Other mods convert RGB to component video.

RGB looks quite superior to composite video, especially in games that use 320 horizontal pixels rather than the more common 256 horizontal pixels.  However, with the RGB amplifier in my system, one can see jailbars in the image that are not present through the composite output.  Jailbars are alternating patterns of light and dark across the screen and they are quite obvious in some games but not in others.  Bonk's Adventure is a game where they are immediately noticeable.

Save Game Backups

HuCards typically used passwords for saving, but a few Japanese supported the Tennoke 2 Backup Unit.  This device plugged into the back of a PC Engine's expansion port.  The Turbo Duo implements 2KB for game memory saving.  Many Turbo CD games only require a fraction of this memory, but some may require most or all of it. Multiple saves can be stored in the system, but there is no real way to transfer saves off the internal memory.  This 2KB SRAM chip is not battery back but instead is kept energized by a large capacitor (I have read the Duo-R has a lot more RAM).  The SRAM will completely drain the capacitor in approximately two weeks if the console is not turned on in that time.  NEC made the Tennokoe Hu-Card to backup saves from the unit.  This device appears to use flash memory, so you don't have to worry about the battery dying.  The current Turbo Everdrive does not support backing up the internal memory, but the next one may).

SCUMM Console Ports - Turbo Duo Loom vs. Sega CD Monkey Island

LucasArts was established by computer game programmers, but by 1990 they were becoming increasingly involved into consoles.  Having the Star Wars and Indiana Jones licenses practically demanded home console games using the licenses.  Except for the port of Maniac Mansion for the NES, LucasArts produced or licensed nothing of note until the SNES.

LucasArts was interested in porting its highly acclaimed adventure games series, using the SCUMM engine, to consoles.  Their adventure games were relatively large by cartridge standards, but the new CD-ROM add-ons for the Sega Genesis and Turbo Grafx 16 could easily contain their adventure games and allow for 16-bit CD Audio.  LucasArts commissioned a port of Loom for the Turbo Duo and a port of The Secret of Monkey Island for the Sega CD.

Loom

Loom - Turbo Super CD Title Screen

Loom for the Turbo systems was intended to run on a Turbo Duo or its equivalent : a Turbo Grafx 16 with the TurboGrafx CD and a Super System Card.  It will also run on the Japanese equivalents.

The Turbo port is  graphically in between the 16 color floppy and the other (mostly) 256-color CD-ROM versions.  If I had to give a hierarchy of ports based solely on the graphics, it would go something like this :

Loom IBM PC 16-color floppy

• Atari ST 16-color floppy
• Amiga 16-color floppy
• Macintosh 16-color floppy

Loom 256-color FM-Towns CD 

Loom 256-color IBM PC CD

Loom 16/256-color Turbo CD

The Turbo CD and FM-Towns CD have similar CD-Audio tracks, which are used for music.  The first set of FM-Towns arrangements (first nine audio tracks) are used for the Turbo CD music.  Musically, this may be the best version, as the FM-Towns also uses second set of music tracks.  The second set sounds more like a synthesizer than the first set and in my opinion is inferior.  Unfortunately, the FM-Towns version plays the first set, then the second set and keeps repeating the second set.  

Loom Turbo Super CD 16 Color Original Background

What is odd is the graphics used on Turbo Loom.  Both the FM-Towns and the IBM PC have full 256-color graphics above the distaff.  Turbo Loom uses a mixture of backgrounds (including portraits) from the 16 and 256 color IBM PC releases.

Consider the sources of the closeups :

Bobbin Threadbare - 16 Color
Hetchel - 256 Color
Elder Atropos - 256 Color
Hetchel as Cynget - 16 Color
Master Goodmold - 256 Color
Fleece Firmflanks - 16 Color
The Dragon - 16 Color
Rusty Nailbender - 256 Color
Master Stoke - 16 Color
Dragon finds Rusty - 256 Color
Bishop Mandible - 16 Color
Cobb - 256 Color
Rusty as a Ghost - 256 Color
Lady Cygna Threadbare - 16 Color
Chaos - 256 Color

Generally, most of Loom island is taken from the 16-color version, with notable exceptions of the Dock, the Loom Sanctuary and Lady Cygna's gravestone closeup.  Crystalguard is entirely 16-color.  The beach, the Shepherd's forest and meadow are taken from the 256 color version, except for the interior of Fleece's hut. The exterior of the Dragon's volcano and its lair are 256 color, as is the Forge and the plain surrounding it. Bishop Mandible's cathedral and the Outside is 16-color except for the very end of the game.

Loom Turbo CD - 256 Color Original Background

The characters are always 256 color, as are most of the animated objects.  The distaff portion of the screen is 16 color regardless of version (although the FM-Towns version has a distaff that does not use the EGA palette).

Why did LucasArts take roughly 50/50 from each version when the Turbo Duo was capable of almost 256 colors (but only a 512 color palette)?  Storage was not an issue with a CD game.  One theory is that the remaining 256 color graphics were not available in time for the Turbo port, so LucasArts had to make do with whatever 256 color art was available at the time.  I am not satisfied with this theory because the FM Towns 256 color version, done in-house by LucasArts, was ready by April, 1991.  The PC CD version was ready by June of 1992.  The Japanese PC Engine Duo version was released in September of 1992 and the US Turbo Duo version followed in December.  I can see no reason why all the 256 color artwork would not have been available for the Turbo port.

An alternative view is the 256 color graphics that they did not use either did not look very good once the color depth had been reduced or they did not look good on a composite monitor.  No NEC console, whether Japanese or US, supported better than composite video at the time Loom was being ported.  What may have looked good through an RGB monitor on the development hardware may have looked like crap on composite.

In instances where the 16 color graphics are being used, the Turbo through a composite monitor looks scarcely better than a Tandy 1000 outputting the 16 color IBM PC floppy graphics to a composite monitor. I recently made a post extolling the underrated virtues of composite video on the SNES and Genesis, but this game was designed on a PC and looks best through a lossless analog connection.  In other words, it looks best on an RGB modded Turbo Duo.  Unfortunately, no NEC console in the TurboGrafx line, not even the SuperGrafx, has native RGB support.

On the PC and other versions, the main graphics window uses 320x136 pixels. The Turbo uses 338x136, but the cursor is limited to the right-most 320 pixels.  Virtually all this horizontal resolution can amazingly be squeezed into the viewable screen of a CRT.  If a screen is non-scrolling, then there will be a noticeable black border on the left side of the screen.  The same is also true of the leftmost side of a scrolling screen.  Vertically, the game does not appear to use any of the vertical space above the main graphics window, but the cursor will descend to line 240 and perhaps line 242, which is the absolute vertical limit of 240p.

Even though the PC Engine did have a mouse available for it, Loom does not support it.  However, it does have some neat options.  It can change the text speed, it can turn the sound effect and music, or just the music, off, and also has a sound test that allows you to listen to any of the CD audio tracks.  The Overture (from Swan Lake) is not otherwise heard in the game.  It also has an option to limit the animation to improve speed, but Loom is not an animation-heavy game (The Secret of Monkey Island is much more animated), so this option would not often be useful.

The saving system is non-intuitive.  Loom will save a game to the backup RAM of the CD unit.  However, it really only saves a checkpoint, the first being when after you reach the beach leading to the Shepherd's forest and Crystalguard.  So if you save prior to leaving Loom Island, you will load back to the very beginning before you acquire your distaff.  You can lose a lot of progress this way because the save points seem quite spread out.  Button I skips cutscenes, perhaps for this reason.

The Secret of Monkey Island

The Secret of Monkey Island Sega CD

The Secret of Monkey Island for the Sega CD is based on the IBM PC CD-ROM version, reducing the on-screen number of colors from 256 colors to a maximum of 64 colors, similar to the Amiga.  The actual game's hierarchy is less complicated :

The Secret of Monkey Island 16-color floppy

• Atari ST 16-color floppy

The Secret of Monkey Island 256-color floppy

• Amiga 32-color floppy

The Secret of Monkey Island 256-color IBM PC CD-ROM

• FM-Towns 256-color CD-ROM (uses 16-color inventory icons, Japanese and English available)
• Sega CD 64-color CD-ROM
• Macintosh 256-color CD-ROM (has graphical filtering option) 

The Sega CD does not use the save RAM inside the CD unit, instead it gives a 4-digit passcode to restore a game.  It is rather amazing that LucasArts could fit all the information needed to restore a game in essentially four bytes.  This passcodes will only restore your game if you have made a substantial achievement like completing one of the three trials.  Your inventory may not be exactly as you had left it, nor may your character be where you left him and the dialogue options may be reset (which is terrible for the sword fighting trial), but you will not get stuck because you don't have an object you need.  You may have to acquire some items again unfortunately.  Button C is used to skip dialogue, and this was probably implemented for this reason.

The Secret of Monkey Island Sega CD SCUMM Bar

Interestingly, this port has support for the Sega Mega Mouse peripheral, making it function much more closely to the computer versions.  This is not mentioned in the US manual or on the US box, but there is a symbol for it on the Japanese box.  The mouse support is present on both the Japanese and US versions of the game.  The game was not released in Europe.

The gameplay itself is not too bad, but it will slow down if there is a lot of animation on the screen.  Scrolling is also jerky when there is a lot of animation on the screen.  When you are selecting dialogue, the cursor disappears, even if you are using a mouse.  The only option of is to change the text speed.  The dialogue options are sometimes redone for this version to decrease the number of dialogue choices that use a second line.

The Secret of Monkey Island Sega CD Portrait

The graphics are dark, especially the backgrounds on Melee Island.  The backgrounds are those used from the Amiga version, 256-colors reduced to 32-colors, but the Sega color palette appears much darker than the Amiga palette.  I guess LucasArts believed that people would simply turn the brightness of their TV up if they felt the screen was too dark.  On the other hand, the character, object and inventory graphics have been converted from the 256 colors of the PC CD version.  The CD audio is more or less identical to the PC CD version and plays the music as it does in the PC CD version.  One positive thing to note is that the Genesis Model 1 and 2 always support RGB output, so the graphics can look pretty nice.

The load times are rather annoying.  There are load times for every time you enter into a new screen, begin dialogue, acquire an item and other characters move on the screen.  Much of this is due to the limited RAM available for the Sega CD.  The average 1992 PC would have had 4-8MB of RAM while the Sega Genesis and CD combined have 832KB.

So, Which is Better?

This is a hard decision, because I cannot honestly recommend either port.  Both have klunky saving and loading, and both have graphical issues.  The Secret of Monkey Island is too dark (but can be improved with RGB), Loom is too inconsistent.  Audio-wise, both are excellent.  There are noticeable slowdowns early in SoMI and lots of CD load times.  It is the load times that kill the Sega CD port.  Loom is a much simpler game and was better suited to the 8/16-bit consoles of the 4th Generation.