(originally posted on Google+)

Three different models were made: the original Model 1, a redesigned unit (smaller) Model 2 and an even smaller unit, Model 3.

Most regard the first-gen Genesis as the best-of-breed. Best sound, OK image quality. 

• Model 1 had stereo output from the front-facing headphone jack, mono from RF & RCA output
• Model 2 had no headphone jack, but stereo output from RF & RCA
• Model 3 had no stereo support at all

Model 1

Model 2

Model 3

(in fact, the whole thing can be even more complicated. See http://www.sega-16.com/forum/showthread.php?7796-GUIDE-Telling-apart-good-Genesis-1s-and-Genesis-2s-from-bad-ones)

But did you know even Model 1 units could differ?

Back in 1990, Accolade released a few unlicensed games for the Genesis. Sega was not pleased about this, so they invented TMSS and put it on all new consoles they manufactured. TMSS forces the game to write the ASCII string “SEGA” to $A14000 within a short period of time, or the VDP will be deactivated. On version 0 of the console, this location is reserved, and writing here may cause a crash, so the game has to first make sure that it’s running on a console that has TMSS.

source: http://en.wikibooks.org/wiki/Genesis_Programming

So if your Sega Genesis, when booting a game, does NOT display “Produced by or under license of Sega Enterprises Ltd.” you have a non-TMSS unit. That means it’s 100 percent guaranteed to be the best audio you will get from a non-modded Sega Genesis because those early models had a dedicated Yamaha FM chip. Really wonderful stuff. 

YouTube has several videos with A/B comparisons:

Keep in mind that Model 1 units (non-TMSS or not) do not offer the best image quality — some Model 2s have superior Sony encoders — but a 32X will fix any problems (like color banding) thanks to its awesome built-in encoder.

At home, we are lucky enough to have a very early Model 1, non-TMSS unit, and I can attest for the unbelievable sound quality. It’s bassy, clear, with zero distortion & noise. Sounds great when plugged into a 5.1 receiver. It’s like a religious experience if you’re into 8-bit or 16-bit game music :)

In the future, these non-TMSS units will be highly sought after. I think ours is already worth about $40. They will go for much more in 5 to 10 years.

How about you? Are you a Sega Genesis fan? I would love to hear from you in the comments!