Reverse engineering the Sega Channel game image file format | Hacker News Reader

Reverse engineering the Sega Channel game image file format

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This is not a dissimilar system to Teletext[1], which transmitted data in the blanking interval of a broadcast TV signal, and could be interpreted by a TV or other hardware with appropriate support. Teletext was pretty widespread throughout Europe in the 1980s and 1990s.

It was typically used to transmit pages of information (news, weather, etc.) that could be viewed directly on the TV, but the BBC's Ceefax[2] Teletext service was also used to distribute software to the BBC Micro, when equipped with the appropriate Teletext Adapter[3].

In a similar fashion to the Sega Channel system, the Teletext system would broadcast looped data, with popular pages (such as news and weather) being repeated frequently so they would load quickly, and less popular pages taking longer to load (or more accurately, to wait for the next time they appeared in the looped data).

I was interested to see that the Sega system used a bitrate of 8Mbps, which sounded pretty high for the mid-90s, but I see that Teletext had a bitrate of almost 7Mbps for PAL broadcasts, despite being roughly 15 years older!

[1] https://en.wikipedia.org/wiki/Teletext

[2] https://en.wikipedia.org/wiki/Ceefax

[3] https://en.wikipedia.org/wiki/BBC_Micro_expansion_unit#Telet...

Teletext had an instantaneous bitrate of 6.9Mbit, but it was only active for a few lines per frame during vertical blanking.

The continuous bandwidth of teletext was only 324 Kbit.

The main innovation of Sega Channel (and similar approaches from about the same time) was allocating a whole TV channel exclusively to data.

You got me interested in how the signal was transmitted, so I looked a bit more into it (see https://segaretro.org/File:SegaChannel_Applications_Scientif... ). It turns out that the 8Mbps number I eyeballed from looking at newspaper coverage of the service was incorrect. When the cable provider received the Sega Channel data stream, they'd split it into two 6Mbps carriers. This allowed them to transmit Sega Channel data without having to dedicate a channel to data, as they could put the carriers between cable channels or in the portion of the spectrum used for cable FM radio. I updated the webpage with the corrected figures.

Yeah, I was wondering how TV-like this channel looked.

It's tempting to wrap it in fake horizontal/vertical blanking so it still looks like a TV signal (and you can send it through existing equipment that's expecting a TV signal). Essentially just Teletext but using every single line.

But what Scientific Atlanta created is much closer to cable internet. The total bandwidth number is notable, each 6Mbit carrier uses 3Mhz of bandwidth, so the two of them add up to 6Mhz, which is how much bandwidth a standard NTSC channel occupies.

I suspect this is because they have rented a single TV channel worth of bandwidth on the Galaxy 7 satellite for disruption to local cable companies.

Splitting into two 3Mhz carriers has the additional advantage of allowing the receiver design to be simpler, it only needs to tune into one at any time.

So my question is - knowing that and looking at the marketing literature, is it possible to somehow recreate the signal to use on actual hardware.

It's possible to RF modulate composite sources over coax for home cable systems (think blonder tongue gear or even consumer gear) - since it is combined to the 6Mhz signal somewhere along the line could one not pipe this signal into a coax cable and then into the hardware to recreate it?

Am looking at setting up my own home analog CATV system and this would be the cherry on the cake. I guess the real question is what was the device expecting in that signal - what was being modulated out over the wire.

Teletext is still very much alive in Germany, pretty much every channel offers it. In fact, I use it most days to quickly check if there are any interesting headlines to follow up online, or for sports results. It's funny that it is frequently faster and easier to navigate than most enshittified news websites.

Same in the Netherlands, but it's updated to the modern age with a website / emulator [0] and app [1], both appropriately styled.

[0] https://nos.nl/teletekst

[1] https://apps.apple.com/nl/app/nos-teletekst/id289246732

It was even crazier in Germany! In 2000, the television station NBC received a radio license for RadioMP3. They broadcasted the charts and entire albums (with covers) via teletext, which could be legally recorded at home. Bit rate 128 kbit/s - simply with a TV capture card. The public broadcaster also transmitted software via “VideoDAT” during its ComputerClub program. However, this required special hardware.

Same in Finland. There is a one guy running the show nowadays. https://yle.fi/a/3-12131551 (article only in Finnish)

I have seen more than one old geezer in NS trains reading the news on the Teletext app on the smartphone :)

Maybe they're into something?

At a company I used to work at there was a service that scraped the teletext XML to get some numbers as a second source to double check what was scraped off a website.

> I was interested to see that the Sega system used a bitrate of 8Mbps, which sounded pretty high for the mid-90s,

This was over cable TV, so not very difficult to obtain these rates compared to general broadcast TV. Cable internet service rolled out in this time period with downstream rates of 40 Mbps per 6 MHz channel.

Thank you for this! You just unlocked some old memories of “Digitiser” which led me here:

https://teletextart.co.uk/artists/paul-rose-mr-biffo

I don't know how I convinced my parents to get me this in elementary but not a single other person I ever met had heard of it. I had no idea it had interesting tech, definitely didn't know it used satellites.

It was a really unreliable service, at least for me in Virginia. We had to call support all the time the short time I had it (1-3 months).

``` They would then send everything to a company called Foley Hi-Tech, who would create the game menu graphics/animations and insert all the monthly content. They ended up with a ~60MB file called a "game image", which was burnt to a CD and sent to a satellite uplink facility in Denver, Colorado. The CD would then be installed in the uplink game server computer, which would continuously transmit the game data in a loop over satellite. Cable headends all over the US would receive the satellite transmission and send it to cable subscribers. The data being sent in a continuous loop is how the service's "interactivity" was achieved at a time when cable TV providers could only transmit data to all subscribers and couldn't receive data (i.e. what game a given subscriber wants to download) ```

> I don't know how I convinced my parents to get me this in elementary but not a single other person I ever met had heard of it.

It was an interesting almost niche thing. My cousins had a unit and the service which I played all the time, but trying to describe it to other children at school years after the fact was difficult because nobody had ever heard of it. Honestly it was a toss-up if they even knew what a Genesis was at that point in life. I didn’t even know what a Sega Channel was called until I had finally found a solid description online with an accompanying picture at some point in high school.

I vividly remember playing Genesis in my living room as a kid when my parents were having friends over from out of town. My dad and his friend came in after a couple beers and my dad explained what I was playing. His friend knew all about Genesis already: "We've got the Sega Channel. I play 50 games a year!" I remember being very fascinated until he started laughing, which to me signaled that he was pulling my leg. I spent the next 30 or so years thinking the "Sega Channel" was a dumb joke from an inebriated friend of my dad's. Until today.

Looking back, maybe my dad nudged him to change the subject so that I wouldn't spend the next month begging for a subscription.

I knew of no one that had Sega Channel and I only knew of its existence because I had a subscription to Sega Power around the time of its launch which was covered in the magazine.

The article claims 250,000 subscribers at its peak but it might as well have been zero on my little world. It's interesting how a service could have a quarter million subscribers and you could have basically no proof that anyone used it at all. That would be unheard of today. If Sega Channel launched today. Those 250k subscribers would be posting on social media, creating dedicated subreddits, uploading review videos to YouTube and live streaming on twitch. Anyone with even a passing interest in it would encounter the content of its users. The internet has really made the world smaller.

Using satellites to transmit the game data wasn't TOO exotic, I imagine they did it because all the cable headends were already set up to receive national channels carried over satellite. Having centralized control of the game server also let them switch out the game images for timed events such as competitions. For regions outside the US, Sega went the simpler route of putting game servers at each headend and mailing out CDs each month.

It makes sense that it was unreliable in certain areas, cable TV was analog back then so cable providers didn't really have to worry about signal noise being a serious problem until Sega Channel came along.

I remember Sega Channel. In 6th grade, my friend had it. I didn't grow up with BBSes and had only learned about the Internet the year before (from the same friend), so the idea of downloading games was pretty wild. The service was way ahead of its time.

IIRC, it was $15/mo. There was a monthly or weekly rotation of games, including some pre-releases. I think at one point we played Vectorman when it was still new in stores or possibly just prior to its official launch.

I've been following the Billy time games release of different sega channel stuff. I remember a friend having it through TWC when it was active and thought it was really cool.

I hope more information comes out about the system setup specifically (like how out of band signaling was used if at all) so the service might actually be able to be recreated ala the way dreampi has allowed webtv to be "reactivated"

Had this back in the 90s for a month or two. It was amazing. A bit finicky. Our neighbors split out cable outside and it stopped working. Apparently needed a dedicated line.

A lot of cable plant maintenance got its hand 'forced' with the introduction of Sega Channel and other interactive services.

Who woulda thunk that a little blue hedgehog paved the way for cable Internet? ;)

Apparently the hardware had 4MB of RAM in case anyone else was curious.

4 MB is the maximum size of the console's ROM address space, so this would've allowed for any Genesis game to be published through this service, with the exception of the few games that used bank switching to go beyond 4 MB (really only Super Street Fighter II). 4 MB was a lot of RAM to be putting into a relatively cheap consumer electronics device in 1994, so I imagine some of the cost of that was subsidized by Sega and had to be covered by the subscription fees.

Oh that’s pretty big.

Obviously full sized games couldn’t fit in the built in 64k of normal RAM, so they had to be put somewhere.

I only got to use Sega Channel once. I knew about it from reading magazines, but when we went to a family Thanksgiving gathering it turned out my cousins had it! I don’t know if they were a test market or not, I don’t remember what year it was so I don’t know if it had been deployed nationally yet.

I don’t know if I realized how it worked. I think cable modems may have started to exist by that point so at the time I probably assumed it was actually two way communication.

I would’ve killed to have it at home. Sega Channel is the first place I ever played Sonic 3-D Blast, the two have always been linked in my memory because of that.

Curious if you could have recorded say, 5 minutes of the channel and they played it back when you wanted to load a game. Assuming new games every month, you could fit a years worth of games in a VHS?

You wouldn't have been able to capture the signal with a VCR as it wouldn't have made it to your TV. You would have had to somehow record the modulated cable signal before it went into your cable box.

If it's anything like teletext, normal VHS wouldn't have the bandwidth, you might have some success with SVHS, although unlike teletext it might not be framed as a normal TV signal so the general tuner / recording circuitry might also have problems.

No, because it wasn't a true channel. It took two 6mhz "channels" on the cable system in that it occupied two 6mhz wide spots whose frequencies are in the cable TV bandplan, but the actual modulation of the RF would not have been recognizable as anything other than noise to NTSC tuners

Not on a regular VHS cassette because it is transmitting data even during the vertical blanking periods. If you had a way to capture the channel signal raw it might work, but that would have been some expensive hardware in 1995.

SNES also had a similar service called StellaView that was only available in Japan. They had live farro and timed legend of Zelda 1 completions iirc.

I used to play some of the roms of these games. The one of Legend of Zelda was pretty slick, redone with A Link to the Past graphics.

https://en.m.wikipedia.org/wiki/Satellaview_games_from_The_L...

This seems to have a lot in common with how the Nabu computers worked, especially the continuous loop of programming to get around the fact that you couldn't upload data back to the cable company. Really cool!

Cool! My dad worked on Sega Channel at General Instrument but we never actually had access to it because ironically our home's cable provider was terrible and didn't offer it.

Nice, I only focused on the Scientific Atlanta version in the article because the game image CD that got uploaded was for that version of the service. From looking at the data on the dev tooling CD, it looks like the General Instrument game CDs just contained a passworded ZIP file with the game ROM files inside. I guess all the processing, scrambling, etc was done on the game server as the data was loaded from the CD to the server's hard drive.

I didn't even know this service existed. Broadcasting games in a loop seems pretty clever, but I suppose ZX Spectrum already had games delivered over radio years before then.

It’s interesting as a technical hack more as the way they figured out to get it over cable before DOCIS and 2-way communication.

Makes a lot of sense. A TV channel is just a stream of data, so just stream all the data you might need over and over and over.

> figured out ... DOCSIS and 2-way communication.

It's not even so much 2 way communication in the local cable system as the fact that cable TV companies were not originally ISPs. Sega Channel was a broadcast over C band no-return-path satellite, it didn't have any Internet related moving parts.

Is it really a hack? Teletext also worked like this, and this service does not need interactivity, so it doesn't need 2-way communication.

The system I'd love to have a better understanding of is the one TiVo deployed for the transmission of data to their boxes in the early 2000s.

They would buy a 30 minute paid programming in the early hours on cable TV networks under the name "Teleworld Paid Programming". The box would tune the channel, record the show, and then decode the data from the recording.

I always thought there was something interesting there, as the process would need to survive the MPEG encode/decode process on the TiVo itself in addition to whatever they needed to do the broadcast.

https://www.youtube.com/watch?v=VfUgT2YoPzI

At one point Teletext on some channels had a two way system of sorts - a chat service where people could send messages over SMS to a paid for service, which would then be broadcast over teletext.

DOCSIS was unrelated, but was absolutely helped by Sega Channel. Sending data down the wire required cleaning up the signal sent out from the cable plant (and into the sega channel unit) a ton relative to analog cable at the time

I don't think of it as a hack any more than Sirius or XM making a "control plane" signal by just interleaving the what's on or channel map in with the music signal.

I remember the TV commercial, but until now I had completely forgotten about it....

PKSPREAD related to PKZIP by Phil Katz somehow?

Given the text:

> Thanks for choosing SA-Ware for your packet spreading requirements!

It's probably short for "packet spreading"

Ah makes sense. I missed that part!

Had this as a kid, for the most part it was great! I remember Vectorman being one game that never seemed to work well.

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