Hi Chris,
I usually describe technology's ever changing nature as being like a river, if your standing on the side it can look to be moving very fast... but if your in the river trying to get some where it ain't moving fast enough.

The S90 ES dates back to 2005, and things were different back then. I only say that to let you know that the situation and solution back then, is different from what is done now...

The S-Series synthesizer's from Yamaha were the same top-of-the-Line tone engine found in the Motif-Series. The difference was the Motif included the Integrated Sampling Sequencer... the ability to record both MIDI and audio right on-board (or at least it was an option with Motif once you added special memory modules, (SIMMs/DIMMs).

So your S90 ES is the sibling to the Motif ES, but without the ability to record MIDI or Audio on board.

It was Winter 2004 that Yamaha purchased Steinberg, and the S90 ES was one of the first to come with a disk for Cubase. With the expansion mLAN board you could add computer-based audio record to the S90 ES (16 audio outputs via FireWire) directly to a Windows PC or Macintosh computer.

Well that was 12 years ago. Your best bets, today, to create audio recording with your S90 ES will be
1) Build a virtual studio in your computer.
2) Get a small handheld recorder which can do a marvelous job of recording audio.

If you are making it a hobby, then it will not matter how much time and effort it takes (it's about the fun and accomplishment); if however it's just a project then you'll probably want to go the quickest easiest method. That we'll leave up to you.

I would not recommend trying to make believe it's 2005 and try to add the mLAN audio record options to your S90 ES... firstly the computer systems that ran that system have moved on from making that a viable path, second, it's just not easy to still find everything you'll need, in 2017.

A recording setup in the computer does not have to be very expensive. Software like Cubase will even let you work with your video and your music - so your Music Production reality with video is not difficult to pull off. You'll need an audio interface (method to get signal into and back out of your computer), a good pair of studio monitor speakers, and perhaps a microphone and headphones.

From there you're on your way. Contact your local Yamaha music dealer, they will be able connect you with the right gear for your budget, and you may be able to join a local user's group that is doing just what you intend - making their own music videos, etc.

Thanks for the reply!

I've got a good set of studio monitors, headphones, and the computer. I just want to clarify - when you say I can build a recording setup with Cubase and the computer, will that be essentially using the computer to record the sound coming from my studio monitors (similar to what a handheld recorder would do), or will I be able to connect the keyboard to the computer via USB for example, and send the audio signal directly to the recording software? My concern is one of sound quality - if I had a grand piano, I'd obviously be fine with recording the literal sound waves created by the piano with a decent microphone, but I wouldn't get as good of a quality recording sound waves from my studio monitors - I'd prefer the sound be transmitted "digitally" into the computer software, so it will play back from my computer to the studio monitors with as good of a quality as it does coming directly from the keyboard to the studio monitors. Forgive me if this is what you were implying - as I mentioned I basically have no idea what I'm talking about in this realm.

I'm not in any sort of a rush - this is purely hobby, so I'm willing to spend some time to get it right.

Thanks again!

I assumed that record quality was of prime concern. The computer route would require you connect the outputs of the S90 ES to an audio interface, which as mentioned would be responsible for getting the audio signal from the S90 ES into the computer, and to get audio from your computer to your Monitor speakers. The audio interface will be doing the "heavy lifting" which here is taking in analog signal and converting it to high resolution audio - 44.1kHz is considered CD-quality, but as your read, you can record at sample resolutions up to 192kHz for a modest investment.

The "only way" to record your the computer will be digitally, and your results will be stellar. The audio interface will act like a direct box taking the signal and transferring it to pristine digital signal... you speakers will be used to monitor your results.

2005 is several years before Yamaha started building USB audio interfaces into the synthesizer's (2011). So you will need to have a way to get audio into the computer... examples of audio interfaces: USB Audio Interfaces